Exploring Noncovalent and Reversible Covalent Interactions As Tools for Developing New Reactions

Exploring Noncovalent and Reversible Covalent Interactions as Tools for Developing New Reactions

Corey Allen McClary

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Chemistry University of Toronto

Exploring Noncovalent and Reversible Covalent Interactions as Tools for Developing New Reactions

Corey Allen McClary

Doctor of Philosophy

Department of Chemistry University of Toronto

2014 Abstract

Noncovalent and reversible covalent interactions have long been exploited in catalysis and supramolecular chemistry. Examples of such noncovalent interactions include hydrogen bonding, halogen bonding and CH-π and π-π interactions. Reversible covalent interactions that have been employed towards these ends comprise the formation of imines, acetals, ketals and boronate esters. This thesis describes the investigation of various noncovalent and reversible covalent interactions, and their possible applications in catalysis and novel reaction development.

Chapter 1 describes the investigation of anion receptors composed of hydrogen- and halogen- bond donor groups. Binding studies of these molecules have indicated that they are capable of interacting with an anion simultaneously through hydrogen and halogen bonding. Receptor design was found to have a profound effect on the strength of the halogen bonding interaction.

Receptors containing halogen-bond donors showed selectivity for halide anions over oxyanions.

In Chapter 2, potential halogen bonding catalysts were synthesized and screened in a series of reactions. Incorporating halogen-bond donors into the catalysts appeared to have no beneficial effect in terms of reactivity. Explanations for these observations are discussed along with suggestions for designing future catalysts that could exploit halogen bonding interactions. ii

Chapter 3 discusses attempts to use hydrogen-bond donor catalysts to effect catalyst-controlled stereoselective additions to 2-nitroglycals. While stereoselective additions were observed in some cases, they were not catalyst-controlled. The results from these experiments suggested that catalysts and reactions developed for simple nitroalkenes could not be easily adapted to 2- nitroglycal substrates.

A review of interactions between boron containing compounds and saccharides is presented in

Chapter 4. Their applications in drug delivery systems, cellular imaging and the sensing and separation of carbohydrates are discussed, in addition to their uses as protecting and activating groups in oligosaccharide synthesis.

Finally in Chapter 5, the development of a regioselective boronic acid-mediated glycosylation reaction is described. This methodology was applied in the synthesis of two key intermediates used in the synthesis of a pentasaccharide derivative isolated from the plant Spergularia ramosa.

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Acknowledgments

Throughout my five years at the University of Toronto, I have been fortunate to have met many wonderful mentors, colleagues and friends. I would like to take this time to thank them for their support and guidance. First, I would like to thank my supervisor, Professor Mark S. Taylor, for the opportunities he has given me while working in his lab. He is an incredible scientist and teacher, and I am grateful to have had him as a supervisor. The research presented in this thesis and the writing of this document would not have been possible without his encouragement and thoughtful comments and suggestions.

I would also like to thank all of the Taylor group members; Doris Lee, Ali Rostami, Golam Sarwar, Sunny Lai, Lee Salsberg, Josh Dubland, Alice Wei, Olga Kvak, Jenny Diep, Caitlin Williamson, Christina Gouliaras, Lina Chan, Stefi Anthonipillai, Michael Chudzinski, Graham Garrett, Thomas Beale, Ross Mancini, Sanjay Manhas, John Su and Alan Vanderkooy for making working in the lab so much fun. A special thanks goes out to Elena Dimitrijević for always being available to talk, whether it was about life or ANTM.

Thank you to Zamin Kanji, Asim Ashraf, Jaishal Kotak and Jeff Geddes for your close friendships. My fondest memories of Toronto have been made in your company! You've certainly made my time here enjoyable. Thank you Zam for always being there for me and for your indispensable advice. You are the big brother I never had.

Finally, I would like to acknowledge the support of my family. My parents have always been so encouraging of my academic endeavors, regardless of how long they took!

Table of Contents

Acknowledgments ...... iv

Table of Contents ...... v

List of Tables ...... viii

List of Figures ...... xi

List of Abbreviations ...... xix

Chapter 1 Anion Receptors Composed of Hydrogen- and Halogen-Bond Donor Groups: Modulating Selectivity With Combinations of Distinct Noncovalent Interactions ...... 1

1.1 Introduction ...... 1

1.2 A brief history of halogen bonding ...... 2

1.3 The Nature of the Halogen Bond ...... 4

1.4 Halogen Bonding in Solution: Inorganic Donors ...... 7

1.5 Halogen Bonding in Solution: Organic Donors ...... 9

1.6 Halogen Bonding in Solution: Anion Recognition ...... 14

1.7 Halogen Bonding in Medicinal Chemistry ...... 20

1.8 Results and Discussion ...... 24

1.9 Initial Receptor Design ...... 25

1.10 Receptors Bearing a Single Halogen-Bond Donor ...... 27

1.11 Receptors Having Two Halogen-Bond Donor Groups ...... 35

1.12 Anion-Arene Interactions in Perfluorinated Receptors ...... 42

1.13 Conclusions ...... 44

1.14 Advances in Halogen Bonding ...... 45

1.15 Experimental Details ...... 47

Chapter 2 Exploration of Halogen-Bond Donors as Catalysts ...... 56

2.1 Introduction ...... 56 v

2.2 Inspiration from Hydrogen Bonding Catalysis ...... 56

2.3 Halogen Bonding Catalysis and Activation ...... 63

2.4 Results and Discussion ...... 69

2.5 Freidel-Crafts Addition of Indoles to Nitroolefins ...... 70

2.6 Hetero-Diels-Alder Reaction ...... 74

2.7 Reduction of Aldimines ...... 76

2.8 N-Acyl Pictet-Spengler Reaction ...... 80

2.9 Advances in Halogen Bonding Catalysis and Activation ...... 82

2.10 Conclusions ...... 84

2.11 Experimental Details ...... 86

Chapter 3 Catalyst Controlled Additions to 2-Nitroglycals ...... 97

3.1 Introduction ...... 97

3.2 Reactions of 2-Nitroglycals ...... 101

3.3 Results and Discussion ...... 104

3.4 Conclusions ...... 111

3.5 Experimental Details ...... 111

Chapter 4 Applications of Organoboron Compounds in Carbohydrate Chemistry ...... 116

4.1 Introduction ...... 116

4.2 Carbohydrate Sensors based on Organoboron Compounds ...... 118

4.3 Application of Boronic Acid-Carbohydrate Interactions in Drug Delivery and Cellular Imaging ...... 124

4.4 Carbohydrate Separation Using Organoboron Compounds ...... 128

4.5 Boronic Acids as Protective Groups for Carbohydrates ...... 132

4.6 Activation of Carbohydrate OH Groups ...... 137

4.7 Conclusions and Outlook ...... 142

Chapter 5 Regioselective Boronic Acid Mediated Glycosylations ...... 144

5.1 Introduction ...... 144

5.2 Traditional Oligosaccharide Synthesis ...... 146

5.3 One-Pot Oligosaccharide Synthesis ...... 151

5.4 Enzymatic Methods ...... 155

5.5 Tin- and Boron-Mediated Approaches ...... 158

5.6 Results and Discussion ...... 159

5.7 Development of the Boronic Acid Mediated Regioselective Glycosylation Reaction ... 165

5.8 Application of the Boronic Acid-Mediated Regioselective Glycosylation Reaction in Difficult Glycosylations ...... 182

5.9 Conclusions ...... 193

5.10 Experimental Details ...... 195

Chapter 6 Final Thoughts ...... 209

Appendix A: 1H, 19F and UV/Vis Titration Data...... 210

Appendix B: 1H, 19F and 13C NMR Spectra...... 255

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List of Tables

Table 1.01. Binding constants of 1.02, 1.03, and 1.04 with Bu4NX (acetone, 295 K). 18

Table 1.02. Associations constants for pseudorotaxane formation between triazolium 19 salts and 1.06 in CDCl3 at 293 K.

Table 1.03. Activity data for hCatL inhibitors. 22

Table 1.04. Association constants (Ka) of receptors 1.14a-1.16a and 1.14b-1.16b with 32 halide anions and the calculated free energy contributions of the halogen bonding interaction (ΔΔGXB).

Table 1.05. Association constants (Ka) of receptors 1.19a and 1.19b with halides and 36 oxoanions and the calculated free energy contributions of the halogen bonding interaction (ΔΔGXB).

Table 1.06. Association constants (Ka) of receptors 1.22a and 1.22b with halides and 41 oxoanions and the calculated free energy contributions of the halogen bonding interaction (ΔΔGXB).

Table 1.07. Association constants (Ka) of receptors 1.19b and 1.23 with halides and 43 oxoanions and the calculated free energy contributions of the arene-anion interaction (ΔΔGAA).

Table 2.01. Rates of HDA reactions in different deuterated solvents. 59

Table 2.02. Reaction conditions for the reduction of 2-phenylquinoline. 65

Table 2.03. Substituent effects on the isomerization of 5-AND by ketosteroid isomerase. 67

Table 2.04. Catalyst and solvent screen for the Freidel-Crafts addition of 1-methylindole 71 to trans-β-nitrostyrene.

Table 2.05. Catalyst and solvent screen for the Freidel-Crafts addition of indole to trans- 72 β-nitrostyrene.

Table 2.06. Solvent screen for the hetero-Diels-Alder reaction. 75

Table 2.07. Catalyst screen for the hetero-Diels-Alder reaction. 76

Table 2.08. Solvent screen for the reduction of aldimines. 77

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Table 2.09. Basic additives used in the reduction of aldimines with Hantzsch ester. 80

Table 2.10. Summary of results for the N-acyl Pictet-Spengler reaction. 81

Table 2.11. Reaction of 1-chloroisochroman 2.41 with ketene silyl acetal 2.42 in the 84 presence of various catalysts.

Table 3.01. Addition of nucleophiles to 2-nitrogalactal. 106

Table 3.02. Addition of nucleophiles to 2-nitroglucal. 107

Table 3.03. Additions of aldehydes and ketones to nitroglycals. 110

Table 5.01. Initial boronic acid and solvent screen for the boronic acid-mediated 166 regioselective glycosylation reaction.

Table 5.02. Solvent screen for the boronic acid-mediated regioselective glycosylation 167 reaction.

Table 5.03. Lewis base screen for the boronic acid-mediated regioselective glycosylation 169 reaction.

Table 5.04. Isolated yields of the boronic acid-mediated regioselective glycosylation 170 reaction using 4 Ǻ M.S.

Table 5.05. Boronic acid screen using optimized conditions for the regioselective 171 glycosylation reaction.

Table 5.06. Identification of optimum reaction conditions for the boronic acid-mediated 172 regioselective glycosylation reaction.

Table 5.07. Development of a catalytic variant of the boronic acid-mediated 173 regioselective glycosylation reaction.

Table 5.08. Boronic acid screen for the regioselective glycosylation reaction to 175 synthesize 5.60.

Table 5.09. Optimization of the boronic acid-mediated regioselective glycosylation 183 between acceptor 5.69 and donor 5.70.

Table 5.10. Boronic acid-mediated regioselective glycosylation of disaccharide 5.76 with 186 donor 5.70.

Table 5.11. Boronic acid- mediated regeioselective glycosylation of disaccharide 5.77 187 with donor 5.79.

Table 5.12. Boronic acid-mediated glycosylation of 5.85 with perbenzylated chloride 191 donor 5.86.

List of Figures

Figure 1.01. Halogen bonding in bromine–dioxane complexes and in bromine–acetone 3 complexes.

Figure 1.02. Depiction of hydrogen and halogen bonding interactions. 4

Figure 1.03. Calculated molecular electrostatic potential energy surfaces of C6F5I. Blue 5 regions indicate areas of partial positive charge. The σ-hole can be seen on the iodine atom.

Figure 1.04. Calculated molecular electrostatic potential surfaces (B3LYP/6-31+G**- 6 LANLdp) of (from left to right) CF4, CF3Cl, CF3Br, and CF3I showing the increase in the size of the σ-hole as a function of halogen atom. Blue coloring represents areas of electron deficiency and red coloring indicates areas of electron density.

Figure 1.05. Representative data for association constants between various Lewis bases 8 and diiodine.

Figure 1.06. Trends in pKBIX for selected Lewis bases. 9

Figure 1.07. Chemical shift differences (Δδ of the fluorine atoms highlighted in bold) of 11 1,6-diiodoperfluorohexane, 1,6-dibromoperfluorohexane, 1,6-dichloroperfluorohexane and perfluorohexane between n-pentane and piperidine.

Figure 1.08. Interactions of iodoperfluorohexane and Lewis bases examined to test the 12 Hunter model. S represents solvent while A represents the halogen-bond acceptor.

Figure 1.09. Interactions of para-substituted perfluoroarenes with tributylphosphine 13 oxide.

Figure 1.10. Heteroditopic receptor for alkali metal halides. 16

Figure 1.11. Structures of halogen-bond receptors 1.02–1.04. 17

Figure 1.12. Halogen-bond halide anion-templated psedorotaxane formation. 18

Figure 1.13. Rotaxane having both halogen- and hydrogen-bond donors. 19

Figure 1.14. Diagram of inhibitor binding to the three pockets of the active site of hCatL. 21 The X represents a substituent that has been varied from H, F, Cl, Br, or I.

Figure 1.15. Diagram of observed perpendicularity between hydrogen and halogen 23 bonds made to the same carbonyl oxygen of a peptide.

Figure 1.16. Design elements of receptors containing hydrogen- and halogen-bond 24 donors and control receptors.

Figure 1.17. Synthesis of 1-(2-hydroxyethyl)-3-(4-nitrophenyl)urea (1.10) and 1-(2- 25 (hydroxymethyl)phenyl)-3-(4-nitrophenyl)thiourea (1.13).

Figure 1.18. Acylation of 1-(2-hydroxyethyl)-3-(4-nitrophenyl)urea (1.10) and 26 attempted acylation of 1-(2-(hydroxymethyl)phenyl)-3-(4-nitrophenyl)thiourea (1.13).

Figure 1.19. Additional receptors bearing single halogen-bond donor groups. 27

Figure 1.20. (a) UV-vis absorption spectra of 1.14a upon addition of Bu4NCl 29 (acetonitrile); (b) change in absorbance (λ = 362 nm) of a solution of 1.14a as a function 19 of [Bu4NCl]. Curve represents the equation of best fit for a 1:1 binding model.; (c) F NMR Job plot for the 1.14a-Cl– complex.

Figure 1.21. Behavior of proton chemical shift of 1.14a upon titration with [Bu4NBr] in 30 acetonitrile-d3 at 295 K. [1.14a] = 2 mM.

Figure 1.22. Behavior of fluorine chemical shift of 1.14a upon titration with [Bu4NBr] in 31 acetonitrile-d3 at 295 K. [1.14a] = 2 mM.

Figure 1.23. Structures of the 1.15a'-Cl–, 1.14a'-Cl–, and 1.16a'-Cl– complexes calculated 33 by DFT (B3LYP/6-31++G(d,p)-LANL2DZdp, gas phase).

Figure 1.24. Synthesis of receptors 1.19a and 1.19b. 35

Figure 1.25. (a) Plot of –∆Gbinding of 1.19a against –∆Gbinding of 1.19b for the series of 39 anions tested. The dotted line corresponds to y = x. (b) Column graph showing the differences in anion selectivity between receptors 1.19a and 1.19b.

Figure 1.26. Structures of symmetrical receptors 1.22a and 1.22b. 40

Figure 1.27. Plot of -∆Gbinding of 1.22a against -∆Gbinding of 1.22b for the series of anions 42 tested. The dotted line corresponds to y = x.

Figure 1.28. Structure of receptor 1.23. 43

Figure 1.29. Ditopic ion transport systems. 46

-1 + Figure 1.30. Association constants, Ka (M ) between 1.25a and 1.25b and Bu4N salts at 47 300 K in DMSO-d6.

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Figure 2.01. Friedel-Crafts alkylation of heteroaromatics with nitroolefins. 58

Figure 2.02. Enantioselective Friedel-Crafts addition of indoles to nitroolefins. 58

Figure 2.03. Hetero-Diels-Alder reactions catalyzed by chiral hydrogen-bond donor 60 catalysts.

Figure 2.04. Reduction of aldimines using diaryl thioureas. 61

Figure 2.05. Acyl-Pictet-Spengler reaction. 61

Figure 2.06. Acyl-Pictet-Spengler reaction catalyzed by chiral thioureas. 62

Figure 2.07. Proposed reaction mechanism of Pictet-Spengler-type cyclizations of 63 hydroxylactams.

Figure 2.08. Iodine-catalyzed Michael reaction of indoles and unsaturated carbonyl 64 compounds.

Figure 2.09. Reduction of 2-phenylquinoline in the presence of haloperfluoroalkanes. 64

Figure 2.10. Conversion of 5-androstene-3,17-dione to 4-androstene-3,17-dione by 66 ketosteroid isomerase.

Figure 2.11. Ritter reaction promoted by haloimidazoliums. 68

Figure 2.12. Organocatalytic aziridine synthesis using F+ salts. 68

Figure 2.13. Potential halogen-bond donor catalysts. 70

Figure 2.14. Hypotheses for the enhanced electrophilicity of the nitroolefins upon 73 interaction with catalysts. (A = urea catalysts, B = sulfonamide catalyst, C = XB catalyst)

Figure 2.15. Basic additives used in the reduction of aldimines with Hantzsch ester. 79

Figure 2.16. Organocatalysis by neutral multidentate halogen-bond donors. 83

Figure 3.01. Enantioselective Michael reaction of malonates with nitroolefins catalyzed 96 by bifunctional thiourea catalyst 3.01.

Figure 3.02. Proline-catalyzed addition of ketones to nitroalkenes. 98

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Figure 3.03. Additions of aldehydes to nitroolefins. 98

Figure 3.04. General scheme for the catalyst-controlled addition to 2-nitroglycals. 98

Figure 3.05. Enantioselective addition of a silyl ketene acetal to 1-chloroisochroman. 99

Figure 3.06. Glycosylation catalyzed by a chiral BINOL-derived phosphoric acid. 100

Figure 3.07. Synthesis of the peracetylated TN antigen. 101

Figure 3.08. Additions of alcohols to 2-nitrogalactal. 102

Figure 3.09. Possible transition states for the addition of alcohols to 2-nitrogalactal. 103

Figure 3.10. Stereoselective Michael-type additions to 2-nitroglycals employing DMAP 103 or PPY.

Figure 3.11. Additions of phenols and malonates to 2-nitroglycals. 104

Figure 3.12. Synthesis of 2-nitrogalactal (3.04). 104

Figure 3.13. Synthesis of 2-nitroglucal (3.08). 105

Figure 3.14. Synthesis of Takemoto's catalyst. 105

Figure 3.15. Resubjecting 3.26α to the standard reaction conditions. 108

Figure 3.16. Resubjecting 3.26α to the standard reaction conditions. 109

Figure 4.01. Key interactions of boronic acids with carbohydrates. 116

Figure 4.02. Anthrylboronic acid sensor for polyols developed by Czarnik and Yoon. 117

Figure 4.03. Introduction of amines into chemosensors for carbohydrates. 118

Figure 4.04. Chiral fluorescent sensors for sugars and sugar derivatives. 119

Figure 4.05. Structures of two representative Ginsenosides. 120

Figure 4.06. Structures of bis-boronic acid receptor 4.04. 121

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Figure 4.07. Boronic acid-based "molecular tweezer" sensor. 121

Figure 4.08. Interactions of borogel with ARS and analytes in an indicator displacement 122 assay.

Figure 4.09. Structure of the Sialyl Lewis X and Sialyl Lewis Y tetrasaccharides. 123

Figure 4.10. Structurse of the bis-boronic acid sensors for sLeX . 124

Figure 4.11. Boronolectin used for the histological analysis of cancer tissue. 125

Figure 4.12. Glucose-dependent equilibria of phenylboronic acid containing polymers. 126

Figure 4.13. Monomer composition of the hydrogel. 127

Figure 4.14. Structure of protected methylacrylamidophenylboronic acid incoporated 128 into the polyacrylamide gels.

Figure 4.15. FACE (left) and BASE (right) separation profiles of AMAC-labeled mono 129 and disaccharides. Lane 1: saccharide mixture; Lane 2: lactose; Lane 3: galactose; Lane 4: N-acetyl glucosamine; Lane 5: Melibiose; Lane 6: Glucose.

Figure 4.16. Glycosylation of polystyrylboronic acid-bound acceptor. 132

Figure 4.17. Boons' polymer-supported oligosaccharide synthesis by a loading-release- 133 reloading strategy.

Figure 4.18. Solid-phase synthesis of -mannosides. 134

Figure 4.19. Regioselective glycosylation of fully unprotected methyl hexopyranosides 135 by transient masking with an arylboronic acid.

Figure 4.20. Regioselective alkylation of methyl fucopyranoside through complexation- 136 induced activation.

Figure 4.21. Selective glycosylation promoted by an internally coordinated organoboron 137 compound.

Figure 4.22. Monofunctionalizations of methyl-α,D-mannopyranoside using borinic ester 138 precatalyst 4.43.

Figure 4.23. Proposed catalytic cycle for diol activation with precatalyst 4.43. 139

Figure 4.24. Selected substrate scope for borinic acid-catalyzed regioselective 140 glycosylation of carbohydrates. xv

Figure 4.25. Synthesis of cardiac glycoside analogs by catalyst-controlled, regioselective 141 glycosylation of digitoxin.

Figure 5.01. Complex oligosaccharide targets Globo H hexasaccharide (5.01) and fucose 144 GM1 (5.02).

Figure 5.02. Typical chemical glycoside synthesis. 145

Figure 5.03. General scheme for glycosylation with non-participating groups at C-2. 146

Figure 5.04. General scheme for glycosylation involving neighboring group 146 participation. Figure 5.05. Alternative strategies for controlling stereochemistry. (a) Crich's sulfoxide method, (b) a tethering approach, (c) expoxidation of a glycal followed by ring opening. 147 Figure 5.06. Synthesis of the disaccharide fragment of a pentasaccharide derivative from Spergularia ramosa. 148 Figure 5.07. Synthesis of the trisaccharide fragment of the pentasaccharide derivative and final glycosylation. 149

Figure 5.08. Armed/Disarmed effects in glycosyl donors. 150

Figure 5.09. Reactivity-based one-pot synthesis of a tetrasaccharide. 152

Figure 5.10. Huang's one-pot synthesis of the Globo-H antigen (5.33). 153

Figure 5.11. Glycosylation using an engineered OleD mutant. 154

Figure 5.12. Enzymatic synthesis of blood group B antigen. 155

Figure 5.13. Mechanism of transglycosylation catalyzed by Agrobacterium sp. β- 156 glucosidase.

Figure 5.14. Stannylene activation method. 157

Figure 5.15. Regioselectivity of tin-mediated glycosylation. 157

Figure 5.16. Typical glycosylation reactions for the regioselective tin-mediated 158 approach.

Figure 5.17. Pentasaccharide derived target of the Du synthesis (5.21) and the proposed 159 deprotected pentasaccharide derivative in our synthesis (5.41).

Figure 5.18. Proposed retrosynthesis of the pentasaccharide derivative 5.41. 160

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Figure 5.19. Synthesis of acceptor disaccharide 5.54 using the borinic acid-catalyzed 162 regioselective glycosylation conditions.

Figure 5.20. Synthesis of disaccharide 5.57 using the borinic acid-catalyzed 163 regioselective glycosylation conditions. Reactions carried out by Stefi Anthonipillai.

Figure 5.21. Proposal for the boronic acid mediated regioselective glycosylation 164 reaction.

Figure 5.22. Initial boronic acid and solvent screen for the boronic acid-mediated 165 regioselective glycosylation reaction.

Figure 5.23. Lewis base screen for the boronic acid-mediated regioselective 167 glycosylation reaction.

Figure 5.24. Boronic acid-mediated regioselective glycosylation reaction without pre- 172 forming of the boronate ester.

Figure 5.25. Boronic acid screen for the synthesis of (Glu-[β-1,3]-Rha) disaccharide 5.60 176 performed by Stefi Anthonipillai.

Figure 5.26. One-pot boronic acid-mediated regioselective glycosylation reaction. 177

Figure 5.27. Synthesis of the donor trisaccharide 5.66. 178

Figure 5.28. Synthesis of the acceptor disaccharide 5.54 and protected pentasaccharide 180 5.67.

Figure 5.29. Borinic acid-catalyzed glycosylation of acceptor 5.69 with donor 5.70. 182

Figure 5.30. Comparison of catalytic and stoichiometric methods. 184

Figure 5.31. Synthesis of disaccharide acceptors 5.76 and 5.77 185

Figure 5.32. Synthesis of boronate ester 5.80. 188

Figure 5.33. Conditions for the Lewis base screen in the attempted glycosylation of 188 boronate ester 5.80.

Figure 5.34. Conditions for the boronic acid screen in the attempted glycosylation of 188 methyl-α,D-glucopyranoside.

Figure 5.35. Glycosylation of glycal acceptor 5.81 bearing a 1,3-diol. 189

Figure 5.36. Attempts at using thioglycosides as acceptors. 190

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Figure 5.37. Glycosylation of 5.85 with perbenzylated chloride donor 5.86 using the 190 borinic acid-catalyzed reaction.

Figure 5.38. Attempted glycosylations using mannosyl chloride donor 5.88. 192

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List of Abbreviations

CH3CN Acetonitrile

MeCN Acetonitrile

Ac Acetyl

AGEs Advanced glycation endproducts

Ala Alanine

ARS Alizarin red S

AMAC 2-Aminoacridone

NH4Cl Ammonium chloride

Ar Aryl

Asp Aspartic acid

Bz Benzoyl

Bn Benzyl

BASE Boron affinity saccharide electrophoresis

BAC Boronate affinity chromatography

Bu Butyl

CAN Ceric ammonium nitrate

δ Chemical shift in parts per million

CHCl3 Chloroform

COSY Correlation spectroscopy

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Da Dalton

CD3CN Deuterated acetonitrile

CDCl3 Deuterated chloroform

C6D6 Deuterated benzene

DMSO-d6 Deuterated dimethyl sulfoxide

DBU 1,8-Diazabicyloundec-7-ene

DABCO 1,4-Diazabicyclo[2.2.2]octane

DBMP 2,6-Dibutyl-4-methylpyridine

DDQ 2,3-Dichloro-5,6-dicyanobenzoquinone

DCE 1,2-Dichloroethane

DCM Dichloromethane

CH2Cl2 Dichloromethane

Et2O Diethyl ether

DMAP 4-Dimethylaminopyridine

DMF Dimethylformamide

DTBMP 2,6-Di-tert-butyl-4-methylpyridine

EI Electron ionization

EWG Electron withdrawing group

ESI Electrospray ionization ee Enantiomeric excess

Equiv Equivalents

EtOAc Ethyl acetate

EDCI•HCl 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride

FMOC Fluorenylmethyloxycarbonyl

FITC Fluorescein isothiocyanate

FACE Fluorophore-assisted carbohydrate electrophoresis

FTIR Fourier transform infrared spectroscopy

Gal Galactose

Glu Glucose

GT Glycosyltransferase

GDP Guanosine diphosphate

XB Halogen bonding

Hz Hertz

HDA Hetero-Diels-Alder

HRMS High resolution mass spectrometry h Hours

HB Hydrogen bonding

HEPES 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid

IDCP Iodonium (di-γ-collidine) perchlorate i-Pr Isopropyl

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LG Leaving group

LFER Linear free energy relationship

MgSO4 Magnesium sulfate

MS Mass spectrometry m/z Mass-to-charge ratio

MALDI-TOF MS Matrix-assisted laser desorption/ionization-time-of- flight mass spectrometry

MHz Megahertz mP-AGE Methacrylamidophenylboronate acrylamide gel electrophoresis

MPBA Methacrylamidophenylboronic acid

MeOH Methanol

Me Methyl

TBME Methyl tert-butyl ether

μL Microliter mL Milliliter mmol Millimole

M Molarity

M.S. Molecular sieves

χ Mole fraction n-Bu n-Butyl

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DIPEA N,N-Diisopropylethylamine

NBS N-Bromosuccinimide

NIS N-Iodosuccinimide

NMR Nuclear magnetic resonance spectroscopy

PMP para-methoxyphenyl ppm Parts per million

Ph Phenyl

PET Photoinduced electron transfer

KN(SiMe3)2 Potassium bis(trimethylsilyl)amide

K2CO3 Potassium carbonate tBuOK Potassium tert-butoxide

PG Protecting group

RRV Relative reactivity value

Rha Rhamnose rt Room temperature

NaHCO3 Sodium bicarbonate

NaOMe Sodium methoxide

TAMSIM Targeted multiplex mass spectrometry imaging

TBS tert-Butyldimethylsilyl tBu tert-Butyl

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+ Bu4N Tetrabutylammonium

Bu4NBr Tetrabutylammonium bromide

Bu4NCl Tetrabutylammonium chloride

TBAF Tetrabutylammonium fluoride

Bu4NI Tetrabutylammonium iodide

THF Tetrahydrofuran

Cl3CCN Trichloroacetonitrile

TCT 2,4,6-Trichloro-1,3,5-triazine (cyanuric chloride)

NEt3 Triethylamine

OTf Triflate

TFA Trifluoroacetic acid

TMS Trimethylsilyl

TTBP 2,3,4-Tri-tert-butylpyrimidine

Tyr Tyrosine

UV-Vis Ultraviolet-visible

UDP Uridine diphosphate

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Chapter 1 Anion Receptors Composed of Hydrogen- and Halogen-Bond Donor Groups: Modulating Selectivity With Combinations of Distinct Noncovalent Interactions1 1.1 Introduction

The importance of noncovalent interactions in chemistry and biology cannot be denied. From controlling the spatial conformation of biological molecules to influencing the physical properties of every substance, the understanding and study of these interactions is paramount to explaining concepts related to chemical reactivity, catalysis, molecular recognition and drug design. Common types of noncovalent interactions include hydrogen bonding (HB), hydrophobic interactions, van der Waals forces, and electrostatic interactions. An often overlooked noncovalent interaction is halogen bonding (XB). Halogen bonding describes the noncovalent interaction between electron deficient halogen atoms and Lewis bases. This may seem like an unexpected interaction as halogen atoms are generally not thought of as electrophilic species due to their high electronegativities. It may seem more intuitive to think of a noncovalent interaction involving a halogen atom to be acting as a hydrogen bond acceptor or interacting with an electron deficient site such as a carbonyl carbon as a Lewis base. Despite the non-intuitive nature of XB such interactions have been studied intensively in the solid and gas phases and to a lesser extent in the solution phase. A recently published review offers a summary of the thermodynamics and applications of XB in solution.2 The use of halogen bonding interactions in crystal engineering, in liquid crystals and in drug design have also been studied. We hypothesized that incorporating a halogen-bond donor into a receptor already containing a hydrogen bond-donor could alter the receptors anion selectivity. Our investigation into anion receptors composed of both hydrogen- and halogen-bond donor groups had several goals; (1) to analyze the relationship between these two noncovalent interactions and determine if they

1 Portions of this chapter have been published: Chudzinski, M. G.; McClary, C. A.; Taylor, M. S. J. Am. Chem. Soc. 2011, 133, 10559-10567. 2 Beale, T. M.; Chudzinski, M. G.; Sarwar, M. G.; Taylor, M. S. Chem. Soc. Rev., 2013, 42, 1669–1680.

2 exhibited orthogonal selectivity for anions, (2) to investigate the effect of receptor design on the strength of these interactions and (3) to clarify solution phase studies of halogen bonding.

1.2 A brief history of halogen bonding

Halogen bonding interactions had been observed as early as the 19th century in Guthrie's experiments on the formation of iodoammonium complexes between iodine powder and ammonia solution, although at the time this interaction was given no formal name.3 The seminal paper by Benesi and Hildebrand in 1949 described their spectrophotometric investigation of the interaction of I2 with aromatic hydrocarbons and the experimental determination of their association constants in various solvents.4 The following year Mulliken published a paper commenting on the interaction of halogen molecules with aromatic and oxygenated solvents and offered a plausible quantum-mechanical explanation for the appearance of the very strong ultraviolet absorption displayed by these complexes.5 Up until this point relatively little was known about the atomic arrangements of the atoms in these complexes. It was the work of Odd Hassel that shed light on the structural aspects of this interatomic charge-transfer bonding. Beginning in the 1950's the group of Hassel used X-ray crystallography to probe the structure of halogen-bonded complexes. Early examples include bromine-dioxane complexes6 and bromine- hexamethylene tetramine complexes.7 Eventually the scope of the structures studied included many Lewis bases (amines, sulfides, dioxanes, dithianes, diselenanes, acetone, acetonitrile, methanol) interacting with halogens, interhalogens and simple organic halides such as iodoform.8,9 These studies lead to several realizations concerning these types of complexes. For example, in the 1:1 bromine–dioxane complex the Br–Br bond length had elongated upon

3 Guthrie, F. J. Chem. Soc., 1863, 16, 247–248. 4 Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc., 1949, 71, 2703–2707. 5 Mulliken, R. S. J. Am. Chem. Soc., 1950, 72, 600–608. 6 Hassel, O.; Hvolef, J. Acta. Chem. Scand., 1954, 6, 873–873. 7 Eia, G.; Hassel, O. Acta. Chem. Scand., 1956, 10, 139–141. 8 Hassel, O.; Rømming, C. Q. Rev. Chem. Soc., 1962, 1–18. 9 Hassel, O. Science, 1970, 170, 497–502.

complex formation to 2.31 Å from 2.28 Å in free Br2, suggesting a weakening of the Br–Br bond. The distance between bromine and oxygen was found to be 2.71 Å, which is less than the sum of van der Waals radii of the bromine and oxygen atoms (3.35 Å), suggesting an attractive interaction. In the 1:1 bromine–acetone complex there was a distance of 2.82 Å between bromine and oxygen, again less than the sum of van der Waals radii of the atoms. Besides indicating an attractive interaction between Lewis bases and halogens, these studies also outlined some geometric preferences of these systems. The Lewis base-halogen-halogen bond angle was always found to be 180° or nearly 180° and the orientation of the Lewis base halogen bond was always found to coincide with the axes of the orbitals of the lone pairs in the noncomplexed donor molecule. (Figure 1.01) Further evidence of the geometric requirements for these interactions was provided by Parthasarathy et. al. in a study where they compiled data from crystallographic studies and found that nucleophiles approaching halogens in C–X (X = Cl, Br, I) bonds preferred to approach at angles of ~ 165°, or nearly head on.10 In the late 1970's and early 1980's Dumas coined the term "halogen bond" (or liaison halogène in French) while determining the

Figure 1.01. Halogen bonding in bromine–dioxane complexes and in bromine–acetone complexes. association constants of various halogen substituted methanes with pyridine and amide derivatives.11,12 Analogous to HB where a hydrogen attached to an electronegative atom or group functions as the hydrogen bond donor and a Lewis base having a lone pair of electrons functions as the hydrogen bond acceptor, in XB the species having a halogen attached to an electron withdrawing group is called the halogen bond donor and the Lewis base with which it interacts is called the halogen bond acceptor. (Figure 1.02)

10 Ramasubbu, N.; Parthasarathy, R.; Murray-Rust, P. J. Am. Chem. Soc. 1986, 108, 4308-4314. 11 Dumas, J. M.; Geron, C.; Kribii, A. R.; Lakraimi, M. Can. J. Chem. 1984, 62, 2634-2640. 12 Dumas, J. M.; Peurichard, H.; Gomel, M. J. Chem. Res. (M) 1978, 649-663.

Figure 1.02. Depiction of hydrogen and halogen bonding interactions.

These early studies on XB relied primarily on spectrophotometric methods to observe the unique band that appeared when these complexes were formed and X-ray crystal structures to probe the atomic structure of these complexes. Due to the observation of a new band in the absorption spectrum upon complex formation, these interactions were initially classified as charge-transfer interactions. As the field progressed into the 1990's, investigations into the nature of the halogen bond were undertaken to explain their geometrical preferences as well as studies of halogen bonding in the gas, solid and solution state. This early research then led to exploration of the use of XB applications such as crystal engineering, chemical separation, liquid crystal assembly and medicinal chemistry.

1.3 The Nature of the Halogen Bond

The complexes studied by UV-vis spectroscopy discussed in the previous section were characterized by their unique bands in the absorption spectra that were not present for just the halogen-bond donor or halogen-bond acceptor alone. These absorption bands are characteristic of charge-transfer (electron donor-acceptor) interactions.13 However, not all examples of XB may be dominated by charge-transfer interactions. In fact, several studies have shown that in some cases an electrostatic component may be the driving force behind these attractive interactions. Gas phases studies carried out by Legon characterized XB complexs by rotational spectroscopy.14 These complexes were generated by rapid mixing of a halogen-bond donor and halogen-bond acceptor followed by supersonic coexpansion of these components into a

13 Plyer, E. K.; Mulliken, R. S. J. Am. Chem. Soc. 1959, 81, 823–826. 14 Legon, A. C. Angew. Chem. Int. Ed. 1999, 38, 2686–2714.

5 vacuum.15 Rotational spectroscopy allowed the determination of several key parameters which describe the XB complexes such as their angular geometries, intermolecular separations, intermolecular binding strengths and the fraction of electronic charge redistribution upon binding. In Legon's gas phase studies of Lewis bases with halogens and interhalogens, it was observed that there were only minor electronic perturbations between the halogen or interhalogen by itself and those present in a XB complex. As significant electronic perturbations would be characteristic of charge-transfer interactions, this suggested that for these particular complexes simple electrostatic interactions were an important contribution to the interaction energy and charge-transfer interactions were not. Politzer has further investigated the role of electrostatic interactions in XB through computed molecular electrostatic potential surface calculations.16,17 These calculations show that for some halogenated molecules R–X, a region of positive electrostatic potential is located on the outermost portion of some covalently bonded halogen atoms centered on the R–X axis.

σ-hole

Figure 1.03. Calculated molecular electrostatic potential energy surfaces of C6F5I. Blue regions indicate areas of partial positive charge. The σ-hole can be seen on the iodine atom.

15 Legon, A. C.; Rego, C. A.; J. Chem. Soc. Faraday Trans. 1990, 86, 1915–1921. 16 Politzer, P.; Lane, P.; Concha, M. C.; Ma, Y.; Murray, J. S. J. Mol. Model, 2007, 13, 305–311. 17 Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. J. Mol. Model, 2007, 13, 291–296.

This area of positive electrostatic potential has been called the σ-hole and provides one explanation for the attraction of the halogen to negative sites in Lewis bases. Natural bond order analysis of CF3X (X = F, Cl, Br, and I) molecules shows that the σ-hole arises due to the distribution of electrons in X. The electron configuration at X for these molecules can be 2 2 2 1 approximated as s px py pz where the z-axis is along the CF3–X bond. The position of the three electron lone pairs creates a belt of negative charge on the surface of X in the x-y plane and leaves an area of partial positive charge on the surface of X on the z axis. Many factors can influence the magnitude of the partial positive charge of the σ-hole. For example, due to the high electronegativity of fluorine atoms they generally do not have σ-holes (as the R group to which it is attached will not be sufficiently electron withdrawing relative to fluorine) and do not participate in halogen bonding. For the remaining halogens, halogen-bond donor ability increases as the electronegativity of the halogen decreases (and magnitude of the partial positive charge at the σ-hole increases) such that the halogen-bond donor ability of the halogens follows the trend Cl < Br < I.

Figure 1.04. Calculated molecular electrostatic potential surfaces (B3LYP/6-31+G**-LANLdp) of (from left to right) CF4, CF3Cl, CF3Br, and CF3I showing the increase in the size of the σ-hole as a function of halogen atom. Blue coloring represents areas of electron deficiency and red coloring indicates areas of electron density.

In addition to the identity of the halogen atom itself, the group to which it is attached also influences the magnitude of the σ-hole. The more electronegative the group attached to the halogen atom, the greater the magnitude of the partial positive charge of the σ-hole. If the R–X bond is along the z-axis, more electronegative or electron withdrawing groups will remove electron density from the area of the σ-hole giving it a greater partial positive charge. For example, a σ-hole is observed on the chlorine atom of CF3Cl, but not in CH3Cl as the CF3 group is more electron withdrawing that the CH3 group. As a further consequence, the order of halogen- bond acceptor strength is correlated to the type of atom to which the halogen is

7 attached. In general, the order of acceptor strength follows X–X > X–C(sp) > X–C(sp2) > X– C(sp3).18

1.4 Halogen Bonding in Solution: Inorganic Donors

Compared to the number of experimentally determined association constants for hydrogen bonding complexes in solution the number for halogen bonding is relatively small. The interaction energy for halogen bonding spans the wide range of almost 0 kJ mol-1 up to ~40 kJ mol-1. The strongest (most thermodynamically stable) halogen bonding interactions are formed between inorganic donors such as halogens (I2, Br2) and interhalogens (ICl, IBr, etc.) and Lewis bases while weaker halogen bonding interactions are formed between organic donors and Lewis bases. Over the years the determination of hundreds of association constants between Lewis bases and iodine has been undertaken. Unfortunately, the conditions (temperature and solvent) under which each experiment was performed were different and sometimes reports for the same association constants varied widely. This made it difficult to draw conclusions concerning the relative abilities of Lewis bases to act as halogen-bond acceptors towards iodine. In order to address this issue, the group of Laurence and co-workers undertook the daunting task of compiling known association constants determined under suitable conditions or redetermining association constants under suitable conditions with the goal of developing a scale useful in determining the relative halogen-bond acceptor abilities of Lewis bases. The result of these studies was a diiodine basicity scale which included 768 organic bases.19 If at all possible the measurements were taken in alkane solvents at 298 K although sometimes solubility or other factors prevented this. In these cases the complexation constants were measured in another solvent in an attempt to give a semi-quantitative relationship between Lewis bases. Equations (1) and (2) give the relationship between association constant, pKBI2 (diiodine basicity), and Gibbs energy.

pKBI2 = log(Ka) (1)

-1 ΔG [kcalmol ] = –RT ln Ka = –1.364 pKBI2 (at 298 K) (2)

18 Metrangolo, P.; Resnati, G. Encyclopedia of Supramolecular Chemistry 2007, 628–635. 19 Laurence, C.; Graton, J.; Berthelot, M.; El Ghomari, M. J. Chem. Eur. J. 2011, 17, 10431–10444.

The nomenclature used in the construction of the diiodine basicity scale is the same used for 20 aqueous proton basicity (pKBH+), and hydrogen-bond basicity (pKBHX) where the strongest diiodine bases have the largest pKBI2 values.

Figure 1.05. Representative data for association constants between various Lewis bases and diiodine.

The general order of diiodine basicity follows the sequence R2NH > R2PH ≈ R2Se > R2S > RI ≈

R2O > RBr > RCl > RF. Diiodine basicity increases going from right to left in a given period and decreases in basicity when ascending columns 16 or 17. These rankings follow those predicted by the hard-soft acid-base theory where iodine (a soft Lewis acid) would prefer to interact with a softer Lewis base. An anomaly in the order of selectivity is seen in group 15 where iodine prefers to bind to amines rather than to phosphanes. The relative abilities of interhalogens as halogen- bond donors could be observed in the study of their interaction with Lewis bases such as pyridine or pyridine N-oxide. The more electronegative the group attached to iodine, the higher the association constant (and therefore pKBI2).

20 Laurence, C.; Brameld, K. A.; Graton, J.; Le Questel, J.-Y.; Renault, J. J. Med. Chem. 2009, 52, 4073–4086.

Figure 1.06. Trends in pKBIX for selected Lewis bases.

The development of the diiodine basicity scale provided a great deal of insight into trends concerning iodine as a halogen-bond donor. However, inorganic iodine and interhalogens possess characteristics notably different than organic halogenated compounds (for example, the polarizable nature of iodine compared to carbon-halogen bonds or the relative electronegativity of carbon compared to halogens). In its original form the diiodine basicity scale was poorly applicable when applied to organic halogen-bond donors (the relative order of halogen-bond acceptor ability of Lewis bases was different for inorganic and organic donors). There was very little correlation between the pKBI2 and pKBXY (where X is the halogen and Y is an organic fragment). There did, however, appear to be reasonable correlations between pKBI2 and pKBXY within families of Lewis bases (for example nitrogen Lewis bases or oxygen Lewis bases). This discrepancy could be alleviated to some extent by including an electrostatic term in the calculation of pKBXY such as the hydrogen-bond basicity parameter pKBHX (since hydrogen- bonding is a mainly electrostatic interaction) or the electrostatic potential, Vs,min. The need to include an electrostatic term for some halogen-bond donors and not for others suggests that the charge-transfer and electrostatic contributions to the strength of these interactions are dependent on the identity of both the halogen-bond donor and acceptor.

1.5 Halogen Bonding in Solution: Organic Donors

Studies of XB interactions involving organic halogen-bond donors were scarce compared to investigations using inorganic donors such as iodine. Early examples include the study of the

10 interactions of haloforms in n-electron donor solvents21 by 1H NMR spectroscopy and trifluoroiodomethane with 2,4,6-trimethylpyridine22 by 19F NMR spectroscopy. These interactions were found to be much weaker than those between iodine and Lewis bases. The group of Laurence performed quantitative studies of cyanogen iodide and iodoalkynes23 as halogen-bond donors using IR spectroscopy and used the change in C–I frequency shift as a 13 basis for a scale of soft basicity Bsoft = Δν(C–I). A follow up study by the same group used C

NMR spectroscopy to monitor the shift of the Cα of a series of iodoalkynes (ICα CβX) upon addition of Lewis base as well as IR spectrophotometry to determine association constants with triphenylphosphine oxide and triphenylarsine oxide. These association constants revealed a 24 linear free energy relationship (LFER) between log(Ka) and the substituent constant σ. Similar to the effect observed for interhalogens, the more electronegative or electron withdrawing the substituent X (larger substituent constant) the greater the association constant. Metrangolo and Resnati studied XB between halo-perfluorocarbons and heteroatom containing hydrocarbons by 19F NMR spectroscopy.25 Fluorine NMR spectroscopy has been recognized as a useful tool to investigate intermolecular recognition processes such as changes in protein conformation26 or cis-trans isomerization27 in the presence of various molecules. This technique is useful due to the sensitivity of the fluorine chemical shifts based on their environment and the sometimes large changes in chemical shifts observed upon complex formation. By monitoring the change in 19F

NMR chemical shift (Δδ) of the RFnX resonance for a particular haloperfluorocarbon in a Lewis

21 Bertrán. J. F.; Rodriguez, M. Org. Magn. Res. 1979, 12, 92–94. 22 Larsen, D. W.; Allred, A. L. J. Phys. Chem. 1965, 69, 2400–2401. 23 Laurence, C.; Quignec-Cabanetos, M.; Dziembowska, T.; Queignec, R.; Wojtkowiak, B. J. Am. Chem. Soc. 1981, 103, 2567–2573. 24 (a) Laurence, C.; Queignec-Cabanetos, M.; Wojtkowiak, B. J. Chem. Soc. Perkin Trans. II 1982, 1605–1610. (b) Laurence, C.; Queignec-Cabanetos, M.; Wojtkowiak, B. Can. J. Chem. 1983, 61, 135–138. 25 Metrangolo, P.; Panzeri, W.; Recupero, F.; Resnati, G. J. Fluorine Chem 2002, 114, 27–33. 26 (a) Bouchard, M.; Paré, C.; Dutasta, J.-P.; Chauver, J.-P.; Gicquaud, C.; Auger, M. Biochemistry, 1998, 37, 3149–3155. (b) Dupureur, C. M.; Hallman, L. M. Eur. J. Biochem. 1999, 261, 261–268. 27 (a) Hoischen, D.; Colmenares, L. U.; Liu, J.; Simmons, C. J.; Britton, G.; Liu, R. S. H. Bioorg. Chem. 1998, 26, 365. (b) Iwasa, T.; Comenares, Hirata, K.; Arime, Y.; Nakagawa, M.; Kikkawa, S.; Takashima, H.; Nosaka, A.; Naito, A.; Saito, H.; Liu, R. S. H.; Tsuda, M. J. Phys. Chem. A 1998, 102, 5602.

11 basic solvent relative to its shift in cyclohexane, a qualitative measure of the strength of the solvent-haloperfluoroalkane halogen bond was determined. The greater the Δδ, the stronger the

XB interaction was assumed to be. As these experiments did not calculate Ka for the intermolecular interactions and Δδ is determined by the chemical shift of the fully complexed donor and the extent of binding (the magnitude of the chemical shift of fully complexed donor can change between different donor acceptor pairs as well as the extent of binding), there is some discrepancy between trends observed with Δδ and Ka. Trends observed in this study include an increase in halogen-bond donor ability (larger Δδ) descending group 17 such that (RCF2I >

RCF2Br > RCF2Cl > RCF3) and also that secondary iodoperfluoroalkanes were better halogen- bond donors than primary iodoperfluoroalkanes. This can be interpreted as a perfluoroalkyl group being a better electron withdrawing group than a single fluorine atom.

Figure 1.07. Chemical shift differences (Δδ of the fluorine atoms highlighted in bold) of 1,6- diiodoperfluorohexane, 1,6-dibromoperfluorohexane, 1,6-dichloroperfluorohexane and perfluorohexane between n-pentane and piperidine.

Hunter's group has developed an electrostatic solvent competition model (referred to as Hunter's model from now on) for noncovalent interactions which treats all functional group interactions as electrostatic. The purpose of this model was to develop a universal scale which applies to all types of noncovalent interactions. It has been successful in explaining trends in hydrogen-bonded complexes in various solvents28 as well as explaining solvent and substituent effects in aromatic interactions.29 Quantitative studies of the complexes formed between perfluorohexyl iodide and hydrogen-bond acceptors30 were undertaken to observe if this model could accurately be applied

28 (a) Cook, J. L.; Hunter, C. A.; Low, C. M. R.; Perez-Velasco A.; Vinter, J. G. Angew. Chem. Int. Ed. 2008, 46, 3706. (b) Cook, J. L.; Hunter, C. A.; Low, C. M. R.; Perez-Velasco, A.; Vinter, J. G. Angew. Chem. Int. Ed. 2008, 47, 6275. 29 Cockroft, S. L.; Hunter, C. A. Chem. Commun. 2006, 3806. 30 Cabot, R.; Hunter, C. A. Chem. Commun. 2009, 2005–2007.

12 towards XB as well. The model is based on equation (3) which predicts the free energy of any pairwise functional group contact in any solvent based on the hydrogen-bond donor (α) and acceptor (β) parameters of the Lewis acid and Lewis base in question, and the hydrogen-bond donor (αs) and acceptor (βs) parameters of the solvent. If the experimental value of α or β is not available it can be calculated using equations (4) or (5) which are based on the calculated AM1 31 -1 molecular electrostatic potential surfaces Emax and Emin. The term 1.4 kcalmol accounts for the cost of bimolecular association in solution.

-1 ΔG = –(α – αs)(β – βs) + 1.4 kcalmol (3)

-1 α = Emax / 12.4 kcalmol (4)

-1 β = Emin / 12.4 kcal mol (5)

The Lewis bases studied are shown in Figure 1.08 along with logKexp calculated from their experimentally determined association constants (by 19F NMR spectroscopy) in benzene at 295

K as well as the logKpred predicted for the interaction based on the calculated value of ΔG using equation (3).

Figure 1.08. Interactions of iodoperfluorohexane and Lewis bases examined to test the Hunter model. S represents solvent while A represents the halogen-bond acceptor.

31 Hunter, C. A. Angew. Chem. Int. Ed. 2004, 43, 5310–5324.

From the data shown in Figure 1.08 it appears as though equation (3) can predict logKexp quite well for tributylphosphine oxide, the acylic amines and pyridine as logKpred agrees within error.

However, for the cyclic amines the agreement between logKexp and logKpred is very poor. The authors suggest that for the cyclic amines there is some contribution of charge-transfer interactions for the formation of halogen-bond complexes in addition to electrostatic interactions. This complicates the classification of XB as it appears some donor-acceptor pairs associate through primarily electrostatic interactions, some through primarily charge-transfer interactions and some through varying combinations of the two. To further investigate the nature of the XB interaction our group has studied the association constants of a series of para-substituted iodotetrafluorobenzenes with Lewis bases by 19F NMR spectroscopy.32 The study of substituent effects on the halogen-bond donor ability of iodoperfluorobenzenes (in complexes with tri-n- butylphosphine oxide acceptor) revealed that electron donating substituents led to lower association constants while electron withdrawing substituents led to higher association constants. Calculations of the electrostatic potential surface at iodine of the various donors showed that the size of the σ-hole is considerably less pronounced in derivatives having electron donating groups.

Figure 1.09. Interactions of para-substituted perfluoroarenes with tributylphosphine oxide.

For the donor-acceptor pair shown in Figure 1.09, LFERs between log(Ka) and Hammett substituent constants (σmeta) or calculated (B3LYP) electrostatic potentials at the iodine atom revealed that trends in substituent effects can be explained based on electrostatic effects.

32 Sarwar, M. G.; Dragisic, B.; Salsberg, L. J.; Gouliaras, C.; Taylor, M. S. J. Am. Chem. Soc. 2010, 132, 1646– 1653.

However, when the Hunter model was applied to other data sets examining the free energies of XB between iodoperfluorooctane or iodoperfluorobenzene with nitrogen or oxygen centered Lewis bases poor predictions of ΔG were observed. This suggested, again, that considering the interaction to be purely electrostatic for all cases would be incorrect. This study also examined solvent effects on halogen bonding interactions and found that the interaction is weak in solvents having acidic protons. This may not be surprising as in these cases the acidic proton could compete with the halogen-bond donor for the Lewis base.

1.6 Halogen Bonding in Solution: Anion Recognition

The previous sections have outlined the role of halogen bonding between organic and inorganic halogen-bond donors and neutral halogen-bond acceptors. Recognition of anions by synthetic receptors is also an active area of research as anions play important roles in the areas of catalysis, medicine and environmental chemistry. Anion receptors have been developed which employ a variety of noncovalent interactions (HB, electrostatic, Lewis acid-base, anion-π and hydrophobic effects) capable of binding anions in solution.33 Selective anion recognition is possible if the design of the receptor is complementary to the structure of the anion.34 For example, cyclic receptors might exclude larger anions based on cavity size while smaller anions are bound or ideal placement of hydrogen-bond donor groups may interact favorably with Y-shaped anions like nitrate. Anions should also be able to interact with halogen-bond donors. The formation of – – the triiodide ion (I3 ) can be considered a XB interaction between iodide anion (I ) and iodine -1 + 35 (I2). The ΔH of binding in aqueous solvent was found to be –4.06 kcalmol (K counterion) by UV-vis and –30.1 kcalmol-1 in the gas phase by mass spectrometry.36 Prior to the work begun in our group, the determination of only a few other halogen-bond association constants had been

33 (a) Beer, P. D.; Gale, P. A. Angew. Chem. Int. Ed. 2001, 40, 486–516. (b) Caltagirone, C; Gale, P. A. Chem. Soc. Rev. 2009, 38, 500–563. (c) Hudnall, T. W.; Chiu, C.-W.; Gabbat, F. P. Acc. Chem. Res. 2009, 42, 388–397. (d) Games, P.; Mooibroek, T. J.; Teat, S. J.; Reedijk, J. Acc. Chem. Res. 2007, 40, 435–444. (e) Hay, B. P.; Bryantsev, V. S. Chem. Commun. 2008, 2417–2428. 34 (a) Choi, K.; Hamilton, A. D. Coord. Chem. Rev. 2003, 240, 101-110. (b) Kang, S. O.; Begum, R. A.; Bowman- James, K. Angew. Chem. Int. Ed. 2006, 45, 7882-7894. 35 Palmer, D. A.; Ramette, R. W.; Mesmer, R. E. J. Solution Chem. 1984, 13, 673-683. 36 Do, K.; Klein, T. P.; Pommerening, C. A.; Sunderlin, L. S. J. Chem. Soc. Mass Spectrometry 1997, 8, 688-696.

15 reported for anions. The determination of association constants by UV-Vis spectroscopy between carbon tetrabromide and halides in CH2Cl2 (tetrapropylammonium counterion) revealed values of 3.0 (ΔG = –0.65 kcalmol-1), 2.8 (ΔG = –0.60 kcalmol-1), and 3.2 (ΔG = –0.69 kcalmol-1) for Cl–, Br–and I– respectively. The group of Resnati and Metrangolo have reported a XB-based receptor for alkali metal halides that is capable of binding both of the counterions in a salt.37 For example, in the case of NaI the ether oxygens of the receptor were able to coordinate to a sodium cation while one of the iodoperfluorophenyl groups interacted with the iodide anion through halogen bonding. A crystal structure of 1.01a with NaI was obtained showing that the anion only interacted with the receptor through a single halogen-bond even though it contained three possible halogen-bond donors. The ESI-MS spectrum of a mixture containing 1.01a and equimolar amounts of I–, Br– and Cl– showed a strong signal at m/z 1230 [1.01a + I–] and two broad peaks with a relative abundance <1% with respect to the first peak at m/z = 1182-1184 [1.01a + Br–] and m/z = 1138 [1.01a + Cl–]. The authors interpreted this as preferential binding of 1.01a with iodide in solution and proposed the strength of halogen-bonds with the same receptor decreased in the order I– > Br– > Cl–. The binding constant of 1.01a with NaI was determined to be 2.6 x 105 M-1 which was approximately 20 times greater than the association constant of 1.01b with NaI, indicating that XB interactions played a role in the receptor's affinity for NaI (the receptor containing X = F was used as a control as fluorine does not behave as a halogen-bond donor).

37 Mele, A.; Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. J. Am. Chem. Soc. 2005, 127, 14972-14973.

Figure 1.10. Heteroditopic receptor for alkali metal halides.

In 2010, our group designed multidentate halogen-bond donor receptors to observe if anions were capable of interacting with multiple donors simultaneously and to observe any trends in selectivity. Association constants were determined between receptors 1.02–1.04 and the tetrabutylammonium salts of the anions shown in Table 1.01 in acetone solvent at 295 K.38 2- Iodoperfluorobenzoic acid was chosen as the XB motif in the receptors due to the electron withdrawing property of the perfluorinated aromatic ring as well as the ability of the receptors to orient multiple halogen-bonds towards an anionic guest. Important conclusions from the study include an observation of significant chelate cooperativity in XB towards anions and a preference for halide anions over oxoanions. The chloride affinity of 1.03 was more than an order of magnitude higher than that of monodentate receptor 1.02, indicating that a bidentate receptor can increase the binding constant. Installation of an additional halogen bond donor showed that tridenate receptor 1.04 had a chloride affinity which was again, an order of magnitude larger than that of 1.03.

38 Sarwar, M. G.; Dragasic, B.; Sagoo, S.; Taylor, M. S. Angew Chem. Int. Ed. 2010, 49, 1674–1677.

Figure 1.11. Structures of halogen-bond receptors 1.02–1.04.

A modified version of 1.03 where the two iodides had been replaced with fluorines was also – studied. This receptor showed no affinity (Ka < 10) for Cl which was expected as fluorine usually does not function as a halogen-bond donor. Receptor 1.04 showed the highest affinity for chloride followed by bromide and finally by iodide. It showed negligible affinity for the oxoanions tosylate, nitrate and bisulfate. This was peculiar as HB receptors with similar geometries showed affinities for those anions,39 suggesting that there was an intrinsic difference between the anion preference of halogen-bond donors and hydrogen-bond donors. The behavior of 1.01a supports the trend in halogen-bond acceptor ability to be I– > Br– > Cl– 40 while other data41 and calculations42 suggest the opposite trend to be the case.

39 McKie, A. H.; Friedland, S.; Hof, F. Org. Lett. 2008, 10, 4653 – 4655. 40 Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem. Int. Ed. 2008, 47, 6114–6127. 41 (a) Zordan, F.; Brammer, L.; Sherwood, P. J. Am. Chem. Soc. 2005, 127, 5979–5989. (b) Minguez Espallargas, G.; Zordan, F.; Brammer, J. Chem. Eur. J. 2009, 15, 7554–7568. 42 Lu, Y. X.; Zou, J.-W.; Wang, Y.-H.; Jiang, Y.-J.; Yu. Q.-S. J. Phys. Chem. A 2007, 111, 10781–10788.

Table 1.01. Binding constants of 1.02, 1.03, and 1.04 with Bu4NX (acetone, 295 K).

- -1 Halogen-bond donor X Ka (M ) 1.02 Cl– 70 1.03 Cl– 1.8 x 103 1.04 Cl– 1.9 x 104 1.04 Br– 3.8 x 103 1.04 I– 7.6 x 102 1.04 TsO– 10 – 1.04 HSO4 < 10 – 1.04 NO3 < 10

Variations of 1.04 which explored the effects of modifying the degree of fluorination of the aromatic ring as well as the identity of the halogen-bond donor atom were also undertaken.43 Replacing any of the fluorines with hydrogen results in a dramatic decrease in chloride affinity highlighting the importance of having very highly electron withdrawing groups attached to the halogen-bond donor. Changing the halogen-bond donor atom from iodine to bromine results in no measurable affinity for chloride which is consistent with bromine being a weaker halogen bond donor than iodine.

Figure 1.12. Halogen-bond halide anion-templated psedorotaxane formation.

The group of Beer has investigated XB with anions through pseudorotaxane host systems.44 These studies suggest that hydorgen and halogen bonding can work in tandem. Figure 1.12

43 Dimitrijević, E.; Kvak, O.; Taylor, M. S. Chem. Commun. 2010, 46, 9025–9027. 44 (a) Serpell, C. J.; Kilah, N. L.; Costa, P. J.; Félix, V.; Beer, P. D. Angew. Chem. Int. Ed. 2010, 49, 5322–5326. (b) Kilah, N. L.; Wise, M. D.; Serpell, C. J.; Thompson, A. L.; White, N. G.; Christensen, K. E.; Beer, P. D. J. Am. Chem. Soc. 2010, 132, 11893–11895.

19 shows the interaction between triazolium salts (1.05•X) and macrocycle 1.06. The binding constants were determined by 1H NMR spectroscopy and are summarized in Table 1.02.

Table 1.02. Associations constants for pseudorotaxane formation between triazolium salts and 1.06 in CDCl3 at 293 K. -1 Triazolium salt Ka (M ) 1.05•Cl- 538 1.05•Br- 1188 1.05•I- 392 - 1.05•BF4 29

- Anions which are incapable of participating in XB (BF4 ) showed low association constants while the halides exhibited an interesting order of selectivity, with the binding constants being greatest for the bromide followed by the chloride and then the iodide. This selectivity may be influenced by the cavity size and may not by an intrinsic preference in halogen-bond acceptors. Rotaxane 1.07 was synthesized via an anion-templated approach.44b The association constants of – -1 -1 -1 1.07•PF6 were found to be 457 M , 1251 M and 2228 M for chloride, bromide and iodide - respectively in 45:45:10 CDCl3:CD3OH/D2O. 1.07•PF6 shows a preference for the larger halide anions (I– > Br– > Cl– ). This preference was attributed to easier accessibility of the binding site to the larger anions or reduced competition for the larger anions by the aqueous solvent used.

Figure 1.13. Rotaxane having both halogen- and hydrogen-bond donors.

These two previous examples are important in that they show XB and HB can be used concurrently for anion recognition. Our studies discussed later in this chapter expand on these ideas and investigate trends in anion selectivities for systems where anion recognition is not influenced significantly by size restrictions of the binding site.

1.7 Halogen Bonding in Medicinal Chemistry

Compounds containing carbon-halogen bonds are important in medicinal chemistry. The replacement of a hydrogen or a hydroxyl group with a fluorine atom is a common strategy for enhancing the biological activity or lipophilicity of analogues of biologically important molecules.45 Halogen atom incorporation can influence steric effects, increase membrane permeability, increase blood-brain barrier permeability or block sites of metabolism.46 The possibility that these halogens also engaged in attractive XB interactions has only recently been studied in detail. In 2004, Auffinger undertook a survey of the Protein Data Bank examining proteins with bound halogenated inhibitors which exhibited intermolecular X–O distances shorter than their respective van der Waals radii.47 This study (and a further study in 2009)48 suggested that XB can be an important interaction in biological systems. Auffinger's study was prompted by the publication of two crsytal structures showing unusually short Br–O contacts. One was a 4-stranded DNA Holliday junction49 and the other was of the enzyme aldose reductase complexed with a halogenated inhibitior.50 The idea that XB interactions can be taken

45 Shibata, N.; Ishimaru, T.; Nakamura, S.; Toru, T. Journal of Fluorine Chemistry, 2007, 469–483. 46 Gentry, C. L.; Egleton, R. D.; Gillespie, T.; Abbruscato, T. J.; Bechowski, H. B.; Hruby, V. J.; Davis, T. P. Peptides, 1999, 20, 1229–1238. 47 Auffinger, F. A.; Hays, F. A.; Westhof, E.; Ho, P. S. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 16789. 48 Lu, Y.; Shi, T.; Wang, H.; Yang, X.; Luo, X. Jaing, H.; Zhu, W. J. Med. Chem. 2009, 52, 2854–2862. 49 Hays, F. A.; Vargason, J. M.; Ho, P. S. Biochemistry, 2003, 42, 9586–9597.

50 Howard, E. I.; Sanishvili, R.; Cachau, R. E.; Mitschler, A.; Chevrier, B.; Barth, P.; Lamour, V.; Van Zandt, M.; Sibley, E.; Bon, C.; et al. Proteins 2004, 55, 792–804.

21 advantage of in designing enzyme inhibitors has recently been investigated by Diederich et al.51 Novel inhibitors of human Cathepsin L (hCatL) were developed which bind covalently to the side chain Cys 25 in the S1 pocket. The sulfide in the catalytic site reacts with the nitrile group of the inhibitor to form a thioimidate which is stabilized by the oxyanion hole. The inhibitors interact through various other lipophilic contacts including a proposed halogen-bonding interaction in the S3 pocket.

O Cys25 S3 Glu63 Gly68 Gly61 N H SH S1 H N N O H N Gln19 X O O O N Tyr72 H N O Asp162 Leu69 SO HN 2 CO2H

Ala214

Met161 S2 Figure 1.14. Diagram of inhibitor binding to the three pockets of the active site of hCatL. The X represents a substituent that has been varied from H, F, Cl, Br, or I.

Inhibitors were designed in which the substituent at X was varied between H, Me, F, Cl, Br, and I to probe the importance of a XB interaction to the carbonyl group of Gly61. The results are summarized in Table 1.03. As fluorine atoms are not generally expected to participate in halogen bonds, it is not surprising that on exchange of X = H for X = F there was no increase in IC50. However, when substituting X = H for halogens that can participate in halogen bonds (X = Cl,

Br, and I) there was a marked decrease in IC50. The decrease in IC50 correlates with the general trend in halogen-bond donor ability (X = I > Br > Cl). Increasing the electron withdrawing capability of the aromatic ring would be expected to increase the strength of the halogen-bond

51 Hardegger, L. A.; Kuhn, B.; Spinnler, B.; Anselm, L.; Ecabert, R.; Stihle, M.; Gsell, B.;Thoma, R.; Diez, J.; Benz, J.; Plancher, J.-M.; Hartmann, G.; Banner, D. W.; Haap, W.; Diederich, F.; Angew. Chem. Int. Ed. 2011, 50, 314–318.

donor. This effect was observed as the IC50 values were lowered even further upon substituting the aromatic ring with a fluorine. To ensure the decrease in IC50 values was not due to a change in lipophilcity upon varying X, the logD of each compound was obtained. The difference in logDs were not sufficient to explain the variations in IC50 values. A crystal structure of hCatL and the non-fluorinated inhibitor where X = Cl was obtained. The O–Cl distance was found to be 3.1 Å (shorter than the sum of van der Waals radii 3.27 Å) and the O–Cl–C angle was determined to be 174° which is close to the ideal 180° angle preferred by halogen bonds. Similar bond lengths and angles were found in the crystal structures of the bromo and iodo derivatives.52

Table 1.03. Activity data for hCatL inhibitors.

X H Me F Cl Br I

IC50 0.29 0.13 0.34 0.022 0.012 0.0065

logD 2.11 2.57 2.36 2.73 2.96 3.23

IC50 0.32 0.35 0.03 0.0065 0.0043

logD 1.98 2.02 2.63 2.75 3

Several other halogen-bond donor compounds have been proposed to function as enzyme inhibitors in part due to attractive XB interactions. The evidence for XB in these systems has been derived from X-ray crystal structures of these inhibitors bound to their target enzymes in which the distance between the halogen-bond donor atom and the halogen-bond acceptor atom was less than the sum of their van der Waals radii. In addition,

52 Diederich, F.; Banner, D. W.; Haap, W. et al.,ChemMedChem, 2011, 6, 2048–2054.

23 the crystal structures showed a preferential R–X•••B (where R is the moiety attached to the halogen bond donor atom, X is the halogen-bond donor atom, and B is a Lewis base) bond angle of between 169° – 180° as is typical for XB interactions. Examples include a cdc2-like kinase inhibitor,53 a cdk9 kinase inhibitor54 and a hepatitis C virus NS3 protease inhibitor.55 An interesting property of halogen bonds observed in crystal structures of protein-ligand complexes involving carbonyl oxygens as the Lewis base is that a large number show noncovalent HB interactions to oxygen while simultaneously participating in XB interactions. HB interactions were observed to be perpendicular to the XB interactions, a phenomenon which was proposed to arise from the distribution of electrostatic potential on oxygen. Ab intio calculations suggested that these halogen bonds are energetically independent of the hydrogen bonds, that is, introducing one of these noncovalent interactions does not decrease the strength of the other.

Figure 1.15. Diagram of observed perpendicularity between hydrogen and halogen bonds made to the same carbonyl oxygen of a peptide.

This study suggested there were intrinsic differences between XB and HB. With this in mind, our group set out to investigate how anion receptors composed of both halogen- and hydrogen- bond donors would behave towards a series of anions. Our goals for this project were (1) to develop receptors having both a urea or thiourea (hydrogen-bond donor) and an iodoperfluoroaryl group (halogen-bond donor), (2) determine the association constants between these receptors and various anions in solution, (3) identify any trends in anion selectivity (as previous reports are

53 (a) Fedorov, O.; Huber, K.; Eisenreich, A.; Filippakopoulos, P.; King, O.; Bullock, A. N.; Szklarczyk, D.; Jensen, L. J.; Fabbro, D.; Trappe, J.; Rauch, U.; Bracher, F.; Knapp, S. Chem. Biol. 2011, 18, 67–78. 54 Baumli, S.; Endicott, J. A.; Johnson, L. N. Chem. Biol. 2010, 17, 931–936. 55 (a) Llinàs-Brunet, M., et al., J. Med. Chem., 2010, 53, 6466-6476. (b) Lemke, C. T.; et al., J. Biol. Chem., 2011, 286, 11434–11443.

24 conflicting) exhibited by the receptors and (4) study how the spatial arrangement of the hydrogen- and halogen-bond donors affects the binding of anions.

1.8 Results and Discussion

Before discussing the results of the project it should be noted that the acquisition of the data assembled in this chapter was a collaborative effort between myself and another graduate student, Michael Chudzinski. In particular he carried out the synthesis of receptors 1.15a, 1.15b, 1.16a, 1.16b, 1.22a and 1.22b and the collection of their binding constants with various anions. He was also responsible for the computational studies. As discussion of only a partial set of the data would not allow the proper conclusions concerning the work to be drawn, his results will also be included with appropriate attribution. To investigate the potential for anion recognition using compounds containing both hydrogen- and halogen-bond donors, a series of urea-based receptors were developed. The goal of the study was to design a receptor with a functional group known to hydrogen bond with anions (a urea)56 and observe how the incorporation of a group capable of acting as a halogen-bond donor (iodoperfluoroarene) influenced receptor anion selectivity and affinity versus the incorporation of a control group (perfluoroarene) which could not participate in XB (Figure 1.16).

O O R1 R1 N N N N Linker Linker H H F H H F HB HB X- I F X- F F

XB F F F F Receptor containing hydrogen- Receptor containing and halogen-bond donors hydrogen-bond donor only

Figure 1.16. Design elements of receptors containing hydrogen- and halogen-bond donors and control receptors.

By comparing the association constants of anions with the hydrogen- and halogen-bond donor receptor to those of the control receptor the contribution of the XB interaction to the free energy

56 Gale, P. A.; Encyclopedia of Supramolecular Chemistry, Marcel Dekker: New York, 2004, 31–41.

25 of anion binding can be estimated. Investigations of the effects of varying both the identity of the linker and anion were undertaken to determine any trends in selectivity.

1.9 Initial Receptor Design

In the design of receptors containing both hydrogen- and halogen-bond donor groups, a urea or thiourea was envisaged as the hydrogen-bond donor moiety due to their prevalence in anion binding chemistry. A facile method of introducing the halogen-bond donating iodoperfluorophenyl group (or the control perfluorophenyl group) was thought to be the formation of an ester linkage between an alcohol and 1-iodotetrafluorobenzoic acid or perfluorobenzoic acid.38 Syntheses of a urea and thiourea containing the requisite hydroxyl group for the desired coupling are outlined in Figure 1.17.

Figure 1.17. Synthesis of 1-(2-hydroxyethyl)-3-(4-nitrophenyl)urea (1.10) and 1-(2- (hydroxymethyl)phenyl)-3-(4-nitrophenyl)thiourea (1.13).

The 4-nitrophenylisocyante (1.08) was chosen for coupling to ethanolamine (1.09) to form the urea as to incorporate a chromophore into the molecule allowing titrations of the receptor with anions to be studied by UV-vis. Acylation of 1-(2-hydroxyethyl)-3-(4-nitrophenyl)urea (1.10) was achieved under standard coupling conditions using 1-ethyl-3-(3- dimethylaminopropyl)carbodiimide hydrochloride (EDCI•HCl) as the coupling reagent to give hydrogen- and halogen-bond donor receptor (1.14a) or hydrogen-bond donor (control) receptor (1.14b). Attempted acylation of 1-(2-(hydroxymethyl)phenyl)-3-(4-nitrophenyl)thiourea (1.13), however, led to neither of the desired products. A complex mixture of products appeared to have formed perhaps due to the known desulfurization reaction of thioureas in the presence of

26 carbodiimides.57 Subsequent attempts at acylating (1.13) using acyl chlorides was successful using benzoyl chloride (albeit in low yield) but unsuccessful using the acyl chlorides of 2,3,4,5- tetrafluoro-6-iodobenzoic acid or pentafluorobenzoic acid (although formation of the acid chloride was verified by quenching with methanol in DIPEA followed by isolation of the methyl ester in good yield). As the urea receptors were easily synthesized, the focus was shifted towards them.

Figure 1.18. Acylation of 1-(2-hydroxyethyl)-3-(4-nitrophenyl)urea (1.10) and attempted acylation of 1-(2-(hydroxymethyl)phenyl)-3-(4-nitrophenyl)thiourea (1.13).

Receptors 1.15a, 1.15b, 1.16a, and 1.16b were synthesized by Michael Chudzinski, and their preparation and characterization can be found here.1 These receptors along with 1.14a and 1.14b

57 Wragg, R. T. Tetrahedron Letters 1970, 45, 3931–3932.

27 were chosen in order to probe the effect, if any, the linker had on anion binding. Titrations of these receptors with various anions are described in the next section.

Figure 1.19. Additional receptors bearing single halogen-bond donor groups.

1.10 Receptors Bearing a Single Halogen-Bond Donor

The binding constants of receptors 1.14a, 1.14b, 1.15a, 1.15b, 1.16a, and 1.16b with Cl–, Br–, and I– in acetonitrile at 295 K were determined using the tetrabutylammonium cation as the counter anion to minimize ion-pairing interactions and to improve solubility. The determination of binding constants was carried out by either fitting changes in solution absorbance or changes in 1H- or 19F-NMR chemical shifts as a function of anion concentration to a 1:1 binding isotherm using standard methods.58 The technique used was based on the magnitude of the association constant. Hirose has described in detail how to choose [H]t (total concentration of host molecule at initial state) and the range of [G]t (total concentration of guest molecule at initial state) for the reliable determination of binding constants. The choice of these values is influenced by the magnitude of the association constant to be measured and is limited by the [H]t which can be -1 measured by the apparatus. For NMR spectroscopy [H]t is in the range of ~0.01 molL and for -1 UV-vis spectroscopy the [H]t is in the range of ~0.0001 molL . Based on the limited choices of

[H]t for each method, large association constants are best determined by UV-vis while lower

58 Hirose, K. J. J. Inclusion Phenom. Macrocyclic Chem. 2001, 39, 193–209.

28 association constants are better determined by 1H- or 19F-NMR spectroscopy. The host:guest stoichiometry for each receptor and anion pair was determined to be 1:1 based on analysis by the method of continuous variation (Job plot). Figure 1.20 shows an example of changes observed in the UV-vis spectra of a receptor (in this case 1.14a) upon addition of Bu4NCl in acetonitrile at

295K (top). The change in absorbance (λ = 362 nm) as a function of [Bu4NCl] fitted with a 1:1 binding isotherm (middle) and the Job Plot for the 1.14a-Cl- complex (bottom) are also shown. For titrations of receptors with anions analyzed by NMR spectroscopy, significant shifts of the urea NH protons in the 1H NMR spectra as well as significant shifts of the fluorines in 19F NMR spectra were observed. The binding constants were calculated by taking the average of that obtained by 1H NMR and 19F NMR spectroscopy.

Figure 1.20. (a) UV-vis absorption spectra of 1.14a upon addition of Bu4NCl (acetonitrile); (b) change in absorbance (λ = 362 nm) of a solution of 1.14a as a function of [Bu4NCl]. Curve represents the equation of best fit for a 1:1 binding model.; (c) 19F NMR Job plot for the 1.14a- Cl– complex.

Association constants determined by changes in the shift of NH protons were generally in agreement with those determined by fluorine shifts but it was sometimes easier to follow the

30 fluorine peaks as the NH peaks would sometimes broaden in the presence of certain anions or be obscured by other proton peaks. Figure 1.21 and Figure 1.22 show an example of the behavior of proton and fluorine chemical shifts respectively of 1.14a upon titration with [Bu4NBr].

Figure 1.21. Behavior of proton chemical shift of 1.14a upon titration with [Bu4NBr] in acetonitrile-d3 at 295 K. [1.14a] = 2 mM.

The signal indicated by (•) represents the aryl urea proton while the signal indicated by (▲) represents the alkyl urea proton. Although following the alkyl urea proton gave identical association constants, the association constants reported were determined using the aryl urea proton. Figure 1.21 shows a change in aryl urea proton chemical shift of > 2 ppm. The Ka for this particular titration (only 7 of 14 1H NMR data points shown) based on aryl urea proton chemical shift was determined to be 2232 M-1. This is in good agreement with the association constant determined by following the fluorine chemical shift (the fluorine ortho to the iodo group was followed, indicated by (•) in Figure 1.22) which was calculated to be 2210 M-1.

Figure 1.22. Behavior of fluorine chemical shift of 1.14a upon titration with [Bu4NBr] in acetonitrile-d3 at 295 K. [1.14a] = 2 mM.

The results of the titrations are summarized in Table 1.04. The value ∆∆GXB , the difference in free energies between the iodinated and fluorinated receptors for a given anion, gives a rough estimate of the contribution of halogen bonding to the overall binding. It can be calculated from association constants using formulas (6) and (7) where values for iodinated receptors are denoted with a subscript "I" and those of the control perfluorinated receptors with "F".

ΔGI = – RTln(Ka( I)) or ΔGF = – RTln(Ka(F)) (6)

∆∆GXB = ∆GI – ∆GF (7)

Table 1.04. Association constants (Ka) of receptors 1.14a–1.16a and 1.14b–1.16b with halide anions and the calculated free energy contributions of the halogen bonding interaction (ΔΔGXB).

-1 Receptor Anion Ka ∆∆GXB (kcalmol ) 1.15a Cl– 8.5 x 103 a,c –0.2 ± 0.1 1.15b Cl– 6.5 x 103 a,c 1.15a Br– 1.7 x 103 b,c –0.1 ± 0.1 1.15b Br– 1.4 x 103 b,c 1.15a I– 1.3 x 103 b,c –0.2 ± 0.1 1.15b I– 1.0 x 103 b,c 1.14a Cl– 8.0 x 103 a –0.9 ± 0.1 1.14b Cl– 1.7 x 103 a 1.14a Br– 2.4 x 103 b –1.1 ± 0.1 1.14b Br– 3.7 x 102 b 1.14a I– 2.2 x 102 b –0.9 ± 0.1 1.14b I– 55 b 1.16a Cl– 8.5 x 103 a,c –0.3 ± 0.1 1.16b Cl– 8.5 x 103 a,c 1.16a Br– 8.5 x 103 b,c –0.4 ± 0.1 1.16b Br– 8.5 x 103 b,c 1.16a I– 8.5 x 103 b,c –0.3 ± 0.1 1.16b I– 8.5 x 103 b,c a Binding constants determined by fitting changes in solution absorbance as a function of anion + concentration to a 1:1 binding isotherm (Bu4N counterion, acetonitrile solvent, 295K carried out b in duplicate, uncertainty in Ka values estimated to be ± 20%). Binding constants determined by fitting changes in 1H- and 19F-NMR chemical shift as a function of anion concentration to a 1:1 + binding isotherm (Bu4N counterion, acetonitrile-d3 solvent, carried out in duplicate, uncertainty in Ka values estimated to be ± 20%). The reported values are the averages of Ka determinations using changes in chemical shift for one urea N-H signal and one fluoro substituent. c Titration carried out by Michael Chudzinski.

Stickly speaking, 1.14b–1.16b are not perfect control receptors as the perfluorinated compounds may still participate in anion-arene interactions in these systems possibly leading to higher affinities than binding through HB to the urea alone. However, experiments discussed later in this chapter show that these anion-arene interactions are non-existent in most cases and minimal in others. Receptors 1.14b-1.16b, therefore, function as suitable control receptors to evaluate the approximate contributions from XB to anion binding. Previous results from our group have indicated that halide anions are good acceptors of halogen bonds.35 However, when comparing

33 the association constants between receptors 1.15a and 1.15b for the various halide anions, very -1 – similar results were obtained leading to calculated values of ∆∆GXB = –0.2 kcalmol (Cl ), –0.1 kcalmol-1 (Br–) and –0.2 kcalmol-1 (I–). This was somewhat of a surprise as it was expected that inclusion of a halogen-bond donor into the receptor would significantly increase the affinity for halide anions. This prediction was found to be true when titrations with 1.14a and 1.14b were undertaken. For the halide anions studied with this set of receptors there was a significant difference in the association constants between the receptor containing a halogen-bond donor and -1 – -1 – the control receptor. This led to values of ΔΔGXB of –0.9 kcalmol (Cl ), –1.1 kcalmol (Br ), and –0.9 kcalmol-1 (I–). This discrepancy between the two sets of receptors was attributed to the different linkers which joined the halogen-bond donor moiety with the hydrogen-bond donor urea.

Figure 1.23. Structures of the 1.15a'-Cl–, 1.14a'-Cl–, and 1.16a'-Cl– complexes calculated by DFT (B3LYP/6-31++G(d,p)-LANL2DZdp, gas phase).

Computational studies were undertaken (by Michael Chudzinski)1 on complexes 1.14a'-1.16a' (to reduce the computational power required to perform the calculations the 4-nitro substituent was removed) with chloride anion. The gas phase geometries of the receptor–Cl– complexes were optimized using density functional theory (DFT) (B3LYP/6-31++G(d,p)-LANL2DZdp). In the energy minimized structures shown in Figure 1.23 the chloride anion interacts with both the hydorgen- and halogen-bond donor groups. However, there is a subtle difference in the C–I•••Cl– bond angle. The calculated bond angle in 1.15a' is 155° whereas the bond angle in 1.14a' was found to be 170°. As mentioned previously, the preferred geometry of a halogen bond is 180°. In 1.15a' the more rigid linker forced a deviation from this ideal angle by 25°, which could account for the failure to observe any significant contribution to binding from halogen bonding (ΔΔGXB) for the 1.15 pair. The more flexible linker in 1.14a' displayed a C–I•••Cl– angle of 170° (closer to the optimal value of 180°) and a significant contribution to binding from XB was observed for the 1.14 pair. Analysis of XB interactions in X-ray crystallography,59 gas-phase microwave spectroscopty,60 and computation61 have indicated that relatively small deviations from the optimal linear arrangement can drastically decrease the strengths of halogen bonds. This effect is much more pronounced in XB than in HB where distortions from the ideal linear geometry, although still incurring energetic penalties, are not as severe.1 This phenomenon is attributed to the relatively localized nature of the region of partial positive charge of the σ-hole relative to a hydrogen atom. Although the angle calculated in 1.16a' was 180° (the ideal angle for XB),

1.16a showed a smaller contribution to binding from XB than 1.14a. The lower value of ΔΔGXB for receptor 1.16a was attributed to increased entropic cost for chloride binding due to the flexible linker tethering the urea and iodoperfluorobenzoate groups. These initial studies revealed important information to take into account when designing receptors containing halogen-bond donors. While rigid receptors are preferred to reduce entropic costs associated with arranging the halogen-bond donor into the required position, careful consideration of the angular preference of this noncovalent interaction must be taken into account to ensure that halogen bonds to the

59 (a) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem. Int. Ed. 2008, 47, 6114–6127. (b) Lommerse, J. P. M.; Stone, A. J.; Taylor, R.; Allen, F. H. J. Am. Chem. Soc. 1996, 118, 3108–3116. (c) Ouvrard, C.; Le Questel, J.-Y.; Berthelot, M.; Laurence, C. Acta Crystallogr, Sect. B 2003, 59, 512–526. 60 Legon, A. C. Angew. Chem. Int. Ed. 1999, 38, 2686–2714. 61 Riley, K. E.; Murray, J. S.; Polizter, P.; Concha, M. C.; Hobza, P. J. Chem. Theory Comput. 2009, 5, 155–163.

35 acceptor are as close to 180° as possible to achieve the maximum interaction strength. The receptors also showed a preference for halide anions in the order Cl– > Br– > I–.

1.11 Receptors Having Two Halogen-Bond Donor Groups

In an effort to increase the contribution of the XB interaction to guest binding, we designed receptors containing two hydrogen-bond donors (the two hydrogens of the urea) and two halogen-bond donors. If the selectivities for various anions differ between hydrogen-bond donors and halogen-bond donors we anticipated that it would be simpler to observe these trends with an increased contribution from XB. This lead to the development of serinol-based receptors 1.17a and 1.17b. The synthesis of these two receptors is outlined in Figure 1.24.

Figure 1.24. Synthesis of receptors 1.19a and 1.19b.

Reaction of isocyanate 1.08 with serinol (1.17) gave 1-(1,3-dihydroxypropan-2-yl)-3-(4- nitrophenyl)urea (1.18). Due to its polarity this compound was difficult to purify so 1.18 was

36 used without purification. 1H NMR spectroscopy indicated that <5% starting material (1.17) remained after isolation. Diacylation of the alcohols using either 2,3,4,5-tetrafluoro-6- iodobenzoic acid or 2,3,4,5,6-pentafluorobenzoic acid gave receptors 1.19a and 1.19b respectively. The affinities of 1.19a and 1.19b for a series of halides and oxoanions are assembled in Table 1.05.

Table 1.05. Association constants (Ka) of receptors 1.19a and 1.19b with halides and oxoanions and the calculated free energy contributions of the halogen bonding interaction (ΔΔGXB).

-1 Receptor Anion Ka ∆∆GXB (kcalmol ) 1.19a BzO– 1.9 x 105 a –0.2 ± 0.1 1.19b BzO– 1.4 x 105 a 1.19a Cl– 2.1 x 104 a –1.3 ± 0.1 1.19b Cl– 2.5 x 103 a 1.19a Br– 6.5 x 103 b –1.5 ± 0.1 1.19b Br– 4.9 x 102 b 1.19a TsO– 1.1 x 103 b –0.1 ± 0.1 1.19b TsO– 9.6 x 102 b 1.19a I– 5.6 x 102 b –1.4 ± 0.1 1.19b I– 60 b 1.19a HSO – 4.4 x 102 b 4 –0.1 ± 0.1 – 2 b 1.19b HSO4 3.8 x 10 1.19a NO – 2.9 x 102 b 3 –0.1 ± 0.1 – 2 b 1.19b NO3 2.6 x 10 a Binding constants determined by fitting changes in solution absorbance as a function of anion + concentration to a 1:1 binding isotherm (Bu4N counterion, acetonitrile solvent, 295K carried out b in duplicate, uncertainty in Ka values estimated to be ± 20%). Binding constants determined by fitting changes in 1H- and 19F-NMR chemical shift as a function of anion concentration to a 1:1 + binding isotherm (Bu4N counterion, acetonitrile-d3 solvent, carried out in duplicate, uncertainty in Ka values estimated to be ± 20%). The reported values are the averages of Ka determinations using changes in chemical shift for one urea N-H signal and one fluoro substituent.

Comparison of the association constants of 1.19a and 1.19b revealed several interesting properties of the receptors. Firstly, there is a pronounced preference for the halogen-bond donor groups to interact with the halide anions over the oxoanions. 1.19a shows association constants for the halides that are approximately one order of magnitude larger than 1.19b, and the -1 -1 calculated ΔΔGXB for the halide anions are all greater than 1 kcalmol (ΔΔGXB = –1.3 kcalmol ,

–1.5 kcalmol-1 and –1.4 kcalmol-1 for chloride, bromide and iodide respectively). For the oxoanions, however, the association constants are almost identical and calculated ΔΔGXB are all less than –0.2 kcalmol-1. On examination of the 19F NMR chemical shift changes of 1.19a upon titration with [Bu4NBr] and [Bu4NI], the maximum changes in chemical shift (Δδmax) observed were –1.7 and –1.9 ppm respectively. For the oxoanions the Δδmax observed were much smaller – – – (Δδmax = –0.32 ppm (TsO ), –0.34 ppm (HSO4 ), and –0.27 ppm (NO3 )). This information, along with the differences in binding constants of the two receptors, support the idea that receptor 1.19a interacts with halide anions through a combination of hydrogen and halogen bonding, while XB interactions are weak or absent in the case of oxoanions. A possible explanation for this selectivity could be a contribution of dispersion or charge transfer to the XB interaction rather than it being a purely electrostatic interaction. This difference in anion preference for hydrogen-bond donor and halogen-bond donors could be interesting in developing receptors which showed altered selectivity patterns. For example, 1.19a is selective for the series – – – – – – – of anions studied in the following order: BzO > Cl > Br > TsO > I > HSO4 > NO3 . The receptor 1.19b is selective for anions in the following different order: BzO– > Cl– > TsO– > Br– > – – – HSO4 > NO3 > I . By comparing these two trends, it can be seen that incorporating a halogen- bond donor into the receptor has increased the receptors selectivity towards halide anions over oxoanions (for example, 1.19a is selective for bromide over tosylate but 1.19b is selective for tosylate over bromide). Previous studies on the anion binding properties of ureas have revealed + the affinity of receptor 1.20 towards anions (in acetonitrile, 298 K, Bu4N counter ion, based on – – – – – log K values) to follow the sequence: CH3COO > C6H5COO > H2PO4 , > NO2 > HSO4 > – 62 NO3 . This order reflects the decrease in basicity of the anions, and log K values correlate linearly with calculated average negative charge on the oxygen atoms. Essentially, the higher the negative charge on the anion, the higher its hydrogen-bond accepting property. This relationship suggests an electrostatic nature to the HB interaction between ureas and anions.62,63 Although not all data points are available, a similar study involving halides as well as oxoanions showed that 1:1 complexes between 1.21 and anions showed decreasing stability following the order:

62 Boiocchi, M.; Del Boca, L.; Gomez, D. E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. J. Am. Chem. Soc. 2004, 126, 16507–16514. 63 Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, 1997.

– – – – – – + CH3COO > Cl > H2PO4 ≈ Br > HSO4 > I (in acetonitrile, 298 K, Bu4N counter ion, based on log K values).64 The available data agrees with the order observed for 1.19b but not with the order observed for 1.19a. This comparison further supports the conclusion that 1.19b is functioning primarily as a hydrogen-bond donor with little or no contribution from XB. The variation in anion selectivity between 1.19a and 1.21 highlights the inherent selectivity of hydrogen and halogen bonds. The difference in selectivity towards halide anions between 1.19a and 1.19b is represented graphically in Figure 1.25.

64 Amendola, V.; Fabbrizzi, L.; Mosca, L.; Schmidtchen F.-P. Chem. Eur. J. 2011, 17, 5972–5981.

Figure 1.25. (a) Plot of –∆Gbinding of 1.19a against –∆Gbinding of 1.19b for the series of anions tested. The dotted line corresponds to y = x. (b) Column graph showing the differences in anion selectivity between receptors 1.19a and 1.19b.

In Figure 1.25 (a) any anions which are not on the line y = x will have some contribution from XB towards anion binding. This graph clearly shows that halide anions deviate significantly from y = x while the other anions show a minimal variance from y = x. Figure 1.25 (b) expresses the same idea, illustrating that major differences in –∆Gbinding are observed only for the halide anions. It should be noted that there appears to be a small contribution of XB for the benzoate anion which, although much less than that observed for the halides, is greater than that observed

40 for the other oxoanions. Additional symmetrical receptors were developed (by Michael Chudzinski)1 to further probe the effect of the linkers as well as investigating the effect of including a weaker hydrogen-bond donor (dialkyl urea versus aryl-alkyl urea or diaryl urea). The anion binding data for receptors 1.22a and 1.22b are assembled in Table 1.06.

Figure 1.26. Structures of symmetrical receptors 1.22a and 1.22b.

Out of all of the receptors investigated in this study, receptor 1.22a shows the largest contribution of XB to the overall thermodynamics of anion binding. The association constant for the iodinated receptor (1.22a) was ~30 times higher than for the perfluorinated control receptor (1.22b). The same general trends were observed for the 1.22a/1.22b receptor pair as for the 1.19a/1.19b pair, in that the incorporation of a halogen-bond donor had the greatest effect on the binding of halide anions. In the case of 1.22a a significant contribution of XB interactions – – towards the oxoanions H2PO4 and BzO was also observed, although this contribution was less than for the halide anions. As XB involving phosphate oxygens has been proposed as a noncovalent interaction responsible for the control of the secondary structure of modified DNA oligomers,65 this study shows that these interactions are indeed attractive (although weakly so) in acetonitrile solution.

65 Auffinger, P.; Hays, F. A.; Westhof, E.; Ho, P. S. Proc. Natl. Acad. Sci. USA 2004, 101, 16789–16794.

Table 1.06. Association constants (Ka) of receptors 1.22a and 1.22b with halides and oxoanions and the calculated free energy contributions of the halogen bonding interaction (ΔΔGXB).

-1 Receptor Anion Ka ∆∆GXB (kcalmol ) 1.22a BzO– 3.3 x 103 a,c –0.7 ± 0.1 1.22b BzO– 1.0 x 103 a,c 1.22a Cl– 2.4 x 103 a,c –2.0 ± 0.1 1.22b Cl– 86 a,c 1.22a H PO – 8.3 x 102 a,c 2 4 –0.8 ± 0.1 – 2 a,c 1.22b H2PO4 2.0x 10 1.22a Br– 7.0 x 103 a,c –2.0 ± 0.1 1.22b Br– 24 a,c 1.22a I– 1.4 x 102 a,c –1.7 ± 0.1 1.22b I– 8.5 a,c 1.22a TsO– 54 b,c –0.2 ± 0.1 1.22b TsO– 38 b,c 1.22a HSO – 30 b,c 4 –0.3 ± 0.1 – b,c 1.22b HSO4 19 1.22a NO – 21 a,c 3 –0.2 ± 0.1 – a,c 1.22b NO3 14 a Binding constants determined by fitting changes in 1H- and 19F-NMR chemical shift as a + function of anion concentration to a 1:1 binding isotherm (Bu4N counterion, acetonitrile-d3 solvent, carried out in duplicate, uncertainty in Ka values estimated to be ± 20%). The reported values are the averages of Ka determinations using changes in chemical shift for one urea N-H b - - signal and one fluoro substituent. Association constants for TsO and HSO4 were determined by 19F-NMR spectroscopy because of broadening of the N-H proton signals in the 1H NMR spectra. c Titration carried out by Michael Chudzinski.

Figure 1.27 represents graphically the difference in selectivity towards anions between 1.22a and 1.22b. Again, anions which are not on the line y = x will have some contribution from XB towards anion binding. The further above the line y = x (ie. –ΔΔGbinding(1.22a) > –ΔΔGbinding(1.22b)) the data point for that particular anion the greater the contribution of XB to the thermodynamics of binding. In this case (as opposed to Figure 1.25 (a)) all of the anions are off the line y = x to some degree. The halide anions, as previously mentioned, exhibit the largest contributions from -1 – – – -1 XB (~ 1 kcalmol ) while NO3 , HSO4 , and TsO show the least (< 0.2 kcalmol ). It is – – interesting to note that both H2PO4 and BzO show a small yet observable contribution from XB (0.3 kcalmol-1 and 0.4 kcalmol-1 respectively.

Figure 1.27. Plot of -∆Gbinding of 1.22a against -∆Gbinding of 1.22b for the series of anions tested. The dotted line corresponds to y = x.

1.12 Anion-Arene Interactions in Perfluorinated Receptors

As mentioned earlier, the perfluoroaryl receptors used as controls in the previous experiments are technically not innocent in terms of their ability to interact with anions through noncovalent interactions. Electron-deficient aryl groups are known to form arene-anion interactions.66 To investigate whether the perfluoroaryl receptors were actually useful control groups, receptor 1.23 was synthesized. Comparison of the anion binding behavior of 1.19b and 1.23 was also useful for investigating potential anion-arene interactions. The results of these experiments are summarized in Table 1.07. Values of ∆∆GAA were calculated according to equation (8) and represent an estimate of the contribution of arene-anion interactions to anion binding.

∆∆GAA = (∆GF - ∆GH) = (∆G1.19b - ∆G1.23) (8)

66 (a) Albrecht, M.; Wessel, C.; de Groot, M.; Rissanen, K.; Lüchow, A. J. Am. Chem. Soc. 2008, 130, 4600–4601. (b) Albrecht, M.; Müller, M.; Mergel, O.; Rissanen, K.; Valkonen, A. Chem. Eur. J. 2010, 16, 5062–5069.

Figure 1.28. Structure of receptor 1.23.

Table 1.07. Association constants (Ka) of receptors 1.19b and 1.23 with halides and oxoanions and the calculated free energy contributions of the arene-anion interaction (ΔΔGAA).

-1 Receptor Anion Ka ∆∆GAA (kcalmol ) 1.19b BzO– 1.4 x 105 a –0.5 ± 0.1 1.23 BzO– 6.6 x 104 a 1.19b Cl– 2.5 x 103 a –0.1 ± 0.1 1.23 Cl– 2.3 x 103 a 1.19b Br– 4.9 x 102 b –0.1 ± 0.1 1.23 Br– 4.6 x 102 b 1.19b TsO– 9.6 x 102 b –0.3 ± 0.1 1.23 TsO– 5.5 x 102 b 1.19b I– 60 b 0 ± 0.1 1.23 I– 55 b 1.19b HSO – 3.8 x 102 b 4 –0.3 ± 0.1 – 2 b 1.23 HSO4 2.3 x 10 1.19b NO – 2.6 x 102 a 3 –0.3 ± 0.1 – 2 a 1.23 NO3 1.6 x 10 a Binding constants determined by fitting changes in solution absorbance as a function of anion + concentration to a 1:1 binding isotherm (Bu4N counterion, acetonitrile solvent, 295 K carried b out in duplicate, uncertainty in Ka values estimated to be ± 20%). Binding constants determined by fitting changes in 1H- and 19F-NMR chemical shift as a function of anion concentration to a + 1:1 binding isotherm (Bu4N counterion, acetonitrile-d3 solvent, carried out in duplicate, uncertainty in Ka values estimated to be ± 20%). The reported values are the averages of Ka determinations using changes in chemical shift for one urea N-H signal and one fluoro substituent.

Data from Table 1.07 suggests that the perfluorinated receptors such as 1.19b were valid control receptors for estimating the contribution to anion binding from XB. The contributions from arene-anion interactions were found to be quite low, especially for the halide anions. The -1 -1 -1 calculated values for ∆∆GAA were –0.1 kcalmol , –0.1 kcalmol , and 0 kcalmol for chloride, bromide and iodide respectively. It was somewhat surprising that moderate values for ∆∆GAA were calculated for the oxoanions (–0.3 kcalmol-1 to –0.5 kcalmol-1) as previous publications suggest these interactions should be weak in acetonitrile solvent.67 However, the significant 19 change in F NMR (∆δmax ~ –0.7 ppm) chemical shift upon anion binding and a computed (by ' – Michael Chudzinski, DFT, B3LYP/6-31++G(d,p), gas phase) structure of a 1.23 -NO3 complex showing a short nitrate-arene contact (dO•••C = 3.25 Ǻ) suggest an anion-arene interaction could be possible.

1.13 Conclusions

These studies have led to several revelations concerning XB and the design of anion receptors. The linker joining the HB moiety to the halogen-bond donor was crucial in observing free energy contributions from XB. Previous studies of XB in the solid and gas phase indicate a strong preference for halogen bonds to adopt a linear EWG–X•••B geometry. Now computational data, along with the solution phase studies detailed above, also suggest that the ability to access this linear geometry is important in determining the strength of halogen bonding interactions in solution. This knowledge will be important in the design of future receptors hoping to capitalize on using these noncovalent interactions in molecular recognition. These studies also indicate that an anion engaged in HB with a urea can also participate in XB with an iodoperfluoroaryl group. -1 Values of ∆ΔGXB were found to be large for the halides (~ 1 kcalmol per halogen bond), small – – -1 for BzO and H2PO4 (< 0.4 kcalmol per halogen bond) and negligible for most other oxoanions -1 - - - ( < 0.2 kcalmol for TsO , HSO4 , and NO3 ). The differences in anion selectivity observed between the iodinated and the perfluorinated receptors suggest the affinities of halogen-bond donors and hydrogen-bond donors towards anions varies. These distinct preferences could provide an approach to modulating the selectivitiy of anion receptors by incorporating different

67 (a) Berryman, O.B.; Sather, A. C.; Hay, B. P. Meisner, J. S.; Johnson, D. W. J. Am. Chem. Soc. 2008, 130, 10895–10897. (b) Gil-Ramírez, G.; Escudero-Adán, E. C.; Benet-Buchholz, J.; Ballester, P. Angew. Chem. Int. Ed. 2008, 47, 4114–4118.

45 types of donor groups. For example, an anion receptor containing hydrogen bond donors might be made more selective towards halide anions by including a halogen-bond donor in the receptor design.

1.14 Advances in Halogen Bonding

Halogen bonding has certainly not been a static field of research since these studies have been undertaken. This section highlights some of the recent advances in the field after the publication of the work described previously in 2011. Further studies examining the correlations between computation and experimental thermodynamics of halogen bonding68 compounded on the idea that for certain halogen-bond donor-acceptor pairs, the Hunter model did not accurately describe the experimental results. The model was particularly poor at predicting free energies of XB involving inorganic halogen-bond donors such as iodine, suggesting that charge-transfer contributions are more significant in the interactions with I2 than with iodoperfluorocarbon (organic) donors. However, an excellent correlation was found between experimentally determined free energies of halogen-bond complexes in alkanes or carbon tetrachloride and calculated (B97-1/6-31+G(d,p)-LANL2DZdp) energies of interaction for the model complexes for both inorganic and organic halogen-bond donors. This result highlights computational chemistry as a useful tool in predicting the strength of halogen bonding interactions if the correct methods are used.

68 Chudzinski, M. G.; Taylor, M. S. J. Org. Chem. 2012, 77, 3483–3491.

Figure 1.29. Ditopic ion transport systems.

Matile and coworkers have developed a series of ditopic ion transport systems shown in Figure 1.29.69 These molecules are capable of transporting anions and cations across a lipid bilayer. The electron rich calix[4]arenes are known to bind ammonium cations and the groups attached through the ether linkage were varied to assess their affect on anion binding. These variable groups were capable of interacting with anions through HB, XB or anion-π interactions. The success of the ion pair transporters was based on their EC50 value, the effective concentration required for half-maximal rates of Me4NCl - Me4NOH symport across egg yolk phosphatidylcholine-based large unilamellar vesicles. The EC50 values for each transporter as well as their Hill coefficients (n) which indicate the degree of cooperativity in the ion transport process are listed in Figure 1.29. Comparing 1.24e and 1.24f indicates that including a halogen- bond donor can increase the efficiency of the transporter although this is not always the case

69 Jentzsch, A. V.; Emery, D.; Mareda, J.; Metrangolo, P.; Resnati, G.; Matile, S. Angew. Chem. Int. Ed. 2011, 50, 11675–11678.

(compare 1.24a and 1.24b). Although 1.24b was shown to bind chloride anions by 19F NMR spectroscopy (KD = 18 mM for Bu4NCl), it was not an efficient transporter. Comparison of 1.24b and 1.24e indicates that the strength of the noncovalent interactions are of crucial importance in obtaining efficient transport and that the stronger halogen bond donors will not always lead to the most efficient transport. Metrangolo and Resnati have reported a 2-iodo-imidazolium 70 receptor which binds oxoanions via charge-assisted halogen bonding in DMSO-d6. The + structure of these receptors as well as their association constants with the Bu4N salts of various anions are shown in Figure 1.30.

Receptor -1 Anion Ka (M ) – 3 1.25a H2PO4 1.1 x 10 – 1.25b H2PO4 26 – 1.25a AcO 2.6 x 102 – 1.25b AcO 23 1.25a Cl– 1.5 x 102 1.25b Cl– 9 1.25a Br– 67 1.25a I– 34

-1 + Figure 1.30. Association constants, Ka (M ) between 1.25a and 1.25b and Bu4N salts at 300 K in DMSO-d6.

This study showed that XB can occur in competitive solvents (although it was charge assisted XB). In opposition to the results obtained in our group, the 2-iodoimidazolium receptor is – – selective for the oxoanions H2PO4 and AcO over the halide anions. These contrasting results indicate additional investigation into halogen-bond donor receptors is necessary in order to clarify the observed trends.

1.15 Experimental Details

General Procedures. All reactions were carried out in oven-dried glassware fitted with rubber septa under nitrogen atmosphere. Stainless steel syringes were used to transfer air- and moisture

70 Cametti, M.; Raatikainen, K.; Metrangolo, P.; Pilati, T.; Terraneo, G.; Resnati, G. Org. Biomol. Chem. 2012, 10, 1329-1333.

48 sensitive liquids. Flash chromatography was performed using silica gel 60 (230-400 mesh) from Silicycle.

Materials. Commercial reagents were purchased from Sigma Aldrich, Alfa Aesar, and TCI and were used as received with the following exceptions: Acetonitrile, THF and dichloromethane were purified by passing through two columns of activated alumina under argon (Innovative Technology, Inc.). Nuclear magnetic resonance (NMR) solvents were purchased from Cambridge Isotope Laboratories.

Instrumentation. Proton nuclear magnetic resonance (1H NMR) spectra, carbon nuclear magnetic resonance (13C NMR) spectra and fluorine nuclear magnetic resonance (19F NMR) spectra were recorded on a 300-MHz or 400-MHz Varian Mercury spectrometer or a 400-MHz Bruker spectrometer. Chemical shifts for protons are reported in parts per million (ppm) downfield from tetramethylsilance and are referenced to residual protium in the NMR solvent

(MeCN-d2: δ 1.94; DMSO-d5: δ 2.50). Chemical shifts for carbon are reported in parts per million downfield from tetramethylsilane and are referenced to the carbon resonances of the 13 solvent (CD3CN: δ 1.24; DMSO: δ 39.43). Partial C spectra are reported for perfluoroarene containing receptors in certain cases. Data are represented as follows; chemical shift (δ, ppm); multiplicity (s-singlet, d-doublet, t-triplet, q-quartet, m = multiplet); coupling constant (J, Hz); integration). Chemical shifts for fluorine were recorded in parts per million (ppm) relative to

CFCl3 using trifluorotoluene (δ –63.72 ppm) as an internal standard. High-resolution mass spectra (HRMS) were obtained on a VS 70-250S (double focusing) mass spectrometer at 70 eV. Infrared (IR) spectra were obtained on a Perkin-Elmer Spectrum 100 instrument equipped with a single-bounce diamond / ZnSe ATR accessory. Data are represented as follows: wavenumber (cm–1); intensity (s-strong, m-medium, w-weak).

General Experimental Procedures

General Procedure A: Preparation of arylureas from p-nitrophenylisocyanate

To a solution of amino alcohol (1.0 equiv) in dry CH2Cl2 (0.02 M) was added p- nitrophenylisocyanate (1.0 equiv). A precipitate was formed during addition. The reaction was stirred at 23 oC for 16 hours and then diluted with pentane. The precipitate was collected by vacuum filtration.

General Procedure B: Esterification of urea-functionalized alcohols

Hydroxyurea (1.0 equiv), benzoic acid derivative (1.5 equiv per -OH group), N,N-dimethyl-4- aminopyridine (DMAP) (20 mol %) and N-(3-dimethylaminopyropyl)-N'-ethylcarbodiimide hydrochloride (EDCI•HCl) (1.5 equiv per -OH group) were weighed into a round bottom flask. o Dry CH2Cl2 (0.07 M) was added and the reaction was stirred at 23 C for 24 hours. The reaction was then diluted with H2O and the organic phase was separated. The aqueous layer was extracted three times with CH2Cl2 and the combined organic layers were washed with brine, dried over

MgSO4, filtered and concentrated in vacuo to give a solid that was purified by column chromatography.

Characterization Data

1-(2-(Hydroxyethyl)-3-(4-nitrophenyl)urea (1.10)

Synthesized according to general procedure A, from ethanolamine (0.603 mL, 10.0 mmol) and p- nitrophenylisocyanate (1.64 g, 10.0 mmol) to yield the title compound as a yellow solid (2.03 g, 1 9.0 mmol, 90% yield). H NMR (400 MHz, DMSO-d6): δ 9.34 (s, 1H), 8.13 (d, J = 9.4 Hz, 2H), 7.60 (d, J = 9.4 Hz, 2H), 6.47 (t, J = 5.6 Hz, 1H), 4.79 (t, J = 4.9 Hz, 1H) 3.46 (dt, J = 5.5, 4.9 13 Hz, 2H), 3.18 (dt, J = 5.6, 5.5 Hz, 2H). C NMR (100 MHz, DMSO-d6): δ 154.4, 147.1, 140.3, 125.1, 116.6, 60.0, 41.8 FTIR (powder, cm-1): v 3501 (w), 3277 (w), 3092 (w), 1669 (m), 1565

(m), 1474 (s), 1245 (s), 1073 (m), 855 (s), 751 (m). HRMS (EI): Calculated for [C9H11N3O4] (M)+ 225.0750; found 225.0743.

1-(1,3-Dihydroxypropan-2-yl)-3-(4-nitrophenyl)urea (1.18)

Synthesized according to general procedure A, from 2-amino-1,3-propandiol (750 mg, 8.2 mmol) and p-nitrophenylisocyanate ( 1.35 g, 8.2 mmol) to yield the title compound as a yellow solid (1.60 g, 6.2 mmol, 76% yield) contaminated with residual starting material (1.17) (< 5%) 1 The compound was used without further purification. H NMR (400 MHz, DMSO-d6): δ 9.41 (s, 1H), 8.13 (d, J = 9.3 Hz, 2H), 7.59 (d, J = 9.3 Hz, 2H), 6.35 (d, J = 8.1 Hz, 1H), 4.77 (m, 2H) 13 3.68-3.58 (m, 1H), 3.46-3.15 (m, obscured by residual H2O peak). C NMR (75 MHz, DMSO- d6): δ 154.1, 147.1, 140.2, 125.1, 116.5, 63.1, 59.8.

2-(3-(4-Nitrophenyl)ureido)ethyl 2,3,4,5-tetrafluoro-6-iodobenzoate (1.14a)

Synthesized according to general procedure B, from 1-(2-hydroxyethyl)-3-(4-nitrophenyl)urea (141 mg, 0.63 mmol) and 2,3,4,5-tetrafluoro-6-iodobenzoic acid (300 mg, 0.94 mmol).

Purification by column chromatography (MeOH/CH2Cl2, 1:99) afforded the title compound as a 1 yellow solid (224 mg, 68% yield); Rf = 0.?? (MeOH/CH2Cl2, 1:99) H NMR (300 MHz,

CD3CN): δ 8.12 (d, J = 9.2 Hz, 2H), 7.82 (s, 1H), 7.61 (d, J = 9.2 Hz, 2H), 5.59 (m, 1H), 4.49 (t, 13 J = 5.7 Hz, 2H) 3.58 (dt, J = 5.7, 5.7 Hz, 2H), C NMR (100 MHz, CD3CN, partial): δ 163.5, 19 155.4, 147.4, 142.6, 125.8, 118.2, 66.7, 39.4. F NMR (282 MHz, CD3CN) δ –115.8 (m, 1F), – 139.7 (m, 1F), –152.5 (m, 1F), –155.2 (m, 1F) FTIR (powder, cm-1): v 1736 (s), 1664 (m), 1559 (m), 1498 (s), 1466 (m), 1330 (m), 1215 (s), 1108 (m). HRMS (EI): Calculated for + [C16H11F4IN3O5] (M+H) 527.9674; found 527.9681.

(3-(4-Nitrophenyl)ureido)ethyl 2,3,4,5,6-pentafluorobenzoate (1.14b)

Synthesized according to general procedure B, from 1-(2-hydroxyethyl)-3-(4-nitrophenyl)urea

(383 mg, 1.70 mmol) and 2,3,4,5,6-pentafluorobenzoic acid (541 mg, 2.55 mmol) in CH2Cl2 to 1 yield the title compound as a yellow solid (537 mg, 75% yield). H NMR (300 MHz, CD3CN): δ 8.11 (d, J = 9.2 Hz, 2H), 7.84 (s, 1H), 7.59 (d, J = 9.2 Hz, 2H), 5.60 (m, 1H), 4.45 (t, J = 5.5 13 Hz, 2H), 3.57 (dt, J = 5.5, 5.5 Hz, 2H). C NMR (100 MHz, CD3CN, partial): δ 159.8, 155.5, 19 147.4, 142.6, 125.8, 118.1, 66.5, 39.4. F NMR (282 MHz, CD3CN): δ –140.6 (m, 2F), –151.5

(m, 1F), –163.4 (m, 2F). FTIR (powder, cm-1): ν 1743 (m), 1699 (m), 1524 (m), 1488 (s), 1321 + (s), 1205 (s), 1102 (m). HRMS (EI): Calculated for [C16H11F5N3O5] (M+H) 420.0613; found 420.0612.

Compound (1.19a)

Synthesized according to general procedure B, from 1-(1,3-dihydroxypropan-2-yl)-3-(4- nitrophenyl)urea (253 mg, 0.99 mmol) and 2,3,4,5-tetrafluoro-6-iodobenzoic acid (950 mg, 2.97 1 mmol) in CH2Cl2 to yield the title compound as a yellow solid (368 mg, 43% yield). H NMR

(400 MHz, CD3CN): δ 8.13 (d, J = 9.2 Hz, 2H), 7.86 (s, 1H), 7.61 (d, J = 9.2 Hz, 2H), 5.69 (m, 13 1H), 4.59 (m, 5H). C NMR (100 MHz, CD3CN, partial): δ 163.2, 154.7, 147.0, 142.9, 125.8, 19 118.4, 66.9, 48.5. F NMR (282 MHz, CD3CN): δ –115.5 (m, 1F), –139.0 (m, 1F), –152.0 (m, 1F), –155.1 (m, 1F). FTIR (powder, cm-1): ν 1740 (m), 1613 (w), 1550 (m), 1502 (s), 1457 (s), + 1328 (m), 1110 (m). HRMS (EI): Calculated for [C24H12F8I2N3O7] (M+H) 859.8631; found 859.8607.

Compound (1.19b)

Synthesized according to general procedure B, from 1-(1,3-dihydroxypropan-2-yl)-3-(4- nitrophenyl)urea (305 mg, 1.20 mmol) and 2,3,4,5,6-pentafluorobenzoic acid (760 mg, 3.59 1 mmol) in CH2Cl2 to yield the title compound as a yellow solid (432 mg, 56% yield). H NMR

(300 MHz, CD3CN): δ 8.12 (d, J = 9.3 Hz, 2H), 7.86 (s, 1H), 7.59 (d, J = 9.3 Hz, 2H), 5.67 (m, 13 1H), 4.55 (m, 5H). C NMR (100 MHz, CD3CN, partial): δ 159.6, 154.9, 147.1, 142.9, 125.8,

19 118.3, 65.9, 48.3. F NMR (282 MHz, CD3CN): δ –140.2 (m, 2F), –150.9 (m, 1F), –163.3 (m, 2F). FTIR (powder, cm-1): ν 1741 (m), 1492 (s), 1457 (s), 1328 (s), 1179 (m), 1111 (m). + HRMS (EI): Calculated for [C24H12F10N3O7] (M+H) 644.0510; found 644.0482.

Compound (1.23)

Synthesized according to general procedure B, from 1-(1,3-dihydroxypropan-2-yl)-3-(4- nitrophenyl)urea (300 mg, 1.18 mmol) and benzoic acid (430 mg, 3.53 mmol) in CH2Cl2 to yield 1 the title compound as a yellow solid (403 mg, 74% yield). H NMR (300 MHz, CD3CN): δ 8.11 (d, J = 9.3 Hz, 2H), 8.04 (d, J = 7.1 Hz, 4H), 7.78 (s, 1H), 7.60 (m, 4H), 7.47 (m, 4H), 5.85 (m, 13 1H), 4.60 (m, 1H), 4.52 (d, J = 4.3 Hz, 4H). C NMR (100 MHz, CD3CN): δ 166.9, 155.0, 147.2, 142.7, 134.2, 130.8, 130.4, 129.5, 125.8, 118.2, 64.9, 48.9. FTIR (powder, cm-1): ν 1717 (m), 1547 (m), 1501 (m), 1327 (m), 1262 (m), 1106 (m). HRMS (EI): Calculated for + [C24H22N3O7] (M+H) 464.1452; found 464.1445.

1H, 19F and UV/Vis Titration Data.

Determination of binding constants by 1H and 19F NMR Spectroscopy.

1H and 19F NMR spectra were recorded on a 300-MHz or 400-MHz Varian Mercury spectrometer or a 400-MHz Bruker spectrometer in deuterated acetonitrile. Care was taken to use dry solvent and to exclude water, as it has a deleterious effect on anion binding. 1H NMR chemical shifts are reported relative to the residual solvent peak (δ = 1.94 ppm). 19F NMR chemical shifts are reported relative to CFCl3 using α,α,α-trifluorotoluene as an internal standard (δ = -63.72 ppm). A solution of halogen bond donor and α,α,α-trifluorotoluene was prepared. This solution was then used as the parent solution to prepare a reference standard and anion solution (as its tetrabutylammonium salt). Titration experiments were carried out by mixing different proportions of the parent solution and the anion solution by syringe. According to the method of Hirose,58 host and guest concentrations were chosen such that the occupancy ratio, β =

[complex]/[host]total would fall in the range 0.2 < β < 0.8. Determinations of Ka were based on changes in chemical shifts of the proton and fluorine substituents of the receptors. For Ka determinations by 1H NMR spectroscopy, the chemical shift of the urea proton adjacent to the aryl group was monitored for 1.14a, 1.14b, 1.15a, 1.15b, 1.16a, 1.16b, 1.19a, and 1.19b. The symmetrical urea protons were monitored for 1.22a and 1.22b. For 19F NMR spectra, the fluorine meta to the ester group was monitored for the perfluorinated receptors while the fluorine ortho to the iodo group was monitored for the iodinated receptors. Upon addition of halogen-bond acceptor, downfield changes in 1H NMR chemical shift were observed as well as downfield shifts of the fluorines corresponding to position 2 and 6 in the pentafluorinated receptors in 19F NMR spectroscopy. For all other fluorine signals in both the iodotetrafluorinated and pentafluorinated receptors, upfield changes in 19F chemical shift were observed. Using the method described by Hughes and coworkers,71 graphs of Δδ (change in 1H or 19F NMR chemical shift) plotted against [acceptor] were curve-fitted to a 1:1 binding isotherm using Igor Pro 5.0 (WaveMetrics, Inc.). In brief, equation (9) describes 1:1 binding that is rapid on the NMR time scale:

Δδ = (δmax - δ0)([C]/[D]0) (9) where Δδ is the observed change in chemical shift, δmax is the chemical shift of the bound halogen bond donor, δ0 is the chemical shift of the free halogen bond donor, [D]0 is the total concentration of donor (bound and unbound) in solution and [C] is the concentration of the donor-acceptor complex and can be determined by solving the quadratic equation (10): 2 [C] + (-[D]0 - [A]0 - 1/Ka)[C] + [D]0[A]0 = 0 (10) where [A]0 is the total concentration of acceptor (bound and unbound) in solution and Ka is the association constant. Nonlinear curve-fitting of the experimental Δδ vs [A]0 to this expression, from experiments of a known [D]0, gave values of δ0, δmax, and Ka, from which [C] could be determined. The curve-fitting was repeated 2 times using two different sets of initial guess values for the afore mentioned parameters. The repeats yielded stable values in all cases. Job plots were created using either 1H NMR or 19F NMR by plotting |Δδ·χ| against χ.

71 Hughes, M. P.; Shang, M.; Smith, B. D. J. Org. Chem. 1996, 61, 4510-4511.

Determination of binding constants by UV/Vis spectroscopy. Optical absorption spectra were collected at room temperature on a Perkin-Elmer Lambda 35 spectrometer in nondeuterated acetonitrile and corrected for background signal with a solvent- filled 1.00 cm quartz cuvette. Stock solutions of host and guest were prepared as described above. Absorption titrations were carried out by sequentially adding aliquots of acceptor solution via syringe to a 2.50 mL solution of host halogen bond donor. Prior to obtaining an absorption spectra the solution was stirred for 30 seconds after each addition. Curve-fitting was performed as described previously for the NMR experiments, with absorption in place of chemical shift. For binding constants calculated based on UV/Vis titrations, changes in absorbance ΔA are plotted against acceptor concentration (M). The curves obtained by fitting to a 1:1 binding isotherm are shown. Job plots were created using either 1H NMR or 19F NMR spectra by plotting |Δδ·χ| against χ.

Chapter 2 Exploration of Halogen-Bond Donors as Catalysts 2.1 Introduction

Analogies are often drawn between XB and HB,72 although the solution phase studies discussed in the previous chapter revealed some fundamental differences in terms of the selectivities of halogen- and hydrogen- bond donor groups for certain classes of anions. Halogen bonds may share some of the characteristics of hydrogen bonds but differ in others. Both noncovalent interactions are capable of acting as strong, specific, and directional interactions to form well- defined supramolecular structures in the solid state, yet the optimal geometries between Lewis bases and hydrogen- and halogen-bond donors may differ. While hydrogen-bond donors are generally hydrophilic (as then can hydrogen bond to water molecules) many halogen-bond donors are hydrophobic.73 As the hydrogen bond has been incorporated into catalysts and exploited as both a reaction promoter and a key noncovalent interaction in enantioselective catalysis, we were wondering if halogen bonds could function in the same fashion. This chapter will focus on the potential application of XB in catalysis.

2.2 Inspiration from Hydrogen Bonding Catalysis

Early investigations into the mechanisms of various enzymes identified hydrogen bonding as a key noncovalent interaction in electrophilic activation and catalysis.74 Realizing that same mode of activation might be applied to small molecule catalysts, chemist began to use simple hydrogen-bond donor molecules as reaction promoters. In addition to taking inspiration from enzyme catalysis, reports of hydrogen bonded co-crystal complexes, such as the seminal papers of Etter which describe the arrangements of hydrogen-bond donors and acceptors in the solid

72 Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Acc. Chem. Res. 2005, 38, 386–395. 73 Andreas, V. J.; Andreas, H.; Jiri, M.; Stefan, M. Acc. Chem. Res. 2013 Ahead of Print. 74 (a) Cleland, W. W.; Kreevoy, M. M. Science, 1994, 264, 1887–1890. (b) Frey, P. A.; Whitt, S. A.; Tobin, J. B. Science, 1994, 264, 1927–1930. (c) Warshel, A.; Papzyan, A.; Kollman, P. A.; Cleland, W. W.; Kreevoy, M. M.; Frey, P. A. Science, 1995, 269, 102–106.

57 state,75 also influenced the development of hydrogen bond catalysts. If development of catalysts containing hydrogen-bond donors stemmed, at least in part, from studies of the interaction in the solid state, could the extensive reports on the use of XB in supramolecular chemistry be used to stimulate the development of catalysts containing halogen-bond donors? The examples of catalysis by hydrogen bond donors examined here is by no means exhaustive. A number of excellent, comprehensive reviews have been published detailing the history and recent advancements in hydrogen bond catalysis.76 The examples described in the following paragraphs were chosen as they required relatively weak hydrogen bond donors to observe activation (this was thought to give the best chance at halogen bond donor activation) or they were catalyzed by thioureas or ureas (as we had previously developed urea based anion receptors composed of both hydrogen- and halogen-bond donors, we were interested in exploring how incorporation of halogen bond donors into catalysts already containing hydrogen bond donors affected reactivity). In 2004 Ricci reported the Friedel-Crafts alkylation of aromatic and heteroaromatic systems with nitroolefins by HB organocatalysts, namely ureas and thioureas.77 A variety of N- methylpyrroles, anilines and indoles were efficient as the aromatic components of the reaction. An example is shown in Figure 2.01. Greater reactivity was observed for the thiourea catalyst 2.05 over the urea catalyst 2.04. This trend in results has been rationalized by the greater hydrogen-bond donor ability of thioureas compared to ureas.78 This parallels the increased 79 acidity of thiourea relative to urea (pKHA thiourea = 21.0; pKHA urea = 26.9). Also, the self- association of thioureas realtive to ureas occurs to a lesser extent due to the decreased hydrogen-

75 (a) Etter, M. C.; Panunto, T. W. J. Am. Chem. Soc. 1988, 110, 5896–5897. (b) Etter, M. C. Acc. Chem. Res. 1990, 23, 120–126. (c) Panunto, T. W.; Urbánczyk-Lipkowska, Z.; Johnson, R.; Etter, M. C. J. Am. Chem. Soc. 1987, 109, 7786–7797. 76 (a) Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107, 5713–5743. (b) Taylor, M. S.; Jacobsen, E. N. Angew. Chem. Int. Ed. 2006, 45, 1520–1543. (c) Pihko, P. M. Angew. Chem. Int. Ed. 2004, 43, 2062–2064. (d) Dalko, P. I.; Moisan, L. Angew. Chem. Int. Ed. 2004, 43, 5138–5175. (e) Schreiner, P. R. Chem. Soc. Rev. 2003, 32, 289–296. 77 Dessole, G.; Herrera, R. P.; Ricci, A. Synlett 2004, 13, 2374–2378. 78 Wittkopp, A.; Schreiner, P. R. Chem. Eur. J. 2003, 9, 407–414. 79 Bordwell, F. G.; Algrim, D. J.; Harrelson, J. A. Jr. J. Am. Chem. Soc. 1988, 110, 5903–5906.

58 bond donor ability of the thiocarbonyl sulfur atom compared to the carbonyl oxygen atom leading to decreased intermolecular hydrogen bonding.80

Figure 2.01. Friedel-Crafts alkylation of heteroaromatics with nitroolefins.

An enantioselective variant of the Friedel-Crafts type addition of indoles to nitroolefins was developed by Jørgensen in 2005 employing the chiral hydrogen-bonding catalyst (1R ,2R)-(+)- N,N'-bis-trifluoromethansulfonyl-1,2-diphenylethylenediamine (2.06).81 Moderate enantioselectivities were observed, up to 64% ee, depending on the identity of the substrate.

Figure 2.02. Enantioselective Friedel-Crafts addition of indoles to nitroolefins.

XB catalysts have also been used to catalyze hetero-Diels-Alder (HDA) reactions. While studying the solvent effect on the HDA reaction between 1-amino-3-silyloxybutadiene (2.07) and

80 Scheerder, J.; Engbersen, J. F. J.; Casnati, A.; Ungaro, R.; Reinhoudt, D. N. J. Org. Chem. 1995, 60, 6448–6454. 81 Zhuang, W.; Hazell, R. G.; Jørgensen, K. A. Org. Biomol. Chem. 2005, 3, 2566–2571.

59 aldehydes, the group of Rawal noticed significantly higher reaction rates in chloroform than other aprotic solvents.82 The observed rates are summarized in Table 2.01.

Table 2.01. Rates of HDA reactions in different deuterated solvents.

Solvent Rate Constant (k) Relative Rate

-5 THF-d8 1.0 x 10 1 -5 benzene-d6 1.3 x 10 1.3 -5 acetonitrile-d3 3.0 x 10 3 chloroform-d 3.0 x 10-4 30 -3 tert-butyl alcohol-d10 2.8 x 10 280 -3 isopropyl alcohol-d8 6.3 x 10 630

The authors concluded that the increased rate observed in solvents capable of acting as hydrogen bond donors could arise from a R—H•••O hydrogen bond between the solvent (R—H) and the carbonyl oxygen atom of the dienophile. Activation of the carbonyl group through HB would make it a better heterodienophile. The possibility of rate increase due to trace amounts of acid was discounted as the measured rate in chloroform was identical after rigorous purification of the solvent and after addition of triethylamine. Also, addition of a catalytic amount of HCl neither increased nor decreased the reaction rate. Using tert-butyl alcohol as the solvent, reactions were even possible with less reactive ketone dienophiles. The reaction was rendered enantioselective using either a TADDOL (α,α,α',α' -tetraaryl-1,3-dioxolan-4,5-dimethanol) catalyst83 (2.10) or a BAMOL (1,1'-biaryl-2,2'-dimethanol) catalyst (2.11).84

82 Huang, Y.; Rawal, V. H. J. Am. Chem. Soc. 2002, 124, 9662–9663. 83 Huang, Y.; Unni, A. K.; Thadani, A. N.; Rawal, V. H. Nature, 2003, 424, 146. 84 Unni, A. K.; Takenaka, N.; Yamamoto, H.; Rawal, V. H. J. Am. Chem. Soc. 2005, 127, 1336–1337.

Figure 2.03. Hetero-Diels-Alder reactions catalyzed by chiral hydrogen-bond donor catalysts.

Schreiner has reported the thiourea-catalyzed transfer hydrogenation of aldimines using electron deficient biaryl thioureas as catalysts and Hantzsch ester as the hydrogen source.85 Hydrogen bonding of the thiourea to the imine nitrogen was proposed to activate the imine and promote hydrogen transfer from the Hantzsch ester to give the amine as product. In the absence of catalyst only a trace amout of product was observed but excellent yields were obtained in the presence of 2.05. Catalyst loadings as low as 0.1 mol% were tolerated and a variety of electron poor or electron rich aryl aldimines as well as aliphatic aldimines were competent as substrates in the reaction, although ketimines were not tolerated.

85 Zhang, Z.; Schreiner, P. R. Synlett, 2007, 9, 1455–1457.

Figure 2.04. Reduction of aldimines using diaryl thioureas.

As a final model reaction, we were interested in looking at the acyl-Pictet-Spengler reaction.86 Unlike normal Pictet-Spengler reactions, acyl-Pictet-Spengler reactions proceed through intermediate N-acyliminium ions. As Pictet-Spengler reactions most often require strong Brønsted acids, high reaction temperatures or highly reactive reagents, this reaction was not envisioned as a likely milieu to observe XB catalysis. The N-acyliminium ions of the acyl-Pictet- Spengler reaction, however, are much more reactive electrophiles.

Figure 2.05. Acyl-Pictet-Spengler reaction.

In 2004 Taylor et al. developed an enantioselective catalytic acyl-Pictet-Spengler reaction using chiral thiourea catalysts.87 Previous results from the Jacobsen group had demonstrated that chiral thiourea catalysts such as 2.14 were capable of inducing enantioselective additions of nucleophiles to N-alkyl88 and N-tert-butoxycarbonyl imines.89

86 (a) Venkov, A. P.; Lukanov, L. K.; Synthesis 1989, 89, 59–61. (b) Venkov, A. P.; Boyadjieva, A. K. Synth. Commun. 1999, 29, 487–494. 87 Taylor, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 2004, 126, 10558–10559. 88 (a) Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 4901–4902. (b) Sigman, M. S.; Vachal, P.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2000, 39, 1279–1281. (c) Vachal, P.; Jacobsen, E. N. J. Am. Chem.Soc. 2002, 124, 10012–10014. (d) Joly, G. D.; Jacobsen, E. N. J. Am.Chem. Soc. 2004, 126, 4102–4103.

Figure 2.06. Acyl-Pictet-Spengler reaction catalyzed by chiral thioureas.

Through extensive catalyst optimization, varying both the alkyl group on the amide nitrogen and the identity of the group on the cyclohexyldiamine moiety, catalyst 2.15 was developed and was shown to catalyze the reaction in 75% yield and 93% ee. Although the initial study didn't detail an examination of the mechanism of the reaction, an additional investigation into a similar transformation by Raheem et al. suggests anion binding plays a crucial role in catalysis.90 The authors proposed the reaction mechanism for the enantioselective Pictet-Spengler-type cyclization of hydroxylactams by hydrogen-bond donor catalysts follows an SN1 pathway involving catalyst assisted dissociation of the halide counterion to form a chiral N-acyliminium ion.90 The chiral thiourea bound ion pair then undergoes an enantioselective cyclization reaction. Experiments showing significant effects of the halide identity as well as solvent effects support this mechanism.

89 (a) Wenzel, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2002, 124, 12964–12965. (b) Wenzel, A. G.; Lalonde, M. P.; Jacobsen, E. N. Synlett 2003, 12, 1919–1922. Takemoto, Y. Org. Lett. 2004, 4, 625–627. 90 Raheem, I. T.; Thiara, P. S.; Peterson, E. A.; Jacobsen, E. N. J. Am. Chem. Soc. 2007, 129, 13404–13405.

Figure 2.07. Proposed reaction mechanism of Pictet-Spengler-type cyclizations of hydroxylactams. 2.3 Halogen Bonding Catalysis and Activation

The use of iodine as an accelerator of organic reactions has been well studied.91 The catalytic activity imparted by iodine in these reactions is often attributed to the formation of HI or TMSI under the reaction conditions. However, there are some cases where control reactions employing HI directly give no reaction or poor yields suggesting molecular iodine may play more of a role than a simple precursor to HI. It is unclear whether this role involves the halogen bond donor ability of iodine. An example of such a reaction is shown in Figure 2.08.92

91 Togo, H.; Iida, S. Synlett, 2006, 2159–2175. 92 Banik, B. K.; Fernandez, M.; Alvarez, C. Tetrahedron Letters, 2005, 46, 2479–2482.

Figure 2.08. Iodine-catalyzed Michael reaction of indoles and unsaturated carbonyl compounds.

The potential use of organic halogen bond donors as catalysts has received considerably less attention than inorganic iodine. A rare example of a reaction being catalyzed by a halogen bond donor has been reported by the group of Bolm. Based on previous studies of molecular aggregation showing attractive interactions between organic bromides and iodides with nitrogen- containing compounds93 their group attempted to activate C=N towards electrophilic hydride addition using haloalkanes. They reported a reduction of 2-phenylquinoline to its 1,2,3,4- tetrahydro derivative using a Hantzsch ester as the reductant and a haloperfluoroalkane as a catalyst.94 The reaction scheme is shown in Figure 2.09 followed by a summary of the catalysts and reaction conditions explored in Table 2.02.

Figure 2.09. Reduction of 2-phenylquinoline in the presence of haloperfluoroalkanes. The results indicate that perfluorinated iodoalkanes were more active catalysts than the corresponding perfluorinated bromoalkanes. This is in agreement with iodine being a better halogen bond donor atom than bromine, all other variables being equal. The catalytic activity

93 (a) Amico, V.; Meille, S. V.; Corradi, E.; Messina, M. T.; Resnati, G. J. Am. Chem. Soc. 1998, 120, 8261–8262. (b) Corradi, E.; Meille, S. V.; Messina, M. T.; Metrangolo, P.; Resnati, G. Angew. Chem. Int. Ed. 2000, 39, 1782– 1786. 94 Bruckmann, A.; Pena, M. A.; Bolm, C. Synlett 2008, 900–902.

was also found to increase with increasing chain length (CF3(CF2)7I > CF3(CF2)6I > CF3(CF2)5I) up until a point (CF3(CF2)9I < CF3(CF2)7I) with perfluorooctyliodide being the optimum catalyst. Mechanistic studies of the reaction, however, revealed some peculiarities. Attempted NMR studies of mixtures of Hantzsch ester and 1-iodoperfluorooctane in the absence of substrate led to decomposition of the haloperfluoroalkane. These decomposition products were not identified.

Table 2.02. Reaction conditions for the reduction of 2-phenylquinoline.

Catalyst Loading Temperature R X Solvent Yield (%) F (mol%) (°C)

none n/a CH2Cl2 25 0 none n/a Toluene 60 0

CF3(CF2)5I 10 CH2Cl2 25 35

CF3(CF2)5I 10 Toluene 60 63

CF3(CF2)5I 10 CH2Cl2 25 20

CF3(CF2)5I 10 Toluene 60 12

CF3(CF2)6I 10 CH2Cl2 25 90

CF3(CF2)7I 10 CH2Cl2 25 98

CF3(CF2)7I 1 CH2Cl2 25 69

CF3(CF2)7Br 10 CH2Cl2 25 38

CF3(CF2)9I 10 CH2Cl2 25 88

The authors suggest the substrate "protects" the perfluoroalkylhalide from decomposition. The reaction was also found to be inhibited by 1,8-bis(dimethylamino)naphthalene (an efficient proton scavenger). This could be explained by competition between the substrate and 1,8- bis(dimethylamino)naphthalene for the halogen-bond donor. However, it may not be possible to completely rule out a contribution of Brønsted acid catalysis to the observed reactivity where the inhibition could be explained by neutralization of the Brønsted acid (perhaps generated through catalyst decomposition) with base.

The group of Herschlag has attempted to evaluate the potential for halogen bonding in the oxyanion hole of ketosteroid isomerase.95 As a model system they evaluated the enzyme ketosteroid isomerase from Pseudomonas putida which converts 5-androstene-3,17-dione (5-

95 Kraut, D. A.; Churchill, M. J.; Dawson, P. E.; Herschlag, D. ACS Chem. Biol.2009, 4, 269.

AND) to 4-androstene-3,17-dione. The mechanism is proposed to proceed through a dienolate intermediate (and therefore dienolate like transition states) which can be stabilized through hydrogen bonding in the oxyanion hole.96 In the native enzyme the transition states are stabilized by the protonated carboxylic acid of Asp103 and the phenol group of Tyr16 (R = OH in Figure 2.10) through hydrogen bonding. To examine the effect substituting a hydrogen-bond donor for a halogen bond donor had on the catalytic activity of the enzyme, a series of semisynthetic analogues were synthesized with various R groups. The results are summarized in Table 2.03.

Figure 2.10. Conversion of 5-androstene-3,17-dione to 4-androstene-3,17-dione by ketosteroid isomerase.

96 (a) Pollack, R. M.; Thomburg, L. D.; Wu, Z. R.; Summers, M. F. Arch. Biochem. Biophys. 1999, 370, 9–15. (b) Ha, N. C.; Choi, G.; Choi, K. Y.; Oh, B. H. Curr. Opin. Struct. Biol. 2004, 11, 674–678. (c) Pollack, R. M. Bioorg. Chem. 2004, 32, 341–353.

The results suggested, at least in this particular case, that the phenol hydrogen-bond donor cannot be replaced with a halogen-bond donor. Compared to the wild type enzyme (R = OH) all of the substitutions (either for halogens, hydrogen or a methyl group) resulted in a decrease in kcat and kcat / KM of approximately 3 to 4 orders of magnitude. The authors proposed that perhaps the oxyanion hole is not flexible enough to accommodate the required geometries or bond lengths required by a halogen bond. Also, the halogen atoms are not attached to electron withdrawing groups so would not be expected to be very good halogen bond donors.

Table 2.03. Substituent effects on the isomerization of 5-AND by ketosteroid isomerase.

-1 -1 -1 R kcat (s ) KM (μM) kcat / KM (M s ) kcat effect kcat / KM effect

OH 1.2 x 104 90 1.3 x 108 1 1 H 4.7 90 6.1 x 104 2500 2100 Me 1.2 30 4.0 x 104 10000 3500 F 4.7 30 1.6 x 105 2500 800 Cl 0.9 15 6.0 x 104 13000 2200 Br 0.7 25 2.8 x 104 17000 4600

Activation of benzhydryl bromide 2.18 with haloimidazolium salts as halogen-bond donors was achieved by Huber when these compounds were used as halide-abstracting agents in a variant of the Ritter reaction (Figure 2.11).97 Formation of a halogen bond between benzhydryl bromide and the haloimidazolium was proposed to either weaken the C—Br bond or lead to its heterolytic cleavage. The deuterated acetonitrile solvent would then act as a nucleophile to form an intermediate nitrilium ion which would be hydrolyzed by trace amounts of water in the solvent to give amide 2.19. In the absence of any activating reagent the yield for the reaction was less than 5%. In the presence of 1 equivalent of 2.20, however, a 54% yield was obtained. Using promoter 2.21 (where the bromine substituents have been replaced with iodine) increased the yield to 85% which would be expected as iodine containing compounds are generally superior halogen-bond donors compared to their bromine counterparts, other factors being equal. The counterion was found to have an effect on reaction yield, as when it was switched from triflate to tetrafluoroborate (promoter 2.22) the yield increased to 97%. In order to test that the observed

97 Walter, S. M.; Kniep, F.; Herdtweck, E.; Huber, S. M. Angew. Chem. Int. Ed. 2011, 50, 7187–7191.

68 reactivity was due to activation by XB and not by trace amounts of Brønsted acid, several control reactions were undertaken. In the presence of one equivalent of HOTf the yield of the reaction was only 25%, which indicated that Brønsted acids can promote the reaction but not as efficiently as the haloimidazoliums.

Figure 2.11. Ritter reaction promoted by haloimidazoliums.

Furthermore, addition of 0.1 equivalents of pyridine to a reaction using 1 equivalent of 2.21 as promoter still gave an 85% yield of product, the same result observed in the absence of pyridine. This strongly suggests the observed reactivity is due to halogen bonding and not due to trace amounts of Brønsted acid. The reaction requires a stoichiometric amount of activator and is therefore not truly catalysis. The authors propose that the bromide anion generated during the reaction binds more tightly to the promoter than does the substrate thereby inhibiting any possible catalyst turnover.

Figure 2.12. Organocatalytic aziridine synthesis using F+ salts.

N-fluoropyridinium salts (such as 2.25) have been shown by the group of Bew to act as catalysts for the addition of ethyldiazoacetate (EDA) to imines to generate N-arylaziridines.98 Although a detailed mechanistic hypothesis was not proposed by the authors, activation of the imine could be due to halogen bonding between the imine nitrogen and the fluorine atom. Although fluorine usually does not act as a halogen bond donor, some exceptions to this rule have been observed.99

2.4 Results and Discussion

The reactions chosen for this study have been described in the introduction (Friedel-Crafts addition of indoles to nitroolefins, hetero-Diels-Alder reaction, reduction of aldimines and the acyl-Pictet-Spengler reaction) and were chosen based on the following reasons; (1) The reactions could be catalyzed by hydrogen-bond donors (as the binding properties of halogen-bond donors are often thought to be similar to those of hydrogen-bond donors perhaps their catalytic abilities would be as well); and (2) the majority of the reactions either required weak hydrogen-bond donors to observe reactivity or proceeded in the absence of catalyst at room temperature but required a catalyst at low temperature. These two factors were viewed as advantageous features as they suggested the activation energies of the reactions were relativley low. As catalysis by halogen-bond donors was not a well established area, reactions which required the catalyst to decrease the activation energy only slightly in order to observe reactivity were thought to offer the best chance of detecting catalysis through XB interactions. A series of molecules were chosen as catalysts based either on their ability to participate in XB in the solution state or on their ability to potentially activate a substrate through both HB and XB. The catalysts are shown in Figure 2.13.

98 Bew, S. P.; Fairhurst, S. A.; Hughes, D. L.; Legentil, L.; Liddle, J.; Pesce, P.; Nigudkar, S.; Wilson, M. A. Org. Lett. 2009, 11, 4552–4555. 99 Metrangolo, P.; Murray, J. S.; Pilati, T.; Politzer, P.; Resnati, G.; Terraneo, G. Cryst. Growth Des. 2011, 11, 4238–4246.

Figure 2.13. Potential halogen-bond donor catalysts.

2.5 Freidel-Crafts Addition of Indoles to Nitroolefins

The first reaction to be investigated was the Freidel-Crafts addition of indoles to nitroolefins. As mentioned previously in the introduction, this reaction can be catalyzed by diaryl ureas and thioureas77 as well as by sulfonamides.81 The general reaction scheme and a summary of the results are shown in Table 2.04 and Table 2.05.

Table 2.04. Catalyst and solvent screen for the Freidel-Crafts addition of 1-methylindole to trans-β-nitrostyrene.

Catalyst Solvent Yield (%)a

none C6D6 0 2.27 C6D6 0

1.04 C6D6 0

none acetone-d6 0

2.27 acetone-d6 0

1.04 acetone-d6 0 none Toluene 0 2.31 Toluene 19 2.32 Toluene 13 2.38 Toluene < 5 2.39 Toluene < 5 none Acetone 0 2.31 Acetone 13 2.32 Acetone 10 2.28 Acetone 0 2.29 Acetone 0 none DCM 0 2.31 DCM 19 2.32 DCM 20 2.28 DCM < 5 2.29 DCM < 5

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

Table 2.05. Catalyst and solvent screen for the Freidel-Crafts addition of indole to trans-β- nitrostyrene.

Catalyst Solvent Yield (%)a none Toluene 0 2.31 Toluene 11 2.32 Toluene 10 2.28 Toluene < 5 2.29 Toluene < 5

none Acetone 0 2.31 Acetone 7

2.32 Acetone 5 2.28 Acetone 0 2.29 Acetone 0 None DCM 0 2.31 DCM 11 2.32 DCM 11

2.28 DCM < 5 2.29 DCM < 5

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

The first catalysts tested were the bidentate and tridentate halogen-bond donor catalysts 2.27 and 1.04. As the iodoperfluorophenyl group has previously been demonstrated to be a relatively strong halogen-bond donor and the catalysts were multidentate, it was hoped catalysis would be observed. Although studies of XB interactions with nitro groups or nitrate anions acting as the halogen bond acceptors suggest a very weak interaction if any, there are several examples of

73 halogen bonds with nitro groups in the solid state.100 In previous studies, the increase in electrophilicity of the nitroalkenes was hypothesized to have resulted from either a bidentate interaction between the catalyst and the nitro group (urea catalyst)77 or a monodentate interaction with the catalyst (sulfonamide catalyst) and the nitro group.81 A similar interaction may be possible with a halogen-bond donor catalyst (Figure 2.14). Unfortunately, neither catalyst 2.27 or 1.04 showed any conversion of starting material to product in C6D6 or acetone-d6. In an effort to observe any activity, catalysts containing both hydrogen-bond donors and halogen-bond donors were tested.

Figure 2.14. Hypotheses for the enhanced electrophilicity of the nitroolefins upon interaction with catalysts. (A = urea catalysts, B = sulfonamide catalyst, C = XB catalyst)

Although reactivity was observed with both catalysts 2.31 and 2.32, there was no significant difference between the catalyst containing a halogen-bond donor (2.31) and the control catalyst lacking a halogen-bond donor (2.32). The formation of product can therefore be attributed to the presence of the urea moiety in the catalysts. The yields observed using catalysts 2.31 and 2.32 are significantly lower than those observed by Ricci using catalyst 2.04.77 A possible explaination for this could be that 2.04 contains two strongly electron withdrawing bis-3,5- trifluoromethylphenyl urea substituents while 2.31 and 2.32 contain only one of these bis-3,5- trifluoromethylphenyl substituents and one less electron withdrawing aryl group. This less electron withdrawing substituent makes these ureas poorer hydrogen-bond donors, and therefore poorer catalysts, than 2.04. In an attempt to observe differences between the halogen bond donor

100 (a) Cabildo, P.; Claramunt, R. M.; Lόpez, C.; García, M. A.; Pérez-Torralba, M.; Pinilla, E.; Torres, M. R.; Alkorta, I.; Elguero, J. Journal of Molecular Structure 2011, 985, 75–81. (b) Lemmerer, A.; Michael, J. P. Acta Cryst. 2011, C67, 288–293.

74 catalysts and the control catalysts, two halogen-bond donors were incorportated into 2.28 and a control catalyst 2.29 was synthesized. However, even poorer yields were observed with no difference between 2.29 and 2.28. These poorer yields relative to the bis-aryl ureas could be due to the replacement of on aryl group with an alkyl substituent decreasing the hydrogen bond donor ability of these ureas. Based on these results, halogen-bond donors (at least the iodoperfluorophenyl donors) do not appear to act as catalysts for the Friedel-Crafts addition of indoles to trans-β-nitrostyrene.

2.6 Hetero-Diels-Alder Reaction

The second reaction investigated was the hetero-Diels-Alder reaction between 1-amino-3- silyloxybutadiene (2.33) and aldehydes. As previous reports had shown solvents possessing hydrogen bond donors were capable of catalyzing the reaction,82 the first experiments performed screened several solvents with the goal of identifying a solvent in which there was no background reaction. Table 2.06. As chloroform (a hydrogen-bond donor) was found to catalyze the reaction quite efficiently (even if it was treated with solid K2CO3) it could not be used as a reaction solvent. However, CDCl3 could be used as an NMR solvent if the reaction was quenched with AcCl and then an aqueous workup performed. At this point no diene was present so CDCl3 could not catalyze the reaction. As a result, toluene and dichloromethane were chosen as possible reaction solvents. Previous studies using hydrogen bond donors to catalyze the reaction had employed toluene as the reaction solvent.83,84

Table 2.06. Solvent screen for the hetero-Diels-Alder reaction.

Solvent Yield (%)a

CHCl3 90 DMSO 0 Toluene 0

CH2Cl2 0 Acetonitrile 0

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

The catalysts 2.27, 1.04, 2.30, and iodoperfluorobenzene were then tested in the reaction. In both toluene and dichloromethane, no reactivity was observed for all of the catalysts using either benzaldehyde or anisaldehyde as the dienophile. The results are summarized in Table 2.06. The results suggest the halogen-bond donors examined in this study are not efficient catalysts for the hetero-Diels-Alder reaction. As a comparison, BAMOL (1,1'-biaryl-2,2'-dimethanol) hydrogen- bond donor catalyst 2.1184 (20 mol%) was capable of catalyzing the HDA reaction between 2.34 and 2.33 in 84% yield using toluene as the reaction solvent.

Table 2.07. Catalyst screen for the hetero-Diels-Alder reaction.

R Catalyst Catalyst Loading (%) Solvent Yield (%)a

OCH3 none N/A Toluene 0

OCH3 2.27 20 Toluene 0

OCH3 1.04 20 Toluene 0

OCH3 2.30 20 Toluene 0

OCH3 iodopentafluorobenzene 40 Toluene 0 H none N/A Toluene 0 H 2.27 20 Toluene 0 H 1.04 20 Toluene 0 H 2.30 20 Toluene 0 H iodopentafluorobenzene 40 Toluene 0

OCH3 none N/A CH2Cl2 0

OCH3 2.27 20 CH2Cl2 0

OCH3 1.04 20 CH2Cl2 0

OCH3 2.30 20 CH2Cl2 0

OCH3 iodopentafluorobenzene 40 CH2Cl2 0

H none N/A CH2Cl2 0

H 2.27 20 CH2Cl2 0

H 1.04 20 CH2Cl2 0

H 2.30 20 CH2Cl2 0

H iodopentafluorobenzene 40 CH2Cl2 0

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard 2.7 Reduction of Aldimines

The third reaction examined was the reduction of aldimines with Hantzsch esters. As CHCl3 was capable of catalyzing the HDA reactions discussed in the previous section, the first experiments

77 performed examined whether any conversion was observed for the reduction of aldimines in several NMR solvents. The results are shown in Table 2.08.

Table 2.08. Solvent screen for the reduction of aldimines.

Catalyst Solvent Yield (%)

none CDCl3 100

none CD3CN insoluble

none DMSO-d6 0

none CH2Cl2 15

1.15a CH2Cl2 55

1.15b CH2Cl2 48 none Toluene 1 1.15a Toluene 3 1.15b Toluene 2 none Acetone 1 1.15a Acetone 2 1.15b Acetone 3 none Cyclohexane 1 1.15a Cyclohexane 2 1.15b Cyclohexane 3

none CH3CN 1

1.15a CH3CN 2

1.15b CH3CN 4 none EtOAc 2 1.15a EtOAc 1 1.15b EtOAc 1

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

As deuterated chloroform was an excellent promoter of the reaction and the reaction components were insoluble in deuterated acetonitrile, DMSO-d6 was chosen as the NMR solvent to determine yields. A further solvent screen (Table 2.08) showed minimal background reaction in toluene, acetone, cyclohexane, acetonitrile and ethyl acetate but also showed no reactivity in the presence of catalyst 1.15a or 1.15b. Dichloromethane showed significant background reaction (15% yield) but also showed reactivity in the presence of catalyst 1.15a (55% yield) and 1.15b (48% yield). The observed background reaction under these conditions is at odds with that reported by Schreiner85 where "trace" amount of product was formed. Schreiner's study also discussed difficulties in reproducing results previously published by Menche concerning the reduction of aldimines with Hantzsch esters using thiourea as a catalyst.101 In these papers an aldehyde and amine were condensed in-situ to form an imine. Menche reported only a trace amount of product in the absence of thiourea, however, Schreiner reported a yield of 78% without catalyst. Adding

NaHCO3 to the reaction suppresed the background reaction but gave no reaction in the presence of thiourea as well. The discrepancies were attributed to auto-oxidation of the aldehyde to the carboxylic acid, which is capable of catalyzing the reaction. Although in the reaction shown in Table 2.08 the imine was preformed, perhaps trace amounts of water could have hydrolyzed the imine to form trace amounts of aldehyde which could be oxidized to the carboxylic acid. This may explain the background reaction observed. In fact, control experiments employing 15 mol% HCl (solution in ether) as catalyst in dichloromethane solvent showed the reaction to be complete in less than 5 minutes and to give >99% yield. As acids appeared to be able to catalyze the reaction, experiments were carried out adding basic additives to eliminate this reaction pathway. The two basic additives studied were 1,8-bis(dimethylamino)naphthalene (proton sponge) (2.37) and the hindered base 2,6-di-tert-butyl-4-methylpyridine (2.38).

101 (a) Menche, D.; Hassfeld, J.; Li, J.; Menche, G.; Ritter, A.; Rudolph, S. Org. Lett. 2006, 8, 741. (b) Menche, D.; Arikan, F. Synlett, 2006, 841.

Figure 2.15. Basic additives used in the reduction of aldimines with Hantzsch ester.

In addition to catalysts 1.15a and 1.15b, iodoperfluorooctane was also tested as a catalyst as it was previously used in the reduction of 2-phenylquinoline discussed earlier94 (Table 2.09). Iodoperfluorooctane as a catalyst gave a 68% yield of product but Bolm's studies94 showed iodoperfluorooctane decomposed in the presence of Hantzsch ester. When proton sponge (2.37) was added to the reaction to suppress any reactivity due to catalysis by trace acid, no background reaction was observed but neither was any reactivity in the presence of any of the catalysts. The addition of the less basic additive 2.38 to the reaction employing iodoperfluorooctane as the catalyst gave a 99% yield, however, the addition of 2.38 increased the extent of background reaction to 50% yield. A similar trend was observed with Schreiner's original thiourea catalyst (2.05) where no reactivity was observed in the presence of 2.37 but activity was retained in the presence of 2.38. These confusing results are in agreement with Bolm's94 observation that iodoperfluorooctane can catalyze the reduction of C=N double bonds. Unfortunately the results can not rule out that the observed reactivity is due to acid catalysis. No significant difference was observed when the halogen bond donor catalyst 1.15a was used compared to the control catalyst 1.15b, suggesting the urea moiety acted as the catalyst. As mentioned previously, 1.15a and 1.15b are poorer catalysts than 2.05 because thioureas are better hydrogen bond donors than ureas and the benzyl ester substituent in 1.15a and 1.15b is less electron withdrawing than the 3,5-bistrifluoromethylphenyl substituent in 2.05.

Table 2.09. Basic additives used in the reduction of aldimines with Hantzsch ester.

Catalyst Additive (y mol%) Solvent NMR Yield (%)

none none CH2Cl2 15

1.15a none CH2Cl2 55

1.15b none CH2Cl2 48

iodoperfluorooctane none CH2Cl2 68

none 2.37 CH2Cl2 0

1.15a 2.37 CH2Cl2 0

1.15b 2.37 CH2Cl2 0

iodoperfluorooctane 2.37 CH2Cl2 0

none 2.38 CH2Cl2 51

1.15a 2.38 CH2Cl2 58

1.15b 2.38 CH2Cl2 53

iodoperfluorooctane 2.38 CH2Cl2 99

2.05 none CH2Cl2 86

2.05 2.37 CH2Cl2 0

2.05 2.38 CH2Cl2 89

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard 2.8 N-Acyl Pictet-Spengler Reaction

The final reaction explored was the N-acyl Pictet-Spengler reaction. This reaction was chosen as the studies outlined in chapter 1 suggested that halogen-bond donors have a unique affinity to bind or interact with halide anions and the N-acyl Pictet-Spengler reaction is proposed to proceed through an acyliminium halide ion pair (Figure 2.05). If the catalysts were capable of interacting with this intermediate they might help promote the reaction. Studies undertaken by Raheem et al.90 on the similar Pictet-Spengler-type cyclizations of hydroxylactams suggested that hydrogen- bond donors could interact with the intermediate N-acyliminium ion (Figure 2.07). The authors

81 proposed that the thiourea catalyst aids in dissociation of the chloride ion from a transient chlorolactam intermediate to form a complex between the thiourea and N-acyliminium chloride. This anion-binding mode of activation was supported by DFT calculations, 1H NMR experiments demonstrating that the thiourea catalyst was capable of binding to chloride and experiments which showed significant halide counterion effects. If a halogen-bond donor was capable of activating the intermediate chlorolactam in a similar manner catalysis by XB may be observed. The results of the experiments involving the N-acyl Pictet-Spengler reaction are summarized in Table 2.10.

Table 2.10. Summary of results for the N-acyl Pictet-Spengler reaction.

Catalyst Catalyst loading Temperature Solvent NMR yield (%)

none N/A 23 °C Et2O 86 (isolated)

1.03 30 mol% – 40 °C Et2O 14

1.04 30 mol% – 40 °C Et2O 10

none N/A – 40 °C Et2O 4

none N/A – 30 °C Et2O 23

1.03 30 mol% – 30 °C Et2O 27

2.30 30 mol% – 30 °C Et2O 25

1.04 30 mol% – 30 °C Et2O 24

1.15a 10 mol% – 40 °C Et2O 28

1.15b 10 mol% – 40 °C Et2O 18

2.31 10 mol% – 40 °C Et2O 30

2.32 10 mol% – 40 °C Et2O 23

iodoperfluorooctane 10 mol% – 40 °C Et2O 13

2.28 10 mol% – 40 °C Et2O 24

2.29 10 mol% – 40 °C Et2O 25

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

At room temperature the N-acyl Pictet-Spengler reaction proceeded in 86% yield in the absence of any catalyst. To determine if halogen-bond donor catalysts offer any rate acceleration the

82 reactions were run at lower temperatures where the background reaction was not as high (–40 °C, 4% yield and –30 °C, 23% yield). An encouraging initial result was obtained when 1.03 was used as the catalyst and gave a 14% yield of 2.40, an increase of 10% compared to the background reaction which gave a 4% yield. However, when 1.04 was used, a catalyst having three halogen-bond donors, only a 10% yield was observed. While this is slightly higher (6%) than the background reaction it is less than that obtained using 1.03, a catalyst containing only two halogen-bond donors. To see if increasing the temperature would show a greater difference in yields between the background reaction and the catalyzed reaction, several reactions were carried out at –30 °C as opposed to –40 °C. The reactions carried out at –30 °C gave very similar yields whether they were carried out in the absence of a catalyst (23%), or in the presence of 1.03 (27%), 2.30 (25%), or 1.04 (24%). Testing the urea-containing catalysts showed a minor increase in yield for those containing a halogen-bond donor for both 1.15a (28%) versus 1.15b (18%, control catalyst) and 2.31 (30%) versus 2.32 (23%, control catalyst). These increases were not observed for the serinol based catalysts 2.28 (24%) and 2.29 (25%). In some cases, slight increases in the reaction yield were observed for catalysts containing halogen-bond donors. This suggests that reactions which proceed through halide ion intermediates might be good targets for observing catalysis by XB in future endeavors. In this particular case, however, efforts to increase the difference in yields between the XB catalysts and the control catalysts were hampered by the significant background reaction at higher (–30 °C) temperatures. Contrary to our initial idea of targeting reactions which proceeded in the absence of catalyst at room temperature but required a catalyst at low temperature, when chosing reactions to study in the future perhaps this characteristic should be avoided.

2.9 Advances in Halogen Bonding Catalysis and Activation

Since the work undertaken in the previous section, one additional example of catalysis by XB has been reported. The report by Huber represents the first example of catalysis using a neutral halogen-bond donor and the first example of carbon-carbon bond formation using halogen bonding catalysis.102 The authors chose to investigate the reaction of ketene silyl acetal 2.42 with

102 Kniep, F.; Jungbauer, S. H.; Zhang, Q.; Walter, S. M.; Schindler, S.; Schnapperelle, I.; Herdtweck, E.; Huber, S. M. Angew. Chem. Int. Ed. 2013, 52, 7028–7032.

1-chloroisochroman 2.41, a reaction that had previously been shown by Jacobsen to be catalyzed enanioselectively by chiral thiourea organocatalysts.103 The proposed mechanism involves the formation of an oxocarbenium thiourea-bound chloride ion pair where the thiourea also assists in chloride ion dissociation.

Figure 2.16. Organocatalysis by neutral multidentate halogen-bond donors.

Selected results from the Huber study are summarized in Table 2.11. There was no reaction in the absence of catalyst or in the presence of perfluorinated 2.44, as expected due to the lack of a halogen-bond donor. Reactivity was, however, observed with catalysts containing halogen-bond donors. Catalyst 2.47 was found to result in the greatest yield of 2.43.

103 Reisman, S. A.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2008, 130, 7198.

Table 2.11. Reaction of 1-chloroisochroman 2.41 with ketene silyl acetal 2.42 in the presence of various catalysts.

Temperature Catalyst Yield (%) (°C) none -78 < 5 2.44 -78 <5 2.45 -78 37 2.46 -78 14 2.47 -78 91

As it is often difficult to rigorously exclude traces of Brønsted acid from the reaction due to potential decomposition of the catalysts, control studies were undertaken to ensure the reactivity was not the result of hidden acid catalysis.104 The reaction was carried out in the presence of 10 mol% HOTf and although some reactivity was observed (27%), it was much less than that observed with 2.47. Catalyst 2.47 could also be re-isolated from test reactions by column chromatography in more than 95% recovery and NMR studies of 2.47 in THF-d8 showed no decomposition after 10 days at room temperature. These results suggest the catalysis is due to halogen bonding and not Brønsted acid catalysis.

2.10 Conclusions

For three of the reactions studied (Freidel-Crafts addition of indoles to nitroolefins, hetero-Diels- Alder reaction and the reduction of aldimines with Hantzsch ester) the halogen-bond-containing catalysts appeared to have a negligible, if any, influence on the reaction outcome. In this respect, the halogen bond donor catalysts were not similar to their hydrogen bond donor counterparts. In each of these cases the proposed mode of activation was the formation of a halogen bond between a neutral functional group (nitroolefin, aldehyde, or imine) and the halogen bond donor catalyst. Although these types of halogen bonds have been observed in either the solid or solution state they do not appear to increase the electrophilicity of these functional groups in these cases. In addition, it was difficult to study the reduction of imines using Hantzsch esters as the reaction was very acid sensitive and the stability of certain catalysts under the reaction

104 Dang, T. T. ; Boeck, F.; Hintermann, L. J. Org. Chem. 2011, 76, 9353–9361.

85 conditions was questionable. However, in the N-acyl Pictet-Spengler reaction a slight improvement in yield (10%) may have been observed for one of the catalysts (1.03) at –40 °C. Unfortunately when the temperature of the reaction was increased to –30 °C in an effort to increase the yield obtained in the catalytic reaction, the background reaction became significant and no difference in yields were observed. The difference in yields at low temperature could be connected to the proposed intermediates through which the N-acyl Pictet-Spengler reaction proceeds. The experiments dicussed in chapter 1 revealed that halogen-bond donor groups interacted preferentially with halide anions compared to oxoanions. If the N-acyl Pictet-Spengler reaction proceeds through an acyliminium chloride ion pair, perhaps binding to the chloride ion was responsible for the increased yield. While the reactions described previously in this chapter were not successful, the reaction described by Huber was. There are several possible explainations for this discrepancy. (1) The results of Huber's studies97, 102 suggested that halogen- bond donor catalysts may interact preferentially with halogens or halides. As the Friedel-Crafts reaciton, hetero-Diels-Alder reaction and the reduction of imines reaction do not involve substrates or intermediates having halogens or halides, activation was not observed. (2) Huber's 2011 study detailed activation by halogen-bond donors where activator turnover was not possible, perhaps due to a strong halogen bonding interaction between the generated bromide anion and the activator. If the bromide anion was a better halogen-bond acceptor than the bromine of the substrate, activator turn over would not occur. In Huber's 2013 study the catalysts employed could be strong enough halogen-bond donors to activate the chloroisochroman and form the oxocarbenium intermediate but weak enough not to end up bound so tightly to the chloride-containing byproduct that turn over was still possible. For the reaction shown in Figure 2.16, this byproduct is TBSCl which is not likely to be as competent a halogen-bond acceptor as chloride. The difference in the nature of the halide byproduct formed (bromide anion versus neutral TBSCl) in Huber's reports from 2011 and 2013 could explain why turnover was observed in one instance but not the other. In the case of the N-acyl-Pictet-Spengler reaction, perhaps the halogen-bond donor moiety of the catalyst was inactivated by being bound to the chloride- containing byproduct in the reaction. This byproduct is most likely 2,6-lutidinium chloride. The anion binding properties of the urea receptors containing halogen-bond donors discussed in chapter 1 indicated that chloride (tetrabutylammonium counterion) was a strong halogen-bond acceptor. Although the counterion in 2,6-lutidinium chloride is different, the chloride anion would still be expected to be a competent halogen-bond acceptor. Previous examples have shown

86 that turnover was possible with urea and thiourea catalysts so if inactivation by chloride anions occured it would most likely be due to the incorporation of the halogen-bond donor. In the development of future reactions, it will be important to consider the halogen-bond acceptor ability of both the substrate and the halogen containing byproduct of the reaction. Catalysis is unlikely to be observed if the reaction byproduct is a superior halogen-bond acceptor to the substrate due to inactivation of the catalyst. In the case of the N-acyl-Pictet-Spengler reaction, if the reaction proceeds through an N-acyliminium chloride ion pair, the competing halogen-bond acceptors would be similar. (3) A crystal structure of a complex formed between catalyst 2.45 and tris(dimethylamino)propenium chloride revealed the C–I•••Cl– bond angle to be ~174°, an angle close to the ideal geometry for a halogen bonding interaction. As discussed in chapter 1, complexes between chloride anions and receptors similar to 2.31, 2.32, 1.15a and 1.15b were calculated to have C–I•••Cl– bond angles of only 155°. As this is significantly removed from the ideal 180° angle preferred by halogen bonds these catalysts many not have been able to interact as strongly with the chloride anion intermediate. (4) Finally, the best catalyst for the reaction of 1-chloroisochroman 2.41 with ketene silyl acetal 2.42 was found to be 2.47. This molecule has six C–I bonds which could function as halogen-bond donors. While the catalyst geometry would not allow all six halogen-bond donors to participate in XB with a single chloride anion, similar crystal structures102 suggest it is capable of interacting with an anion in a bidentate fashion. If multiple binding sites are present in the catalyst the catalyst loading is effectively increased.

2.11 Experimental Details

General Procedures. All reactions were carried out in oven-dried glassware fitted with rubber septa under nitrogen atmosphere. Stainless steel syringes were used to transfer air- and moisture sensitive liquids. Flash chromatography was performed using silica gel 60 (230-400 mesh) from Silicycle.

Materials. Commercial reagents were purchased from Sigma Aldrich or Alfa Aesar and were used as received with the following exceptions: Acetonitrile, THF and dichloromethane were purified by passing through two columns of activated alumina under argon (Innovative Technology, Inc.). Nuclear magnetic resonance (NMR) solvents were purchased from Cambridge Isotope Laboratories.

(MeCN-d2: δ 1.94; DMSO-d5: δ 2.50; CHCl3: δ 7.26). Chemical shifts for carbon are reported in parts per million downfield from tetramethylsilane and are referenced to the carbon resonances 13 of the solvent (CD3CN: δ 1.24; DMSO: δ 39.43; CDCl3: δ 77.16). Partial C spectra are reported for perfluoroarene containing receptors in certain cases. Data are represented as follows; chemical shift (δ, ppm); multiplicity (s-singlet, d-doublet, t-triplet, q-quartet, m = multiplet); coupling constant (J, Hz); integration). Chemical shifts for fluorine were recorded in parts per million (ppm) relative to CFCl3 using trifluorotoluene (δ –63.72 ppm) as an internal standard. High-resolution mass spectra (HRMS) were obtained on a VS 70-250S (double focusing) mass spectrometer at 70 eV. Infrared (IR) spectra were obtained on a Perkin-Elmer Spectrum 100 instrument equipped with a single-bounce diamond / ZnSe ATR accessory. Data are represented as follows: wavenumber (cm–1); intensity (s-strong, m-medium, w-weak).

General procedure C: Freidel-Crafts addition of indoles to trans-β-nitrostyrene.

Trans-β-nitrostyrene (0.10 mmol) and catalyst (0.02 mmol) were weighed into a 2-dram vial and 150 μL of solvent were added. Then indole or 1-methylindole (0.15 mmol) was added and the reaction stirred at 23 °C for 24 hours. Then 1 mL of 20 mmolL-1 4,4'-di-tert-butylbiphenyl (in acetone) was added to the reaction and an aliquot was taken, concentrated, dried under vacuum 1 1 then dissolved in CDCl3 and submitted for H NMR analysis. The H NMR yields were determined by comparing the integration of product peaks to those of the internal standard. Spectral data for compounds 2.03a and 2.03b were consistent with those reported in the literature.81

General procedure D: Hetero-Diels-Alder reaction.

The catalyst (x mol%) was weighed into a 2-dram vial and solvent (500 μL) was added. Anisaldehyde or benzaldehyde (0.20 mmol) and Rawal's diene (0.10 mmol) were added to the reaction and the reaction was stirred at 23 °C for 20 hours. The reactions were then diluted with

88 dichloromethane (500 μL) and cooled to –78 °C. Acetyl chloride (0.20 mmol) was added to the reaction and it was stirred for 30 minutes at –78 °C. The reaction was warmed to 23 °C then 1 mL of 20 mmolL-1 4,4'-di-tert-butylbiphenyl (in acetone) was added to each reaction. An aliquot 1 was taken from the reaction, concentrated, dissolved in CDCl3 and submitted for H NMR analysis. The 1H NMR yields were determined by comparing the integration of product peaks to those of the internal standard. Spectral data for compounds 2.36a and 2.36b were consistent with those reported in the literature.82

General procedure E: Reduction of imines with Hantzsch ester.

N-benzylideneaniline (0.10 mmol), Hantzsch ester (0.11 mmol), catalysts (10 mol%), and additive (y mol%) were weighed into a scintillation vial then 0.5 mL of solvent was added. The reactions were stirred at 23 °C for 20 hours. Then 1 mL of 20 mmolL-1 of 4,4'-di-tert- butylbiphenyl (in ethyl acetate) was added to the reaction. An aliquot was taken from the reaction and the solvent was removed immediately. The sample was dissolved in DMSO-d6 and submitted for 1H NMR analysis. The 1H NMR yields were determined by comparing the integration of product peaks to those of the internal standard. Spectral data for compound 2.13 were consistent with those reported in the literature.85

General procedure F: N-acyl Pictet-Spengler reaction.

Imine (2.39) (0.10 mmol) and catalyst (x mol%) were dissolved in Et2O (0.5 mL) then cooled to –78 °C in an acetone/dry ice bath. Acetyl chloride (0.10 mmol) and 2,6-lutidine (0.10 mmol) were added at –78 °C. The reaction was warmed to the indicated temperature (using a cooling bath) and stirred for 24 h. NaOMe solution (0.5 M in methanol) (0.10 mmol) was added to the reaction which was then allowed to warm to 23 °C and stir for 2 h. 3 mL of H2O was added to the reaction and then 1 mL of 20 mmolL-1 4,4'-di-tert-butylbiphenyl (in ethyl acetate) was added to the reaction. A sample of the organic layer of the reaction was removed, concentrated, dried under vacuum and analyzed by 1H NMR spectroscopy. The 1H NMR yields were determined by comparing the integration of product peaks to those of the internal standard. Spectral data for compound 2.40 were consistent with those reported in the literature.87

Characterization Data Compound (1.03)

Synthesized using a modified literature procedure.105 1,2-Phenylenedimethanol (0.724 mmol), 2,3,4,5-tetrafluoro-6-iodobenzoic acid106 (2.172 mmol) and 4-dimethylaminopyridine (DMAP) (0.145 mmol) were weighed into a round bottom flask and dissolved in dichloromethane (4 mL). N,N'-Diisopropylcarbodiimide (DIC) (2.172 mmol) was added dropwise via syringe. The reaction was stirred at 23 °C for 24 h and then quenched with NH4Cl. The aqueous layer was extracted three times with dichloromethane. The organic layers were washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified by column chromatography (diethyl ether:pentane; 1:4) to give a colorless oil (455 mg, 85% yield). Spectral data for compound 1.03 agree with those reported in the literature.105 1H NMR (300 MHz, 19 CDCl3): δ 7.54 (dd, J = 7.2, 4.4 Hz, 2H), 7.43 (dd, J = 7.2, 4.4 Hz, 2H), 5.57 (s, 4H). F NMR

(282 MHz, CDCl3): δ –116.3 (m, 1F), –140.1 (m, 1F), –152.7 (m, 1F), –155.5 (m, 1F).

105 Sarwar, M. G.; Bojan, D.; Sagoo, S.; Taylor, M. S. Angew. Chem. Int. Ed. 2010, 49, 1674–1677. 106 Richardson, R. D.; Zayed, J. M.; Altermann, S.; Smith, D.; Wirth, T. Angew. Chem. Int. Ed. 2007, 46, 6529– 6532.

Compound (2.27)

Synthesized using a modified literature procedure.105 2,2-dimethylpropane-1,3-diol (1.0 mmol), 2,3,4,5-tetrafluoro-6-iodobenzoic acid106 (3.0 mmol) and 4-dimethylaminopyridine (DMAP) (0.2 mmol) were weighed into a round bottom flask and dissolved in dichloromethane (4 mL). N,N'- diisopropylcarbodiimide (DIC) (3.0 mmol) was added dropwise via syringe. The reaction was stirred at 23°C for 24 h and then quenched with NH4Cl. The aqueous layer was extracted three times with dichloromethane. The organic layers were washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified by column chromatography (diethyl ether:pentane; 1:4) to give a white solid (628 mg, 77% yield). Spectral data for 105 1 compound 2.27 agree with those reported in the literature. H NMR (300 MHz, CDCl3): δ 19 4.26 (s, 4H), 1.13 (s, 6H). F NMR (282 MHz, CDCl3): δ –116.0 (m, 1F), –139.9 (m, 1F), – 152.7 (m, 1F), –155.5 (m, 1F).

Compound (1.04)

Synthesized using a modified literature procedure.105 2,3,4,5-tetrafluoro-6-iodobenzoic acid106 (2.5 mmol) was weighed into a round bottom flask and dissolved in acetonitrile (4 mL) to give a clear colorless solution. 1,8-Diazabicyclo[5,4,0] undec-7-ene (DBU) (2.5 mmol) was added and the reaction stirred at 23 °C for 10 minutes. 2,4,6-Tris(bromomethyl) mesitylene (0.5 mmol) was

91 added to the flask and the reaction was heated to 50 °C and stirred at this temperature for 26 h.

The reaction was cooled to 23 °C and 100 mL of saturated aqueous NH4Cl was added. The aqueous layer was extracted three times with dichloromethane. The organic layers were washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified by column chromatography (ethyl acetate:pentane; 3:7) to give a white solid (392 mg, 70% yield). Spectral data for compound 1.04 agree with those reported in the literature.105 1H 19 NMR (300 MHz, CDCl3): δ 5.57 (s, 6H), 2.55 (s, 9H). F NMR (282 MHz, CDCl3): δ –116.5 (m, 3F), –140.5 (m, 3F), –152.9 (m, 3F), –155.6 (m, 3F).

Compounds (2.28) and (2.29)

2-amino-1,3-propane diol (2.5 mmol) was slurried in 5 mL of dichloromethane (5 mL). 3,5- bis(trifluoromethyl)phenyl isocyanate (2.5 mmol) was then added dropwise and the reaction was stirred at 23 °C for 20 h. The reaction was diluted with pentane and the white precipitate was collected by suction filtration to give intermediate A as a white powdery solid (705 mg, 81% 1 yield). The compound was used without further purification. H NMR (300 MHz, DMSO-d6): δ 9.39 (s, 1H), 8.05 (s, 2H), 7.56 (s, 1H), 6.29 (d, J = 8.1 Hz, 1H), 4.83 (t, J = 5.2 Hz, 2H), 3.75- 13 3.37 (m, peaks obscured by residual H2O peak). C NMR (75 MHz, DMSO-d6): (partial) δ 19 - 154.6, 142.6, 60.1, 52.8. F NMR (282 MHz, DMSO-d6): δ –63.12 (6F). FTIR (powder, cm 1): ν 3287 (w), 1688 (s), 1594 (s), 1543 (m),1272 (s), 1127 (s), 1005 (m).

Intermediate A (0.578 mmol), 2,3,4,5-tetrafluoro-6-iodobenzoic acid106 (1.734 mmol), 4- dimethylaminopyridine (DMAP) (0.289 mmol), and N-(3-dimethylaminopropyl)-N′- ethylcarbodiimide hydrochloride (EDCI•HCl) (1.734 mmol) were weighed into a scintillation vial and 3 mL of dry CH2Cl2 was added. The reaction was stirred at 23 °C for 48 h. The reaction mixture was poured into 30 mL of water. The aqueous layer was extracted three times with dichloromethane and the organic layers were combined, washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified by column chromatography (MeOH/DCM; 5:95) to give 2.28 as a white solid (281 mg, 51% yield). 1H NMR (300 MHz,

CDCl3): δ 7.85 (s, 2H), 7.54 (s, 1H), 6.85 (s, 1H), 5.19 (d, J = 7.5 Hz, 1H), 4.78–4.47 (m, 5H). 13 C NMR (126 MHz, DMSO-d6): (partial) δ 162.28 , 154.49 , 142.19 , 130.76 (q, JC-F = 32.7

Hz), 123.41 (q, JC-F = 272.7 Hz), 122.88, 122.66, 117.74 , 114.08 , 65.69 , 47.19 (Signals for the 19 aromatic carbons bonded directly to fluorine are not reported). F NMR (282 MHz, CDCl3): δ –64.1 (6F), –112.7 (2F), –137.4 (2F), –149.1 (2F), –152.5. FTIR (powder, cm-1): ν 1741 (s), 1656 (m), 1571 (m), 1505 (s), 1458 (s), 1385 (s), 1274 (s), 1125 (s). HRMS (ESI): Calculated + for [C26H11F14I2N2O5] (M+H) 950.8528; found 950.8539.

Intermediate A (0.578 mmol), 2,3,4,5,6-pentafluorobenzoic acid (1.734 mmol), 4- dimethylaminopyridine (DMAP) (0.289 mmol), and N-(3-dimethylaminopropyl)-N′- ethylcarbodiimide hydrochloride (EDCI•HCl) (1.734 mmol) were weighed into a scintillation vial and 3 mL of dry CH2Cl2 was added. The reaction was stirred at 23 °C for 48 h. The reaction

93 mixture was poured into 30 mL of water. The aqueous layer was extracted three times with dichloromethane and the organic layers were combined, washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified by column chromatography (MeOH/DCM; 2:98) to give 2.29 as a white solid (327 mg, 77% yield). 1H NMR (300 MHz, 13 DMSO-d6): δ 9.38 (s, 1H), 8.02 (s, 2H), 7.58 (s, 1H), 6.78 (s, 1H), 4.57–4.50 (m, 5H). C NMR

(126 MHz, DMSO-d6): (partial) δ 158.42 , 154.73 , 142.25 , 130.78 (q, JC-F = 32.6 Hz), 123.38

(q, JC-F = 272.6 Hz), 117.53 , 113.95 , 107.21 , 65.29 , 47.09. (Signals for the aromatic carbons 19 bonded directly to fluorine are not reported). F NMR (282 MHz, CDCl3): δ –140.6 (4F), – 150.2 (2F), –163.0 (4F). FTIR (powder, cm-1): ν 1723 (s), 1654 (s), 1500 (s), 1277 (s), 1224 + (m), 1134 (s), 993 (s). HRMS (ESI): Calculated for [C26H11F16N2O5] (M+H) 735.0407; found 735.0414.

Compound (2.30)

Prepared according to a modified literature procedure.107 To a solution of 1,3-bis(diisopropyl- 2,6-phenyl) imidazole-2-ylidene (0.413 mmol) in THF (2.5 mL) a solution of iodine (0.413 mmol) in THF (5 mL) was added dropwise. A white precipitate was formed and the reaction was stirred at 23 °C for 1.5 h. The solvent was carefully decanted and the remaining white solid (Intermediate B) was washed with several portions of THF and dried under vacuum to give a 1 white powdery solid (72 mg, 28% yield). H NMR (400 MHz, CD3CN): δ 7.92 (s, 2H), 7.74–

7.66 (m, 2H), 7.52 (d, J = 7.8 Hz, 4H), 2.32 (m, 4H), 1.30 (d, J = 6.9 Hz, 12H), 1.23 (d, J = 6.8 Hz, 12H). Intermediate B (0.0986 mmol) was weighed into a round bottom flask and suspended in 7.5 mL of water and 2.5 mL of methanol. NaBPh4 (0.20 mmol) was added and the reaction was refluxed for 20 h.The white solid was collected by suction filtration and washed with several portions of water then dried under vacuum to give 2.32 as a white solid (45 mg, 55% yield).

107 Arduengo III, A. J.; Tamm, M.; Calabrese, J. C. J. Am. Chem. Soc. 1994, 116, 3625–3626.

Spectral data for compound 1.04 agree with those reported in the literature.107 1H NMR (400

MHz, CD3CN): δ 7.94 (s, 2H), 7.78–7.66 (m, 2H), 7.53 (d, J = 7.8 Hz, 4H), 7.31–7.26 (m, 8H),

7.08–6.95 (m, 8H), 6.87 (d, J = 7.2 Hz, 4H), 2.32 (q, J = 6.8 Hz, 4H), 1.31 (d, J = 6.8 Hz, 12H),

1.24 (d, J = 6.8 Hz, 12H).

Compounds (2.31) and (2.32)

o-Aminobenzyl alcohol (5.88 mmol) was slurried in 100 mL of dichloromethane. 3,5- bis(trifluoromethyl)phenyl isocyanate (5.88 mmol) was then added dropwise and the reaction was stirred at 23 °C for 24 h. The reaction was diluted with pentane and the white precipitate was collected by suction filtration to give intermediate C as a white solid (2.103 g, 95% yield). The 1 compound was used without further purification. H NMR (300 MHz, DMSO-d6): δ 10.10 (s, 1H), 8.45 (s, 1H), 8.14 (s, 1H), 7.82 (d, J = 8.1 Hz, 1H), 7.63 (s, 1H), 7.35-7.20 (m, 2H), 7.08– 7.03 (m, 1H), 5.52 (s, 1H), 4.54 (s, 2H). FTIR (powder, cm-1): ν 3302 (w), 1714 (s), 1647 (m), 1556 (m), 1269 (s), 1125 (s), 897 (s).

Intermediate C (1.042 mmol), 2,3,4,5-tetrafluoro-6-iodobenzoic acid106 (1.562 mmol), 4- dimethylaminopyridine (DMAP) (0.313 mmol), and N-(3-Dimethylaminopropyl)-N′- ethylcarbodiimide hydrochloride (EDCI•HCl) (1.562 mmol) were weighed into a scintillation vial and 3 mL of dry CH2Cl2 was added. The reaction was stirred at 23 °C for 48 h. The reaction

95 mixture was poured into 30 mL of water. The aqueous layer was extracted three times with dichloromethane and the organic layers were combined, washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified by column chromatography (MeOH/DCM; 2:98) to give 2.31 as a white solid (328 mg, 46% yield). 1H NMR (300 MHz,

DMSO-d6): δ 9.61 (s, 1H), 8.48 (s, 1H), 8.08 (s, 2H), 7.63–7.55 (m, 2H), 7.49 (d, J = 7.6 Hz 13 1H). 7.41–7.36 (m, 1H), 7.25–7.19 (m, 1H), 5.46 (s, 2H) C NMR (126 MHz, DMSO-d6):

(partial) δ 162.46 , 153.27 , 141.97 , 136.55 , 130.81 (q, JC-F = 32.6 Hz), 129.76 , 129.36 , 128.08

, 125.25 , 125.05 , 123.41 (q, JC-F = 272.7 Hz), 117.91 , 114.42 , 65.42 . (Signals for the aromatic 19 carbons in the fluorinated phenyl ring are not reported) F NMR (282 MHz, DMSO-d6): δ – 64.3 (6F), –117.5 (1F), –140.8 (1F), –152.7 (1F), –156.0. FTIR (powder, cm-1): ν 1714 (s), 1648 (s), 1555 (s), 1505 (s), 1270 (m), 1125 (s), 1052 (m). HRMS (ESI): Calculated for + [C23H12F10IN2O3] (M+H) 680.9727; found 680.9733.

Intermediate C (1.042 mmol), 2,3,4,5,6-pentafluorobenzoic acid (1.562 mmol), 4- dimethylaminopyridine (DMAP) (0.313 mmol), and N-(3-Dimethylaminopropyl)-N′- ethylcarbodiimide hydrochloride (EDCI•HCl) (1.562 mmol) were weighed into a scintillation vial and 3 mL of dry CH2Cl2 was added. The reaction was stirred at 23 °C for 48 h. The reaction mixture was poured into 30 mL of water. The aqueous layer was extracted three times with dichloromethane and the organic layers were combined, washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified by column chromatography (MeOH/DCM; 2:98) to give 2.34 as a white solid (381 mg, 64% yield). 1H NMR (300 MHz,

DMSO-d6): δ 9.62 (s, 1H), 8.40 (s, 1H), 8.06 (s, 2H), 7.67–7.56 (m, 2H), 7.49 (d, JH-H = 7.6 Hz 13 1H). 7.46–7.34 (m, 1H), 7.30–7.16 (m, 1H), 5.44 (s, 2H). C NMR (126 MHz, DMSO-d6):

(partial) δ 158.49 , 153.28 , 141.97 , 136.80 , 130.84 (q, JC-F = 32.7 Hz), 130.04 , 129.42 , 128.00

, 125.23 , 124.98 , 123.39 (q, JC-F = 272.6 Hz), 117.80 , 114.35 , 107.28, 65.51. (Signals for 19 aromatic carbons bonded directly to fluorine are not reported). F NMR (282 MHz, DMSO-d6): δ –64.4 (6F), –141.3 (2F), –151.4 (1F), –163.8 (2F). FTIR (powder, cm-1): ν 1714 (s), 1647 (s),

1555 (s), 1504 (s), 1270 (m), 1125 (s), 1052 (m). HRMS (EI): Calculated for [C23H12F11N2O3] (M+H)+ 573.0667; found 573.0671.

Compounds (1.15a) and (1.15b)

The compounds have been previously reported. The procedure for their synthesis and their spectral data can be found here.1

Chapter 3 Catalyst Controlled Additions to 2-Nitroglycals 3.1 Introduction

In the previous chapter, one of the test reactions studied in an attempt to observe halogen bonding catalysis was the Friedel-Crafts alkylation of heteroaromatics with nitroolefins. This reaction had previously been shown to be catalyzed by thiourea and sulfonamide hydrogen-bond donor catalysts (Figure 2.01 and Figure 2.02).77,81 Enantioselective Michael additions to nitroalkenes have also been shown to be catalyzed by chiral hydrogen-bond donor catalysts. For example, Takemoto has reported the enantioselective Michael reaction of malonates with nitroolefins catalyzed by bifunctional thiourea catalyst 3.01 (Figure 3.01).108

Figure 3.01. Enantioselective Michael reaction of malonates with nitroolefins catalyzed by bifunctional thiourea catalyst 3.01.

Additions of aldehydes and ketones to nitroalkenes catalyzed by proline or prolinol derivatives have also been reported.109 Although List's report of the proline-catalyzed addition of ketones to nitroalkenes gave poor enantioselectivities (up to 23% ee depending on the identity of the nitroolefin and ketone used), the yields were excellent (Figure 3.02).109a A later report showed that using prolinol-derived catalyst 3.03 can drastically improve the yield of the addition of aldehydes to nitroolefins compared to using L-proline as the catalyst (Figure 3.03).109b

108 Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am. Chem. Soc. 2005, 125, 12672–12673. 109 (a) List, B.; Pojarliev, P.; Martin, H. J. Org. Lett. 2001, 16, 2423–2425. (b) Hayashi, Y.; Gotoh, H.; Hayashi, T.; Shoji, M. Angew. Chem. Int. Ed. 2005, 44, 4212–4215. (c) Mase, N.; Thayumanavan, R.; Tanaka, F.; Barbas III, C. F. Org. Lett. 2004, 15, 2527–2530.

Figure 3.02. Proline-catalyzed addition of ketones to nitroalkenes.

Figure 3.03. Additions of aldehydes to nitroolefins.

This chapter details our investigations into using unique nitroolefin substrates, 2-nitroglycals, in Michael additions and conjugate additions. The goals of this project were (1) to determine if these substrates reacted similarly to simple nitroalkenes under conditions described previously and (2) determine if HB catalysts could influence the stereoselectivity of the additions.

Figure 3.04. General scheme for the catalyst-controlled addition to 2-nitroglycals.

The development of catalyst controlled glycosylation reactions is interesting as it would allow the configuration at the anomeric position to be set based on the enantiomer of the catalyst used. Typically, the anomeric selectivity of a glycosylation reaction is heavily influenced by the protecting groups (anomeric and neighboring group effects) and leaving group present in the glycosyl donor. As changing one of these parameters invariably requires an alternate synthesis of the glycosyl donor, catalyst controlled glycosylation offers the advantage that a common substrate could be converted into either the α or β anomer by simply changing the catalyst. Very few examples of chiral catalysts significantly influencing the stereochemical outcome of glycosylation reactions have been reported. The enatioselective thiourea-catalyzed addition of carbon nucleophiles (silyl ketene acetals) to 1-chloroisochroman represents a model reaction for

99 such an approach.103 Although 1-chloroisochroman is a simple substrate compared to most glycosyl donors, the thiourea-catalyzed reaction is proposed to proceed through an oxocarbenium ion, a class of intermediates through which many glycosylation reactions are also proposed to proceed. If HB catalysts are capable of mediating enantioselective additions to simple oxocarbenium ion, perhaps the methodology could be extended to more complex systems.

Figure 3.05. Enantioselective addition of a silyl ketene acetal to 1-chloroisochroman.

Fairbanks has reported the use of a chiral Brønsted acid which influenced the stereochemical outcome of glycosylation reactions employing trichloroacetimidate donors (Figure 3.06).110 Although changing the enantiomer of the BINOL-derived phosphoric acid catalyst from (S) to (R) did not change the selectivity of the glycosylation from β to α, the preference for the β anomer was diminished. This study was conceptually important as it demonstrates that the diastereoselectivity of a glycosylation reaction can be altered by a chiral activator.

110 Cox, D. J.; Smith, M. D.; Fairbanks, A. J. Org. Lett. 2010, 12, 1452–1455.

100

Figure 3.06. Glycosylation catalyzed by a chiral BINOL-derived phosphoric acid.

In particular, development of catalyst controlled additions to 2-nitroglycals would be useful as the products of these reactions can subsequently be transformed into the corresponding 2-amino- or 2-acetamido-derivatives.111,116 The 2-N-acetamido-2-deoxyglycosides are prevalent in a variety of biomolecules such as glycoconjugates,112 glycosaminoglycans,113 and blood group oligosaccharides.114 Using 2-N-acetamido-2-deoxyglycosyl donors directly to perform glycosylation reactions is sometimes problematic due to oxazoline formation. Although a variety

111 Das, J.; Schmidt, R. R. Eur. J. Org. Chem.1998, 1609–1613. 112 Dwek, R. A. Chem. Rev. 1996, 96, 683–720. 113 Casu, B. Adv. Carbohydr. Chem. Biochem. 1985, 43, 51–134. 114 Lemiuex, R. U. Chem. Soc. Rev. 1978, 7, 423–452.

101 of methods have been developed to circumvent this difficulty115, (such as using nonparticipating 2-azido donors), alternate methods are still desirable.

3.2 Reactions of 2-Nitroglycals

The Michael-type addition of O-, N-, S-, C-, and P-nucleophiles to 2-nitrolglycals has been reviewed by Schmidt and Vankar.116 The addition of O-nucleophiles has been used to synthesize 117 core structures of the mucin type including the TN and STN antigens as well as O-glycosyl amino acids.118 This methodology was particularly useful in these cases as the nitro group present in the addition product could be reduced to the amine and acylated to give the desired 2- N-acetamido-2-deoxyglycosides.

Figure 3.07. Synthesis of the peracetylated TN antigen.

These addition reactions are generally base-promoted and the stereochemical outcome is heavily influenced by the substrate (2-nitrogalactal versus 2-nitroglucal), base and nucleophile. For example, addition of MeOH to 2-nitrogalactal using NaOMe as the base furnished the α anomer as the major product (α:β = 8:1). However, if the weaker base NEt3 was used, the β anomer was

115 Bongat, A. F. G.; Demchenko, A. V. Carbohydr. Res. 2007, 342, 374–406. 116 Schmidt, R. R.; Vankar, Y. D. Acc. Chem. Res. 2008, 41, 1059–1073. 117 Winterfeld, G. A.; Schmidt, R. R. Angew. Chem. Int. Ed. 2001, 40, 2654–2657. 118 Winterfeld, G. A.; Khodair, A. I.; Schmidt, R. R. Eur. J. Org. Chem. 2003, 1009–1021.

102 the major product formed (α:β = 1:8).111 In general, for alcohol nucleophiles, strong bases favored the formation of the α products while weak bases favored β products.

Figure 3.08. Additions of alcohols to 2-nitrogalactal.

The authors proposed that transition states TSα and TSβ might play a role in the observed stereochemistry (Figure 3.09). The nitro group was always observed in the equatorial position in the products after protonation (Figure 3.09).116 Yu has also reported a stereoselective Michael- type addition of 2-nitroglycals using 4-(N,N-dimethylamino)pyridine (DMAP) or 4- pyrrolidinopyridine (PPY).119 These reactions were β-selective for both 2-nitrogalactal (3.04) and 2-nitroglucal (3.08). The authors proposed a mechanism involving attack by DMAP or PPY at C1 from the α side of the glycal to furnish an α-glycosyl pyridinium species. SN2 substitution of this intermediate by a nucleophile would lead to the β products.

119 Xue, W.; Sun, J.; Yu, B. J. Org. Chem. 2009, 74, 5079–5082.

103

Figure 3.09. Possible transition states for the addition of alcohols to 2-nitrogalactal.

Figure 3.10. Stereoselective Michael-type additions to 2-nitroglycals employing DMAP or PPY.

In addition to alcohols, both phenols120 and malonates121 could be added to 2-nitroglycals. Addition of phenols to 2-nitrogalactal in the presence of tBuOK as base led to the formation of the α product, whereas addition of malonates to 2-nitrogalactal gave the β product.

120 Khodair, A. I.; Winterfeld, G. A.; Schmidt, R. R. Eur. J. Org. Chem. 2003, 1847–1852. 121 Pachamuthu, K.; Gupta, A.; Das, J.; Schmidt, R. R.; Vankar, Y. D. Eur. J. Org. Chem. 2002, 1479–1483.

104

Figure 3.11. Additions of phenols and malonates to 2-nitroglycals.

3.3 Results and Discussion

To investigate whether catalyst controlled additions to nitroglycals were possible, the nitroglycals 3,4,6-tri-O-benzyl-2-nitro-D-galactal (3.04) and 3,4,6-tri-O-benzyl-2-nitro-D-glucal were synthesized according to literature procedures122 as shown in Figures 3.12 and 3.13.

Figure 3.12. Synthesis of 2-nitrogalactal (3.04).

122 Das, J.; Schmidt, R.R. Eur. J. Org. Chem. 1998, 1609–1613

105

Figure 3.13. Synthesis of 2-nitroglucal (3.08).

Takemoto's catalysts ((S,S)-3.22 and (R,R)-3.22), L-proline and (S)-2- (diphenyl((trimethylsilyl)oxy)methyl)pyrrolidine (3.03) were chosen as the hydrogen-bond donor catalysts to be tested as they had previously been shown to catalyze additions to nitroolefins. L- proline and 3.03 were commercially available while (S,S)-3.22 and (R,R)-3.22) were prepared according to a literature procedure as outlined in Figure 3.14.123

Figure 3.14. Synthesis of Takemoto's catalyst.

The first reactions studied were the the addition of methanol, 4-bromophenol and diethylmalonate to 2-nitrogalactal (3.04). In the absence of a base or catalyst, no reaction was observed for all three compounds. To assess whether Takemoto's catalyst, which contains a

123 Berkessel, A.; Seelig, B. Synthesis 2009, 2113–2115.

106 tertiary amine, was functioning as a simple base or as a HB catalyst, the outcome of the reactions in the presence of triethylamine was determined as a comparison. Using methanol as the nucleophile gave 3.23 in 91% yield as a 1:6 α:β mixture (Table 3.01, entry 1). This is in good agreement with the value reported for this reaction under these conditions (90% yield, α:β = 1:8). When (S,S)-3.22 was used as the catalyst and methanol as the nucleophile, 3.23 was obtained in 69% yield (α:β = 1:15). While the α:β ratio appeared to have increased compared to the reaction where NEt3 was used as a base, using (R,R)-3.22 as catalyst gave a similar result (70% yield, α:β = 1:12) to using (S,S)-3.22. This suggests the stereoselectivity of the reaction is not significantly influenced by the identity of the catalyst. The addition of 4-bromophenol gave 3.24 in 50% yield as a 2:1 α:β mixture (Table 3.01, entry 4) when NEt3 was used as the base. Although this reaction had not previously been reported, the addition of phenols in the presence of strong bases like tBuOK are generally α-selective. The weaker base, triethylamine, appears to still be α-selective but a significant amount of the β anomer was formed. Similar to the results obtained using methanol as the nucleophile, almost identical yields and α:β ratios were obtained regardless of the enantiomer of catalyst employed (Table 3.01, entries 5 and 6). No reaction was observed when diethylmalonate was used as the nucleophile (entry 3).

Table 3.01. Addition of nucleophiles to 2-nitrogalactal.

Catalyst NMR Yield Entry Solvent NucH (equiv) α:β (mol %) (%)

1 NEt3 (1500) THF MeOH (10) 91 1:6 2 (S,S)-3.22 (10) THF MeOH (10) 69 1:15 3 (R,R)-3.22 (10) THF MeOH (10) 70 1:12

4 NEt3 (1500) THF 4-bromophenol (1.1) 50 2:1 5 (S,S)-3.22 (10) THF 4-bromophenol (1.1) 65 2:1 6 (R,R)-3.22 (10) THF 4-bromophenol (1.1) 72 2:1

7 NEt3 (1500) THF diethylmalonate (1.1) 0 N/A 8 (S,S)-3.22 (10) THF diethylmalonate (1.1) 0 N/A 9 (R,R)-3.22 (10) THF diethylmalonate (1.1) 0 N/A

107

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

Table 3.02. Addition of nucleophiles to 2-nitroglucal.

Catalyst NMR Yield Entry Solvent NucH (equiv) α:β (mol %) (%)a 1 (S,S)-3.22 (10) Toluene MeOH (10) 0 N/A 2 (S,S)-3.22 (10) Toluene MeOH (100) 90b 0:1 3 (R,R)-3.22 (10) Toluene MeOH (10) 0 N/A 4 (R,R)-3.22 (10) Toluene MeOH (100) 90b 0:1 5 (S,S)-3.22 (10) Toluene 4-bromophenol (1.1) 30 4:1 6 (R,R)-3.22 (10) Toluene 4-bromophenol (1.1) 33 4:1

7 NEt3 (10) Toluene 4-bromophenol (1.1) 31 4:1 8 2.05 (10) Toluene 4-bromophenol (1.1) 0 N/A

9 2.05+NEt3 (10) Toluene 4-bromophenol (1.1) 25 4:1 10 (S,S)-3.22 (10) Toluene 4-methoxyphenol (1.1) 71 4:1 11 (R,R)-3.22 (10) Toluene 4-methoxyphenol (1.1) 71 4:1 12 (S,S)-3.22 (10) Toluene perfluorophenol (1.1) 0 N/A 13 (R,R)-3.22 (10) Toluene perfluorophenol (1.1) 0 N/A 14 (S,S)-3.22 (10) Toluene diethylmalonate (1.1) 0 N/A 15 (R,R)-3.22 (10) Toluene diethylmalonate (1.1) 0 N/A 16 (S,S)-3.22 (10) Toluene nitromethane (1.1) 0 N/A 17 (R,R)-3.22 (10) Toluene nitromethane (1.1) 0 N/A

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

b The reaction was performed at 50 °C instead of 23 °C

Exploration of the addition of nucleophiles to 2-nitroglucal revealed the same trends. For the addition of 4-bromophenol, using NEt3, (S,S)-3.22 or (R,R)-3.22 as the catalyst gave very similar yields and identical α:β ratios (Table 3.02, entries 5-7). In the presence of thiourea catalyst 2.05,

108 which lacks the tertiary amine moiety, no addition of 4-bromophenol was observed (Table 3.02, entry 8). Using both 2.05 and NEt3 as catalysts restored reactivity, but the yield and α:β ratio was similar to that obtained using solely NEt3. Varying the identity of the phenol nucleophile led to increased yields for electron rich 4-methoxyphenol (Table 3.02, entries 10 and 11) but no reaction for the electron deficient perfluorophenol. In the case where 4-methoxyphenol was used as the nucleophile, the reaction outcome was not influenced by the stereochemistry of the catalyst. At room tempearture neither (S,S)-3.22 nor (R,R)-3.22 was capable of catalyzing the addition of methanol to 3.08 (Table 3.02, entries 1 and 3). Under forcing conditions (100 equivalents of nucleophile, 50 °C) the product was obtained in 90% yield, however, complete β selectivity was observed for both (S,S)-3.22 and (R,R)-3.22. (Table 3.02, entries 2 and 4). The results summarized in Tables 3.01 and 3.02 suggest that Takemoto's catalyst is acting as a simple tertiary amine base and had no influence on the stereochemical outcome of the reaction. As hetero-Michael additions (sulfa-Michael reactions and oxa-Michael reactions) are often reversible,124 a set of reactions was carried out to determine if the addition of 4-bromophenol to 2-nitroglucal was reversible under thermodynamic control. Isolated, pure 3.26α was resubjected to the reaction conditions and the products of the reaction were analyzed by 1H NMR spectroscopy. The crude reaction showed a ratio of 3.26α:3.26β:3.08:3.27 of 3:1:15:15. This suggested that under the reaction conditions, the product 3.26 is in equilibrium with the starting material 3.08.

124 (a) Nising, C. F.; Bräse, S. Chem. Soc. Rev. 2012, 41, 988–999. (b) Wang, X-F.; Hua, Q.L.; Cheng, Y.; An, X.- L.; Yang, Q.-Q.; Chen, J.-R.; Xiao, W.-J. Angew. Chem. Int. Ed. 2010, 49, 8379–8383. (c) Marigo, M.; Schulte, T.; Franzen, J.; Jørgensen, K. A. J. Am. Chem. Soc. 2005, 127, 15710–15711.

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Figure 3.15. Resubjecting 3.26α to the standard reaction conditions.

A similar result was obtained if 3.26β was resubjected to the reaction conditions. In this case, the crude reaction mixture also showed a ratio of 3.26α:3.26β:3.08:3.27 of 3:1:15:15 by 1H NMR spectroscopy. As the reaction was found to be reversible under the conditions studied, the selectivity observed did not appear to be catalyst controlled

Figure 3.16. Resubjecting 3.26α to the standard reaction conditions.

Prolinol derivative 3.03 was used as a catalyst in an attempt to add ketones or aldehydes to 2- nitroglycals. However, no reaction was observed under any of the conditions tested for both a variety of aldehydes and for acetone. Changing the solvent, heating the reaction or adding benzoic acid did not lead to product formation. The sets of conditions tested are summarized in Table 3.03.

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Table 3.03. Additions of aldehydes and ketones to nitroglycals.

Aldehyde or Ketone NMR Yield Entry T (°C) Solvent (equiv) (%)a 1 hexanal (5) 23 pentane 0

2 hexanal (5) 23 Et2O 0 3 hexanal (5) 23 DCM 0 4 hexanal (5) 40 DCM 0 5 hexanal (5) 23 Toluene 0 6 hexanal (5) 50 Toluene 0 7 hexanal (5) 23 Tolueneb 0 8 2-ethylbutyraldehyde (5) 23 Toluene 0 9 2-ethylbutyraldehyde (5) 23 Tolueneb 0 cyclohexane 10 23 Toluene 0 carboxaldehyde (5) cyclohexane 11 23 Tolueneb 0 carboxaldehyde (5) 12 Acetone (10) 23 Toluene 0 13 Acetone (100) 50 Toluene 0 14 Acetaldehyde (10) 23 Toluene 0

a Yield determined by 1H NMR spectroscopy using 4,4'-di-tert-butylbiphenyl as a quantitative internal standard

b Benzoic acid (10 mol%) was added to the reaction

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3.4 Conclusions

The experiments performed in this chapter suggested that catalysts and reactions developed for simple nitroalkenes could not be easily adapted to 2-nitroglycal substrates. The conditions for the attempted additions of diethyl malonate and aldehydes or ketones to 2-nitroglycals were based on previously described reactions which used simple nitroolefins as Michael-acceptors. However, using 2-nitroglycals as Michael-acceptors led to no reaction. The addition of methanol and phenols to 2-nitroglycals was possible, as previously described by Schmidt.116,120,121,122 The stereoselectivity of the reaction was, however, not influenced by chiral bifunctional thiourea catalysts (S,S)-3.22 nor (R,R)-3.22. In the case of the addition of 4-bromophenol, this was most likely due to the reversibility of the addition under the reaction conditions.

3.5 Experimental Details

Instrumentation. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a 300-MHz or 400-MHz Varian Mercury spectrometer or a 400-MHz Bruker spectrometer. Chemical shifts for protons are reported in parts per million (ppm) downfield from tetramethylsilance and are referenced to residual protium in the NMR solvent (CHCl3: δ 7.26). Data are represented as follows; chemical shift (δ, ppm); multiplicity (s-singlet, d-doublet, t- triplet, q-quartet, m = multiplet); coupling constant (J, Hz); integration).

Assignment of 1H NMR signals. 1H NMR signal assignments were based on analysis of both coupling constants and COSY experiments. The two-dimensional COSY spectrum can be found in Appendix B.

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General procedure G: Addition of nucleophiles to 2-nitroglycals.

(S,S)-3.22 or (R,R)-3.22 (x mol%) and nucleophile (y equiv) were weighed out or transferred into a 2-dram vial equipped with a stir bar. Solvent (0.2 mL) was then added. A solution of 2- nitroglycal (0.1 mmol) in solvent (0.2 mL) was added to the reaction. The reaction was stirred at the indicated temperature for 20 hours. At this time, 1 mL of 10 mmolL-1 4,4'-di-tert- butylbiphenyl (in EtOAc) was added to the reaction. Water (0.5 mL) was added to the reaction and the aqueous layer extracted with DCM. A sample of the organic layer was removed, concentrated and dried under vacuum. It was then submitted for 1H NMR analysis where the yields were determined by comparing the integration of product peaks to those of the internal standard. Spectral data for compounds 3.23α, 3.25β, 3.25α and 3.25β are in agreement with those reported in the literature.111,119 Spectral data for 3.24α, 3.24β, 3.26α and 3.26β are reported below. The anomers of 3.27 could not be separated so the assignmenta of peaks belonging to 3.27α and 3.27β were based on coupling constant analysis and comparison to 3.26α and 3.26β.

Resubjecting 3.26α to standard reaction conditions.

3.26α (0.043 mmol) and (S,S)-3.22 (10 mol%) were weighed into a 1-dram vial. Toluene (0.2 mL) was added and the reaction was stirred at 23 °C for 20 hours. A sample of the reaction was removed, concentrated and dried under vacuum. It was then submitted for 1H NMR analysis where the ratios of the various products were determined by comparing their respective signals.

Resubjecting 3.26β to standard reaction conditions.

3.26β (0.015 mmol) and (S,S)-3.22 (10 mol%) were weighed into a 1-dram vial. Toluene (0.2 mL) was added and the reaction was stirred at 23 °C for 20 hours. A sample of the reaction was removed, concentrated and dried under vacuum. It was then submitted for 1H NMR analysis where the ratios of the various products were determined by comparing their respective signals

General procedure H: Addition of aldehydes or ketones to 2-nitroglycals.

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The 2-nitroglycal (0.2 mmol), (S)-2-(diphenyl((trimethylsilyl)oxy)methyl)pyrrolidine (3.03, 20 mol%), benzoic acid (if applicable, 0.02 mmol) and solvent (0.4 mL) were added to a 2-dram vial. The aldehyde or ketone (x equiv) was added and the reaction stirred at the indicated temperature for 20 hours. A sample of the reaction was then removed, concentrated, dried under vacuum and analyzed by 1H NMR spectroscopy. No products were ever observed by 1H NMR or TLC.

Characterization Data

Compound (3.24α)

A mixture of anomers of 3.24 was synthesized according to general procedure G using (S,S)- 3.22 as catalyst and 1.1 equivalents of 4-bromophenol. The crude product was purified by column chromatography (EtOAc/pentance 1:4) to give 16.4 mg (26 %) of 3.24α and 8.7 mg (14 1 %) of 3.24β. H NMR (400 MHz, CDCl3): δ 7.34–7.10 (m, 17H), 6.83 (d, J = 8.9 Hz, 2H, ArH), 5.78 (d, J = 4.1 Hz, 1H, H-1), 5.06 (dd, J = 10.7, 4.1 Hz, 1H, H-2), 4.80 (d, J = 11.1 Hz

,1H, OCH2Ph), 4.77–4.67 (m, 2H, OCH2Ph), 4.55 (dd, J = 10.6, 3.0 Hz, 1H, H-3), 4.43 (d, J =

11.1 Hz, 1H, OCH2Ph), 4.34 (m, 2H, OCH2Ph), 4.07 (m, 1H, H-5), 4.01 (dd, J = 3.1, 1.1 Hz, 1H, H-4), 3.57–3.42 (m, 2H, H-6, H-6').

Compound (3.24β)

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ArH), 5.29 (d, J = 8.1 Hz, 1H, H-1), 5.15 (dd, J = 10.6, 8.1 Hz, 1H, H-2), 4.93 (d, J = 11.4 Hz

,1H, OCH2Ph), 4.67 (d, J = 11.6, 1H, OCH2Ph), 4.61 (d, J = 11.6, 1H, OCH2Ph), 4.53 (d, J =

11.4 Hz, 1H, OCH2Ph), 4.49 (d, J = 4.1 Hz, 2H, OCH2Ph), 4.16 (dd, J = 10.7, 2.6 Hz, 1H, H-3), 4.07 (m, 1H, H-4), 3.80 (t, J = 6.5 Hz, 1H, H-5), 3.67 (d, J = 6.5 Hz, 2H, H-6, H-6').

Compound (3.26α)

A mixture of anomers of 3.26 was synthesized according a modified procedure of general procedure G using (S,S)-3.22 as catalyst and 1.1 equivalents of 4-bromophenol. The reaction was carried out on a 0.2 mmol scale as opposed to a 0.1 mmol scale. The crude product was purified by column chromatography (DCM/toluene 1:1) to give 27 mg (21 %) of 3.24α and 10 mg (8 %) 1 of 3.24β. H NMR (400 MHz, CDCl3): δ 7.38–7.10 (m, 17H), 6.88 (d, J = 8.9 Hz, 2H, ArH),

5.85 (d, J = 3.2 Hz, 1H, H-1), 4.91 (m, 2H, OCH2Ph), 4.79 (d, J = 10.8 Hz ,2H, OCH2Ph), 4.72–

4.68 (m, 2H, H-2, H-3), 4.57 (d, J = 9.6 Hz, 1H, OCH2Ph), 4.54 (d, J = 8.5 Hz, 1H, OCH2Ph),

4.43 (d, J = 11.9 Hz, 1H, OCH2Ph), 3.99–3.91 (m, 1H, H-4), 3.89–3.80 (m, 1H, H-5), 3.74 (dd, J = 11.0, 3.4 Hz, 1H, H-6), 3.60 (dd, J = 11.0, 2.0 Hz, 1H, H-6').

Compound (3.26β)

5.22 (d, J = 8.1 Hz, 1H, H-1), 4.77–4.66 (m, 3H, OCH2Ph, H-2), 4.52 (d, J = 11.1 Hz ,2H,

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OCH2Ph), 4.43 (d, J = 12.0 Hz, 2H, OCH2Ph), 4.24 (dd, J = 10.4, 8.8 Hz, 1H, H-3), 3.72–3.56 (m, 4H, H-4, H-5, H-6, H-6').

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Chapter 4 Applications of Organoboron Compounds in Carbohydrate Chemistry125 4.1 Introduction

The reversible interaction between diols and boronic acids to form stable cyclic boronate esters has been known since the early 1950s.126 Various equilibria of boronic acid-diol complexes are shown in Figure 4.01. These complexes can exist having either a tricoordinate boron atom (non- coordinating solvents) or a tetracoordinate boron atom, as is the case in basic aqueous solutions. The defining feature of tricoordinate boron complexes is the sp2 hybridized boron atom which contains a vacant p orbital. This Lewis acidic site allows for the complexation of Lewis bases which not only causes the boron to become sp3 hybridized, but also affects the binding properties and reactivity of the molecule. The acidic character of boronic acids is not that of a Brønsted acid, but rather that of a Lewis acid. Coordination of water to boron causes ionization and proton transfer to another water molecule to form hydronium ions. The elucidation of the tetrahedral nature of the phenylboronate anion was made by Lorand and Edwards in 1959.127 Binding of a polyol to a boronic acid has been shown to increase the Lewis acidity of boron and therefore decrease its pKa. This phenomenon is attributed to the relief of angle strain on conversion of a trigonal cyclic boronate to a tetrahedral cyclic boronate. For example, the pKa of phenylboronic 128 acid was determined to be 8.8 but the the pKa of its fructose ester was lowered to 4.6.

125 Portions of this chapter have been published. McClary, C. A.; Taylor, M. S. Carb. Res. 2013, submitted. 126 H. G. Kuivila, A. H. Keough, E. J. Soboczenski, J. Org. Chem. 1954, 19, 780. 127 Lorand, J. P.; Edwards, J. O. J. Org. Chem. 1959, 24, 769 128 Springsteen, G.; Wang, B. Tetrahedron, 2002, 58, 5291–5300.

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Figure 4.01. Key interactions of boronic acids with carbohydrates.

Carbohydrates may contain multiple hydroxyl groups potentially giving rise to a multitude of boronic ester regioisomers. However, complexation with certain diol motifs is more favorable than others. In general, complexation is more favorable with 1,2- versus 1,3-diols and more favorable with cis- versus trans-1,2-diols in cyclic systems.129 As a large number of saccharides contain these motifs which form strong complexes with boronic acids, the interactions described above can be used to investigate the sensing, separation, and synthesis of carbohydrate molecules. This review highlights how the interaction between boron containing compounds and saccharides has been exploited in various areas of carbohydrate chemistry.

129 (a) van den Berg, R.; Peters, J. A.; van Bekkum, H. Carbohydr. Res. 1994, 253, 1–12.

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4.2 Carbohydrate Sensors based on Organoboron Compounds

Since the seminal report by Czarnik and Yoon detailing the first example of a fluorescent chemosensor capable of signaling the binding of polyols,130 a great deal of interest has been shown in developing sensors which can selectively recognize different saccharides in aqueous media. These type of receptors are of interest as they offer distinct advantages over hydrogen- bonding based receptors (for which polar protic solvents such as water compete with guest binding) and their analytes are important in a wide range of areas including the monitoring of blood glucose levels, quality control and medicinal applications.

Figure 4.02. Anthrylboronic acid sensor for polyols developed by Czarnik and Yoon.

Although the anthrylboronic acid receptors were the first example of fluorescent chemosensors to signal binding of polyols, the photoinduced electron transfer (PET) from the boronate anion was not efficient. (Iin the presence of saccharide / I0 in the absence of saccharide = ~ 0.7). An important contribution to the field was made by Shinkai and coworkers when their group introduced more efficient sensors containing amines.131 In the absence of a bound polyol the nitrogen interacts only weakly with the boronic acid (forming a boron–nitrogen complex) and is still available to act as a PET quencher. Binding of a polyol increases the Lewis acidity of the boron atom and increases the strength of the boron–nitrogen interaction. This results in less efficient PET quenching by the amine and an increase in fluorescence.

130 Yoon, J.; Czarnik, A. W. J. Am. Chem. Soc. 1992, 114, 5874–5875. 131 (a) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. J. Chem. Soc. Chem. Commun. 1994, 477–478. (b) James, T. D.; Sandanayake, K. R. A. S.; Iguchi, R.; Shinkai, S. J. Am. Chem. Soc. 1995, 117, 8982–8987.

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Figure 4.03. Introduction of amines into chemosensors for carbohydrates.

The use of boronic acids in saccharide sensors has been reviewed recently and in depth by several groups.132 The following examples highlight some of the most recent developments in the field. After Shinkai's contribution of sensors capable of detecting saccharides via changes in fluorescence, focus shifted to developing methods for selective saccharide detection133 and introduction of more efficient fluorophores. These goals were realized through the synthesis of receptors containing multiple boronic acid residues separated by different linker groups. Spatial or stereochemical complementarity between the boronic acid moieties in the receptor and diols in the analyte imparted selectivity to the receptors. The groups of James and Shinkai have developed a number of chiral fluorescent molecular sensors to enantioselectively discriminate between various sugars and sugar derivatives.134 The most recent examples are shown in Figure 4.04.

132 (a) Fossey, J. S.; D'Hooge, F.; Van Den Elsen, J. M. H.; Pereira Morais, M. P.; Pascu, S. I.; Bull, S. D.; Marken, F.; Jenkins, A. T. A.; Jiang, Y.-B.; James, T. D. The Chemical Record, 2012, 12, 464–478. (b) Guo, Z.; Shin, I.; Yoon, J. Chem. Commun. 2012, 48, 5956–5967. (c) Bull, D. B.; Davidson, M. G.; Vand Den Elsen, J. M. H.; Fossey, J. S.; Jenkins, A. T. A.; Jiang, Y.-B.; Kubo, Y.; Marken, F.; Sakurai, K.; Zhao, J.; James, T. D. Acc. Chem. Res. 2013, 46, 312–326. (d) Nishiyabu, R.; Kubo, Y.; James, T. D.; Fossey, J. S. Chem. Commun. 2012, 47, 1106– 1123.

133 (a) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Angew. Chem. Int. Ed. 1994, 33, 2207–2209. (b) James, T.D.; Shinmori, H.; Shinkai, S. Chem. Commun. 1997, 71–72. (c) Swamy, K.M.K.; Jang, Y.J.; Park, M.S.; Koh, H.S.; Lee, S.K.; Yoon, Y.J.; Yoon, J. Tetrahedron Lett. 2005, 46, 3453–3456.

134 (a) Han, F.; Chi, L.; Liang, X.; Ji, S.; Liu, S.; Zhou, F.; Wu, Y.; Han, K.; Zhao, J.; James, T. D. J. Org. Chem. 2009, 74, 1333–1336. (b) Zhao, J.; Fyles, T. M.; James, T. D. Angew. Chem. Int. Ed. 2004, 116, 3543–3546 (c) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Nature, 1995, 374, 345–347.

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Figure 4.04. Chiral fluorescent sensors for sugars and sugar derivatives.

The diboronic acid receptors R,R-4.01 and S,S-4.01 exhibited substantially different association constants with various sugar alcohols. Enantioselectivity factors (KR : KS) as high as 1:2000 were observed in the case of D-mannitol.135,136 These sensors were also highly chemoselective for six- hydroxyl sugar alcohols over five- or four-hydroxyl sugar alcohols. The chirality of the receptor also had an effect on the chemoselectivity between two different biological molecules. For example, the chemoselectivity of S,S-4.01 for D-glucaric acid/D-sorbitol was determined to be

135 Zhao, J.; Davidson, M. G.; Mahon, M. F.; Kociok-Köhn, G.; James, T. D. J. Am. Chem. Soc. 2004, 126, 16179– 16186. 136 Zhao, J.; James, T. D. J. Mater. Chem. 2005, 15, 2896.

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260:1 but with R,R-4.01 the chemoselectivity was found to have increased to 790:1. Eantioselective discrimination of D- and L- tartaric acid was capable with Phenothiazine sensors R,R-4.02, S,S-4.02 and R,R-4.03, S,S-4.03 as well as the enantioselective recognition of disaccharides and ginsenosides Re and Rb1.137 These studies have shown that including chiral elements in the design of receptors can lead to significantly altered characteristics.

Figure 4.05. Structures of two representative Ginsenosides.

Another factor which is critical in tuning the selectivity of the receptor is the size of the linker present between the two boronic acid groups. The groups of Anslyn and Sessler have developed a porphyrin based receptor for ginsenosides shown in Figure 4.06.138 It was designed based on the idea that the boronic acids would interact with the saccharide moieties on the edges of the molecule and the porphyrin could interact with the steroid core through hydrophobic interactions. Receptor 4.04 showed association constants of 3900 ± 210 M-1 and 2500 ± 350 M-1 for Ginsenoside Re and Ginsenoside Rb1, respectively, in DMSO/aqueous HEPES buffer. However, S,S-4.03 displayed increased affinity for these two analytes (6.79 ± 0.5 x 105 M-1 and 2.88 ± 0.51 x 105 M-1, respectively, in methanol/water pH 7.4). This tight binding was tentatively attributed to a better fit of the ginsenosides into the bis-boronic acid binding pocket of S,S-4.03.

137 Wu, Y.; Guo, H.; Zhang, X.; James, T. D.; Zhao, J. Chem. Eur. J. 2011, 17, 7632–7644.

138 Hargrove, A.E.; Reyes, R.N.; Riddington, I.; Anslyn, E.V.; Sessler, J.L. Org. Lett. 2010, 12, 4804–4807.

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Figure 4.06. Structures of bis-boronic acid receptor 4.04.

The group of James has also developed a fluorescent "molecular tweezer" receptor for the sensing of carbohdryates. The two boronic acids allow two point binding to saccharides and the two pyrene units increase hydrophobicity in the binding pocket. The molecular tweezer selectively opens in the presence of certain saccharides with the stability order of D-glucose >> D-galactose > D-mannose.139

Figure 4.07. Boronic acid-based "molecular tweezer" sensor.

Libraries of boronic acid sensors have been applied towards pattern-based discrimination of saccharides based on array sensing schemes and multicomponent indicator displacement sensor assays. These assays employ catechol derivatives as indicator dyes which are capable of

139 Phillips, M. D.; Fyles, T. M.; Barwell, N. P.; James, T. D. Chem. Commun. 2009, 6557–6559.

123 engaging in reversible covalent interactions with boronic acids. Displacement of the indicator dye upon binding of the analyte with the boronic acid triggers a change in color or fluorescence quantum yield of the solution due to release of the dye. Using a suite of bis-boronic acid receptors having various linkers, the group of Anslyn was able to discriminate between structurally similar gensinosides and ginsengs.140 The group of James has developed a dye displacement assay for saccharide detection using boronate hydrogels.141 This colorimetric, competitive-binding assay uses polyacrylamide hydrogels containing boronic acid moieties (borogels). The borogels are first exposed to an indicator, in this case alizarin red-S (ARS) where the catechol functionality binds reversibly to boron. Binding results in a hypsochromic shift which is manifested in a color change from red to orange. The gels are then washed with phosphate buffer to remove any residual dye that has not been covalently bound to boron. Exposure of the borogels bound to ARS to fructose caused the release of the dye into solution as fructose displaced the dye through transesterification. This technique was used to determine the relative amounts of boron-binding species (saccharides) in fruit juice samples.

Figure 4.08. Interactions of borogel with ARS and analytes in an indicator displacement assay.

140 Zhang, X.; You, L.; Anslyn, E. V.; Qian, X. Chem. Eur. J. 2012, 18, 1102–1110.

141 Ma, W.M.J.; Morais, M.P.P.; D'Hooge, F.; van den Elsen, J.M.H.; Cox, J.P.L.; James, T.D.; Fossey, J.S. Chem. Commun. 2009, 532534.

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4.3 Application of Boronic Acid-Carbohydrate Interactions in Drug Delivery and Cellular Imaging

Many carbohydrate-based biomarkers have been correlated with the development and progression of certain diseases. Such biomarkers include the sialyl Lewis X (sLeX), sialyl Lewis Y (sLeY) and sialyl Lewis a (sLea) antigens. The development of gastrointestinal, pancreatic and breast cancers have been linked with the over-expression of sLeX-containing mucins on cell surfaces.142

Figure 4.09. Structure of the Sialyl Lewis X and Sialyl Lewis Y tetrasaccharides.

In an effort to develop sensors capable of selectively identifying cells expressing the sLeX antigen, the group of Wang has taken advantage of the ability of boronic acids to bind compounds with vincinal diols. They proposed a series of molecules containing two boronic acid moieties capable of binding to the dihydroxyl structural motifs in sLeX, interspaced with a linker.143 The length and nature of the linker were varied with the goal of achieving a complimentary spatial arrangement between the boronic acid groups of the sensor and the diol structures in sLeX. In order for the sensor to be able to report the binding event, the Wang group adopted the use of the anthracene-based PET fluorescence system previously developed by Shinkai.144

142 (a) Iida, S.; Tsuiji, H.; Nemoto, Y.; Sano, Y.; Reddish, M. A.; Irimura, T. Oncol. Res. 1998, 10, 407. (b) Ye, C.; Kiriyama, K.; Mistuoka, C.; Kannagi, R.; Ito, K.; Watanabe, T.; Kondo, K.; Akiyama, S.; Takagi, H. Int. J. Cancer 1995, 61, 455. (c) Weston, B. W.; Hiller, K. M.; Mayber, J. P.; Manousos, G. A.; Bendt, K. M.; Liu, R.; Cusack, J. C. Jr. Cancer Res. 1999, 59, 2127–2135. 143 (a) Yang, W.; Fan, H.; Gao, X.; Gao, S.; Karnati, V.V.R.; Ni, W.; Hooks, W.B.; Carson, J.; Weston, B.; Wang, B. Chem.Biol. 2004, 439–448. (b) Yang, W.; Gao, S.; Gao, X.; Karnati, V.V.R.; Ni, W.; Wang, B.; Hooks, W.B.; Carson, J.; Weston, B. Bioorg. Med. Chem. Lett. 2002, 12, 2175–2177. 144 James, T.D., Sandanayake, K.R.A.S., Iguchi, R., and Shinkai, S. J. Am. Chem. Soc. 1995, 117, 8982–8987.

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Figure 4.10. Structurse of the bis-boronic acid sensors for sLeX .

After testing a suite of compounds having various R groups by measuring the change in fluorescence intensity upon binding with sLeX (methanol/0.1M phosphate buffer) the authors discovered that a simple phenyl linker (4.06) gave the greatest fluorescence change upon mixing with sLeX. Changing the linker had a profound effect on the selectivity of sensor. 4.06 showed no affinity for sLeY whereas 4.07 showed high selectivity for sLeY and no selectivity for sLeX. 4.08 had poor affinity for both sLeX and sLeY. To test whether 4.06 could be used to label cells expressing sLeX, a series of cell lines were incubated with 4.06 and then examined under fluorescent microscopy. The HEPG2 cell line was chosen as a cell line known to selectively express sLeX on the cell surface, whereas HEP3B expresses only the Lewis Y antigen and COS7 expresses none of the fucosylated antigens associated with carcinoma progression. Cell lines and compounds 4.06, 4.07 and 4.08 were incubated and then examined under fluorescent microscopy. 4.06 labelled the HEPG2 cell line (expressing sLeX), 4.07 labelled the HEP3B cell line (expressing sLeY) and 4.08 labelled neither. The COS7 cell line was not labelled by any of the sensors. These sensors represent the first examples of small organic molecules used to fluorescently label cells based on recognition of cell-surface carbohydrate structures. The Wang group further explored the use of their bis-boronic acid sensors by adapting them for TArgeted multiplex Mass Spectrometry IMaging (TAMSIM).145 This technique employs molecules capable of binding an analyte that are also outfitted with a laser-reactive photo-cleavable

145 Dai, C.; Cazares, L.J.; Wang, L.; Chu, Y.; Wang, S.L.; Troyer, D.A.; Semmes, O.J.; Drake, R.R.; Wang, B. Chem. Commun. 2011, 47, 10338–10340.

126 molecular tag. Irradiation of a bound affinity molecule causes the release of the molecular tag which is detectable by mass spectrometry. This reveals the location of the analyte in a sample.146

Figure 4.11. Boronolectin used for the histological analysis of cancer tissue.

As the receptor with a phenyl linker (4.06) had previously been shown to be selective for sLeX, a phenyl ring modified with a trityl thioether tag was chosen as the linker in 4.09. The ability of the boronolectin-trityl reporter conjugate to identify tumor regions in tissue was then tested by incubating frozen renal tissues containing both normal and tumor regions. The slides containing the tissue were then washed with PBS (phosphate buffered saline) to remove any unbound material and the sample was analyzed directly by MALDI-TOF without addition of matrix material. Irradiation with the laser triggered fragmentation and loss of the substituted trityl cation. The signal for the trityl peak was observed in the tumor region of the tissue where sLeX was expressed on the cells but not in the regions containing healthy tissue.

146 (a) Thiery, G.; Shchepinov, M. S.; Southern, E. M.; Audebourg, A.; Audard, V.; Terris, B.; Gut, I. G. Rapid Commun. Mass Spectrom., 2007, 21, 823–829. (b) Thiery, G.; Anselmi, E.; Audebourg, A.; Darii, E.; Abarbri, M.; Terris, B.; Tabet, J.-C.; Gut, I. G. Proteomics,2008, 8, 3725-3734. (c) Lemaire, R.; Stauber, J.; Wisztorski, M.; Van Camp, C.; Desmons, A.; Deschamps, M.; Proess, G.; Rudlof, I.; Woods, A. S.; Day, R.; Salzet, M.; Fournier, I. J. Proteome Res. 2007, 6, 2057–2067.

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The group of Miyahara has developed a self-regulated insulin delivery system based on stimuli- responsive "smart gels".147 These gels can be loaded with insulin and the insulin can be released or retained based on certain external stimuli. Their system relies on the binding of glucose to phenylboronic acid moieties in the polymer gel as such a stimuli. This type of approach could be used to treat Diabetes as insulin could be released from the gel when glucose levels were high and retained when glucose levels were low. In this effect, the system works as a mimic of a biofeedback system.

Figure 4.12 shows the equilibrium between different boronic acid species and their glucose complexes. When the polymer gel is exposed to an increased concentration of glucose the equilibrium is shifted towards 4.15 and the gel swells and becomes hydrated. In this state compounds loaded onto the gel may be released. If the concentration of glucose is reduced, however, the equilibrium is shifted towards 4.13 and 4.14. This causes the gel to shrink and become dehydrated. This dehydration causes a skin layer148 to form on the gel which prevents any of the compound loaded onto the gel from being released.

Figure 4.12. Glucose-dependent equilibria of phenylboronic acid containing polymers.

The gels were prepared through radical copolymerization of several acrylamide monomers (Figure 4.13). The choice of substituents on the boronic acid containing monomer was critical in order to control the pKa of the boronic acid. The pKa of phenyl boronic acid is approximately 9, too weakly acidic to generate significant amounts of charged phenylboronate 4.15 under

147 Matsumoto, A.; Ishii, T.; Nishida, J.; Matsumoto, H.; Kataoka, K.; Miyahara, Y. Angew. Chem. Int. Ed. 2012, 51, 2124–2128. 148 Yoshida, R.; Sakai, K.; Okano, T.; Sakurai, Y. Adv. Drug Delivery Rev. 1993, 11, 85–108.

128 physiological pH and temperature (pH 7.4 and 37 °C). The addition of a strongly electron withdrawing fluorine substituent to the phenyl ring reduced the pKa of the boronic acid to 7.2 in AmECFPBA.

Figure 4.13. Monomer composition of the hydrogel.

The gels were loaded with fluorescein isothiocyanate (FITC)-labeled (bovine) insulin by allowing them to be hydrated in a solution of insulin and then sealed with a skin layer by decreasing the glucose concentration. Subsequent additions of glucose caused the FITC-labeled insulin to be released. Switching between high and low glucose concentrations allowed the release of insulin to be controlled over multiple cycles. The results clearly show that as the glucose concentration is increased, insulin is released as the skin layer becomes hydrated. As the glucose concentration is decreased, the gel becomes dehydrated, the skin layer reforms and insulin release stops. This study highlights how the reversible covalent interaction between sugars and boronic acids can be used to control the properties of a synthetic smart gel. Its application towards the continuous control of insulin release under physiological conditions is an important step towards developing new treatment options for patients suffering from diabetes.

4.4 Carbohydrate Separation Using Organoboron Compounds

As many carbohydrates have similar polarities and exist as neutral molecules lacking chromophores or fluorophores, their detection, separation and analysis can sometimes be difficult. Performing these operations on mixtures of carbohydrates or glycated proteins is relevant to the diagnosis and study of various diseases such as diabetes149, cancer150,

149 Uribarri, J.; Vlassara, H. Rev. Endocr. Metab. Disord. 2004, 5, 181–188. 150 van Heijst, J. W. J.; Niessen, H. W. M.; Hoekman, K.; Schalkwijk, C. G. Ann N Y Acad. Sci. 2005, 1043, 725– 733.

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Alzheimer's disease151 and autoimmune diseases152. Previous methods used for the separation of carbohydrate mixtures include boronate affinity chromatography (BAC)153 and fluorophore- assisted carbohydrate electrophoresis (FACE).154 BAC, which employs cross-linked agarose covalently modified with 3-aminophenylboronic acid, can be used to measure the degree of glycosylation of hemoglobin, a variable important in diagnosing diabetes mellitus. This method suffers from the drawback that all types of glycation modifications, including N- and O-linked glycosylated products, are retained on the column. Further separation of these mixtures is often required in order to identify each component. FACE is efficient at separating oligosaccharides on a size and charge basis, but this method requires conjugation of the analyte with a fluorophore (such as disodium 8-amino-napthalene-1,3,6-trisulfonate (ANTS) or 2-aminoacridone (AMAC)) and is therefore only applicable to reducing sugars. The method is also not very good at separating molecules with similar masses and charge and, if neutral labels such as AMAC are employed, unexpected migration patterns are sometimes displayed.155 To address these problems the groups of van den Elsen, James and Fossey have developed several separation methods involving the use of polyacrylamide gels containing boronic acids.

Figure 4.14. Structure of protected methylacrylamidophenylboronic acid incoporated into the polyacrylamide gels.

151 Vitek, M. P.; Bhattacharya, K.; Glendening, J. M.; Stopa, E.; Vlassara, H.; Bucala, R.; Manogue, K.; Cerami, A. Proc. Natl. Acad. Sci. USA 1994, 91, 4766–4770. 152 Kurien, B. T.; Scofield, R. H. Autoimmune Rev. 2008, 7, 567–573. 153 (a) Klenk, D. C.; Hermanson, G. T.; Krohn, R. I.; Fujimoto, E. K.; Mallia, A. K.; Smith, P. K.; England, J. D.; Wiedmeyer, H.-M.; Little, R. R.; Goldstein, D. E. Clin. Chem. 1982, 28, 2088-2094. (b) Mallia, A. K.; Hermanson, G. T.; Krohn, R. I.; Fujimoto, E. K.; Smith, P. K. Anal. Lett. 1981, 14, 649–661. 154 (a) Jackson, P. Biochem. J. 1990, 270, 705-713. (b) Starr, C. M.; Masada, R. I.; Hague, C.; Skop, E.; Klock, J. C. J. Chromatogr. A 1996, 720, 295–321. 155 (a) Calabro, A.; Benavides, M.; Tammi, M.; Hascall, V. C.; Midura, R. J. Glycobiology 2000, 10, 273–281. (b) Mahoney, D. J.; Aplin, R. T.; Calabro, A.; Hascall, V. C.; Day, A. J. Glycobiology 2001, 11, 1025–1033.

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One such method is boronate affinity saccharide electrophoresis (BASE).156 Incorporation of methacrylamidophenylboronic acid (MPBA) (4.16) into the polyacrylamide gel drastically altered the retention and separation of various saccharides in electrophoresis exeriments due to the affinity between boronic acids and diols. Figure 4.15 shows a normal FACE separation profile of AMAC-labeled mono and disaccharides (left) in a 20% polyacrylamide get at pH 9.0 compared to the separation profile of an otherwise identical gel containing 0.5% MPBA (right). The separation between the carbohydrates is significantly improved in the boronic acid containing gel, especially the separation between mono- and disaccharides.

Figure 4.15. FACE (left) and BASE (right) separation profiles of AMAC-labeled mono and disaccharides. Lane 1: saccharide mixture; Lane 2: lactose; Lane 3: galactose; Lane 4: N-acetyl glucosamine; Lane 5: Melibiose; Lane 6: Glucose. Reproduced with permission from ref. 155. Copyright 2008 Wiley-VCH, GmbH.

This method was also applied to the separation of glucose oligomers. Using standard FACE protocols, AMAC-labeled glucose migrates more slowly than AMAC-labeled maltose, which does not correlate to their respective molecular size. This inverted migration pattern was corrected on switching to BASE. The BASE approach was adapted for the analysis of protein glycation (coined methacrylamido phenylboronate acrylamide gel electrophoresis (mP-AGE) or

156 Jackson, T. R.; Springall, J. S.; Rogalle, D.; Masumoto, N.; Li, H. C.; D'Hooge, F.; Perera, S. P.; Jenkins, A. K. A.; James, T. D.; Fossey, J. S.; van den Elsen, J. M. H. Electrophoresis 2008, 29, 4185–4191.

131 protein-boron assisted saccharide electrophoresis (pro-BASE)) to detect proteins that had been glycated with various carbohydrates, either as their Amadori products or advanced glycation endproducts (AGEs).157 When Sbi-III-IV, a recombinant immunoglobulin-binding protein shown to inhibit the innate immune system,158 was incubated with D-(+)-gluconic acid δ-lactone and then analyzed by mass spectrometry a peak was observed corresponding to the expected mass of the protein (16587 Da) in addition to a larger peak (16705 Da, +178 Da) corresponding to a δ-gluconolactone modification at the N-terminus. Although the gluconoylated Sbi protein could be observed by MS, when conventional SDS-PAGE was used to attempt a separation of the two species the bands were almost indistinguishable. However, when mP-AGE analysis was performed the mobility of the modified protein was greatly affected, appearing at a position expected for a protein of ~60000 Da. This increased retention is attributed to the interaction between the gluconoylated protein and the boronic acid residues in the acrylamide gel and not due to a large increase in actual mass. This was supported by experiments where the concentration of MPBA incorportated into the gel was varied and the retention of the modified proteins varied while the unmodified proteins remained constant. Studies of glycation of Human serum albumin with various mono and disaccharides using mP-AGE analysis also revealed interesting trends in terms of what kind of glycated or glycosylated proteins could be separated using this methodology. It was observed that enzymatically glycosylated proteins were not well separated using mP-AGE while non-enzymatically glycated products (especially those protein adducts with δ-gluconolactone and glucose) were more easily separated. This was attributed to favorable interactions with functional groups (anomeric 1,2-diols which, upon formation of a boronate could be stabilized by an adjacent amino group) present in Amadori-type glycation but absent in glycosylated proteins.

157 Pereira Morais, M. P.; Mackay, J. D.; Bhamra, S. K.; Buchanan, J. G.; James, T. D.; Fossey, J. S.; van den Elsen, J. M. H. Proteomics 2010, 10, 48–58. 158 Burman, J.D.; Leung, E.; Atkins, K. L.; O'Seaghdha, M. N.; Lango, L.; Bernadó, P.; Foster, T. J.; Isenman, D. E.; van den Elsen, J. M. H. J. Biol. Chem. 2008, 283, 17579–17593.

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4.5 Boronic Acids as Protective Groups for Carbohydrates

Boronic acids represent attractive protective groups for carbohydrates for several reasons. Their ability to preferentially form cyclic boronates at 1,2-cis-diols as well as 4,6-diols of carbohydrates allows for the regioselective protection of two hydroxyl groups, as the partial π character of the B–O bonds reduces the nucleophilicity of the boron-bound alkoxy groups, leaving the others available to be functionalized. Assembly and deprotection of carbohydrate- derived boronic esters are operationally simple and require only mild conditions. The use of boronic esters as protective groups in carbohydrate chemistry has been previously reviewed.159 In 2000, the group of Boons made use of boronic acids not only as protective groups for oligosaccharide synthesis but also as a point of attachment to a solid support.160 In an effort to address the difficulty of forming 1,2-cis-linked glycosides on solid supports due to the tendency of donors lacking a participating group at C-2 to form anomeric mixtures, they developed a sequence of manipulations where these anomeric mixtures could be easily cleaved from the support, purified, and then reloaded to continue the synthesis. Their initial studies focused on developing conditions to load saccharides onto the polymeric support and identifying appropriate glycosylation conditions. As difficult or poor yielding loading and cleavage steps would limit the usefulness of such an approach, simple conditions which allowed the efficient attachement and removal of the substrate from the support were desirable. Polystyrylboronic acid fit this description perfectly as loading of a saccharide onto the polymer (via formation of a boronate ester at the 4,6-diol) requires only heating in pyridine and removal from the support is achieved by treatment with a mixture of acetone and water. The authors were able to identify efficient glycosylation conditions using thioglycosides as donors under NIS/TMSOTf activation conditions. Donors having either a participating group or non-participating group at C-2 underwent efficient glycosylation, although a mixture of anomers was obtained if the group at C- 2 was non-participating (Figure 4.16).

159 (a) Ferrier, R. J. Adv. Carbohydr. Chem. Biochem. 1978, 35, 31–80. (b) Duggan, P. J.; Tyndall, E. M. J. Chem. Soc., Perkin Trans. 1, 2002, 1325–1339.

160 Belogi, G.; Zhu, T.; Boons, G.-J. Tetrahedron Lett. 2000, 41, 6965–6968.

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Figure 4.16. Glycosylation of polystyrylboronic acid-bound acceptor.

To challenge their loading-release-reloading strategy, they developed a synthesis of a trisaccaride in which one of the glycosylation reactions gave a mixture of anomers.161 Beginning with polymer-bound methyl-galactopyranoside (4.19) the free hydroxyl group at C-2 was glycosylated using a thioglycoside donor (4.24) in the presence of NIS/TMSOTf to give

(4.25αβ). The FMOC protecting group was removed using NEt3 in CH2Cl2 without removing the disaccharide from the support to give (4.26αβ). Cleavage of the dissacharide from the resin, followed by purification of the α-anomer and reloading, yielded a polymer-bound disaccharide (4.27α). Further glycosylation at the free hydroxyl group with thioglycoside donor (4.28), followed by cleavage from the resin with acetone/H2O gave the free trisaccharide (4.29α). This example highlights how the use of polystyrylboronic acid as a polymeric support for oligosaccharide synthesis can function as both a protective group and an easily loadable/cleavable support useful in situations where intermediate products might need to be removed and purified.

161 Belogi, G.; Zhu, T.; Boons, G.-J. Tetrahedron Lett. 2000, 41, 6969–6972.

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Figure 4.17. Boons' polymer-supported oligosaccharide synthesis by a loading-release-reloading strategy.

Crich has elegantly shown that the 4,6-O-polystyrylborinate esters of mannosyl donors can be employed for the solid-phase synthesis of -mannosides.162 As one of the more difficult glycosidic bonds to form, much effort has been put towards developing strategies for - mannoside formation163 and adapting them to solid-supported synthesis.164 The Crich lab has developed a -mannosylation reaction involving the low-temperature activation of a 4,6-O- benzylidene-protected mannopyranosyl sulfoxide with triflic anhydride.165 These reactions are proposed to proceed through the -mannosyl triflate followed by SN2 displacement of the triflate

162 Crich, D.; Smith, M. J. Am. Chem. Soc. 2002, 124, 8867–8869.

163 (a) Barresi, F.; Hindsgaul, O. Can. J. Chem. 1994, 72, 1447-1465. (b) Ito, Y.; Ogawa, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 1765–1767. (c) Lichtenthaler, F. W.; Schneider-Adams, T. J. Org. Chem. 1994, 59, 6728–6734. (d) Stork, G.; La Clair, J. J. J. Am. Chem. Soc. 1996, 118, 247–248. (e) Hodosi, G.; Kovać, P. J. Am. Chem. Soc. 1997, 119, 2335–2336. (f) Lemanski, G.; Ziegler, T. Tetrahedron 2000, 56, 563579. (g) Weingart, R.; Schmidt, R. R. Tetrahedron Lett. 2000, 41, 8753-–8758. 164 Ito, Y.; Ogawa, T. J. Am. Chem. Soc. 1997, 119, 5562–5566. 165 (a) Crich, D.; Sun, S. J. Org. Chem. 1997, 62, 1198–1199. (b) Crich, D.; Sun, S. Tetrahedron 1998, 54, 8321– 8348. (c) Crich, D.; Sun, S. J. Am. Chem. Soc. 1998, 120, 435–436.

135 with an alcohol acceptor to give the -mannoside. The 4,6-O-benzylidene group is crucial for obtaining -selectivity as it serves to torsionally disarm the intermediate mannosyl triflate and disfavor the formation of free oxacarbenium ions. The authors found that the 4,6-O- phenylboronate esters, formed from either a phenylboronic acid or polystyrylboronic acid, exhibited similar torsionally disarming properties as the benzylidene group. Thiomannoside (4.30) was loaded onto the solid support (4.18) through formation of its 4,6-O-phenylboronate ester to give (4.31) by heating in pyridine. Activation of the bound glycosyl donor by 1- benzenesulfinyl pyridine (BSP, 4.32) and Tf2O in the presence of 2,4,6-tri-tert-butylpyrimidine (TTBP) at –60 °C was followed by addition of a glycosyl acceptor (4.33). After the glycosylation reaction was completed, the immobilized -mannoside could be cleaved from the support by heating in a mixture of acetone and water to give the free -mannoside (4.34) (Figure 4.18).

Figure 4.18. Solid-phase synthesis of -mannosides.

Kaji has reported a regioselective glycosylation of fully unprotected methyl hexopyranosides using arylboronic acids as transient masking groups for 1,2-cis-diols or 4,6-diols of hexoses.166 The simple one-pot glycosylation reaction first involves the selective formation of a boronate ester between the monosaccharides and an arylboronic acid. For methyl-fucopyranoside and methyl-rhamnopyranoside the formation of boronate esters with 1,2-cis-diols is preferred while

166 Kaji, E.; Nishino, T.; Ishige, K.; Ohya, Y.; Shirai, Y. Tetrahedron Letters, 2010, 51, 1570–1573.

136 the formation at the 4,6-diol is preferred for methyl-galactopyranosides and methy- glucopyranosides. Subsequent glycosylation with a thioglycoside (NIS/TMSOTf activation) or glycosyl bromide (Ag(I) silica-alumina activation) donor led to the regioselective formation of disaccharides. In the case of methyl-fucopyranosides and rhamnopyranosides there was complete regioselectivity for the unprotected hydroxyl group. For galactopyranosides and glucopyranosides (for which there were two unprotected hydroxyl groups) the major regioisomer appeared to result from glycosylation of the less sterically hindered hydroxyl group. After 167 glycosylation had occured, the boronate ester was oxidatively cleaved with NaBO3•4H2O.

Figure 4.19. Regioselective glycosylation of fully unprotected methyl hexopyranosides by transient masking with an arylboronic acid.

Several groups have taken advantage of stability of boronate esters formed between boronic acids and diols for the regioselective protection and functionalization of carbohydrates. Highlights of this methodology include well defined sites of boronate ester formation, ease of assembly and cleavage of the protecting group, the torsionally disarming effect of 4,6-O- phenylboronate esters and the advantages associated with using polystyrylboronic acid as a solid support.

167 Kabalka, G. W.; Shoup, T. M.; Goudgaon, N. M. J. Org. Chem. 1989, 54, 5930

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4.6 Activation of Carbohydrate OH Groups

In contrast to reactions described in the previous section, boronic acids and borinic acids may also be used as activating groups for diol motifs found in carbohydrates. Although complexation between a boronic acid and a carbohydrate to give a tricoordinate cyclic boronate ester decreases the nucleophilicity of the oxygen atoms due to partial π bond formation between the oxygen and boron atoms, complexation of an internal or external Lewis base to the boron atom interrupts this oxygen atom electron delocalization into the empty p orbital on boron. Diols complexed to tetrahedral boron therefore have increased electron density on their oxygen atoms leading to enhanced nucleophilicity. The group of Aoyama has taken advantage of the ability of boronic acids to preferentially form cyclic boronates with vincinal cis-diols or 4,6-diol moieties in monosaccharides and combined it with the ability of tetrahedral boronate complexes to activate diols. This has led to the development of regioselective alkylation and glycosylation reactions. Initial investigations involved the formation of a cyclic boronate ester between phenylboronic acid and the cis-diol of methyl-,L-fucopyranoside followed by activation of the oxygen atoms within the cyclic boronate with an external Lewis base, triethylamine.168 This complexation of the Lewis base to the boron atom was proposed to proceed through a tetrahedral boron intermediate, activating the bound alkoxyl groups and allowing alkylation with n-butyliodide to occur at the less sterically hindered equatorial position of the cis-diol.

Figure 4.20. Regioselective alkylation of methyl fucopyranoside through complexation-induced activation.

This idea was further extended to glycosylation reactions, using the borinic acid derived promoter 4.41. The key tetracoordinate boronate intermediate was proposed to result from protodeboronation of 4.41 to the boronic acid, followed by condensation with the

168 Oshima, K.; Kitazono, E.-I.; Aoyama, Y. Tetrahedron Lett. 1997, 38, 5001.

138 monosaccharide substrate and coordination of the pendant hydroxyl group.169 Exposure of the activated diol to a glycosyl bromide and Ag(I) promoter allowed the regioselective glycosylation of the equatorial positions of a variety of vincinal-cis-diols as well as the simultaneous 3,6-O- double glycosylation of galacto- and mannopyranosides. The methodology allows the use of thioglycosides as acceptors enabling subsequent use of the di or trisaccharides formed as donors.

Figure 4.21. Selective glycosylation promoted by an internally coordinated organoboron compound.

Based on these studies of tetracoordinate boron complexes, our group has developed methodologies for the organoboron-catalyzed regioselective functionalization of carbohydrates. Whereas the previous examples employed external or pendant Lewis bases to generate an activated tetrahedral boronate complex, our methodology relies on the use of a borinic acid precatalyst which forms the activated tetrahedral complex with a diol without the need for complexation of an additional Lewis base. The methodology is useful for the regioselective monoacylation170, sulfonylation171, alkylation172 and glycosylation173 of carbohydrate derivatives. Throughout all four reaction types, a trend was observed in which functionalization occurs at the equitorial position of the vincinal diols.

169 (a) Oshima, K.; Aoyama, Y. J. Am. Chem. Soc. 1999, 121, 2315. (b) Oshima, K.; Yamauchi, T.; Shimomura, M,; Miyauchi, S.; Aoyama, Y. Bull. Chem. Soc. Jpn. 2002, 75, 1319.

170 Lee, D.; Taylor, M.S. J. Am. Chem. Soc. 2011, 133, 3724–3727. 171 Lee, D.; Williamson, C.L.; Chan, L.; Taylor, M.S. J. Am. Chem. Soc. 2012, 134, 8260–8267. 172 Chan, L.; Taylor, M.S. Org. Lett., 2011, 13, 3090–3093. 173 Gouliaras, C.; Lee, D.; Chan, L.; Taylor, M.S. J. Am. Chem. Soc. 2011, 133, 13926–13929.

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Figure 4.22. Monofunctionalizations of methyl-α,D-mannopyranoside using borinic ester precatalyst 4.43.

Investigations into the kinetics of the sulfonylation reaction support a mechanism consistent with that depicted in Figure 4.23. The aminoethyl diphenylborinate precatalyst enters into the catalytic cycle by bis-sulfonylation of the ethanolamine ligand, followed by binding to the cis- diol to form the activated tetrahedral boronate intermediate. The reaction of the electrophile with this tetrahedral intermediate was determined to be the rate determining step. Displacement of the boron bound product with another molecule of substrate regenerates the catalyst resting state. The observed trends in regioselectivity appear to be influenced by several effects. Preferential functionalization of the equatorial position over the axial position in a 1,2-cis-diol suggests a steric component to the selectivity although computational studies suggest an electronic effect as well. Computational studies of diol bound tetrahedral borinate intermediates display calculated Fukui indices to be greater on the oxygen atom that becomes functionalized. This glycosylation reaction represents the first small-molecule-catalyzed (nonenzymatic) regioselective glycosylation reaction. The methodology tolerates a wide variety of armed and disarmed

140 glycosyl donors as well as both donor bromides and chlorides. Kinetic studies of the reaction displayed first order kinetics in both acceptor and donor suggesting a mechanistic pathway 174 involving significant SN2 character and minimal formation of oxocarbenium intermediates.

The stereochemical outcome of the reaction also supports an SN2-like mechanism as the products show inversion of stereochemistry at the anomeric center.

Figure 4.23. Proposed catalytic cycle for diol activation with precatalyst 4.43.

Application of the borinic acid catalyzed glycosylation reaction towards the functionalization of complex polyol natural products represents an attractive avenue for late stage modification of molecules. This methodology was used for the regioselective monoglycosylation of digitoxin to gain access to novel purpurea-type cardiac glycosides.175 The regioselectivity of the reaction was

174 (a) Wulff, G.; Röhle, G.; Krüger, W. Angew. Chem. Int. Ed. 1970, 9, 455–456. (b) Huang, M.; Garrett, G. E.; Birlirakis, N.; Bohé, L.; Pratt, D. A.; Crich, D. Nature Chem. 2012, 4, 663–667. 175 Beale, T.M.; Taylor, M.S. Org. Lett., 2013, 15, 1358–1361.

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Figure 4.24. Selected substrate scope for borinic acid-catalyzed regioselective glycosylation of carbohydrates. consistent with previous studies where the equatorial position of the 1,2-cis-diol of digitoxin was glycosylated out of the possible five free hydroxyl groups. The newly formed glycosidic bonds were found to be of -configuration. A variety of peracetlyated glycosyl bromides were tolerated in the reaction. The generation of tetrahedrally coordinated boron compounds as activators for the functionalization of 1,2-cis-diols represents a use distinctly different than their use as carbohydrate sensors or protective groups. The development of these organoboron promoted

142 regioselective funtionalizations of carbohydrate derivatives represents a unique opportunity to avoid the traditional use of protecting groups in oligosaccharide synthesis. Advantages of the methodology include the wide array of electrophiles tolerated by the reaction as well as consistent and predictable functionalization patterns. Preliminary results suggest these methods will be useful even in the complex setting of naturally occurring polyols.

Figure 4.25. Synthesis of cardiac glycoside analogs by catalyst-controlled, regioselective glycosylation of digitoxin. 4.7 Conclusions and Outlook

The applications of organoboron compounds in carbohydrate chemistry and glycobiology span a wide number of areas. Ranging from their use in oligosaccharide synthesis to their incorporation

143 into sensors and drug delivery systems, organoboron compounds have widespread utility. Advantages of this class of compounds include their selectivity for binding diols and their ease of inclusion into complex molecules. The examples discussed in this review are sure to stimulate the development of new, highly selective sensors for biological molecules as knowledge concerning receptor design continues to grow. The improvement of existing methodology employing organoboron compounds as protective groups or catalysts will enable the functionalization of more complex and densely functionalized carbohydrate substrates.

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Chapter 5 Regioselective Boronic Acid Mediated Glycosylations 5.1 Introduction Carbohydrates play an important role in biological systems. Their wide array of functions, ranging from energy sources and structural materials to signaling molecules, is in part due to the large variance and diversity in their structures.176 Unlike other biopolymers such as proteins and nucleic acids, the constituent monomers of oligosaccharides may be organized in nonlinear patterns.177 As each monosaccharide contains multiple positions at which the chain can be elongated, a great deal of complexity can be established with relatively few units. Taking into account that each glycosidic linkage could exist in two anomeric configurations, many sugars can exist in either the furanose or pyranose forms and subunits can exhibit modifications (sulfation, phosphorylation or acylation)178, the possible number of permutations increases dramatically. Even greater variation is possible as oligosaccharides are often found as glycoconjugates with proteins (glycopeptides) and lipids (glycolipids). The situation is further complicated in a biological setting as naturally synthesized glycoproteins are often present only in small quantities and as heterogeneous mixtures of various glycoforms.179 This makes the isolation and purification of a particular glycoprotein or its oligosaccharide component from a biological source difficult. As these glycoconjugates and carbohydrates often have interesting biological properties and medical applications, such as their use in developing carbohydrate- based vaccines180 and the identification of cancerous tumor cells181, methods for obtaining pure samples are required. To address these needs, a variety of tools for the laboratory synthesis of

176 Gamblin, D. P.; Scanlan, E. M.; Davis, B. G. Chem. Rev. 2009, 109, 131–163. 177 Kobata, A. Acc. Chem. Res. 1993, 26, 319–324. 178 Davis, B. G. Chem .Rev. 2002, 102, 579–601. 179 (a) Rademacher, T. W.; Parekh, R. B.; Dwek, R. A. Annu. Rev. Biochem. 1988, 57, 785. (b) Dwek, R. A. Chem. Rev. 1996, 96, 683. 180 Seeberger, P. H.; Werz, D. B. Nature 2007, 446, 1046–1051. 181 (a) Hakomori, S.; Zhang, Y. Chem. Biol. 1997, 4, 97–104. (b) Feizi, T.; Nature, 1985, 314, 53–57. (c) Livingston, P. O. Curr. Opin. Immunol. 1992, 4, 624–629.

145 carbohydrates and their glycoconjugates has been developed, including both chemical and enzymatic methods.182 Recent advances in carbohydrate chemistry have allowed for the synthesis of very complex oligosaccharides such as the Globo H hexasaccharide (5.01)183 and 184 Fucose GM1 (5.02) .

Figure 5.01. Complex oligosaccharide targets Globo H hexasaccharide (5.01) and fucose GM1 (5.02).

Two of the main challenges associated with glycosylation reactions are the control of stereochemistry and regiochemistry. This chapter details investigations into regioselective glycosylation reactions mediated by boronic acids and their application towards the synthesis of a pentasaccharide derivative isolated from the plant Spergularia ramosa.

182 Smoot, J. T.; Demchenko, A. V. Adv. Carbohydr. Chem. Biochem.2009, 62, 161–250. 183 (a) Burkhart, F.; Zhang, Z.; Wacowich-Sgarbi, S.; Wong, C.-H. Angew. Chem. Int. Ed. 2001, 40, 1274–1277. (b) Jeon, I.; Iyer, K.; Danishefsky, S. J. J. Org. Chem. 2009, 74, 8452–8455. (c) Zhu, T.; Boons, G.-T. Angew. Chem. Int. Ed. 1999, 38, 3495–3497. 184 (a) Allen, J. R.; Danishefsky, S. J. J. Am. Chem. Soc. 1999, 121, 10875–10882. (b) Mong, T.; Lee, H.-K.; Duron, S. G.; Wong, C.-H. PNAS 2003, 100,797–801.

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5.2 Traditional Oligosaccharide Synthesis Typical glycoside synthesis involves the glycosylation of a suitably protected glycosyl acceptor, usually containing a single free hydroxyl group, with a fully protected glycosyl donor having a leaving group at the anomeric position. The reaction proceeds in the presence of a suitable promoter (Figure 5.02). A large number of glycosyl donors have been developed185 including bromides and chlorides (Koenigs-Knorr glycosylation),186 trichloroacetimidates,187 fluorides,188 aryl and alkyl thioglycosides189 and pentenyl glycosides.190

Figure 5.02. Typical chemical glycoside synthesis.

185 (a) Toshima, K.; Tatsuta, K. Chem. Rev. 1993, 93, 1503–1531. (b) Zhu, X.; Schmidt, R. R. Angew. Chem. Int. Ed. 2009, 48, 1900–1934. 186 (a) Koenigs, W.; Knorr, E.; Ber. 1901, 34, 957-981. (b) Igarashi, K. Adv. Carbohydr. Chem. Biochem. 1977, 32, 243–283. 187 Schmidt, R. R.; Michel, J.; Angew. Chem. Int. Ed. 1980, 19, 731–732. 188 Mukaiyama, T. Angew. Chem. Int. Ed. 2004, 43, 5590–5614. 189 Garegg, P. J. Adv. Carbohydr. Chem. Biochem. 1997, 52, 179–205. 190 (a) Fraser-Reid, B.; Madsen, R. In Preparative Carbohydrate Chemistry; Hanessian, S. Ed.; Dekker: New York, 1996: p. 341. (b) Mootoo, D. R.; Date, V.; Fraser-Reid, B. J. Am. Chem. Soc. 1988, 110, 2662.

147

During the course of a glycosylation reaction, two factors must be considered: the regioselectivity of the reaction (what position becomes glycosylated) and the stereoselectivity of the reaction (what stereoisomer is formed at the anomeric position). In most glycosylation reactions that lack a participating group at C-2, the promoter assisted departure of the leaving group is proposed to lead to the formation of an oxocarbenium ion intermediate in an SN1 reaction. The sp2 hybridized nature of the anomeric carbon would allow attack of a nucleophile from either face of the ring to give both 1,2-trans and 1,2-cis products. Although the 1,2-cis product (in this particular example shown) would be favored thermodynamically due to the anomeric effect,191 a mixture of both products usually occurs.

Figure 5.03. General scheme for glycosylation with non-participating groups at C-2. The problem of stereoselectivity can often be overcome through judicious control of the reaction conditions, leaving group and protecting groups used. For example, installation of ester protecting groups at the position adjacent to the leaving group often leads to the formation of 1,2-trans glycosides due to the intermediacy of an acyloxonium ion intermediate which can only be attacked by a nucleophile from the face opposite the ester group (Figure 5.04).192

Figure 5.04. General scheme for glycosylation involving neighboring group participation.

191 Tvaroska, I.; Bleha, T. Adv. Carbohydr. Chem. Biochem. 1989, 47, 45–123. 192 (a) Goodman, L. Adv. Carbohydr. Chem. Biochem. 1967, 22, 109–175. (b) Nukada, T.; Bérces, A.; Zgierski, M. Z.; Whitfield, D. M. J. Am. Chem. Soc. 1998, 120, 13291–13295. (c) Demchenko, A. V. Handbook of Chemical Glycosylation: Advances in Stereoselectivity and Therapeutic Relevance, 2008, Wiley-VCH, Weinheim.

148

While controlling the identity of the protecting group at C-2 is one strategy for manipulating the stereoselectivity of a glycosylation reaction, there are certainly other strategies used to influence stereocontrol. In particular, alternative strategies are required if a β-mannoside linkage is desired. This 1,2-cis glycosidic linkage is difficult to form as both a participating and non-participating protecting group at C-2 would favor a 1,2-trans glycosidic linkage if the reaction proceeded through an oxocarbenium ion. Methods for the formation of β-mannosides include the sulfoxide method developed by Crich162,165 (discussed previously in section 4.5) and tethering approaches.193 The sulfoxide method is proposed to proceed through an α-glycosyl triflate which is displaced in an SN2 reaction to give the β-mannoside. In the tethering approaches the aglycon is delivered intramolecularly from one face of the anomeric position. Using an insoluble silver promoter in a Koenigs-Knorr reaction with an α-mannosyl bromide can lead to the formation of the β-mannoside product via an SN2 reaction, however, this methodology is not general with respect to the glycosyl acceptor.194

Figure 5.05. Alternative strategies for controlling stereochemistry. (a) Crich's sulfoxide method, (b) a tethering approach, (c) use of insoluble Ag promoter.

193 (a) Barresi, F.; Hindsgaul, O. J. Am. Chem. Soc. 1991, 113, 9376–9377. (b) Ito, Y.; Ogawa, T. Angew. Chem. Int. Ed. 1994, 33, 1765–1767. (c) Stork, G.; Kim, G. J. Am. Chem. Soc. 1992, 114, 1087–1088. 194 van Boeckel, C. A. A.; Beetz, T.; van Aelst, S. F. Tetrahedron 1984, 40, 4097–4107.

149

While the strategy outlined in Figure 5.02 works quite well for the formation of disaccharides, additional thought and planning is necessary if the resultant disaccharide is to be extended into a trisaccharide or larger oligosaccharide. In the example above, a protecting group (PG2) which is orthogonal to the other protecting groups in the molecule could be removed to reveal a new glycosyl acceptor. In a different scenario, PG3 could be removed and the free anomeric hydroxyl group converted to a leaving group to form a new glycosyl donor. In these cases, installation of orthogonal protecting groups at defined positions is not always a trivial task. To address this issue of regioselectivity, multiple protecting group manipulations are often employed. This requires additional steps to install and remove the protective groups which is inefficient in terms of time and atom economy. Figure 5.06 and Figure 5.07 outline a typical convergent synthesis using traditional methods. The target is a pentasaccharide derivative of a triterpenoid saponin isolated from Spergularia ramosa. This synthesis was completed by Du and coworkers.195

Figure 5.06. Synthesis of the disaccharide fragment of a pentasaccharide derivative from Spergularia ramosa.

195 Gu, G.; Du, Y. J. Chem. Soc., Perkin Trans., 1 2002, 2075–2079.

150

Figure 5.07. Synthesis of the trisaccharide fragment of the pentasaccharide derivative and final glycosylation.

The goal of the convergent synthesis is to divide the pentasaccharide into a disaccharide fragment 5.11 and a trisaccharide fragment 5.20 followed by a final glycosylation to assemble the two pieces. Although each individual step in the synthesis is relatively efficient, nine steps

151 out of the fourteen step synthesis are protective group manipulations. These protection/deprotection steps are required in order to differentiate between the various hydroxyl groups and control the site of reactivity during glycosylation reactions. For example, the synthetic sequence beginning from arabinose derivative 5.03 (the synthesis of which requires two additional steps from readily available arabinose)196 necessitates several steps to install orthogonal protecting groups at C-2,C-3, and C-4 in 5.07. Later on in the synthesis, another series of protective group manipulations is required to distinguish the hydroxyl groups at C-1 and C-2 in 5.16. The following sections highlight some state of the art techniques employed to avoid the use of protective groups in oligosaccharide synthesis, minimizing the number of steps in a synthesis as well as reducing the amount of waste generated.

5.3 One-Pot Oligosaccharide Synthesis The group of Wong has developed a reactivity-based one-pot glycosylation strategy based on the relative reactivity values (RRVs) of a variety of protected glycosyl donors.197 The strategy was initially based on Fraser-Reid's armed/disarmed concept198 where the nature of the protecting groups present in an O-pentenyl glycosyl donor significantly influenced their relative reactivities towards glycosylation. The presence of electron withdrawing protecting groups (OAc, OBz) "disarmed" or decreased the reactivity of the donors relative to less strongly electron withdrawing groups such as benzyl (OBn) protecting groups which were considered to be "armed".

Figure 5.08. Armed/Disarmed effects in glycosyl donors.

196 Finch, P.; Iskander, G. M.; Siriwardena, A. H. Carbohydr. Res. 1991, 210, 319–325. 197 Zhang, Z.; Ollmann, I. R.; Ye, X.-S.; Wischnat, R.; Baasov, T.; Wong, C.-H. J. Am. Chem. Soc. 1999, 121, 734– 753. 198 (a) Mootoo, D. R.; Konradsson, P.; Udodong, U.; Fraser-Reid, B. J. Am. Chem. Soc. 1998, 110, 5583–5584. (b) Fraser-Reid, B.; Wu, Z.; Udodong, U. E.; Ottosson, H. J. Org. Chem. 1990, 55, 6068–6070.

152

For example, Figure 5.08 shows the reaction between an armed O-pentenyl glycoside donor (5.22) and a disarmed O-pentenyl glycoside donor (5.23) in the presence of a mild activator, iodonium (di-γ-collidine) perchlorate (IDCP). The armed donor reacts preferentially over the disarmed donor to give 5.24 as the major product. The disarmed donors could be activated under more harsh reaction conditions or in the presence of a stronger promoter system such as (N- iodosuccinimide (NIS)/triflic acid (TfOH). Over the years a large number of investigations studied how various protecting groups either armed or disarmed glycosyl donors through both electronic and torsional effects.199 Several groups developed scales of relative reactivity for series of glycosyl donors.200 Wong et al assigned a RRV to a large number of thioglycoside donors by performing competition experiments between them.197 The larger the RRV, the higher the reactivity of the thioglycoside. The RRV data was compiled into a computer program called OptiMer which could be used as a guide to help select the appropriate building blocks for the one-pot assembly of a specified oligosaccharide. An example of the assembly of a tetrasaccharide via the programmable one-pot method is shown is Figure 5.09. The one-pot procedure began by coupling 5.25 with 5.26 using NIS/TfOH as promoters. As the RRV of 5.25 is the larger of the two, 5.25 acted as the glycosyl donor and no self-condensation of 5.26 occurs. Subsequent glycosylations occurred by adding increasingly less reactive donors, thereby ensuring that the correct thioglycoside acted as the donor or acceptor. The final acceptor 5.28 has an RRV of 0 as it cannot act as a donor. Several other oligosaccharides have been synthesized (wholly or in part) using the one-pot reactivity based methodology, including the tumor- associated antigen N3 minor,201 heparin-like oligosaccharides,202 embryonic stem cell surface

2 203 204 184b carbohydrates Lc4 and IV Fuc-Lc4, a Lewis Y carbohydrate hapten, and fucose GM1.

199 Premathilake, H. D.; Demchenko, A. V. Top. Curr. Chem. 2011, 301, 189–221. 200 (a) Douglas, N. L.; Ley, S. V.; Lucking, U.; Warriner, S. L. J. Chem. Soc. Perkin Trans. 1 1998, 51–65. (b) Fraser-Reid, B.; Wu, Z.; Andrews, C. W.; Skowronski, E. J. Am. Chem. Soc. 1991, 113, 1434–1435. (c) Wilson, B. G.; Fraser-Reid, B. J. Org. Chem. 1995, 60, 317–320. (d) Fridman, M.; Solomon, D.; Yogev, S.; Baasov, T. Org. Lett, 2002, 4, 281–283. 201 Lee, J.-C.; Wu, C.-Y.; Apon, J. V.; Siuzdak, G.; Wong, C.-H. Angew. Chem. Int. Ed. 2006, 45, 2753–2757. 202 Polat, T.; Wong, C.-H. J. Am. Chem. Soc. 2004, 129, 12795–12800. 203 Hsu, Y.; Lu, X.-A.; Zulueta, M. M. L.; Tsai, C.-M.; Lin, K.-I.; Hung, S.-C.; Wong, C.-H. J. Am. Chem. Soc. 2012, 134, 4549–4552.

153

Figure 5.09. Reactivity-based one-pot synthesis of a tetrasaccharide.

This methodology simplifies oligosaccharide synthesis as it eliminates the need to temporarily protect the anomeric position, remove the protecting group, then introduce a new leaving group. One drawback of this approach is the limitations imposed by the RRVs. The order in which the glycosyl acceptors can be added is determined by the RRVs. For example, an acceptor with a high RRV cannot be coupled with a donor having a low RRVas self-condensation of the acceptor would occur. To avoid this drawback the group of Huang developed a strategy in which a p-tolyl thioglycoside glycosyl donor is preactivated at low temperature by a stoichiometric amount of p- toluenesulfenyl triflate (p-TolSOTf), which is generated in situ from p-toluenesulfenyl chloride (p-TolSCl) and silver triflate (AgOTf).205 A reactive intermediate was generated in less than 5 minutes and a p-tolyl thioglycoside was then added as an acceptor to form a disaccharide where only the activated thioglycoside was observed to act as a glycosyl donor. The authors did not directly comment on the nature of the activated intermediate but noted that Crich has observed

204 Mong, K.-K. T.; Wong, C.-H. Angew. Chem. Int. Ed. 2002, 41, 4087–4090. 205 Huang, X.; Huang, L.; Wang, H.; Ye, X.-S. Angew. Chem. Int. Ed. 2004, 43, 5221–5224.

154 glycosyl triflates as intermediates by low-temperature NMR experiments in the activation of sulfoxides with triflic anhydride in the presence of 2,6-di-tert-butyl-4-methylpyridine (DTBMP).206 This series of steps could be repeated in the same pot by preactivating the disaccharide at low temperature then adding another acceptor. An example of the preactivation strategy employed in the synthesis of the tumor-associated carbohydrate antigen Globo-H is shown in Figure 5.10.207

Figure 5.10. Huang's one-pot synthesis of the Globo-H antigen (5.33). While the efficiency of these one-pot syntheses cannot be denied, one of their drawbacks is the synthesis of the requisite building blocks themselves. Obtaining glycosyl donors or acceptors with the appropriate protecting groups can sometimes require multiple steps. In addition, as the reaction sequences are one-pot with a final separation required at the end, any glycosylation reactions that are not highly stereoselective will produce a mixture of diastereomers. If this occurs in more than one step the potential final separation of stereoisomers could be difficult.

206 (a) Crich, D.; Sun. S. Tetrahedron 1998, 54, 8321–8348. (b) Crich, D.; Cai, W. J. Org. Chem. 1999, 64, 4926– 4930. 207 Wang, Z.; Zhou, L.; El-Boubbou, K.; Ye, X.-S.; Huang, X. J. Org. Chem. 2007, 72, 6409–6420.

155

5.4 Enzymatic Methods

Enzymatic glycosylation reactions offer an attractive alternative to traditional glycosylation reactions as they alleviate the need for multi-step protecting group manipulations and they are considered "green" as the reactions are often performed in water. Drawbacks of this methodology are mostly associated with the specificity of enzymes for a particular substrate, which means that an enzyme may not be available for every desired transformation. Despite these drawbacks, glycosyltransferases (GTs, enzymes which catalyze the transfer of activated nucleotide sugar glycosyl donors to acceptor molecules) and glycosidases (enzymes which normally catalyze the hydrolysis of glycosidic linkages but which may, under certain conditions, allow glycosylation to occur with a nucleophile other than water) have been employed in the laboratory synthesis of oligosaccharides.208 In recent years genetic engineering techniques have expanded the availability of GTs.209 Using an engineered triple mutant of the oleandomycin (OleD) GT the group of Thorson was able to glucosylate a variety of acceptors.210 It was interesting to note that the mutant OleD was capable of glucosylating acceptors that the wild type enzyme was not.

Figure 5.11. Glycosylation using an engineered OleD mutant. Wang and coworkers developed an enzymatic synthesis of the blood-group B antigen 5.40 using cloned bacterial glycosyltransferases.211 WbnJ, a β-1,3-galactosyltransferase, was able to

208 (a) Wong, C.-H.; Halcomb, R. L.; Ichikawa, Y.; Kajimoto, T. Angew. Chem. Int. Ed. 1995, 34, 521–546. (b) Drueckhammer, D. G.; Hennen, W. J.; Pederson, R. L.; Barbas III, C. F.; Gautheron, C. M.; Krach, T.; Wong, C.-H. Synthesis 1991, 499–525. 209 Hancock, S. M.; Vaughan, M. D.; Withers, S. G. Curr. Opin. Chem. Biol. 2006, 10, 509–519. 210 Gantt, R. W.; Goff, R. D.; Williams, G. J.; Thorson, J. S. Angew. Chem. Int. Ed. 2008, 47, 8889–8892. 211 Yi, W.; Shao, J.; Zhu, L.; Li, M.; Singh, M.; Lu, Y.; Lin, S.; Li, H.; Ryu, K.; Shen, J.; Guo, H.; Yao, Q.; Bush, C. A.; Wang. P. G. J. Am. Chem. Soc. 2005, 127, 2040–2041.

156 perform a galactosylation of acceptor 5.37 with UDP-Gal to give 5.38 in 85% yield. Disaccharide 5.38 was then fucosylated using the α-1,2-fucosyltransferase WbnK yielding 5.39 in 75% yield. The final step in the sequence involved a galactosylation mediated by α-1,3- galactosyltransferase WbnI to provide the tetrasaccharide 5.40 in 93% yield.

Figure 5.12. Enzymatic synthesis of blood group B antigen. Glycosidases212 and mutant glycosidases (termed glycosynthases) have also been used for oligosaccharide synthesis.213 The normal function of the glycosidase enzymes is to hydrolyze glycosidic linkages. However, if the donor is intercepted by an acceptor other than water, glycosylation can occur. The mechanism for the transglycosylation reaction catalyzed by Agrobacterium sp. β-glucosidase is shown in Figure 5.13.214 The unmutated enzyme requires a substrate with a β-configured leaving group. As the product of the reaction is also a substrate for the enzyme in the hydrolysis reaction, the yield of glycosylation is generally low. However,

212 Scigelova, M.; Singh, S.; Crout, D. H. G. J. Mol. Catal. B Enzy. 1999, 6, 483–494. 213 Cobucci-Ponzano, B.; Moracci, M. Nat. Prod. Rep. 2012, 29, 697–709. 214 (a) McCarter, J. D.; Withers, S. G. Curr. Opin. Struct. Biol. 1994, 4, 885–892. (b) Sinnot, M. L. Chem. Rev. 1990, 90, 1171.

157 mutation of the nucleophilic glutamic acid 358 residue to an amino acid with a non-nucleophilic side chain (alanine) renders the enzyme catalytically inactive towards substrates with β- configured leaving groups as it is not possible to form the required α-glycosyl enzyme intermediate (Figure 5.13B).

Figure 5.13. Mechanism of transglycosylation catalyzed by Agrobacterium sp. β-glucosidase.

The mutated enzyme can, however, act on an α-configured donor (generally a glycosyl fluoride). The product of the reaction is no longer a substrate for the enzyme as the absence of a nucleophilic residue at position 358 precludes the reformation of the α-glycosyl enzyme intermediate. Although glycosidases and glycosynthases are more widely available than glycotransferases, they are sometimes limited by lower specificities and yields.

158

5.5 Tin- and Boron-Mediated Approaches

Traditional oligosaccharide synthesis relies on protective group manipulations to mask the reactivity of certain hydroxyl groups. In opposition to this deactivation strategy, glycosylation of a particular hydroxyl group could be achieved in the presence of activating agents which would enhance its reactivity relative to other hydroxyl groups present. Regioselective glycosylations of unprotected carbohydrates have been achieved using both tin- and boron-based approaches. The use of stannylated sugars as acceptors capable of undergoing regioselective glycosylations was pioneered by Ogawa215 and investigated by several other groups since.216 A general scheme for the stannylene activation method is shown in Figure 5.14.

Figure 5.14. Stannylene activation method.

The oxygen atoms bound to tin have enhanced nucleophilicity and are activated towards glycosylation. In general, the major products of the reactions proceeding via stannylene acetals or stannylene ethers contained (1→6)-glycosidic linkages, although some cases of (1→3)- glycosidic linkages were reported. The glycosylations are usually carried out using Koenigs- Knorr conditions (glycosyl halide donor with a silver salt promoter) but the use of thioglycoside donors is also possible.216d The regioselectivity of the glycosylations depends both on the identity and relative reactivities of stannylated sugars in solution.216a,216c

Figure 5.15. Regioselectivity of tin-mediated glycosylation.

215 Ogawa, T.; Katano, K.; Matsui, M. Carbohydr. Res. 1978, 64, C3–C9. 216 (a) Kaji, E.; Harita, N. Tetrahedron Letters, 2000, 41, 53–56. (b) Kartha, R. K. P.; Kiso, M.; Hasegawa, A.; Jennings, H. J. J. Chem. Soc. Perkin Trans. 1 1995, 3023–3026. (c) Kaji, E.; Shibayama, K.; In, K. Tetrahedron Letters, 2003, 44, 4881–4885. (d) Garegg, P. J.; Maloisel, J.-L.; Oscarson, S. Synthesis 1995, 409–414. (e) Cruzado, C.; Bernabe, M.; Martin-Lomas, M. Carbohydr. Res. 1990, 296–301.

159

Figure 5.16. Typical glycosylation reactions for the regioselective tin-mediated approach.

Although these tin-based methodologies represent a unique glycosylation strategy and may offer an advantage over traditional oligosaccharide synthesis, they suffer from several shortcomings; (1) a stoichiometric amount of toxic organotin reagent is required; (2) the scope is limited and the yields are often poor; and (3) formation of the activated stannylene acetal or ether requires an additional step. This is followed by a glycosylation step which sometimes requires low temperatures and can take several days for completion. Activation of carbohydrates using organoboron compounds was discussed in detail in Section 4.6 of this work. These methods offer an improvement over those described previously involving organotin reagents as the organoboron compounds are significantly more environmentally benign and they can sometimes be used in catalytic quantities to promote glycosylation reactions.

5.6 Results and Discussion

Our group's development of regioselective glycosylation reactions using borinic acid derived catalysts (described previously in Section 4.6, Figures 4.23, 4.24, 4.25 and 4.26)173,175,217 represents an advantage over methods described in the previous section. Highlights of this

217 Dimitrijević, E.; Taylor, M. S. Chem. Sci. 2013, 4, 3298–3303.

160 methodology include a consistent regioselectivity over a wide range of substrates for the equatorial position of the 1,2-cis-diol of the acceptor, the use of a relatively benign reagent (the borinic acid derivatives are much more environmentally friendly than organotin reagents), the use of a substoichiometric amount of borinic acid catalyst and a convenient reaction setup which does not require strict exclusion of moisture, long reaction times or high temperatures. However, in some rare situations the catalytic procedure gave poor yields. In particular, we had envisioned using the diarylborinic acid-derived catalysts in a regioselective glycosylation reaction for two key steps in the synthesis of a pentasaccharide derivative isolated from Spergularia ramosa.218 A related target (5.21) had previously been prepared by Du and coworkers195 (described in Section 5.02, Figure 5.05 and Figure 5.06) using traditional oligosaccharide synthesis in 14 steps, 9 of which were protective group manipulations.

Figure 5.17. Pentasaccharide derived target of the Du synthesis (5.21) and the proposed deprotected pentasaccharide derivative in our synthesis (5.41).

We hoped to emphasize how regioselective glycosylation could be used to reduce the number of steps in a synthetic sequence and diminish the use of protective group manipulations. Our proposed retrosynthesis is outlined in Figure 5.18. We envisioned obtaining the pentasaccharide derivative 5.41 through a one step global deprotection of the protected pentasaccharide 5.42. As the regioselective glycosylation reaction negates the typical need to use various orthogonal protecting groups to manipulate hydroxyl groups, a single deprotection step is feasible if only one type of protecting group is used. In this synthesis we anticipated using only ester protective groups. Protected pentasaccharide derivative 5.42 could be disassembled into two important fragments; an acceptor disaccharide 5.43, and a donor trisaccharide 5.44. The preparation of 5.42

218 De Tommasi, N.; Piacente, S.; Gacs-Baitz, E.; De Simone, F.; Pizza, C.; Aquino, R. J. Nat. Prod. 1998, 61, 323–327.

161 requires a glycosylation reaction that is selective for the C-2 hydroxyl group of 5.43. The acceptor disaccharide (5.43) is the expected product of the glycosylation of the

Figure 5.18. Proposed retrosynthesis of the pentasaccharide derivative 5.41. arabinopyranoside 5.45 with the fucosyl bromide 5.46. The glycosylation was anticipated to occur at the equatorial OH group of the 1,2-cis-diol (highlighted in blue) of 5.45. Although this catalyst-controlled glycosylation (and the methodology in general) would install the donor moiety at the correct position, it would also leave two unprotected hydroxyl groups. We were interested in investigating whether it would be possible to differentiate between these two positions by either selectively installing a protecting group or performing a glycosylation under substrate control. The donor trisaccharide 5.44 was disconnected into a monosaccharide xylosyl

162 trichloroacetimidate donor 5.48 and disaccharide 5.47, of which only a single hydroxyl group was free. This appropriately protected acceptor could be obtained by a C-2-selective protection of 5.49, the expected product of the borinic acid-catalyzed regioselective glycosylation of rhamnose acceptor 5.51 with glucosyl bromide 5.50. Again, the glycosylation should occur at the equatorial position of the 1,2-cis-diol (highlighted in blue) of 5.51. In the forward direction, the proposed synthesis would require only six steps where only two are protective group manipulations. Compared to the fourteen step synthesis of Du (which didn't include a final deprotection) our proposed retrosynthesis would offer a significant reduction in the total number of steps. As the regioselective glycosylations are critical to the success of our plan we began by attempting to synthesize disaccharides 5.43 and 5.49. A glycosylation of para-methoxyphenyl (PMP) protected acceptor 5.52 with fucosyl bromide 5.53a was attempted using the optimum conditions developed previously for regioselective glycosylations using 2-aminoethyl diphenylborinate as catalyst. The PMP group was chosen as a protecting group for the reducing terminus because it is stable to deacylation conditions and will not react as a glycosyl donor under any activating conditions used in the remainder of the synthesis (Koenigs–Knorr conditions or TMSOTf/NIS). However, the PMP group could be removed selectively using ceric ammonium nitrate (CAN) if desired.219 Although 5.54 was obtained with the desired regioselectivity (corresponding to glycosylation at the equatorial position of the 1,2-cis-diol) and as the only disaccharide product, it was isolated in only 20% yield. Starting material 5.52 was recovered from the reaction but bromide 5.53a had decomposed under the reaction conditions. This particular glycosylation represents a difficult donor–acceptor combination where regiocontrol is a challenge. This is not completely surprising as, generally, hexoses lacking an oxygen substituent at C-6 and pentoses are poor donors, and rhamnose and arabinose are poor acceptors. The donors listed are difficult substrates for the reaction as they are more reactive relative to those bearing an additional electron withdrawing oxygen substituent. They are more easily hydrolyzed by trace amounts of water and in some cases may undergo nonselective glycosylation reactions leading to a mixture of regioisomeric products. In addition, certain acceptors are poor substrates for the reaction. In particular, rhamnose, arabinose and fucose derived acceptors typically give poor yields. Therefore it was somewhat expected that the

219 Mori, M.; Ito, Y.; Ogawa, T. Carbohydr. Res. 1989, 192, 131–146.

163 combination of donor 5.53a and acceptor 5.52 would be a difficult glycosylation. In an attempt to increase the yield, the chloride 5.53b was used as a donor instead of the bromide. Although this modification had previously been found to increase the yield of glycosylation reactions involving peracetylated fucosyl chlorides, peracetylated arabinosyl chlorides and perbenzylated glucosyl chlorides, no reactivity was observed in this case. Initial investigations into the synthesis of a disaccharide similar to 5.49 using the previously developed method were also disappointing. The reactions shown in Figure 5.19 were conducted by a Master's student, Stefi Anthonipillai.

Figure 5.19. Synthesis of acceptor disaccharide 5.54 using the borinic acid-catalyzed regioselective glycosylation conditions.

Stefi was responsible for developing and optimizing several steps in the synthesis and her results are indicated in the discussion that follows.220

220 Anthonipillai, Stefi. 2012. Progress Towards the Synthesis of a Pentasaccharide Derivative Found in Spergularia ramosa. (Master's thesis).

164

Figure 5.20. Synthesis of disaccharide 5.57 using the borinic acid catalyzed regioselective glycosylation conditions. Reactions carried out by Stefi Anthonipillai.

Although moderate yields were obtained under certain conditions (65% yield, X = Br, T = 80 °C), there was certainly room for improvement. This could be considered a moderately difficult glycosylation as rhamnose acceptors are typically difficult substrates for the reaction. It should be noted that using the glycosyl chloride donor failed to increase the yield in this reaction. With the two key steps in our synthesis giving disheartening results using the catalytic regioselective glycosylation methodology developed in our lab, a new method for the synthesis of these two disaccharides was undertaken. The initial inspiration behind the development of our catalytic glycosylation reaction using borinic acid derivatives came from Aoyama's reports which detailed glycosylation or alkylation reactions involving the stoichiometric formation of a boronic ester followed by activation via a tetracoordinate adduct (discussed in Section 4.06, Figure 4.20 and Figure 4.21). In the case of the glycosylation reactions the unique borinic acid derivative 4.41 was employed. We hypothesized that simple boronic acids could be complexed with 1,2-cis-diols in carbohydrates and then activated towards glycosylation (as opposed to alkylation) through complexation of an external Lewis base.

165

Figure 5.21. Proposal for the boronic acid-mediated regioselective glycosylation reaction.

This methodology would require a stoichiometric amount of boronic acid to preform the requisite boronate ester (a disadvantage when compared to the catalytic method) but if higher yields of the valuable products could be obtained, the boronic acid mediated regioselective glycosylation reaction might represent a viable alternative. This methodology also offers an advantage over the catalytic method in terms of reaction optimization. As the identity of both the boronic acid and Lewis base was found to influence the reaction yields (discussed below), these parameters can be easily varied to identify the most favorable reaction conditions. A diverse set of both boronic acids and Lewis bases are commercially available where only a single borinic acid precatalyst can be purchased.

5.7 Development of the Boronic Acid Mediated Regioselective Glycosylation Reaction

To investigate the potential of this approach (and to develop a method of generating key disaccharide intermediate 5.54 in higher yields) optimization of the reaction shown in Figure 5.22 was undertaken. The results of an initial boronic acid and solvent screen are shown in Table 5.01. Acetonitrile was chosen as the inital solvent as it had proven most successful in the borinic acid catalyzed reactions and triethylamine was chosen as the inital Lewis base as it had been used in Aoyama's studies. A survey of a variety of aryl (electron donating and electron withdrawing) and alkyl boronic acids gave yields of 5.54 37–49% (Table 5.01, entries 1–5).

166

Figure 5.22. Initial boronic acid and solvent screen for the boronic acid-mediated regioselective glycosylation reaction.

This was encouraging as these yields were almost double those obtained using the catalytic protocol. Surprisingly, varying the quantity of triethylamine used in the reaction had little effect on the yield when 2-methoxyphenylboronic acid was used (Table 5.01, entries 5-9). The dependence on triethylamine stoichiometry seemed to be boronic acid dependent, as this parameter affected the yields for certain boronic acids (3,5-bistrifluoromethylphenylboronic acid, compare Table 5.01, entries 1 and 10) but not others (cyclohexyl boronic acid, compare Table 5.01, entries 3, 11, and 12). It was thought that perhaps acetonitrile was a sufficiently strong Lewis base to complex to the boronate ester and promote the reaction itself. In dichloromethane, the yield of the background reaction in the absence of triethylamine was found to be much lower (Table 5.01, entry 13).

167

Table 5.01. Initial boronic acid and solvent screen for the boronic acid mediated regioselective glycosylation reaction.

Entry Boronic Acid Solvent Equivalents NEt3 Isolated Yield (%)

1 3,5-bis(trifluoromethyl)phenyl MeCN 1 45

2 2-methoxyphenyl MeCN 1 46 3 cyclohexyl MeCN 1 49 4 8-quinolinyl MeCN 1 37 5 2-methoxyphenyl MeCN 1 46 6 2-methoxyphenyl MeCN 0.5 53 7 2-methoxyphenyl MeCN 0.25 51 8 2-methoxyphenyl MeCN 0.1 45 9 2-methoxyphenyl MeCN 0 41

10 3,5-bis(trifluoromethyl)phenyl MeCN 0.1 10

11 cyclohexyl MeCN 0.1 40 12 cyclohexyl MeCN 0 47 13 cyclohexyl DCM 0 4

To further explore the relationship between triethylamine equivalents and reaction yield, a more extensive solvent screen was undertaken. To reduce the time required to set up reactions, the boronate ester 5.58 was pre-formed and used as the starting material for the screens. The results of the solvent screen are summarized in Table 5.02. Dichloromethane was identified as a solvent in which there was no background reaction in the absence of Lewis base (Table 5.02, entry 6), but a moderate yield was obtained in the presence of 1 equivalent of triethylamine (Table 5.02, entry 3). Increasing the equivalents of triethylamine to five caused a dramatic increase in yield up to 76%, however, further increases had negligible effect on the reaction yield (Table 5.02, entry 8). Using dichloroethane as the solvent gave similar results to using dichloromethane (Table 5.02, compare entries 3 and 9), but a slight decrease in yield was observed if the reaction was heated in DCE to 80 °C.

168

Table 5.02. Solvent screen for the boronic acid-mediated regioselective glycosylation reaction.

Entry Solvent Temperature Equivalents NEt3 Isolated Yield (%) 1 THF 23 oC 1 <10 2 Ether 23 oC 1 <10 3 DCM 23 oC 1 36 4 pentane 23 oC 1 0 5 Toluene 23 oC 1 <10 6 DCM 23 oC 0 0 7 DCM 23 oC 5 66 8 DCM 23 oC 10 60 9 DCE 23 oC 1 43 10 DCE 80 oC 1 32

After establishing dichloromethane as a suitable solvent for the reaction, a study on the effect of the identity of the Lewis base on reaction yield was undertaken. As isolation of the disaccharide by column chromatography was time consuming and determination of yields by H1 NMR spectroscopy was not possible due to overlap of starting material, product, and side product peaks, an alternative method of analysis was explored. The PMP group is a chromophore so the reaction could be analyzed by HPLC using vanillin as an internal standard. The general reaction scheme is shown in Figure 5.23 and the results are summarized in Table 5.03.

Figure 5.23. Lewis base screen for the boronic acid-mediated regioselective glycosylation reaction.

169

A survey of a large number of Lewis bases including amines, phosphine oxides, sulfoxides and fluoride did not reveal any compounds which gave a result superior to that when triethylamine was used (compare Table 5.03, entry 1 to entries 2-23). Neither varying the the equivalents of donor (compare Table 5.03, entry 1 and entry 25) nor using Ag2CO3 as a promoter in place of

Ag2O (compare Tabe 5.03, entry 1 and 29) resulted in an increase in yield. However, adding 4 Ǻ M.S. to the reaction appeared to drastically improve the yield (Table 5.03, entry 28).

170

Table 5.03. Lewis base screen for the boronic acid-mediated regioselective glycosylation reaction. Entry Boronic Acid Solvent Temperature Lewis Base HPLC Yield (%) o 1 4-Methoxyphenyl DCM 23 C NEt3 79 o 2 4-Methoxyphenyl DCM 23 C Bu3P=O broad peak 3 4-Methoxyphenyl DCM 23 oC DABCO 0 4 4-Methoxyphenyl DCM 23 oC TBAF (1M in THF) 0 o 5 4-Methoxyphenyl DCM 23 C Bu2S=O 0 o 6 4-Methoxyphenyl Toluene 23 C NEt3 <5 o 7 4-Methoxyphenyl Toluene 23 C Bu3P=O <5 8 4-Methoxyphenyl Toluene 23 oC DABCO 0 9 4-Methoxyphenyl Toluene 23 oC TBAF (1M in THF) 0 o 10 4-Methoxyphenyl Toluene 23 C Bu2S=O 0 o 11 3,5-difluorophenyl DCM 23 C NEt3 39 o 12 3,5-difluorophenyl DCM 23 C Bu3P=O <5 13 3,5-difluorophenyl DCM 23 oC DMAP 0 o 14 3,5-difluorophenyl DCM 23 C Ph3P=O 0 o 15 3,5-difluorophenyl DCM 23 C NPh3 0 o t 16 4-Methoxyphenyl DCM 23 C BuNH2 17 17 4-Methoxyphenyl DCM 23 oC benzylamine 8 18 4-Methoxyphenyl DCM 23 oC diisopropylamine 53 19 4-Methoxyphenyl DCM 23 oC 2-aminopyridine 24 20 4-Methoxyphenyl DCM 23 oC N-benzylmethylamine 0 21 4-Methoxyphenyl DCM 23 oC Aniline 0 22 4-Methoxyphenyl DCM 23 oC diphenylamine 10 23 4-Methoxyphenyl DCM 23 oC Hunig's base 21 24 4-Methoxyphenyl MeCN 70 oC None 27 o a 25 4-Methoxyphenyl DCM 23 C NEt3 74 o a 26 4-Methoxyphenyl MeCN 23 C NEt3 27 o a 27 4-Methoxyphenyl MeCN/PhMe 23 C NEt3 44 o b 28 4-Methoxyphenyl DCM 23 C NEt3 98 o c 29 4-Methoxyphenyl DCM 23 C NEt3 69

a b c 3 equivalents of the donor were used. 4 Å M.S. were added to the reaction. Ag2CO3 was used as the

silver (I) source instead of Ag2O.

171

However, when the reaction was repeated on a larger scale and the product isolated, 70% yield was obtained (Table 5.04, entry 1). It seems as though determination of yields by HPLC may overestimate the success of the reaction. This may be due to the broad peak shape exhibited by the product. It was, however, useful in identifying improved reaction conditions as a 70% yield was the highest observed up until this point.

Table 5.04. Isolated yields of the boronic acid-mediated regioselective glycosylation reaction using 4 Ǻ M.S.

Entry Temperature X Isolated Yield (%) 1 23 oC Br 70 2 23 oC Cl 0 3 23 oC Br 65a 4 80 oC Br 42b

a 1.5 equivalents of donor were used. b DCE was used as solvent.

Using the newly optimized conditions employing 4 Å M.S., several modifications were rescreened to see if improved yields were observed. Exchanging the glycosyl bromide for the chloride (Table 5.04, entry 2) gave no product and increasing the equivalents of donor to 1.5 had no beneficial effect on yield. Changing the solvent to DCE and increasing the temperature decreased the yield to 42%. Performing a final boronic acid screen using the optimized conditions and the two step procedure revealed pentafluorophenylboronic acid as the optimum boronic acid (79% yield, Table 5.05, entry 5). Having to pre-form the boronate ester is time consuming and adds steps to the synthesis. We therefore wanted to develop a one-pot procedure. The boronate ester formation was found to be efficient in DCM in the presence of 4 Å M.S. at room temperature (although longer reaction times were required).

172

Table 5.05. Boronic acid screen using optimized conditions for the regioselective glycosylation reaction.

Isolated Entry Boronic Acid Yield (%) 1 4-Methoxyphenyl 66 2 8-quinolinyl 18

3 3,5-bis(trifluoromethyl)phenyl 21

4 2,6-difluorophenyl 74 5 pentafluorophenyl 79

As the boronate ester could be formed in DCM (the solvent required for the second step of the reaction), a one-pot procedure was developed. This version of the reaction no longer required the removal of the solvent prior to the glycosylation step. After the formation of the boronate ester was complete, the donor, promoter and Lewis base were simply added to the reaction. Using the one-pot procedure was actually found to increase the yield slightly to 87% in the presence of three equivalents of triethylamine (Table 5.06, entry 3). Decreasing the equivalents of triethylamine was found to decrease the yield (Table 5.06, entries 1 and 2). As a final investigation into identifying optimum reaction conditions the Lewis bases tributylphosphine oxide (Table 5.06, entry 4) and pyridine N-oxide (Table 5.06, entry 5) were tested as well as phenylboronic acid (Table 5.06, entry 6) and 2-carboxyphenylboronic acid (Table 5.06, entry 7). None of these modifications offered any improvement in isolated yield.

173

Table 5.06. Identification of optimum reaction conditions for the boronic acid-mediated regioselective glycosylation reaction.

Entry Boronic Acid Lewis Base x Isolated Yield (%)

1 pentafluorophenyl NEt3 1 71

2 pentafluorophenyl NEt3 2 79

3 pentafluorophenyl NEt3 3 87

4 pentafluorophenyl Bu3P=O 3 27 pyridine N- 5 pentafluorophenyl 3 0 oxide

6 phenyl NEt3 10 47

7 2-carboxyphenyl NEt3 3 20

After the identification of these optimum reaction conditions, an attempt to further simplify the one-pot procedure was made. If all of the reagents could be added simultaneously, with boronate ester formation happening in situ followed by activation, the procedure would be more straightforward.

Figure 5.24. Boronic acid-mediated regioselective glycosylation reaction without pre-forming of the boronate ester.

174

Carrying out the reaction without pre-forming the boronate ester lead to an isolated yield of 77%. No other regioisomers were observed suggesting that the boronate ester was being successfully formed in situ prior to activation by the Lewis base. As the isolated yield was 10% lower than that obtained using the one-pot procedure (Table 5.06, entry 3), the latter method was used in the total synthesis. Although reactions using a catalytic amount of boronic acid were found to be inferior to reactions run using borinic acid catalysts in the initial investigation,173 attempts at using a catalytic amount of boronic acid and Lewis base were not made. The results summarized in Table 5.07 detail our endeavors into developing a catalytic variant of the boronic acid- mediated regioselective glycosylation reaction.

Table 5.07. Development of a catalytic variant of the boronic acid-mediated regioselective glycosylation reaction.

Entry Boronic Acid x Solvent Molecular Sieves Isolated Yield (%) 1 8-quinolinyl 10 MeCN No 23 2 2-methoxyphenyl 20 MeCN No 9 3 4-methoxyphenyl 20 MeCN No 41 4 4-methoxyphenyl 20 MeCN Yes 17 5 4-methoxyphenyl 20 DCM No 36 6 4-methoxyphenyl 20 DCM Yes 29 7 8-quinolinyl 20 DCM No 19 8 8-quinolinyl 20 DCM Yes 5 9 pentafluorophenyl 20 DCM No 55 10 2,6-difluorophenyl 20 DCM No 35 11 pentafluorophenyl 20 MeCN No 51 12 2,6-difluorophenyl 20 MeCN No 32

The catalytic ability of different boronic acids appears to vary greatly (compare Table 5.07, entries 1,2 and 3). Whereas the addition of molecular seives were crucial to obtaining high yields in the stoichiometric variant of the boronic acid-mediated regioselective glycosylation reaction,

175 they were found to have a detrimental effect in the catalytic version (compare Table 5.07, entries 3 and 4, 5 and 6, and 7 and 8). This may be because excluding water from the reaction prevents or inhibits catalyst turnover. These observations are consistent with other boronic acid catalyzed reactions.221 A report discussing the regioselective silylation of carbohydrate derivatives using a boronic acid/Lewis base co-catalyst system found that molecular sieves hindered catalysis.222 In the absence of molecular sieves the highest yield was obtained using (20 mol%) pentafluorophenylboronic acid and triethylamine in dichloromethane (Table 5.07, entry 9, 55%). This yield was significantly lower than those obtained using the stoichiometric method. Although the one-pot procedure could not be simplified further, by either making it catalytic in boronic acid and Lewis base or by forming the boronate ester in situ, the initial problem of developing a method of synthesizing 5.54 in higher yield had been acheived. Compared to the 20% yield from the catalytic borinic acid method, the stoichiometric boronic acid-mediated regioselective glycosylation method afforded the product in 87% yield. While developing the conditions to synthesize 5.54, we were also optimizing reaction conditions to obtain 5.57 in higher yield. The methyl rhamnopyranoside acceptor 5.59 was used in place of allyl rhamnopyranoside acceptor 5.55 as it was more readily available.

221 (a) Zheng, H.; Lejkowski, M.; Hall, D. G. 2011, 2, 1305–1310. (b) Al-Zoubi, R. M.; Marion, O.; Hall, D. G. 2008, 47, 2876–2879. 222 Lee, D.; Taylor, M. S. 2013, 11, 5409–5412.

176

Table 5.08. Boronic acid screen for the regioselective glycosylation reaction to synthesize 5.60.

Isolated Entry Boronic Acid Yield (%) 1 4-methoxyphenyl 68

2 3,5-bis(trifluoromethyl)phenyl 68

3 8-quinolinyl 78 4 2-methoxyphenyl 89 5 phenyl 39a 6 ferrocenyl 27a 7 2-biphenyl 69a

a The reaction was carried out by Lina Chan, another graduate student in the group.

As the catalytic reaction already gave a moderate yield of 5.57 (Figure 5.20), the stoichiometric boronic acid-mediated reaction gave excellent yields using 2-methoxyphenylboronic acid with only one equivalent of triethylamine in acetonitrile. In the total synthesis the (Glu-[β-1,3]-Rha) disaccharide eventually needs to function as a glycosyl donor. Having a methyl protected hydroxy group at C-1 (as in 5.60) would not be useful as this protecting group is difficult to remove and is not a group which is typically activated in glycosylation reactions. Initially, we had planned on using an allyl group as the protecting group at C-1. Allyl glycosides are often used as intermediates in multistep oligosaccharide synthesis223 due to their ability to act as orthogonal protecting groups which are stable to many common reaction conditions. The allyl group could then be removed by either isomerizing it to the propenyl glycoside with potassium tert-butoxide followed by hydrolysis in the presence of mercury (II) chloride or mercury oxide224

223 Khamsi, J.; Ashmus, R. A.; Schocker, N.S.; Michael, K. Carbohydr. Res. 2012, 357, 147–150. 224 Gigg, R.; Payne, S.; Conat, R. Carbohydr. Res. 1983, 2, 207–233.

177 or via allyl transfer with palladium (II) chloride in the presence of sodium acetate.225 The free hydroxyl group could then be converted to a reactive donor such as a trichloroacetimidate. However, we speculated that using an n-pentenyl group as the C-1 protecting group might be more efficient. The n-pentenyl group is both a stable protecting group and a leaving group that can be activated directly in glycosylation reactions.226 With this in mind, acceptor 5.59 and donor 5.56a were used to synthesis 5.60 using similar reaction conditions to those developed above. These reactions were carried out by Stefi Anthonipillai and are summarized in Figure 5.25. 4-Methoxyphenylboronic acid was identified as the optimum boronic acid for this substrate combination. The fact that the optimal boronic acid is different for each donor–acceptor pair (even for systems as similar as those used to synthesize 5.60 and 5.61) prevents the design of a general set of conditions for the reaction but it does offer a parameter which is easily varied and has a pronounced effect on reaction yield. This is in contrast to the catalytic borinic acid- catalyzed reaction as the number of commercially available borinic acid derivatives is very limited.

Figure 5.25. Boronic acid screen for the synthesis of (Glu-[β-1,3]-Rha) disaccharide 5.60 performed by Stefi Anthonipillai.

Although an acceptable yield of 5.61 was obtained using the two-step procedure described in Figure 5.25, the one-pot procedure described in Table 5.06 would be more efficient from an

225 Ogawa, T.; Nakabayashi, S. Carbohydr. Res. 1981, 93, C1–C5. 226 (a) Fraser-Reid, B.J.; Konradsson, P.; Mootoo, D.R.; Udodong, U. Chem. Commun. 1988, 823–825. (b) Mootoo, D.R.; Date, V.; Fraser-Reid, B.J. J. Am. Chem. Soc. 1988, 110, 2662-2663. (c) Ferrier, R.J.; Hay, R. W.; Vethaviyasar, N. Carbohydr. Res. 1973, 27, 55–61.

178 operational standpoint. Fortuitously, the same conditions used to synthesize 5.54 using the one- pot procedure also gave 5.61 in an excellent yield of of 86%.

Figure 5.26. One-pot boronic acid-mediated regioselective glycosylation reaction.

The development of the one-pot boronic acid-mediated regioselective glycosylation reaction has allowed the synthesis of the two key disaccharide fragments required for our synthesis of the pentasaccharide derivative isolated from Spergularia ramosa. The remainder of the synthesis was completed by Stefi Anthonipillai and Ross Mancini, two graduate students in the group. A summary of the complete synthesis is shown in Figures 5.27 and 5.28. The first step in our synthesis of the donor trisaccharide 5.66 is the boronic acid-mediated regioselective glycosylation of acceptor 5.59 with glucosyl donor 5.56a to give disaccharide 5.61 in 86% yield using the one-pot procedure developed above.

179

Figure 5.27. Synthesis of the donor trisaccharide 5.66.

To differentiate between the two hydroxyl groups in 5.61 a selective benzoylation was developed by Stefi Anthonipillai. Careful control of the equivalents of benzoyl chloride used in the reaction gave benzoylated disaccharide 5.62 where the protection was completely selective for the C-2 hydroxyl group. These conditions were based on a selective monopivaloylation of a rhamnose

180 derivative developed by Chan and coworkers.227 Glycosylation of 5.62 with trichloroacetimidate donor 5.63 under conditions developed by Schmidt228 (TMSOTf in DCM) gave the pentenyl glycoside 5.64 in 78% yield (This reaction was performed by Stefi Anthonipillai). Stefi's attempts to activate the pentenyl group under Fraser-Reid's conditions and use 5.64 as a donor directly were largely unsuccessful in a model system. It was therefore decided that the pentenyl group would be hydrolyzed and then converted to a more traditional trichloroacetimidate donor. Although this modification adds two steps to our synthesis, the final glycosylation (discussed later) was greatly simplified using the trichloroacetimidate donor. Ross Mancini optimized the pentenyl hydrolysis reaction and was able to obtain the deprotected trisaccharide 5.65 with a free OH group at the anomeric position in 70% yield. Conversion of 5.65 to the trichloroacetimidate 5.66 under standard condtions proceeded smoothly in 79% yield (reaction performed by Ross Mancini). As discussed previously, the acceptor disaccharide 5.54 was synthesized in 87% yield using a one-pot boronic acid-mediated regioselective glycosylation reaction. The TMSOTf promoted glycosylation between 5.54 and 5.66 was completely selective for the C-2 OH group of the acceptor to give 5.67 in >95% yield (reaction performed by Ross Mancini). The final step of the synthesis is the global deprotection and efforts towards this end are underway. In summary, in our attempt to synthesis a pentasaccharide derivate isolated from Spergularia ramosa, we encountered two difficult donor-acceptor combinations which gave poor yields using the borinic acid-catalyzed regioselective glycosyaltion methodology. In an effort to improve the yields of these glycosylations, a stoichiometric variant of the reaction was successfully developed (It should be noted that the catalytic reaction can be used to great effect on certain complex substrates such as the cardiac glycosides explored by Beale).175 In fact the stoichiometric version was found to perform poorly in this situation. These two methods are therefore complementary and their success is reaction dependent.) The one-pot boronic acid-mediated regioselective glycosylation reaction provided access to two key intermediates (5.61 and 5.54) which were used by Stefi Anthonipillai and Ross Mancini to prepare the protected pentasaccharide 5.67.

227 Jiang, L.; Chan, T.-H. J. Org. Chem. 1998, 63, 6035–6038. 228 Nicolaou, K. C.; Ueno, H. In Preparative Carbohydrate Chemistry, Hanessian, S. Ed.; Dekker: New York, 1996: p. 315.

181

Figure 5.28. Synthesis of the acceptor disaccharide 5.54 and protected pentasaccharide 5.67.

Comparing the synthesis of the protected pentasaccharide derivates synthesized by our group (5.67) and by Du and coworkers (5.21), we were able to shorten the synthetic sequence to 7 steps (3 of which were protective group manipulations) from 14 steps (9 of which were protective group manipulations). It was shown that the products of the regioselective reactions (which contained two free hydroxyl groups) could be functionalized (acylated or glycosylated) selectively under substrate control using certain conditions. We were also able to show that the one-pot boronic acid-mediated regioselective glycosylation reaction could drastically reduce the

182 number of steps in an oligosaccharide synthesis and eliminate the need for extensive protective group manipulations employed in traditional oligosaccharide synthesis. For complex carbohydrate targets containing the requiste motif (1,2-trans glycosidic linkage at the equitorial position of a 1,2-cis-diol in the acceptor) the methodology described above represents an expedient and novel pathway to these intermediates.

5.8 Application of the Boronic Acid-Mediated Regioselective Glycosylation Reaction in Difficult Glycosylations

While the initial drive to develop the stoichiometric boronic acid-mediated glycosylation reaction was to solve the issues related to the total synthesis of the pentasaccharide derivative, we were also interested in examining its performance in other glycosylations that were difficult using the borinic acid catalyzed methodology. As mentioned previously, certain donor-acceptor pairs represent difficult substrate combinations for the catalytic glycosylation reaction. In general, 6-deoxyhexopyranosyl and pentopyranosyl halides are poor donors (fucose and arabinose derivatives) and rhamnopyranosides, arabinopyranosides and fucopyranosides (to a lesser extent) are poor acceptors. We identified several problematic combinations and applied the stoichiometric boronic acid variant. The first reaction studied was the glycosylation of methyl- α,L-fucopyranoside acceptor 5.69 with peracetylated fucosyl bromide 5.70 to form the (Fuc-[- 1,3]-Fuc) disaccharide 5.71. Using the optimized conditions for the borinic acid-catalyzed method gave a 70% yield of a mixture of regioisomers (1.5:1) (Figure 5.29). While the yield of this reaction is moderate, the formation of regioisomers is certainly undesirable. If the stoichiometric boronic acid-mediated reaction was capable of drastically increasing the yield of certain reactions, could it also suppress the formation of undesirable regioisomers in cases where the regioselectivity was not high? Conditions for the stoichiometric reaction were screened varying the boronic acid and the equivalents of triethyamine used.

183

Figure 5.29. Borinic acid-catalyzed glycosylation of acceptor 5.69 with donor 5.70.

The stoichiometric method was found to be completely regioselective giving 5.71 as the only product. As 5.70 is an armed donor lacking an electron withdrawing oxygen substituent at C-6, perhaps the low regioselectivity observed in the catalytic reaction is due to uncatalyzed background reaction. In the case of the stoichiometric reaction, the boronate ester could prevent glycosylation at the undesired positions. Details of the optimization of the glycosylation reaction between 5.69 and 5.70 are described in Table 5.09. Screening several boronic acids (Table 5.09, entries 1–5) in the presence of one equivalent of triethylamine as Lewis base led to the identification of phenylboronic acid (62% yield), 2,6-difluorophenylboronic acid (60% yield), and pentafluorophenylboronic acid (46% yield) as possible leads towards obtaining an efficient regioselective reaction. When the amount of triethylamine used was increased to three equivalents, while using the lead boronic acids, interesting results were obtained (Table 5.09, entries 5–8). In the case of 2,6-difluorophenylboronic acid no increase in yield was observed upon increasing the concentration of triethylamine (58% yield). However, for both phenylboronic acid and pentafluorophenylboronic acid a significant increase in yield was observed (81% yield). Heating the reaction to 40 °C using phenylboronic acid and three equivalents of triethylamine decreased the yield (71% yield, Table 5.09, entry 9). Using ten equivalents of triethylamine increased the yield to 87% when phenylboronic acid was used (Table 5.09, entry 10) but decreased the yield to 74% when pentafluorophenylboronic acid was used (Table 5.09, entry 11). It appears that the optimal concentration of triethylamine is highly dependent on the boronic acid used.

184

Table 5.09. Optimization of the boronic acid-mediated regioselective glycosylation between acceptor 5.69 and donor 5.70.

Entry Boronic Acid Equivalents NEt3 Isolated Yield (%) 1 Phenyl 1 62 2 4-Methoxyphenyl 1 10 3 2,6-difluorophenyl 1 60 4 Pentafluorophenyl 1 46 5 2-biphenyl 1 20 6 2,6-difluorophenyl 3 58 7 Phenyl 3 81 8 Pentafluorophenyl 3 81 9 Phenyl 3 71 (40 oC) 10 Phenyl 10 87 11 Pentafluorophenyl 10 74

Entry 10 represents the best conditions for the synthesis of 5.71. A series of glycosylation reactions were studied using both the borinic acid-catalyzed methodology and the stoichiometric boronic acid-mediated methodology in order to compare the two approaches. The results are summarized in Figure 5.30. While two of the attempted glycosylations gave mixtures of regioisomers using either the catalytic or stoichiometric reaction (glycosylation of acceptors 5.72 and 5.73 with donor 5.70), a slight difference was observed between the two methods when the glycosylation of acceptor 5.72 with donor 5.74 was studied. Using the catalytic method the bromide donor 5.74a gave a mixture of regioisomeric products (This reaction was performed by Christina Gouliaras, another graduate student in our group). A single regioisomer was obtained in 86% yield when the donor was switched to the less reactive glycosyl chloride 5.74b.173

185

Figure 5.30. Comparison of catalytic and stoichiometric methods.

However, a single regioisomer was obtained in 94% yield by employing the stoichiometric activation method with the glycosyl bromide donor 5.74a. Although this yield is similar to that obtained using the catalytic method, a stoichiometric amount of boronic acid is required making

186 it less useful in a practical sense. However, the tolerance of a more reactive glycosyl donor is noteworthy. We have conducted glycosylation reactions where the stoichiometric method has proven superior to (either by providing improved yields or regioselectivities), similar to and inferior to the catalytic method. The approach which will work best seems to differ on a case by case basis. The stoichiometric method does, however, offer an alternative in instances where the catalytic method gives poor results.

In the reactions described above a limited set of glycosyl acceptors and donors were studied. All of the acceptors were monosaccharide derivatives containing a 1,2-cis-diol and having a methyl, pentenyl, allyl, or para-methoxyphenyl protecting group at C-1. The donors were either peracetylated bromides or chlorides. The next section documents our investigations into what other types of acceptors or donors might be competent partners in the stoichiometric glycosylation reaction. The first set of acceptors studied were disaccharides containing a 1,2-cis- diol but having more than three free hydroxyl groups. Two test disaccharides were synthesized as shown in Figure 5.31.

Figure 5.31. Synthesis of disaccharide acceptors 5.76 and 5.77.

Attempts at glycosylating acceptor 5.76 with donor 5.70 using the boronic acid-mediated methodology gave a major trisaccharide product, 5.78. In some cases a minor side product was isolated, but never in sufficient quantities to characterize by NMR spectroscopy (<5% yield, assuming that it was a regioisomeric trisaccharide). Increasing the amount of donor used in the reaction from 1.1 equivalents to 2 equivalents improved the yield slightly from 28% to 36%, but a further increase to 5 equivalents resulted in the formation of a complex mixture of products.

187

Table 5.10. Boronic acid-mediated regeioselective glycosylation of disaccharide 5.76 with donor 5.70.

Equivalents Silver (I) Entry Boronic Acid Solvent Isolated Yield (%) Donor (y) Source

1 pentafluorophenyl DCM 1.1 Ag2O 28

2 pentafluorophenyl DCM 2 Ag2O 36

3 pentafluorophenyl DCM 5 Ag2O complex mixture a 4 pentafluorophenyl DCM 2 Ag2O complex mixture Ag(I)/ 5 pentafluorophenyl DCM 1.1 7 Al2O3/SiO2

6 pentafluorophenyl DCM 1.1 Ag2CO3 31 b 7 pentafluorophenyl DCM 1.1 Ag2O 0

8 pentafluorophenyl CH3CN 1.1 Ag2O 27 a The second step of the reaction was performed at 40 °C. b The reaction was carried out using the fucosyl chloride as the donor.

Heating the reaction to 40 °C using 2 equivalents of donor also led to a complex mixture of products (Table 5.10, entry 4). Using other silver (I) sources did not significantly improve the yield (Table 5.10, entries 5 and 6) and employing the glycosyl chloride as a donor instead of the glycosyl bromide gave no reaction (Table 5.10, entry 7). Performing the reaction in acetonitrile instead of dichloromethane gave similar results. Attempts at glycosylating the other disaccharide gave even poorer results. Under every set of conditions tested, a complex mixture of products was observed which could not be separated or identified. The conditions tested are summarized in Table 5.11.

188

Table 5.11. Boronic-acid mediated regeioselective glycosylation of disaccharide 5.77 with donor 5.79.

Equivalents Equivalents Entry Boronic Acid Solvent Isolated Yield (%) NEt3 (x) Donor (y) 1 pentafluorophenyl DCM 3 1.1 complex mixture 2 pentafluorophenyl DCM 3 2 complex mixture 3 pentafluorophenyl DCM 10 1.1 complex mixture

4 4-methoxyphenyl CH3CN 1 1.1 complex mixture 5 phenyl DCM 3 1.1 complex mixture

In all of the previous cases where a regioselective reaction was observed, the site of glycosylation was the equatorial position of a 1,2-cis-diol. While boronic acids can form cyclic boronate esters with these motifs, they are also capable of forming cyclic boronate esters with 1,3-diols. The synthesis of the 4,6-O-phenyl boronate ester of methyl-α,D-glucopyranoside has been previously described.229 To assess whether 6-membered boronate esters (formed between 1,3-diols and boronic acids) could be activated similarly to 5 membered boronate esters (formed between 1,2-cis-diols and boronic acids), 5.80 was synthesized and then tested in glycosylation reactions.

229 Meiland, M.; Heinze, T.; Guenther, W.; Liebert, T. Tetrahedron Letters 2009, 50, 469–472.

189

Figure 5.32. Synthesis of boronate ester 5.80.

Under the set of conditions shown in Figure 5.33, no reaction was observed when NEt3, H2O, iPrOH, DMAP, tributylphosphine oxide, triphenylphosphine, N,N,N',N'-tetramethylethylene and quinuclidine were used as the Lewis base.

Figure 5.33. Conditions for the Lewis base screen in the attempted glycosylation of boronate ester 5.80.

Using 8-quinolinyl, ferrocenyl, cyclohexyl, 2,6-difluorophenyl, and pentafluorophenyl boronic acid also resulted in no reaction (Figure 5.34). It was thought that activation of the boronate ester would lead to glycosylation at the less sterically hindered 6-OH position. However, in the case of methyl-α,D-glucopyranoside this did not seem to be the case.

Figure 5.34. Conditions for the boronic acid screen in the attempted glycosylation of methyl- α,D-glucopyranoside.

190

Althought these reactions were performed early on in the optimization of the glycosylation leading to 5.54 and the "best" conditions had not yet been developed, the results were not promising. Later, after development of the one-pot conditions where DCM was used as the solvent, pentafluorophenylboronic acid as the boronic acid and 4 Ǻ M.S. were added to the reaction, this problem was revisited. Only a single example of the regioselective glycosylation of a 1,3-diol was observed when D-glucal was used as the acceptor (Figure 5.31). Glycosylation occured at the less hindered primary position. If the reaction was performed without forming the boronate ester no product was formed.

Figure 5.35. Glycosylation of glycal acceptor 5.81 bearing a 1,3-diol.

As Aoyama had reported that thioglycoside acceptors had been used in their regioselective glycosylation reaction,140b we anticipated being able to use them as well. If this could be achieved, the disaccharide product formed after the initial regioselective glycosylation reaction could later be used as a glycosyl donor under conditions which would activate the thioglycoside. Attempts to use thioglycoside acceptors 5.83 and 5.84 in our methodology, unfortunately, led to decomposition of the starting material and none of the desired products were ever observed. (Figure 5.32) This may be due to interaction of the sulfur with silver (I) oxide.

191

Figure 5.36. Attempts at using thioglycosides as acceptors.

Of the additional acceptors studied only moderate success was observed with disaccharide 5.76. We then turned our attention to examining what other kinds of donors could be used. In the catalytic reaction, a large scope was not demonstrated for the reaction of the perbenzylated glucosyl chloride donor 5.86. The stereoselectivity of this glycosylation is also interesting as there is the possibility of obtaining the 1,2-cis or 1,2-trans linkage as the protecting group at C-2 is not participating. Perbenzylated glucosyl chloride 5.86 was used in the stoichiometric reaction and the yields were compared to the borinic acid-catalyzed methodology. The result of the catalytic reaction for the glycosylation of the acceptor methyl-α,L-fucopyranoside (5.85) with donor 5.86230 is shown in Figure 5.37.

Figure 5.37. Glycosylation of 5.85 with perbenzylated chloride donor 5.86 using the borinic acid-catalyzed reaction.

230 Zhang, Z.; Magnusson, G. Carbohydr. Res. 1996, 925, 41–55.

192

Although a regioselective glycosylation was observed under the previously developed conditions, the yield was only moderate at 60%. The stoichiometric reaction conditions tested are summarized in Table 5.14.

Table 5.12. Boronic acid-mediated glycosylation of 5.85 with perbenzylated chloride donor 5.86

Entry Boronic Acid Isolated Yield (%) 1 4-methoxyphenyl 80 2 4-methoxyphenyl 71a 3 4-methoxyphenyl 83b 4 3,5-bistrifluoromethylphenyl 65 5 4-methoxyphenyl 83c 6 pentafluorophenyl 65c

a Step 2 of the reaction was performed at 40 °C. b Step 2 of the reaction was performed at 0 °C cReaction was performed using the one-pot procedure where DCM was used as solvent for both steps.

The stoichiometric reaction was able to provide the product in 83% yield, a significant increase in yield over the catalytic reaction. It would appear that perbenzylated chlorides can be used as donors in the stoichiometric procedure. Attempts to use perbenzylated mannosyl chloride donor 5.88231 to furnish β-mannoside linkages using the stoichiometric protocol gave a complex mixture of products. Although it was not possible to separate the components of the product mixture, it appeared as though both α and β linked disaccharides were formed. The presence of the α linked products suggests that, in this particular case, an SN1 pathway proceeding through an oxocarbenium intermediate may be possible. While this outcome contradicts the proposed SN2

231 Gόmez, A. M.; Pedregosa, A.; Casillas, M.; Uriel, C.; Lόpez, J. C. Eur. J. Org. Chem. 2009, 3579–3588.

193 mechanism for the boronic acid-mediated glycosylation reaction, previous results from our group have shown that mannosyl halide donors do not give soley the expected β-mannosides in the borinic acid-catalyzed reaction.232 When the glycosylation of 5.72 with 2,3,4,6-tetra-O-acetyl-α- D-mannopyranosyl bromide was attempted only the orthoester was observed. This could also suggest a contribution from an SN1 pathway when mannosyl halides are used as donors.

Figure 5.38. Attempted glycosylations using mannosyl chloride donor 5.88.

5.9 Conclusions

This chapter details the optimization of a stoichiometric boronic acid-mediated regioselective glycosylation reaction. The driving force behind developing this methodology was to improve the yields in glycosylation reactions for which the borinic acid-catalyzed methodology gave poor

232 Gouliaras, Christina. 2011. Regioselective Activation of Glycosyl Acceptors by a Diarylborinic Acid Catalyst. (Master's thesis).

194 results. This methodology was succesfully applied towards the synthesis of two key intermediates in the synthesis of a pentasaccharide derivative isolated from Spergularia ramosa. The remainder of the synthesis was completed by Stefi Anthonipillai and Ross Mancini. Significantly fewer steps were required compared to the previous synthesis reported by Du and coworkers. (7 steps compared to 14 steps). This work shows that the boronic acid-mediated regioselective glycosylation reaction can be usesul in the synthesis of complex oligosaccharide targets. Although the boronic acid methodology suffers from the disadvantage of requiring a stoichiometric quantity of boronic acid (compared to the catalytic borinic acid methodology), it can provide improved yields in certain cases. In particular, improved reaction yields were observed for armed donors (such as peracetylated bromides lacking an oxygen substituent at C-6 or 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl chloride) and rhamnose/fucose derived acceptors. Unfortunately, general reaction conditions which worked well for all of the donor/acceptor combinations were elusive. Yields were highly dependent on the identity of the boronic acid used and the concentration of triethylamine used. Fortunately, a large variety of boronic acids are commercially available and screening of this reaction parameter allowed the identification of conditions which gave acceptable yields. While regioselective glycosylations were observed for monosaccharides containing 1,2-cis-diols and bearing a methyl, allyl, or pentenyl protecting group at the anomeric position, extension of the methodology to different types of acceptors was largely unsuccessful. A single example of a regioselective glycosylation of a 1,3-diol was observed when D-glucal was used as the acceptor. Glycosylation occurred at the less sterically hindered primary position. Although perbenzylated glucosyl chloride was an efficient donor in the stoichiometric reaction, perbenzylated mannosyl chloride failed to cleanly give the β- mannosides as products. The boronic acid-mediated regioselective glycosylation reaction provides high yields of trans-1,2 linked disaccharides from donors having a gluco or galacto configuration (but not the manno configuration) and acceptors having a 1,2-cis-diol. The product disaccharides are useful intermediates in the synthesis of more complex oligosaccharides as it has been shown they can be selectively functionalized (benzoylated or glycosylated) under substrate control.

195

5.10 Experimental Details

Materials. Commercial reagents were purchased from Sigma Aldrich, Alfa Aesar, and carbosynth and were used as received with the following exceptions: Acetonitrile, THF and dichloromethane were purified by passing through two columns of activated alumina under argon (Innovative Technology, Inc.). Nuclear magnetic resonance (NMR) solvents were purchased from Cambridge Isotope Laboratories.

Instrumentation. Proton nuclear magnetic resonance (1H NMR) spectra and carbon nuclear magnetic resonance (13C NMR) spectra were recorded on a 300-MHz or 400-MHz Varian Mercury spectrometer, a 400-MHz Bruker spectrometer or a 500-MHz Varian Unity spectrometer. Chemical shifts for protons are reported in parts per million (ppm) downfield from tetramethylsilance and are referenced to residual protium in the NMR solvent (DMSO-d5: δ 2.50;

CDCl3: δ 7.26). Chemical shifts for carbon are reported in parts per million downfield from tetramethylsilane and are referenced to the carbon resonances of the solvent (DMSO: δ 39.43;

CDCl3: 77.16). Data are represented as follows; chemical shift (δ, ppm); multiplicity (s-singlet, d-doublet, t-triplet, q-quartet, m = multiplet); coupling constant (J, Hz); integration). As High- resolution mass spectra (HRMS) were obtained on a VS 70-250S (double focusing) mass spectrometer at 70 eV. Infrared (IR) spectra were obtained on a Perkin-Elmer Spectrum 100 instrument equipped with a single-bounce diamond / ZnSe ATR accessory. Data are represented as follows: wavenumber (cm–1); intensity (s-strong, m-medium, w-weak).

Assignment of 1H NMR signals. 1H NMR signal assignments were based on analysis of both coupling constants and COSY experiments. The two-dimensional COSY spectrum can be found in Appendix B.

196

General procedure I: Synthesis of glycosyl bromides from peracetylated sugars

A round-bottomed flask was equipped with a stir bar and put under an argon atmosphere. The peracetylated sugar (1.0 equiv) was dissolved in dichloromethane (1.5 M), transferred to the round bottom flask and the reaction was cooled to 0 °C in an ice bath. A solution of HBr (33 wt. %) in acetic acid (4.5 equiv) was added dropwise. The reaction was slowly allowed to warm to 23 °C and then was stirred at this temperature for 4 h. The reaction was then diluted with DCM and then poured slowly into ice-cold water. The aqueous layer was extracted with DCM three times. The combined organic layers were washed with water, saturated aqueous NaHCO3, and brine, then dried over MgSO4, filtered and concentrated. The crude product was purified by flash column chromatography (EtOAc/pentane; 2:8). The product is unstable on silica so the column must be done quickly. The glycosyl bromide products were stored inside a glovebox in a fridge.

General procedure J: Deprotection of acetyl groups

Acetylated sugar (1 equiv) was weighed into a round-bottomed flask and dissolved in anhydrous methanol (0.023 M) . NaOMe (0.2 equiv) was added to the reaction and it was stirred at 23 °C for 24 h. A spatula tip of DOWEX 50WX2-100 was washed five times with methanol and then added to the reaction and stirred for 5 minutes. The reaction was filtered and concentrated to give the deprotected sugar. The product required no further purification.

General procedure K: Two-step boronic acid-mediated regioselective glycosylation reaction.

The acceptor (0.1 mmol) and boronic acid (0.1 mmol) were weighed into a 2-dram vial, outfitted with a stir bar, and slurried in toluene (1.0 mL). The reaction was heated to 100 °C and stirred at this temperature for 3 hours. The reaction was concentrated in vacuo to give a solid which was dried under vacuum. The donor (0.11 mmol), silver (I) promoter (0.1 mmol), and molecular sieves (100 mg), if applicable, were weighed into the vial and the solvent used in the second step was added (1.0 mL). The Lewis base (x mmol) was then added to the reaction and it was stirred at the indicated temperature for 20 hours. The reaction was quenched with 10 drops of methanol and then diluted with DCM. The reaction was filtered through a plug of celite and the filtrate was concentrated to give the crude reaction mixture which was subsequently purified by column chromatography.

197

General procedure L: Boronic acid-mediated regioselective glycosylation of pre-formed boronate esters 5.58 or 5.80.

The boronate ester (0.1 mmol), donor (0.11 mmol), silver oxide (0.1 mmol), and molecular sieves (100 mg), if applicable, were weighed into a 2-dram vial. The reaction solvent (1 mL) was added and followed by the Lewis base (x mmol). The reaction was stirred at the indicated temperature for 20 hours at which point the reaction was quenched with 10 drops of methanol and then diluted with DCM. The reaction was filtered through a plug of celite and the filtrate was concentrated to give the crude reaction mixture which was subsequently purified by column chromatography.

General procedure M: Determination of yields of 5.54 by HPLC.

Boronate ester 5.58 (0.04 mmol), 5.53a (0.044 mmol), silver (I) source (0.04 mmol) and molecular sieves, if applicable, were weighed into a 2-dram vial outfitted with a stirbar. In cases where the Lewis base (0.12 mmol) was a solid it was also weighed into the vial. Solvent (0.3 mL) was added to the reaction followed by the Lewis base (0.12 mmol) if it was a liquid or solution. The reactions were stirred at the indicated temperature for 20 hours. Reactions were diluted with DCM and filtered through celite. The reactions were washed with 0.5 mL 1M HCl followed by 0.5 mL of water. The organic layer was separated and 100 μL of 200 mmolL-1 vanillin internal standard (as a solution in EtOAc) was added. 0.5 mL of the reaction solution was taken and diluted with isoproanol and then analyzed by HPLC. A Luna 5u CN 100A column was used with an eluent of 20% isoproanol/80% hexane at a flow rate of 1mLmin-1 for 30 minutes. The yields were determined by comparing the intergrations of the peak for vanillin (7.25 min) to that of 5.54 (20.6 min). A calibration curve was constructed to account for variations in instrument response to the two different analytes.

General procedure N: One-pot boronic acid-mediated regioselective glycosylation reaction.

The acceptor (0.1 mmol), boronic acid (0.1 mmol) and molecular sieves (100 mg) were weighed into a 2-dram vial, outfitted with a stir bar, and DCM was added (1.0 mL). The reaction was stirred at 23 °C for 6.5 hours. The donor (0.11 mmol), silver (I) promoter (0.1 mmol), and molecular sieves (100 mg), if applicable, were weighed into the vial and the Lewis base (x mmol) was then added. The reaction was stirred at the indicated temperature for 20 hours. The

198 reaction was quenched with 10 drops of methanol and then diluted with DCM. The reaction was filtered through a plug of celite and the filtrate was concentrated to give the crude reaction mixture which was subsequently purified by column chromatography.

General procedure O: Catalytic borinic acid-catalyzed reaction.

The acceptor (0.1 mmol), donor (0.11 mmol), 2-aminoethyl diphenylboronate (0.01 mmol) and silver (I) oxide (0.1 mmol) were weighed into a 2-dram vial. Acetonitrile (0.75 mL) was added and the reaction stirred at 23 °C for 20 hours. The reaction was quenched by adding 10 drops of methanol. The reaction was diluted with 5 mL of DCM and then filtered through celite and concentrated. The crude product was purified by column chromatography.

General procedure P: Catalytic boronic acid-catalyzed reaction.

The acceptor (0.1 mmol), donor (0.11 mmol), boronic acid (0.01 or 0.02 mmol) and silver (I) oxide (0.1 mmol) were weighed into a 2-dram vial. Solvent (0.75 mL) was added followed by the Lewis Base (0.01 or 0.02 mmol) and the reaction stirred at 23 °C for 20 hours. The reaction was quenched by adding 10 drops of methanol. The reaction was diluted with 5 mL of DCM and then filtered through celite and concentrated. The crude product was purified by column chromatography.

Characterization Data para-Methoxybenzene-α-L-arabinopyranoside (5.52)

2,3,4-Tri-O-acetyl-α,L-arabinopyranosyl bromide was synthesized according to general procedure J using 1,2,3,4-tetra-O-acetyl-α/β-L-arabinopyranose233 (20 mmol) as starting material 1 (89% yield). H NMR (400 MHz, CDCl3): δ 6.70 (d, J = 3.8 Hz, 1H, H-1), 5.46–5.36 (m, 2H,

233 Timmons, S. C.; Jakeman, D. L. Carbohydr. Res. 2008, 343, 865–874.

199

' H-3, H-4), 5.09 (dd, J = 3.8, 11.7 Hz, 1H, H-2), 4.21 (dd, J = 13.5, 1.5 Hz, 2H, H-5, H-5 ), 2.15

(s, 3H, OCOCH3), 2.11 (s, 3H, OCOCH3), 2.03 (s, 3H, OCOCH3). Rf = 0.8 (EtOAc/pentanes; 1:3) Intermediate D was synthesized by weighing 2,3,4-tri-O-acetyl-β,L-arabinopyranosyl bromide (8.63 mmol), para-methoxyphenol (34.50 mmol), and CsCO3 (17.25 mmol) into a round bottom flask. The reaction was slurried in acetonitrile (30 mL), heated to 60 °C and stirred at this temperature for 21 h. The reaction was diluted with EtOAc and the organic layer was washed twice with water and once with brine. The organic layer was then dried over MgSO4, filtered and concentrated. The crude product was purified by column chromatography 1 (EtOAc/pentane; 1:3) to give a colorless oil (831 mg, 49% yield). H NMR (400 MHz, CDCl3):

δ 6.96 (d, J = 9.1 Hz, 2H, ArH), 6.82 (d, J = 9.1 Hz, 2H, ArH), 5.41 (dd, J = 9.1, 6.6 Hz, 1H, H-

2), 5.30 (m, 1H, H-4), 5.12 (dd, , J = 9.1, 3.5 Hz, 1H, H-3), 4.94 (d, J = 6.6 Hz, 1H, H-1), 4.10 ' (dd, J = 12.9, 3.7 Hz, 1H, H-5), 3.77 (s, 3H, OCH3), 3.70 (dd, J = 12.9, 1.9 Hz, 1H, H-5), 2.15

(s, 3H, OCOCH3), 2.10 (s, 3H, OCOCH3), 2.06 (s, 3H, OCOCH3). Rf = 0.4 (EtOAc/pentanes; 1:3)

5.52 was synthesized according to general procedure K using Intermediate D (7.23 mmol) 1 giving a white solid (1.685 g, 91% yield). H NMR (400 MHz, DMSO-d6): δ 6.94 (d, J = 9.1

Hz, 2H, ArH), 6.85 (d, J = 9.1 Hz, 1H, ArH), 5.21 (s, 1H, C2OH), 4.88 (s, 1H, C3OH ), 4.68 (d, J

= 6.9 Hz, 1H, H-1), 4.66 (s, 1H, C4OH), 3.76–3.64 (m, 5H, H-4, H-5, OCH3), 3.59–3.49 (m, 2H, ' 13 H-2, H-5), 3.43 (d, J = 8.8 Hz, 1H, H-3). C NMR (75 MHz, DMSO-d6): δ 151.20, 119.07, 117.84, 114.43, 101.94, 72.52, 70.40, 67.62, 58.13, 55.36. FTIR (powder, cm-1): ν 3534 (w), 3367 (m), 1508 (m), 1209 (m), 1070 (s), 1031 (s), 1011 (s). HRMS (ESI, m/z): Calculated for + [C12H16NaO6] (M+Na) 279.0839; found 279.0839.

200

2,3,4-Tri-O-acetyl-α-D-fucopyranosyl bromide (5.53a)

5.53a was synthesized according to general procedure J using 1,2,3,4-tetra-O-acetyl-α/β-D- fucopyranose (5.87 mmol) as starting material (72% yield). Spectral data are in agreement with

234 1 previous reports. H NMR (300 MHz, CDCl3): δ 6.68 (d, J = 3.9 Hz, 1H, H-1), 5.49–5.32

(m, 2H, H-3, H-4), 5.01 (dd, J = 10.5, 3.9 Hz, 1H, H-2), 4.40 (q, J = 6.7 Hz, 1H, H-5), 2.16 (s,

3H, OCOCH3), 2.10 (s, 3H, OCOCH3), 2.00 (s, 3H, OCOCH3), 1.21 (d, J = 6.7 Hz, 3H, CH3). Rf = 0.8 (EtOAc/pentanes; 1:3).

2,3,4-Tri-O-acetyl-α-D-fucopyranosyl chloride (5.53b)

Synthesized according to a modified literature procedure.235 Cyanuric acid (0.933 mmol) was weighed into a schlenk tube and put under an argon atmosphere. DMF (262 μL) was added to give a clear beige solution. The reaction was stirred at 23 °C for 30 minutes. A solution of 2,3,4- tri-O-acetyl-α/β-D-fucose (0.848 mmol) in DCE (4.2 mL) was added followed by DBU (0.933 mmol) and the reaction was heated to 60 °C. The reaction was stirred at this temperature for 4 hours. The reaction was cooled to 23 °C and then diluted with diethyl ether. The reaction was filtered and the filtrate was concentrated to give a colorless oil. Purified by quickly passing through a plug of silica. Spectral data were in agreement with previous reports.204 1H NMR (300

MHz, CDCl3): δ 6.34 (d, J = 3.9 Hz, 1H, H-1), 5.47–5.33 (m, 2H, H-3, H-4), 5.23 (dd, J = 10.6,

3.9 Hz, 1H, H-2), 4.11 (q, J = 7.1 Hz, 1H, H-5), 2.16 (s, 3H, OCOCH3), 2.10 (s, 3H, OCOCH3),

2.00 (s, 3H, OCOCH3), 1.19 (d, J = 6.6 Hz, 3H, CH3). Rf = 0.6 (EtOAc/pentanes; 1:3)

234 Liu, L. D.; Liu, H. W. Tetrahedron Lett. 1989, 1, 35–38. 235 Chang, C.-W.; Chang, S.-S.; Chao, C.-S.; Mong, K.-K. T. Tetrahedron Letters 2009, 50, 4536–4540.

201 para-Methoxybenzene-3-O-(2',3',4'-tri-O-acetyl-α-D-fucopyranosyl)-α-L- arabinopyranoside (5.54)

Synthesized according to general procedure N, from 5.52 and 5.53a to give a white solid (87% 1 yield). H NMR (400 MHz, CDCl3): δ 7.02 (d, J = 9.0 Hz, 2H, ArH), 6.82 (d, J = 9.0 Hz, 2H,

ArH), 5.27–5.21 (m, 2H, H-2', H-4'), 5.06 (dd, J = 10.6, 3.3 Hz, 1H, H-3'), 4.76 (d, J = 8.0 Hz,

1H, H-1'), 4.70 (d, J = 7.2 Hz, 1H, H-1), 4.11 (dd, J = 12.9, 2.5 Hz, 1H, H-5a), 4.06–3.97 (m,

2H, H-2, H-4), 3.85 (q, J = 6.4 Hz, 1H, H-5'), 3.77 (s, 3H, OCH3), 3.72 (dd, J = 8.9, 3.4 Hz, 1H,

H-3), 3.57 (dd, J = 12.9, 2.0, 1H, H-5b), 2.80 (d, J = 2.3 Hz, 1H, C4-OH), 2.44 (d, J = 2.8 Hz,

1H, C2-OH), 2.20 (s, 3H, OCOCH3), 2.07 (s, 3H, OCOCH3), 2.00 (s, 3H, OCOCH3), 1.22 (d, J = 13 6.4 Hz, 1H, CH3) . C NMR (100 MHz, CDCl3): δ 170.75, 170.32, 170.25, 155.63, 151.04, 119.03, 114.65, 102.48, 101.93, 81.94, 71.01, 70.49, 70.12, 69.63, 69.13, 67.50, 65.27, 55.76, 20.99, 20.81, 20.77, 16.26. FTIR (powder, cm-1): ν 3483 (w), 1742 (s), 1506 (s), 1065 (s), 829 + (m), 741(m). HRMS (ESI, m/z): Calculated for [C24H26N1O13] (M+NH4) 546.2187; found

546.2199. Rf = 0.5 (Acetone/DCM; 1:4) para-Methoxybenzene-3,4-O-para-methoxyphenylboronate-α-L-arabinopyranoside (5.58)

5.52 (0.8 mmol) and 4-methoxyphenylboronic acid were weighed into a round bottom flask and slurried in toluene (2 mL). The reaction was heated to 100 °C and stirred at that temperature for 12 hours. The reaction was cooled to 23 °C and the solvent removed in in vacuo to give a white solid (>95% yield). The compound was used without further purification. 1H NMR (400 MHz,

CDCl3): δ 7.80 (d, J = 8.6 Hz, 2H, ArH), 6.98 (d, J = 9.0 Hz, 2H, ArH), 6.92 (d, J = 8.6 Hz, 2H, ArH), 6.82 (d, J = 9.1 Hz, 2H, ArH), 4.98 (d, J = 7.0 Hz, 1H, H-1), 4.73 (dt, J = 7.9, 5.4 Hz, 1H,

H-4), 4.57 (t, J = 7.9 Hz, 1H, H-3), 4.18 (dd, J = 12.7, 5.4 Hz, 1H, H-5a), 4.06–3.98 (m, 2H, H-2,

H-5b), 3.84 (s, 3H, OCH3), 3.77 (s, 3H, OCH3), 2.62 (d, J = 3.2 Hz, 1H, C2-OH).

202 n-Pentenyl-α-L-rhamnopyranoside (5.59)

5.59 was synthesized according to a modified literature procedure.220 A solution of 1,2,3,4-tetra- O-acetyl-α/β-L-rhamnopyranoside (2.89 mmol) in DCM (2 mL) was cooled to 0 °C in an ice bath. At this temperature, BF3•Et2O (8.67 mmol) and 4-penten-1-ol (4.33 mmol) were added dropwise. The reaction was warmed to 23 °C and stirred for 12 hours. The reaction mixture was diluted with DCM and washed with NaHCO3 (aq), brine, and water. The organic layer was then dried over MgSO4, filtered and concentrated. The crude product was purified by column chromatography (EtOAc/pentane; 3:7) to give n-Pentenyl-2,3,4-tri-O-acetyl-α-L- rhamnopyranoside as a colorless oil (31% yield). Spectral data were in agreement with 220 1 previous reports. H NMR (400 MHz, CDCl3): δ 5.71-5.61 (m, 1H, O-CH2-CH2-CH2-CH), 5.13 (dd, J = 10.0, 3.2 Hz, 1H, H-3), 5.06 (dd, J = 3.6, 2.0 Hz, 1H, H-2), 4.92-4.87 (m, 2H, O- CH2-CH2-CH2-CH-CH2, H-4), 4.84 (dd, 1H, O-CH2-CH2-CH2-CH-CH2), 4.56 (d, J = 1.6 Hz, 1H, H-1), 3.76-3.69 (m, 1H, H-5), 3.57-3.52 (m, 1H, O-CH2), 3.32-3.27 (m, 1H, O-CH2), 2.02- 1.95 (m, 2H, O-CH2-CH2- CH2), 1.99 (s, 3H, COCH3), 1.90 (s, 3H, COCH3), 1.83 (s, 3H,

COCH3), 1.60-1.53 (m, 2H, OCH2-CH2), 1.06 (d, J = 6.0 Hz, 3H, CH3). Rf = 0.8 (EtOAc/pentanes; 3:7) 5.59 was synthesized according to general procedure K using n-Pentenyl- 2,3,4-tri-O-acetyl-α-L-rhamnopyranoside (1.84 mmol) giving a white solid (406 mg, 95%

236 1 yield). Spectral data were in agreement with previous reports. H NMR (300 MHz, CDCl3): δ

5.90-5.68 (m, 1H, O-CH2-CH2-CH2-CH-CH2), 5.02 (dd, 1H, O-CH2-CH2-CH2-CH-CH2), 5.00–

4.95 (m, O-CH2-CH2-CH2-CH-CH2), 4.73 (d, J = 1.5 Hz, H-1), 4.42 (br s, 1H, O–H), 4.12 (br s, 1H, O–H), 4.04 (br s, 1H, O–H), 3.89 (m, 1H, H-2), 3.80–3.72 (m, 1H, H-3), 3.66-3.57 (m, 2H,

H-5, O-CH2-CH2-CH2-CH-CH2), 3.47-3.35 (m, 2H, H-4, O-CH2-CH2-CH2-CH-CH2), 2.10-2.05

(m, 2H, O-CH2-CH2-CH2-CH-CH2), 1.67-1.60 (m, 2H, O-CH2-CH2-CH2-CH-CH2), 1.27 (d, J =

236 Sarkar, S.; Lombardo, S. A.; Herner, D. N.; Talan, R. S.; Wall, K. A.; Sucheck, S. J. J. Am. Chem. Soc. 2010, 132, 17236–17246.

203

13 6.0 Hz, 3H, CH3). C NMR (75 MHz, CDCl3): δ 138.03, 115.17, 99.88, 73.06, 71.94, 71.23,

68.17, 67.16, 30.36, 28.71, 17.66. Rf = 0.2 (EtOAc/pentanes; 7:3)

Methyl-3-O-(2',3',4',6'-tetra-O-acetyl-β-D-glucopyranosyl)-α-L-rhamnopyranoside (5.60)

Synthesized according to general procedure K, from methyl-α-L-rhamnopyranoside and 5.56a 1 to give a white solid (89% yield). H NMR (300 MHz, CDCl3): δ 5.23 (t, J = 9.5 Hz, 1H, H-3'), 5.05 (dd, J = 10.0, 9.5 Hz, 1H, H-4'), 5.03 (dd, J = 9.5, 8.0 Hz, 1H, H-2'), 4.70 (d, J = 1.6 Hz, 1H

H-1), 4.68 (d, J = 8.2 Hz, 1H, H-1'), 4.21–4.15 (m, 2H, H-6a', H-6b'), 3.99 (m, 1H, H-2), 3.79–

3.59 (m, 4H, H-5', H-4, H-3, H-5), 3.36 (s, 3H, OCH3), 2.68 (d, J = 3.2 Hz, 1H, C2–OH), 2.29

(d, J = 2.5 Hz, 1H, C4–OH), 2.08 (s, 3H, OCOCH3), 2.05 (s, 3H, OCOCH3), 2.03 (s, 3H, 13 OCOCH3), 2.01 (s, 3H, OCOCH3). C NMR (75 MHz, CDCl3): δ 170.82, 170.27, 170.11, 169.53, 101.38, 100.47, 83.30, 72.46, 72.14, 71.73, 71.03, 69.70, 68.48, 67.61, 61.86, 54.94, 20.87, 20.78, 20.73, 17.72. FTIR (powder, cm-1): ν 2905 (w), 1748 (s), 1368 (m), 1223 (s), + 1035 (s), 972 (m). HRMS (ESI, m/z): Calculated for [C21H32NaO14] (M+Na) 531.1684; found

531.1690. Rf = 0.3 (EtOAc/pentanes; 7:3) n-Pentenyl-3-O-(2',3',4',6'-tetra-O-acetyl-β-D-glucopyranosyl)-α-L-rhamnopyranoside (5.61)

Synthesized according to general procedure N, from 5.59 and 5.56a to give a white solid isolated 220 1 in 86%. Spectral data were in agreement with previous reports. H NMR (400 MHz, CDCl3)

δ 5.80–5.70 (m, 1H, OCH2CH2CH2CHCH2), 5.17 (dd, 1H, J = 9.6 Hz, 9.2 Hz, H-3'), 5.04–4.90

(m, 4H, H-2', OCH2CH2CH2CHCH2, H-4'), 4.72 (d, 1H, J = 1.6 Hz, H-1), 4.67 (d, 1H, J = 8.0

204

Hz, H-1'), 4.18 (dd, 1H, J = 12.2, 6.4 Hz, H-6'), 4.09 (dd, 1H, J = 12.2, 2.4 Hz, H-6'), 3.92 (dd,

1H, J = 1.6, 2.8 Hz, H-2), 3.77–3.58 (m, 5H, H-5', H-3, OCH2, H-5, H-4), 3.39–3.33 (m, 1H,

OCH2), 2.07–2.02 (m, 2H, OCH2CH2CH2), 2.01 (s, 3H, OCOCH3), 1.99 (s, 3H, OCOCH3), 1.97

(s, 3H, OCOCH3), 1.95 (s, 3H, OCOCH3), 1.65–1.60 (m, 2H, OCH2CH2), 1.24 (d, 3H, J = 6.0 13 Hz, CH3). C NMR (100 MHz, CDCl3) δ 170.8, 170.3, 170.3, 169.6, 138.1, 115.1, 101.4, 99.4, 83.5, 72.7, 72.1, 71.7, 70.9, 70.0, 68.7, 67.8, 67.0, 62.2, 30.4, 28.7, 20.9, 20.8, 20.7, 17.7.

(pentane/ethyl acetate 3:7). HRMS (ESI, m/z): Calculated for [C25H38O14]: 562.2262; Found:

562.2285. Rf = 0.5 (EtOAc/pentanes; 7:3)

Methyl-3-O-(2',3',4'-tri-O-acetyl-β-L-fucopyranosyl)-α-L-fucopyranoside (5.71)

Synthesized according to general procedure N, from 5.69 and 5.70 to give a white solid (87% 1 yield). H NMR (400 MHz, CDCl3): δ 5.23 (dd, J = 3.5, 1.1 Hz, 1H, H-4'), 5.19 (dd, J = 10.5, 7.8 Hz, 1H, H-2'), 5.03 (dd, J = 10.5, 3.5 Hz, 1H, H-3'), 4.76 (d, J = 3.9 Hz, 1H, H-1), 4.71 (d, J = 7.8 Hz, 1H, H-1'), 4.00–3.87 (m, 2H, H-2, H-5), 3.85 (dt, J = 3.3, 1.7 Hz, 1H, H-4), 3.82 (dd, J

= 6.5, 1.1 Hz, 1H, H-5'), 3.78 (dd, J = 9.6, 3.3 Hz, 1H, H-3), 3.42 (s, 3H, OCH3), 2.54 (d, J = 1.7

Hz, 1H, C4-OH), 2.18 (s, 3H, OCOCH3), 2.06 (m, 4H, OCOCH3, C2-OH), 1.99 (s, 3H, 13 OCOCH3), 1.31 (d, J = 6.6 Hz, 3H, CH3), 1.20 (d, J = 6.4 Hz, 3H, CH3'). C NMR (100 MHz,

CDCl3): δ 170.74, 170.38, 170.29, 102.04, 99.55, 81.12, 71.20, 71.04, 70.13, 69.48, 69.31, 68.03, 65.35, 55.49, 21.05, 20.83, 20.77, 16.35, 16.33. FTIR (powder, cm-1): ν 2940 (w), 1747

(s), 1369 (m), 1215 (s), 1046 (s), 909 (m). HRMS (ESI, m/z): Calculated for [C19H30NaO12] + (M+Na) 473.1629; found 473.1633. Rf = 0.25 (Acetone/DCM; 1:4)

205

Methyl-6-O-tert-butyldimethylsilyl-α-D-mannopyranoside (5.72)

Methyl-a,D-mannopyranoside (20 mmol) and tert-butyldimethylsilyl chloride (24 mmol) were weighed into a round bottom flask and put under an Argon atmosphere. The solids were dissolved in pyridine (0.7 M) and the reaction was stirred at 23 oC for 24 hours. The resulting solution was poured into saturate aqueous NaHCO3 and the aqueous layer was extracted three times with dichloromethane. The combined organic layers were dried over MgSO4, filtered and concentrated in vacuo. The resulting colorless oil was azeotroped with toluene to remove residual pyridine to give a white solid. The crude material was purified by gradient column chromatography (silica). Spectral data are in agreement with previous reports.170 1H NMR (400

MHz, CDCl3): δ 4.71 (d, J = 1.5 Hz, 1H, H-1), 3.94–3.76 (m, 5H, H-6a, H-6b, H-2, H-3 and H-

4), 3.58 (ddd, J = 8.9, 5.7, 5.7 Hz, 1H, H-5), 3.39 (s, 1H, C4-OH), 3.27 (s, 3H, OCH3), 2.95 (s,

C3-OH), 2.59 (d, J = 3.4 Hz, 1H, C2-OH), 0.91 (s, 9H, Si(C(CH3)3)(CH3)2), 0.11 (s, 3H,

Si(C(CH3)3)(CH3)2), 0.11 (s, 3H, Si(C(CH3)3)(CH3)2). Rf: 0.3: (ethyl acetate/pentanes 7:3).

Methyl-6-O-tert-butyldimethylsilyl-3-O-(2',3',4'-tri-O-acetyl-α-L-arabinopyranosyl)-α- Dmannopyranoside (5.75)

Synthesized according to general procedure N, from 5.72 and 5.74a to give a white solid (94% 173 1 yield). Spectral data are in agreement with previous reports. H NMR (300 MHz, CDCl3): δ 5.29–5.27 (m, 1H, H-4'), 5.24 (dd, J = 10.2, 7.5 Hz, 1H, H-2'), 5.04 (dd, J = 10.1, 3.5 Hz, 1H, H- 3'), 4.75 (d, J = 1.6 Hz, 1H, H-1), 4.49 (d, J = 7.5 Hz, 1H, H-1'), 4.08 (dd, J = 13.3, 2.5 Hz, 1H,

H-5a'), 3.94 (dd, J = 10.9, 3.2 Hz, 1H, H-3), 3.87–3.74 (m, 3H, H-2, H-4, H-6a), 3.74 (dd, J = 9.1,

3.3 Hz, 1H, H-6b), 3.69 (dd, J = 13.3, 1.5 Hz, 1H, H-5b'), 3.58–3.53 (m, 1H, H-5), 3.37 (s, 3H,

OCH3), 3.34 (d, J = 0.7 Hz, 1H, C4-OH), 2.19 (d, J = 3.1 Hz, 1H, C2-OH), 2.15 (s, 3H,

OCOCH3), 2.09 (s, 3H, OCOCH3), 2.03 (s, 3H, OCOCH3), 0.90 (s, 9H, Si(C(CH3)3)(CH3)2),

0.08 (s, 6H, Si(C(CH3)3)(CH3)2). Rf: 0.4 (ethyl acetate/pentanes 1:1).

206

Methyl-3-O-(β-L-fucopyranosyl)-α-L-fucopyranoside (5.76)

Synthesized according to general procedure K from 5.71 (0.86 mmol), 95% yield, white solid. 1 H NMR (500 MHz, CD3OD-d4): δ 4.66 (d, J = 3.9 Hz, 1H, H-1), 4.45 (d, J = 7.6 Hz, 1H, H-1', ), 3.97–3.88 (m, 3H, H-2, H-4, H-5), 3.79 (dd, J = 10.0, 3.2 Hz, 1H, H-3), 3.65 (qd, J = 6.4, 1.0 Hz, 1H, H-5'), 3.60 (dd, J = 3.4, 1.0 Hz, 1H, H-4'), 3.57 (dd, J = 9.8, 7.6 Hz, 1H, H-2'), 3.48 (dd,

J = 9.8, 3.4 Hz, 1H, H-3'), 3.38 (s, OCH3, 3H), 1.25 (d, J = 6.4 Hz, 3H, CH3'), 1.23 (d, J = 6.6 13 Hz, 3H, CH3). C NMR (126 MHz, CD3OD-d4): δ 106.25, 101.27, 81.83, 74.76, 73.18, 72.86, 72.79, 71.88, 68.89, 66.86, 55.54, 16.89, 16.62. FTIR (powder, cm-1): ν 3367 (w), 1591 (w),

1299 (w), 1156 (m), 1074 (s), 1047 (s). HRMS (ESI, m/z): Calculated for [C13H24NaO9] (M+Na)+ 347.1313; found 347.1314.

Methyl-3-O-(α-L-arabinopyranosyl)-α-D-mannopyranoside (5.77)

Synthesized according to general procedure K from 5.75 (1.42 mmol), 91% yield, white solid. 1 H NMR (400 MHz, CD3OD-d4): δ 4.67 (d, J = 1.7 Hz, 1H, H-1), 4.30 (d, J = 7.1 Hz, 1H, H-1'),

4.00–3.94 (m, 2H, H-2, H-5a'), 3.91 (dd, J = 12.5, 2.7 Hz, 1H, H-5b'), 3.83–3.75 (m, 3H, H-4', H-

6a, H-6b), 3.69 (t, J = 9.5 Hz, 1H, H-4), 3.65–3.48 (m, 4H, H-2', H-3', H-3, H-5), 3.37 (s, 3H, 13 OCH3), 0.92 (s, 9H, Si(C(CH3)3)(CH3)2), 0.10 (s, 6H, Si(C(CH3)3)(CH3)2). C NMR (100 MHz,

CD3OD-d4): δ 102.98, 102.21, 80.71, 74.84, 74.10, 72.45, 69.78, 69.47, 67.41, 66.50, 64.35, 55.07, 26.41, 19.25, -5.12. FTIR (powder, cm-1): ν 3400 (m), 2929 (m), 1254 (m), 1130 (m), + 1067 (s), 1046 (s). HRMS (ESI, m/z): Calculated for [C19H38NaO9Si] (M+Na) 463.1970; found 463.1971.

207

Methyl-3-O-(3'-O-(2'',3'',4''-tri-O-acetyl-β-L-fucopyranosyl)-β-L-fucopyranosyl)-α-L- fucopyranoside (5.78)

Synthesized according to general procedure N, from 5.76 and 5.70 to give a white solid (36% 1 yield). H NMR (500 MHz, CDCl3): δ 5.23 (dd, J = 3.5, 1.1 Hz, 1H, H-4''), 5.18 (dd, J = 10.5, 7.9 Hz, 1H, H-2''), 5.03 (dd, J = 10.5, 3.5 Hz, 1H, H-3''), 4.77 (d, J = 3.9 Hz, 1H, H-1), 4.75 (d, J = 7.9 Hz, 1H, H-1''), 4.48 (d, J = 7.8 Hz, 1H, H-1'), 4.08–4.01 (m, 1H, H-2), 3.92 (q, J = 6.8 Hz,

1H, H-5), 3.90–3.88 (m, 1H, H-4), 3.87–3.77 (m, 5H, H-3, H-2', H-4', H-5'', C2'-OH)), 3.63–3.60

(m, 1H, H-5'), 3.57 (dd, J = 9.4, 3.3 Hz, 1H, H-3'), 3.42 (s, 3H, OCH3), 3.25 (s, 1H, C2-OH), 2.97

(s, 1H, C4'-OH), 2.79 (s, 1H, C4-OH), 2.18 (s, 3H, OCOCH3), 2.06 (s, 3H, OCOCH3), 1.98 (s, 3H

OCOCH3), 1.33 (d, J = 6.5 Hz, 3H, CH3'), 1.29 (d, J = 6.8 Hz, 3H, CH3''), 1.20 (d, J = 6.4 Hz, 13 3H, CH3''). C NMR (126 MHz, CDCl3): δ 170.75, 170.60, 170.29, 104.56, 101.82, 99.70, 82.52, 81.57, 71.71, 71.00, 70.72, 70.55, 70.42, 70.15, 69.54, 69.19, 68.14, 65.72, 55.46, 21.05, + 20.83, 20.77, 16.59, 16.34, 16.32. HRMS (ESI, m/z): Calculated for [C25H40NaO16] (M+Na)

619.2213; found 619.2215. Rf: 0.35 (acetone/DCM 2:3).

Compound (5.82)

Synthesized according to general procedure N, from 5.81 and 5.79 to give a white solid (71% 1 yield). H NMR (400 MHz, CDCl3): δ 6.30 (dd, J = 6.0, 1.7 Hz, 1H, H-1), 5.22 (t, J = 9.5 Hz, 1H, H-3'), 5.09 (t, J = 9.5 Hz, 1H, H-4'), 5.03 (dd, J = 9.6, 8.0 Hz, 1H, H-2'), 4.74 (dd, J = 6.1,

2.2 Hz, 1H, H-2), 4.65 (d, J = 8.0 Hz, 1H, H-1'), 4.27–4.20 (m, 3H, H-3, H-6a', H-6b'), 4.11 (dd, J

= 11.4, 2.7 Hz, 1H, H-6a), 3.95 (dd, J = 11.4, 4.4 Hz, 1H, H-6b), 3.87 (m, 1H, H-5), 3.78–3.68

(m, 2H, H-4, H-5'), 2.80 (s, 1H, C2-OH), 2.10 (s, 3H, OCOCH3), 2.06 (s, 3H, OCOCH3), 2.03 (s, 13 3H, OCOCH3), 2.01 (s, 3H, OCOCH3), 1.65 (s, 1H, C3-OH). C NMR (75 MHz, CDCl3): δ

208

170.93, 170.38, 169.72, 169.53, 144.31, 103.04, 101.11, 72.78, 72.21, 71.12, 70.57, 69.90, 68.38, 68.24, 62.06, 61.79, 20.93, 20.85, 20.76, 20.74. FTIR (powder, cm-1): ν 3288 (w), 1747 (s),

1717 (m), 1646 (w), 1365 (w), 1219 (s). HRMS (ESI, m/z): Calculated for [C20H28NaO13] (M+Na)+ 499.1422; found 499.1422.

Methyl-3-O-(2',3',4',6'-tetra-O-benzyl-β-D-glucopyranosyl)-α-L-fucopyranoside (5.87)

Synthesized according to general procedure N, from 5.85 and 5.86 to give a white solid in 83% 1 yield. H NMR (400 MHz, CDCl3): δ 7.35–7.24 (m, 18H, ArH), 7.17–7.10 (m, 2H, ArH), 4.96–

4.74 (m, 6H, 5 CH2Ph, H-1), 4.63–4.44 (m, 4H, 3 CH2Ph, H-1'), 4.00 (ddd, J = 9.7, 3.8, 3.6 Hz,

1H, H-2), 3.95–3.85 (m, 2H, H-5, H-3), 3.75 (td, J = 3.0, 1.3 Hz, 1H, H-4), 3.71 (d, J = 3.8 Hz,

1H, C2-OH), 3.70–3.55 (m, 4H, H-3', H-4', H-5', H-6a'), 3.53–3.49 (m, 1H, H-6b'), 3.51 (t, J = 8.3

Hz, 1H, H-2'), 3.42 (s, 3H, OCH3), 2.21 (d, J = 1.3 Hz, 1H, C4-OH), 1.28 (d, J = 6.6 Hz, 3H, 13 CH3). C NMR (100 MHz, CDCl3): δ 138.43, 138.10, 137.87, 137.83, 128.66, 128.58, 128.57, 128.54, 128.18, 128.11, 128.08, 128.06, 128.05, 127.94, 127.88, 127.86, 102.39, 99.74, 85.01, 81.90, 81.73, 77.78, 75.88, 75.37, 75.23, 74.79, 73.67, 70.87, 68.69, 67.43, 65.36, 55.47, 16.16. FTIR (powder, cm-1): ν 3508 (m), 1455 (w), 1359 (m), 1057 (s), 753 (w), 697 (m). HRMS + (ESI, m/z): Calculated for [C41H52N1O10] (M+NH4) 718.3591; found 718.3619. Rf: 0.35

(acetone/DCM 2:3). Rf: 0.3 (acetone/DCM; 1:19).

209

Chapter 6 Final Thoughts

The projects presented in this thesis detail the investigations of various noncovalent and reversible covalent interactions and their applications in catalysis and novel reaction development. While each venture did not always proceed as planned, the experiments revealed important information concering the advantages and limitations of each system studied. This final chapter outlines the important conclusions drawn from the results of each project.

The studies of anion receptors composed of both hydrogen- and halogen-bond donors were discussed in chapter 1. While the experiments were not directly related to an application of these noncovalent interactions towards catalysis or reaction development, they revealed essential knowledge concerning the anion binding properties of these receptors. This information could be useful in designing receptors or catalysts incorporating both HB and XB. Experiments varying the linker attaching the hydrogen-bond donor to the halogen-bond donor suggested that access to a linear geometry for EWG-X•••B was required to observe contributions from XB to anion binding. For the urea receptors studied, ∆ΔGXB was found to be large for the halides, small for – – BzO and H2PO4 , and negligible for most other oxoanions. This unique selectivity is different than that displayed by hydrogen-bond donors. This opens opportunities for developing selective anion receptors based on the identity and number of donor groups present in the molecule.

In chapter 2, the use of halogen-bond donors as catalysts was explored. Although no increase in reaction yield was observed when using a XB catalyst for the Freidel-Crafts addition of indoles to nitroolefins, the hetero-Diels-Alder reaction and the reduction of aldimines with Hantzsch ester, a slight increase in yield was observed in some cases for the N-acyl-Pictet-Spengler reaction. This increase in yield was proposed to result from the interaction of the catalyst with an intermediate N-acyl-iminium chloride ion pair. As halogen-bond donors were found to have a preference for halides over oxoanions, targeting reactions that require abstraction of a halogen or proceed through halide intermediates might be more successful in future endeavors.

Attempts at using bifunctional thiourea catalysts to influence the stereoselective addition of nucleophiles to 2-nitroglycals were unsuccessful. All of the additions, under the conditions studies, appeared to be under substrate control as very similar results were obtained regardless of

210 the enantiomer of catalyst used. These experiments showed that methodology developed for simple nitroalkenes was not easily transferred to the more complex 2-nitroglycal substrates.

The development of the boronic acid-mediated regioselective glycosylation reaction was detailed in chapter 5. A major advantage of this methodology is that higher reaction yields are obtained, in some cases, compared to the catalytic borinic acid-catalyzed reaction. The reactions are also easily optimized, as a wide variety of boronic acids and Lewis bases are commercially available. The stoichiometric variant of the reaction was particularly useful when performing glycosylations with reactive donors (for example, 6-deoxyhexopyranosyl halides or perbenzylated glucosyl chloride). The methodology was applied towards the synthesis of two key intermediates in our synthesis of a pentasaccharide derivative isolated from Spergularia ramosa.

211

Appendix A: 1H, 19F and UV/Vis Titration Data

-1 Figure A1. 1.14a---n-Bu4NCl (CH3CN, 295K): Ka = 8000 ± 800 M (UV/Vis)

Plots of A(λ=362nm) against [n-Bu4NCl] (M) for the UV/Vis titration of 1.14a. Top: trial #1 -1 -1 (Ka = 8198.0 M ). Bottom: trial #2 (Ka = 7775.1 M ).

212

1 - Figure A2. H-NMR Job Plot for the 1.14a-Cl complex (CD3CN, 295K, [1.14a + n-Bu4NCl] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.14a with Bu4NCl.

19 - Figure A3. F-NMR Job Plot for the 1.14a-Cl complex (CD3CN, 295K, [1.14a + n-Bu4NCl] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.14a with Bu4NCl.

213

-1 1 Figure A4. 1.14a---n-Bu4NBr (CD3CN, 295K): Ka = 2400 ± 240 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NBr] (M) for the H-NMR titration of 1.14a. Top: trial #1 (Ka -1 -1 = 2475.4 M ). Bottom: trial #2 (Ka = 2232.9 M ).

1 - Figure A5. H-NMR Job Plot for the 1.14a-Br complex (CD3CN, 295K, [1.14a + n-Bu4NBr] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.14a with Bu4NBr.

214

-1 Figure A6. 1.14a---n-Bu4NBr (CD3CN, 295K): Ka = 2300 ± 230 M

19 Plots of || (ppm) against [n-Bu4NBr] (M) for the F-NMR titration of 1.14a. Top: trial #1 (Ka -1 -1 = 2425.7 M ). Bottom: trial #2 (Ka = 2210.2 M ).

19 - Figure A7. F-NMR Job Plot for the 1.14a-Br complex (CD3CN, 295K, [1.14a + n-Bu4NBr] = 5 mM)

215

19 Plot of |·χ| against χ for the F-NMR titration of 1.14a with Bu4NBr.

-1 Figure A8. 1.14a---n-Bu4NI (CD3CN, 295K): Ka = 210 ± 21 M

1 Plots of || (ppm) against [n-Bu4NI] (M) for the H-NMR titration of 1.14a. Top: trial #1 (Ka = -1 -1 213.0 M ). Bottom: trial #2 (Ka = 208.8 M ).

1 - Figure A9. H-NMR Job Plot for the 1.14a-I complex (CD3CN, 295K, [1.14a + n-Bu4NI] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.14a with Bu4NI.

216

-1 Figure A10. 1.14a---n-Bu4NI (CD3CN, 295K): Ka = 220 ± 22 M

19 Plots of || (ppm) against [n-Bu4NI] (M) for the F-NMR titration of 1.14a. Top: trial #1 (Ka = -1 -1 221.8 M ). Bottom: trial #2 (Ka = 216.5 M ).

19 - Figure A11. F-NMR Job Plot for the 1.14a-I complex (CD3CN, 295K, [1.14a + n-Bu4NI] = 5 mM)

217

19 Plot of |·χ| against χ for the F-NMR titration of 1.14a with Bu4NI.

-1 Figure A12. 1.14b---n-Bu4NCl (CH3CN, 295K): Ka = 1700 ± 170 M (UV/Vis)

Plots of A(λ=364nm) against [n-Bu4NCl] (M) for the UV/Vis titration of 1.14b. Top: trial #1 -1 -1 (Ka = 1716.6 M ). Bottom: trial #2 (Ka = 1709.9 M ).

1 - Figure A13. H-NMR Job Plot for the 1.14b-Cl complex (CD3CN, 295K, [1.14b + n-Bu4NCl] = 5 mM)

218

1 Plot of |·χ| against χ for the H-NMR titration of 1.14b with Bu4NCl.

19 - Figure A14. F-NMR Job Plot for the 1.14b-Cl complex (CD3CN, 295K, [1.14b + n-Bu4NCl] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.14b with Bu4NCl.

-1 1 Figure A15. 1.14b---n-Bu4NBr (CD3CN, 295K): Ka = 350 ± 35 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NBr] (M) for the H-NMR titration of 1.14b. Top: trial #1 (Ka -1 -1 = 327.5 M ). Bottom: trial #2 (Ka = 379.9 M ).

219

1 - Figure A16. H-NMR Job Plot for the 1.14b-Br complex (CD3CN, 295K, [1.14b + n-Bu4NBr] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.14b with Bu4NBr.

-1 19 Figure A17. 1.14b---n-Bu4NBr (CD3CN, 295K): Ka = 370 ± 37 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NBr] (M) for the F-NMR titration of 1.14b. Top: trial #1 (Ka -1 -1 = 364.5 M ). Bottom: trial #2 (Ka = 382.1 M ).

220

19 - Figure A18. F-NMR Job Plot for the 1.14b-Br complex (CD3CN, 295K, [1.14b + n-Bu4NBr] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.14b with Bu4NBr.

-1 1 Figure A19. 1.14b---n-Bu4NI (CD3CN, 295K): Ka = 46 ± 5 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NI] (M) for the H-NMR titration of 1.14b. Top: trial #1 (Ka = -1 -1 46.0 M ). Bottom: trial #2 (Ka = 45.9 M ).

221

1 - Figure A20. H-NMR Job Plot for the 1.14b-I complex (CD3CN, 295K, [1.14b + n-Bu4NI] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.14b with Bu4NI.

-1 19 Figure A21. 1.14b---n-Bu4NI (CD3CN, 295K): Ka = 64 ± 6 M ( F-NMR)

222

19 Plots of || (ppm) against [n-Bu4NI] (M) for the F-NMR titration of 1.14b. Top: trial #1 (Ka = -1 -1 64.3 M ). Bottom: trial #2 (Ka = 63.5 M ). 19 - Figure A22. F-NMR Job Plot for the 1.14b-I complex (CD3CN, 295K, [1.14b + n-Bu4NI] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.14b with Bu4NI.

-1 Figure A23. 1.19a---n-Bu4NCl (CH3CN, 295K): Ka = 21000 ± 2100 M

Plots of A(λ=369nm) against [n-Bu4NCl] (M) for the UV/Vis titration of 1.19a. Top: trial #1 -1 -1 (Ka = 20033 M ). Bottom: trial #2 (Ka = 21125 M ).

223

1 - Figure A24. H-NMR Job Plot for the 1.19a-Cl complex (CD3CN, 295K, [1.19a + n-Bu4NCl] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19a with Bu4NCl.

19 - Figure A25. F-NMR Job Plot for the 1.19a-Cl complex (CD3CN, 295K, [1.19a + n-Bu4NCl] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19a with Bu4NCl.

224

-1 1 Figure A26. 1.19a---n-Bu4NBr (CD3CN, 295K): Ka = 6600 ± 700 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NBr] (M) for the H-NMR titration of 1.19a. Top: trial #1 (Ka -1 -1 = 5472.0 M ). Bottom: trial #2 (Ka = 7682.3 M ).

1 - Figure A27. H-NMR Job Plot for the 1.19a-Br complex (CD3CN, 295K, [1.19a + n-Bu4NBr] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19a with Bu4NBr.

225

-1 19 Figure A28. 1.19a---n-Bu4NBr (CD3CN, 295K): Ka = 6300 ± 600 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NBr] (M) for the F-NMR titration of 1.19a. Top: trial #1 (Ka -1 -1 = 5283.3 M ). Bottom: trial #2 (Ka = 7378.4 M ).

19 - Figure A29. F-NMR Job Plot for the 1.19a-Br complex (CD3CN, 295K, [1.19a + n-Bu4NBr] = 5 mM)

226

19 Plot of |·χ| against χ for the F-NMR titration of 1.19a with Bu4NBr.

-1 1 Figure A30. 1.19a---n-Bu4NI (CD3CN, 295K): Ka = 580 ± 60 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NI] (M) for the H-NMR titration of 1.19a. Top: trial #1 (Ka = -1 -1 589.8 M ). Bottom: trial #2 (Ka = 573.6 M ).

1 - Figure A31. H-NMR Job Plot for the 1.19a-I complex (CD3CN, 295K, [1.19a + n-Bu4NI] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19a with Bu4NI.

227

-1 19 Figure A32. 1.19a---n-Bu4NI (CD3CN, 295K): Ka = 540 ± 50 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NI] (M) for the F-NMR titration of 1.19a. Top: trial #1 (Ka = -1 -1 536.8 M ). Bottom: trial #2 (Ka = 537.2 M ).

19 - Figure A33. F-NMR Job Plot for the 1.19a-I complex (CD3CN, 295K, [1.19a + n-Bu4NI] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19a with Bu4NI.

228

-1 Figure A34. 1.19a---n-Bu4NOBz (CH3CN, 295K): Ka = 186000 ± 19000 M (UV/Vis)

Plots of A(λ=369nm) against [n-Bu4NOBz] (M) for the UV/Vis titration of 1.19a. Top: trial #1 -1 -1 (Ka = 172930 M ). Bottom: trial #2 (Ka = 198570 M ).

1 - Figure A35. H-NMR Job Plot for the 1.19a-BzO complex (CD3CN, 295K, [1.19a + n- Bu4NOBz] = 5 mM)

229

1 Plot of |·χ| against χ for the H-NMR titration of 1.19a with Bu4NOBz.

19 - Figure A36. F-NMR Job Plot for the 1.19a-BzO complex (CD3CN, 295K, [1.19a + n- Bu4NOBz] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19a with Bu4NOBz.

-1 1 Figure A37. 1.19a---n-Bu4NOTs (CD3CN, 295K): Ka = 1200 ± 120 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NOTs] (M) for the H-NMR titration of 1.19a. Top: trial #1 (Ka -1 -1 = 1202.2 M ). Bottom: trial #2 (Ka = 1183.7 M ).

230

1 - Figure A38. H-NMR Job Plot for the 1.19a-TsO complex (CD3CN, 295K, [1.19a + n- Bu4NOTs] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19a with Bu4NOTs.

-1 19 Figure A39. 1.19a---n-Bu4NOTs (CD3CN, 295K): Ka = 1000 ± 100 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NOTs] (M) for the F-NMR titration of 1.19a. Top: trial #1 -1 -1 (Ka = 1031.4 M ). Bottom: trial #2 (Ka = 972.3 M ).

231

19 - Figure A40. F-NMR Job Plot for the 1.19a-TsO complex (CD3CN, 295K, [1.19a + n- Bu4NOTs] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19a with Bu4NOTs.

-1 1 Figure A41. 1.19a---n-Bu4NHSO4 (CD3CN, 295K): Ka = 470 ± 47 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NHSO4] (M) for the H-NMR titration of 1.19a. Top: trial #1 -1 -1 (Ka = 474.2 M ). Bottom: trial #2 (Ka = 471.1 M ).

232

1 - Figure A42. H-NMR Job Plot for the 1.19a-HSO4 complex (CD3CN, 295K, [1.19a + n- Bu4NHSO4] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19a with Bu4NHSO4.

-1 19 Figure A43. 1.19a---n-Bu4NHSO4 (CD3CN, 295K): Ka = 400 ± 40 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NHSO4] (M) for the F-NMR titration of 1.19a. Top: trial #1 -1 -1 (Ka = 403.8 M ). Bottom: trial #2 (Ka = 391.0 M ).

233

19 - Figure A44. F-NMR Job Plot for the 1.19a-HSO4 complex (CD3CN, 295K, [1.19a + n- Bu4NHSO4] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19a with Bu4NHSO4.

-1 1 Figure A45. 1.19a---n-Bu4NNO3 (CD3CN, 295K): Ka = 300 ± 30 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NNO3] (M) for the H-NMR titration of 1.19a. Top: trial #1 -1 -1 (Ka = 311.0 M ). Bottom: trial #2 (Ka = 287.9 M ).

234

1 - Figure A46. H-NMR Job Plot for the 1.19a-NO3 complex (CD3CN, 295K, [1.19a + n- Bu4NNO3] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19a with Bu4NNO3.

-1 19 Figure A47. 1.19a---n-Bu4NNO3 (CD3CN, 295K): Ka = 270 ± 27 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NNO3] (M) for the F-NMR titration of 1.19a. Top: trial #1 -1 -1 (Ka = 285.6 M ). Bottom: trial #2 (Ka = 257.3 M ).

235

19 - Figure A48. F-NMR Job Plot for the 1.19a-NO3 complex (CD3CN, 295K, [1.19a + n- Bu4NNO3] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19a with Bu4NNO3.

-1 Figure A49. 1.19b---n-Bu4NCl (CH3CN, 295K): Ka = 2500 ± 250 M

Plots of A (λ=369nm) against [n-Bu4NCl] (M) for the UV/Vis titration of 1.19b. Top: trial #1 -1 -1 (Ka = 2584.4 M ). Bottom: trial #2 (Ka = 2496.2 M ).

236

1 - Figure A50. H-NMR Job Plot for the 1.19b-Cl complex (CD3CN, 295K, [1.19b + n-Bu4NCl] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19b with Bu4NCl.

19 - Figure A51. F-NMR Job Plot for the 1.19b-Cl complex (CD3CN, 295K, [1.19b + n-Bu4NCl] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19b with Bu4NCl.

237

-1 1 Figure A52. 1.19b---n-Bu4NBr (CD3CN, 295K): Ka = 460 ± 46 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NBr] (M) for the H-NMR titration of 1.19b. Top: trial #1 (Ka -1 -1 = 467.1 M ). Bottom: trial #2 (Ka = 447.8 M ).

1 - Figure A53. H-NMR Job Plot for the 1.19b-Br complex (CD3CN, 295K, [1.19b + n-Bu4NBr] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19b with Bu4NBr.

238

-1 19 Figure A54. 1.19b---n-Bu4NBr (CD3CN, 295K): Ka = 520 ± 52 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NBr] (M) for the F-NMR titration of 1.19b. Top: trial #1 (Ka -1 -1 = 540.6 M ). Bottom: trial #2 (Ka = 492.6 M ).

19 - Figure A55. F-NMR Job Plot for the 1.19b-Br complex (CD3CN, 295K, [1.19b + n-Bu4NBr] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19b with Bu4NBr.

239

-1 1 Figure A56. 1.19b---n-Bu4NI (CD3CN, 295K): Ka = 59 ± 6 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NI] (M) for the H-NMR titration of 1.19b. Top: trial #1 (Ka = -1 -1 58.6 M ). Bottom: trial #2 (Ka = 59.7 M ).

1 - Figure A57. H-NMR Job Plot for the 1.19b-I complex (CD3CN, 295K, [1.19b + n-Bu4NI] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19b with Bu4NI.

240

-1 19 Figure A58. 1.19b---n-Bu4NI (CD3CN, 295K): Ka = 61 ± 6 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NI] (M) for the F-NMR titration of 1.19b. Top: trial #1 (Ka = -1 -1 65.5 M ). Bottom: trial #2 (Ka = 56.1 M ).

19 - Figure A59. F-NMR Job Plot for the 1.19b-I complex (CD3CN, 295K, [1.19b + n-Bu4NI] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19b with Bu4NI.

241

-1 Figure A60. 1.19b---n-Bu4NOBz (CH3CN, 295K): Ka = 145000 ± 15000 M (UV/Vis)

Plots of A(λ=369nm) against [n-Bu4NOBz] (M) for the UV/Vis titration of 1.19b. Top: trial #1 -1 -1 (Ka = 139410 M ). Bottom: trial #2 (Ka = 150500 M ).

1 - Figure A61. H-NMR Job Plot for the 1.19b-BzO complex (CD3CN, 295K, [1.19b + n- Bu4NOBz] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19b with Bu4NOBz.

242

19 - Figure A62. F-NMR Job Plot for the 1.19b-BzO complex (CD3CN, 295K, [1.19b + n- Bu4NOBz] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19b with Bu4NOBz.

-1 1 Figure A63. 1.19b---n-Bu4NOTs (CD3CN, 295K): Ka = 980 ± 100 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NOTs] (M) for the H-NMR titration of 1.19b. Top: trial #1 -1 -1 (Ka = 1002.1 M ). Bottom: trial #2 (Ka = 955.7 M ).

243

1 - Figure A64. H-NMR Job Plot for the 1.19b-TsO complex (CD3CN, 295K, [1.19b + n- Bu4NOTs] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19b with Bu4NOTs.

-1 19 Figure A65. 1.19b---n-Bu4NOTs (CD3CN, 295K): Ka = 940 ± 94 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NOTs] (M) for the F-NMR titration of 1.19b. Top: trial #1 -1 -1 (Ka = 933.4 M ). Bottom: trial #2 (Ka = 952.2 M ).

244

19 - Figure A66. F-NMR Job Plot for the 1.19b-TsO complex (CD3CN, 295K, [1.19b + n- Bu4NOTs] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19b with Bu4NOTs.

-1 1 Figure A67. 1.19b---n-Bu4NHSO4 (CD3CN, 295K): Ka = 350 ± 35 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NHSO4] (M) for the H-NMR titration of 1.19b. Top: trial #1 -1 -1 (Ka = 377.0 M ). Bottom: trial #2 (Ka = 325.3 M ).

245

1 - Figure A68. H-NMR Job Plot for the 1.19b-HSO4 complex (CD3CN, 295K, [1.19b + n- Bu4NHSO4] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19b with Bu4NHSO4.

-1 19 Figure A69. 1.19b---n-Bu4NHSO4 (CD3CN, 295K): Ka = 400 ± 40 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NHSO4] (M) for the F-NMR titration of 1.19b. Top: trial #1 -1 -1 (Ka = 394.1 M ). Bottom: trial #2 (Ka = 399.9 M ).

246

19 - Figure A70. F-NMR Job Plot for the 1.19b-HSO4 complex (CD3CN, 295K, [1.19b + n- Bu4NHSO4] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19b with Bu4NHSO4.

-1 1 Figure A71. 1.19b---n-Bu4NNO3 (CD3CN, 295K): Ka = 250 ± 25 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NNO3] (M) for the H-NMR titration of 1.19b. Top: trial #1 -1 -1 (Ka = 252.6 M ). Bottom: trial #2 (Ka = 250.2 M ).

247

1 - Figure A72. H-NMR Job Plot for the 1.19b-NO3 complex (CD3CN, 295K, [1.19b + n- Bu4NNO3] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.19b with Bu4NNO3.

-1 19 Figure A73. 1.19b---n-Bu4NNO3 (CD3CN, 295K): Ka = 260 ± 26 M ( F-NMR)

19 Plots of || (ppm) against [n-Bu4NNO3] (M) for the F-NMR titration of 1.19b. Top: trial #1 -1 -1 (Ka = 270.6 M ). Bottom: trial #2 (Ka = 257.2 M ).

248

19 - Figure A74. F-NMR Job Plot for the 1.19b-NO3 complex (CD3CN, 295K, [1.19b + n- Bu4NNO3] = 5 mM)

19 Plot of |·χ| against χ for the F-NMR titration of 1.19b with Bu4NNO3.

-1 Figure A75. 1.23---n-Bu4NCl (CH3CN, 295K): Ka = 2300 ± 230 M

Plots of A(λ=369nm) against [n-Bu4NCl] (M) for the UV/Vis titration of 1.23. Top: trial #1 (Ka -1 -1 = 2325.8 M ). Bottom: trial #2 (Ka = 2221.0 M ).

249

1 - Figure A76. H-NMR Job Plot for the 1.23-Cl complex (CD3CN, 295K, [1.23 + n-Bu4NCl] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.23 with Bu4NCl.

-1 1 Figure A77. 1.23---n-Bu4NBr (CD3CN, 295K): Ka = 460 ± 46 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NBr] (M) for the H-NMR titration of 1.23. Top: trial #1 (Ka = -1 -1 450.3 M ). Bottom: trial #2 (Ka = 474.0 M ).

250

1 - Figure A78. H-NMR Job Plot for the 1.23-Br complex (CD3CN, 295K, [1.23 + n-Bu4NBr] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.23 with Bu4NBr.

-1 1 Figure A79. 1.23---n-Bu4NI (CD3CN, 295K): Ka = 55 ± 6 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NI] (M) for the H-NMR titration of 1.23. Top: trial #1 (Ka = -1 -1 58.2 M ). Bottom: trial #2 (Ka = 51.7 M ).

251

1 - Figure A80. H-NMR Job Plot for the 1.23-I complex (CD3CN, 295K, [1.23 + n-Bu4NI] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.23 with Bu4NI.

-1 Figure A81. 1.23---n-Bu4NOBz (CH3CN, 295K): Ka = 66000 ± 6600 M (UV/Vis)

Plots of A(λ=369nm) against [n-Bu4NOBz] (M) for the UV/Vis titration of 1.23. Top: trial #1 -1 -1 (Ka = 54733 M ). Bottom: trial #2 (Ka = 77517 M ).

252

1 - Figure A82. H-NMR Job Plot for the 1.23-BzO complex (CD3CN, 295K, [1.23 + n-Bu4NOBz] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.23 with Bu4NOBz.

-1 1 Figure A83. 1.23---n-Bu4NOTs (CD3CN, 295K): Ka = 550 ± 55 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NOTs] (M) for the H-NMR titration of 1.23. Top: trial #1 (Ka -1 -1 = 532.4 M ). Bottom: trial #2 (Ka = 563.7 M ).

253

1 - Figure A84. H-NMR Job Plot for the 1.23-TsO complex (CD3CN, 295K, [1.23 + n-Bu4NOTs] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.23 with Bu4NOTs.

-1 1 Figure A85. 1.23---n-Bu4NHSO4 (CD3CN, 295K): Ka = 230 ± 23 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NHSO4] (M) for the H-NMR titration of 1.23. Top: trial #1 -1 -1 (Ka = 230.2 M ). Bottom: trial #2 (Ka = 237.9 M ).

254

1 - Figure A86. H-NMR Job Plot for the 1.23-HSO4 complex (CD3CN, 295K, [1.23 + n- Bu4NHSO4] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.23 with Bu4NHSO4.

-1 1 Figure A87. 1.23---n-Bu4NNO3 (CD3CN, 295K): Ka = 160 ± 16 M ( H-NMR)

1 Plots of || (ppm) against [n-Bu4NNO3] (M) for the H-NMR titration of 1.23. Top: trial #1 (Ka -1 -1 = 160.8 M ). Bottom: trial #2 (Ka = 156.1 M ).

255

1 - Figure A88. H-NMR Job Plot for the 1.23-NO3 complex (CD3CN, 295K, [1.23 + n-Bu4NNO3] = 5 mM)

1 Plot of |·χ| against χ for the H-NMR titration of 1.23 with Bu4NNO3.

256

Appendix B: 1H, 19F and 13C NMR Spectra

1 H NMR (300 MHz, CDCl3)

19 F NMR (282 MHz, CDCl3)

257

1.10

258

1.14a

259

1.14b

260

1.19a

261

1.19b

262

1.23

263

1 H NMR (300 MHz, CDCl3)

19 F NMR (282 MHz, CDCl3)

264

1 H NMR (300 MHz, CDCl3)

19 F NMR (282 MHz, CDCl3)

265

1 H NMR (300 MHz, DMSO-d6)

13 C NMR (75 MHz, DMSO-d6)

266

1 H NMR (300 MHz, CDCl3)

13 C NMR (126 MHz, DMSO-d6)

267

19 F NMR (282 MHz, CDCl3)

1 H NMR (300 MHz, CDCl3)

268

13 C NMR (126 MHz, DMSO-d6)

19 F NMR (282 MHz, CDCl3)

269

1 H NMR (400 MHz, CD3CN)

1 H NMR (300 MHz, DMSO-d6)

270

1 H NMR (300 MHz, DMSO-d6)

13 C NMR (126 MHz, DMSO-d6)

271

19 F NMR (282 MHz, DMSO-d6)

1 H NMR (300 MHz, DMSO-d6)

272

13 C NMR (126 MHz, DMSO-d6)

19 F NMR (282 MHz, DMSO-d6)

273

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

274

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

275

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

276

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

277

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

278

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

279

1 H NMR (400 MHz, DMSO-d6)

1 H COSY (400 MHz, DMSO-d6)

280

13 C NMR (75 MHz, DMSO-d6)

1 H NMR (300 MHz, CDCl3)

281

1 H COSY (300 MHz, CDCl3)

1 H NMR (300 MHz, CDCl3)

282

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

283

13 C NMR (100 MHz, CDCl3)

1 H NMR (400 MHz, CDCl3)

284

1 H COSY (400 MHz, CDCl3)

1 H NMR (300 MHz, CDCl3)

285

13 C NMR (100 MHz, CDCl3)

1 H NMR (300 MHz, CDCl3)

286

1 H COSY (300 MHz, CDCl3)

13 C NMR (75 MHz, CDCl3)

287

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

288

13 C NMR (100 MHz, CDCl3)

1 H NMR (300 MHz, CDCl3)

289

1 H COSY (300 MHz, CDCl3)

1 H NMR (500 MHz, CD3OD-d4)

290

1 H COSY (500 MHz, CD3OD-d4)

13 C NMR (126 MHz, CD3OD-d4)

291

1 H NMR (400 MHz, CD3OD-d4)

1 H COSY (400 MHz, CD3OD-d4)

292

13 C NMR (100 MHz, CD3OD-d4)

1 H NMR (500 MHz, CDCl3)

293

1 H COSY (500 MHz, CDCl3)

13 C NMR (126 MHz, CDCl3)

294

1 13 H- C HSQC (500 MHz, CDCl3)

1 H NMR (400 MHz, CDCl3)

295

1 H COSY (400 MHz, CDCl3)

13 C NMR (75 MHz, CDCl3)

296

1 H NMR (400 MHz, CDCl3)

1 H COSY (400 MHz, CDCl3)

297

13 C NMR (100 MHz, CDCl3)