MECHANISTIC INSIGHTS TOWARDS NEW REACTIONS AND MATERIALS
A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF CHEMISTRY
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Matthew Karl Kiesewetter
October 2010
© 2011 by Matthew Karl Kiesewetter. All Rights Reserved. Re-distributed by Stanford University under license with the author.
This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/
This dissertation is online at: http://purl.stanford.edu/dk657vx4442
ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Robert Waymouth, Primary Adviser
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Eric Kool
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Barry Trost
Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education
This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives.
iii ABSTRACT
Catalysis is the enabling science of polymer synthesis, and new catalytic mechanisms yield new materials. The development of organocatalysts for polymer synthesis has, particularly in the last decade, spawned an impressive array of new catalysts, processes and mechanistic insights. While the focus of most recent research in organocatalysis has concentrated on enantioselective synthesis of small molecules, organocatalysis offers a number of opportunities in polymer synthesis and was among the earliest methods of catalyzing the synthesis of polyesters. The enthalpy of ring opening of cyclic esters or carbonates drives the majority of organocatalytic polymerization reactions catalyzed by a still-evolving array of organocatalysts. Organocatalysts are thought to effect polymerization of cyclic esters by several mechanisms. Some proceed via a monomer activated mechanism whereby the catalyst activates the cyclic ester towards transesterification to the polymer chain. Others operate by an alcohol activation mechanism where the alcoholic end group of the growing polymer chain is activated to induce transesterification. Some are thought to be operative by a combination of these mechanisms. The unique reactivity offered by organocatalysts has provided access to precisely controlled macromolecular architectures and well-defined (co)polymers including a wide array of functionality. The notion that rate must be sacrificed to implement organocatalysts is fading with the discovery of transesterification organocatalysts that rival in reaction rate even the most active metal-containing catalysts. Cyclopentadienyl ruthenium complexes with quinaldic acid-type ligands are robust allylation catalysts in alcoholic solvents, but they are sensitive to dissolved oxygen, requiring reactions to be conducted in an inert atmosphere. Moving from alcoholic to neat aqueous solvents decreases the rate of deallylation but allows the reaction to be conducted in air without loss of catalyst activity over the course of the reaction. These complexes are also effective for the formation of allyl ethers, allowing the synthesis of poly(2,5-dihydrofuran) from the condensation polymerization of 2- cis-butene-1,4-diol. This material was previously deemed inaccessible via traditional polycondensation catalysts.
iv PREFACE
Chapter 1 is a review of organocatalysis as it pertains to the polymer synthesis with a particular emphasis on ring opening polymerization. It is intended to give the reader a background in concepts pertaining to metal-free polymer synthesis and a comparison between those methods. This article was published as a Perspective in Macromolecules and was co-authored by Eun Ji Shin, Dr. James L. Hedrick and Professor Robert M. Waymouth. Chapter 2 pertains to the peculiar reactivity of a single N-heterocyclic carbene and the specialized polymer architectures that can be generated from this catalyst and specialized initiators. The initial work on this system was performed by Andrew Mason, Prof. Philippe Dubois and Dr. Olivier Coulembier. The synthesis of the macromolecular initiators, initial characterization (MALDI, and Mw versus time plots) was performed by Andrew Mason and Dr. Olivier Coulembier. This chapter is largely reproduced from an Angewandte Chemie communication. Chapter 3 concerns the marked increase in reactivity of 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) relative to another guanidine catalyst 1,4,6- triazabicyclo[3.3.0]oct-4-ene (TBO). The reactivity of the two species in amidation and polymerization reactions is explored and a kinetic analysis is performed. The synthesis of TBO and the polymerization reactions with this catalyst were performed by Dr. Marc Scholten. The kinetic analyses and reagent screening with TBD were formed by undergraduate researchers Ryan Weber and Nicole Kirn, but the experimental designs were my own. This chapter is largely reproduced from a J. Org. Chem. publication. Chapter 4 outlines the synthesis of the first in a new class of molecular transporters made by the ring-opening polymerization of functionalized carbonates. The initial syntheses of 4.4, 4.3 and 4.6a-c were performed by Dr. Fredrik Nederberg. I devised the synthesis of 4.4b and 4.6d-e and synthesized all of 4.6 used in the biological assays for publication. I performed all oligomerization reactions for materials used in the final J. Am. Chem. Soc. publication. The experiments and syntheses pertaining to quinine were performed by undergraduate researcher Justin
v Edward (with the exception of the MTC-quinine synthesis which I performed), but the experimental designs were my own. The syntheses of 4.8, 4.11, and 4.12 were my own design and execution. All other syntheses were performed by Christina Cooley or Brian Trantow as were all biological experiments. The text of the original communication was a collaborative effort between all co-authors (see the printed communication: J. Am. Chem. Soc. 2009, 131, 16401). Chapter 5 details the kinetic analysis of a deprotection of allyl alcohol reaction by a cyclopentadienyl ruthenium kynurenic acid allylation catalyst that is stable in air and water. A solid supported version of the catalyst is also discussed. All the work in this chapter was performed by me. At the time of printing, this chapter was submitted to Organometallics for publication. Chapter 6 reports the synthesis of a heretofore unobtainable polymer from cis- 2-butene-1,4-diol formed with a Ru allylation catalyst. All of the experiments described in this chapter were my own design although some experiments (small molecule allylation reactions, lactide polymerizations and a repeated catalyst synthesis) were performed by Justin Edward. At the time of printing, this chapter was in final preparation for submission to J. Am. Chem. Soc. Chapter 7 outlines out attempts to characterize the anion radicals of several N- heterocyclic carbenes. The work in the chapter was performed by me.
vi ACKNOWLEDGEMENTS It is always the solvent. I have many people to whom I owe a great deal of credit. To not miss a single person would exceed the confines a reasonable acknowledgements section. First, I would like to thank my committee for all the hard work they have done for me: Prof. Wender (my de facto co-advisor on Chapters 4 and 5) and Prof. Contag, and especially Profs. Kool and Trost for serving on my committee for 4 years. I would like to express my gratitude to Prof. Robert (Bob) Waymouth. His enthusiasm for ‘cool’ science and his desire to always take a project to the next level are unbelievably motivating. His sense of humor is a source of joy in the lab and has always driven me to come up with some new ‘cute’ comment for Bob. Further, Bob possesses the best quality in an advisor: you can always hear him coming. As I construct the section on my wonderful lab mates, I come to realize the trait that I most cherish in a person is their ability to take a joke. I will leave to others the commentary about what that says about me. However, the quality of coworkers that I have shared lab space with over the years far defies the statistics of random coworker generators. Dr. Nahrain Kamber became a good friend in the couple years that we were bench neighbors, and her influence has outlasted her physical presence in my life. Dr. David Pearson and I have the ability to find humor and trouble in far more obscure places than the average duo. I will never forget tremendous fun we have had or the Variac that paid the ultimate price. Eun Ji (Eunj) Shin is one of the kindest people that I have ever met, and consequently one of the people that I tease the least. Once I finally got her to speak, we quickly became good friends. I will miss our therapy sessions. It has been fun watching people grow as chemists over the years. I would like to thank Hayley(meister) Brown for knowing all the rules. I trust that Kristen Brownell’s relationship with gravity will improve even more with time, and Jeff Simon (the youngest Waymite) has the drive to succeed in graduate school. Even though they do not need it, I wish these people the best of luck at getting what they want in life.
vii Over my years at Stanford, I have had the opportunity to work with several undergraduate researchers from whom I may have learned more than I taught. Ryan Weber was (and continues to be) a gifted scientist, and I was forced to learn so much just to stay a couple of paces ahead of him. Ryan so enjoyed his time with us that he sent his friend, Ray Gipson, to us the next summer. Nicole Kirn became probably my best friend from among all my undergraduate cohorts. I am sad that Nicole did not like our country as much as we liked having her in it. Justin Edward is a typical Stanford pre-med, but I tried not to hold it against him. It was a great joy to watch Justin evolve from a Freshman, virtually without chemistry skills, into the great scientist that he has become. Justin’s subtle synthetic technique and quiet disposition mirrors my own, but he is just so cute that you cannot get too upset. There are many friends from outside my Stanford world that continue to play an important role in my life. Certainly one of my best friends, Nate Miller has been a source of entertainment and companionship since our undergraduate days at Illinois State (ISU). Things are never calm or quiet when Nate is around, and, unfortunately, few people around the two of us have any idea what we are talking about. They, perhaps, do not realize that neither do we. Audrey Butcher has been a long time friend, and I have no doubt that we will continue to be each other’s cheerleader. Thanks Maekers (Shirleyan, Val, Christine and Ruth) for all the afternoons and evenings I was a fixture in your living room. Many of my friends from prior to college were my teachers: Deb Voorhees (how do you always win at Scrabble?), Carl Byers (I think we should all be more like Mr. Byers), Mary Sutter (who eased my transition from Bloomington High to ISU) and Mike Gebhard (the source of all my Egyptian slave labor analogies). I owe a special thanks to Angie Lawrence who took the time to feed my addiction to chemistry even after I finished her sophomore chemistry class. Before Mrs. Lawrence I wanted to teach science, but afterwards there was only chemistry. I will never be able to repay Distinguished Prof. Cheryl Stevenson (ISU) for the impact that she has had on my life. Cheryl’s scientific enthusiasm sparked an interest in me to pursue the degree of which this document is the culmination. The
viii convergence of the right people at the right time produced for me at least a moderately successful undergraduate research career that, I think without qualification, took me to Stanford and the impacts of which will last far beyond this place. I will long remember those times when we puled about the latest reviews as we quaffed those kind nepenthes. The day that she retired from teaching, Illinois State lost the best instructor in the institution. I also owe a great deal of thanks to my co-advisor from Illinois State, Prof. Richard (Dick) Reiter. Dick is, without overstating the matter, the world’s expert on EPR spectrum simulations. The patience that he displays while looking at those spectra is simply inspiring, and exactly what he sees when he looks a spectrum is at the heart of the indescribable thing that he possesses which makes him so great. I want to acknowledge him for his input on the EPR simulations in Chapter 7; it is much appreciated. Lastly, I would like to thank my family. My big sister, Michelle Kiesewetter, was a constant support to me when we were growing up. She pretended that she did not care about my well being, but I caught her doing just that every now and then. I noticed, and I appreciate it. My parents, Pamela and Terrance Kiesewetter, were always amazingly supportive of my adventures in kindergarten science and my lingering fascination with spontaneous, highly exothermic, reactions. My dad, especially, supported my budding scientific curiosity as a child. I remember when he showed me the glow of a bottle of tonic in the sunlight – I was hooked. My family’s knack for finding humor in almost every situation has served well in life even if it has gotten me into trouble every now and then. The last thing on my mind when I went of to California for graduate school was getting married. On my first visit to the Waymouth lab, I met my future wife. I recall that I was immediately smitten and that Elizabeth was not. We have had an incredible adventure together so far, and the Drs. Kiesewetter are headed off to new adventures in far colder environs. The places one goes in life and the milestones one passes do not seem as important as the people with whom they are shared. Liz and I agree that there may be no better place to live in this country than this little spot of the San Francisco peninsula, and we have certainly cherished our time here. Stanford will
ix always hold a special place in our hearts. Perhaps we will return to this place some day. NMR is not a verb.
Matt Kiesewetter Stanford, CA September 24, 2010
x TABLE OF CONTENTS
ABSTRACT iv PREFACE v ACKNOWLEDGEMENTS vii LIST OF TABLES xvi LIST OF FIGURES xvii LIST OF SCHEMES xx SYMBOLS AND ABBREVIATIONS xxii
CHAPTER 1: Organocatalysis: Opportunities and Challenges for 1 Polymer Synthesis 1.1 Introduction 2 1.2 Electrophilic Monomer Activation 5 1.3 Nucleophilic Monomer Activation. 5 1.4 Initiator or Chain-end Activation by a General Base 6 1.5 Bifunctional Activation of Monomer and Initiator/Chain-End 6 1.6 Organic Acids 7 1.7 Pyridine Bases and Nucleophiles 9 1.8 Phosphine and Carbene Bases and Nucleophiles 12 1.9 Strong Neutral Bases: Phosphazenes. 17 1.10 Nitrogen Bases 18 1.11 Bifunctional Activation 19 1.11.1 Thiourea / Amines 19 1.11.2 1,5,7-triazabicyclododecene (TBD) 21 1.12 Chemoselectivity, Substrate Tolerance 23 1.13 Architectural Control 25 1.14 Conclusions 28 1.15 References 31
xi CHAPTER 2: A Distinctive Organocatalytic Approach to Complex Macromolecular Architectures 45 2.1 Introduction 44 2.2 Results and Discussion 45 2.3 Conclusion 49 2.4 Experimental Section 49 2.4.1 General Considerations 49 2.4.2 Cyanoethylation of hydroxy-terminated PEG 50 2.4.3 Cyanoethylation of amine-terminated PEG 50 2.4.4 General procedure for nitrile reduction 51 2.4.5 Synthesis of diamine initiator 51 2.4.6 Synthesis of tetra-amine initiator 51 2.4.7 General polymerization procedure 52 2.4.8 ε-caprolactam initiated polymerization of lactide 53 2.5 References 56
CHAPTER 3: Cyclic Guanidine Organic Catalysts: What Is Magic About Triazabicyclodecene? 59 3.1 Introduction 60 3.2 Results and Discussion 63 3.2.1 Kinetics and Mechanism 66 3.2.2 Effect of Catalyst Structure 68 3.2.3 Lactide Polymerization 71 3.3 Conclusion 72 3.4 Experimental Section 73 3.4.1 General Considerations 73 3.4.2 Procedure for Kinetic Experiments 73 3.4.2.1 Kinetic and Thermodynamic Data 74 3.4.3 Typical Substrate Screening Experiment 77 3.4.4 Synthesis of n-butylacetamide (solution) 78
xii 3.4.5 Synthesis of n-butylacetamide (neat) 79 3.4.6 Synthesis of (S)-n-butyl-2-hydroxypropanamide 79 3.4.7 Synthesis of the Mosher Ester 79 3.4.8 Synthesis of (S)-naproxen methyl ester 79 3.4.9 Synthesis of rac-naproxen methyl ester 80 3.4.10 Synthesis of (S)-naproxen butyl amide 80 3.4.11 Synthesis of 1,4,6-triazabicyclo[3.3.0]-oct-4-ene (TBO) 81 3.4.12 Synthesis of acyl-TBO 84
3.4.13 Polymerization of L-LA Using TBO catalyst 84 3.4.14 Computational Details 84 3.5 References 87
CHAPTER 4: Oligocarbonate Molecular Transporters: Oligomerization- Based Syntheses and Cell-Penetrating Studies 93 4.1 Introduction 94 4.2 Results and Discussion 95 4.3 Conclusion 101 4.4 Preliminary Results for Future Directions 102 4.5 Experimental Section 106 4.5.1 General Considerations 106 4.5.2 Synthesis of dansyl initiator 107 4.5.3 2-(tritylthio)ethanol 107 4.5.4 MTC-ethylguanidine-BOC 108 4.5.5 Synthesis of Oligomers 109 4.5.6 Synthesis of PMTC-guanidines 111 4.5.7 Synthesis of Luciferin Oligomers 113 4.5.8 Synthesis of Dansyl-r8 114 4.5.9 Octanol-Water Partitioning 116 4.5.10 Cellular Uptake Assays by Flow Cytometry 118 4.5.11 Cellular Uptake Assay at 4°C or in the Presence of Sodium Azide 118
xiii 4.5.12 Cellular Uptake Assay, High Potassium [K+] Buffer 118 4.5.13 Fluorescence Microscopy Studies 122 4.5.14 Cellular Assays for Luciferin Release 124 4.5.15 Hydrolytic Stabilities of the Dansylated Conjugates 124 4.5.17 Hydrolytic Stabilities and Luciferin Release of the Luciferin 126 4.5.18 Synthesis of MTC-pyrene 126 4.5.19 Synthesis of MTC-quinine 127 4.5.20 Synthesis of PMTC-quinine 128 4.5.21 Initiation of Oligomerization from Quinine 129 4.5.22 Initiation of Oligomerization from Taxol 129 4.5.23 MTC/amine “Click” Reaction 131 4.6 References 132
CHAPTER 5: Kinetics of an Air and Water Stable Ruthenium(IV) Catalyst for the Deprotection of Allyl Alcohols 135 5.1 Introduction 136 5.2 Results and Discussion 136 5.3 Conclusion 146 5.4 Experimental Section 146 5.4.1 General considerations 146 5.4.2 Synthesis of kynurenic acid allyl ester 146 5.4.3 Synthesis of methoxy substituted kynurenic acid allyl ester 147 5.4.4 Synthesis of RuIV catalyst 147 5.4.5 Synthesis of ligand for solid supported catalyst 148 5.4.6 Attachment of ligand to solid support 148 5.4.7 Synthesis of solid supported catalyst 149 5.4.8 Kinetic analysis to give the Ru loading PS bead 149 5.4.9 Determining Equilibrium 150 5.4.2 Kinetic Data 152 5.5 References 154
xiv
CHAPTER 6: Poly(2,5-dihydrofuran) from 2-cis-butene-1,4-diol and a Ruthenium Allylation Catalyst 157 6.1 Introduction 158 6.2 Results and Discussion 161 6.3 Conclusions 166 6.4 Experimental Section 167 6.4.1 General Considerations 167 6.4.2 General polymerization experiment 167 6.4.3 2-trans-1,4-butenediol 169 6.4.6 Reaction of CpRuIV-allyl with 2-cis-butene-1,4-diol 171 6.4.7 ROP of LA from the PDHF macroinitiator 171 6.4.8 2-cis-butene-1,4-diol Polymerization Experiment with Drying 172
6.4.8 Allylation of CD3OD with 2-cis-butene-1,4-diol 173
6.4.8 Allylation of CD3OD with 2-cis-pentene-1-ol 173 6.5 References 173
CHAPTER 7: Alkali Metal Reductions of N-Heterocyclic Carbenes and Their HCl Salts 176
7.1 Introduction 177 7.2 Results and Discussion 179 7.2.1 Reductions of NHCs and their HCl Salts 179 7.2.2 Decomposition Products 183 7.3 Conclusion 185 7.4 Experimental Section 185 7.4.1 General Considerations 185 7.4.2 Example Reduction Experiment and Quenching 186 7.5 References 187
xv LIST OF TABLES
Table 2.1 Polymerization of Lactide from Amine and Alcohol Initiators. 54 Table 3.1 Substrate Screening for the TBD Catalyzed Amidation of Esters 65 Table 3.2 Polymerization of L-lactide (L-LA) with TBO 72 Table 4.1 Oligomerization from Taxol with 4.3 103 Table 4.2 Synthesis and Characterization of 4.3 oligomers 110 Table 4.3 Toxicity and Stability of Oligoguanidines 4.6a-c 131 Table 4.4 Stability and Release of Luciferin oligomers 4.7a-b 132
xvi LIST OF FIGURES
Figure 1.1 Representative Organic Catalysts and Initiators 8 Figure 1.2 MTC-OR Monomers Accessible from MTC-OH 24 Figure 2.1 Reversible E-H insertion reactions of 2.1 45
Figure 2.2 Comparison of rates of LA polymerization with PEO-(NH2)2 and
PEO-(OH)2 catalyzed by 2.1 48 Figure 2.3 MALDI-TOF of PLA initated from 4-pyrene-methylamine 53
Figure 2.4 Chart of Molecular weight and Mw/Mn versus conversion 54 1 Figure 2.5 H-NMR (CDCl3) of PLA2-PEO-PLA2 54 Figure 2.6 First-order plots for the rac-lactide polymerization 55 Figure 2.7 GPC of lactide polymerization carried out with an equimolar
mixture of PEO-(NH2)2 and PEO-(OH)2 56 Figure 3.1. Nucleophilic and basic organocatalysts for ROP 60 Figure 3.2. ROP of lactide with TBD is even faster than with NHC's 61 Figure 3.3 Model studies demonstrating the acylating ability of TBD 62 Figure 3.4 Hydrogen-bonding mechanism suggested by theoretical studies 63 Figure 3.5 Catalytic amidation of esters 64 Figure 3.6 Proposed mechanism for formation of n-butylacetamide from benzyl acetate and butylamine 67 Figure 3.7 Generation of Acyl-TBD and acylation with amines and alcohols 68 Figure 3.8 B3LYP/6-31G* calculated geometries of ATBD and ATBO 71 -1 Figure 3.9 Determining kobs(TBD) from kobs vs [alcohol] 74
Figure 3.10 Determining k2(TBD) from kobs vs. [amine]o 74
Figure 3.11 kobs vs. [TBD]o demonstrates first order in TBD 75
Figure 3.12 Determining k-1(TBD) 75 Figure 3.13 One of the [ROH] vs time used to construct Figure 3.9 76 Figure 3.14 Temperature dependent equilibrium between benzyl acetate/TBD and benzyl alcohol/acyl-TBD 76
Figure 3.15 Determining kobs(TBO) 77
Figure 3.16 Determining k2(TBO) from ln([acylTBO]/[ acylTBO]o) vs time 77
xvii Figure 3.17 1H-NMR spectra of the TBD-catalyzed reaction of
benzyl acetate with n-butylamine in toluene-d8 t minutes after the addition of butylamine. 78 1 Figure 3.18 H-NMR spectrum of acyl-TBO in CDCl3 82 13 Figure 3.19 C-NMR spectrum of acyl-TBO in toluene-d8 83 Figure 4.1 Flow cytometry determined cellular uptake 98 Figure 4.2 Fluorescence microscopy images 99 Figure 4.3 Assay for measurement of intracellular luciferin delivery 100 Figure 4.4 Observed bioluminescence from HepG2 cells 101 Figure 4.5 Overlay of RI and UV detector signals of PMTC-guanidine-boc 110 Figure 4.6 1H-NMR spectrum of 4.5b 111 Figure 4.7 Stacked 1H-NMR of MTC-guanidine-boc, PMTC-guanidine-boc, and PMTC-guanidine 112 1 Figure 4.8 H-NMR (D2O) of 4.7a (n=8) 114 1 Figure 4.9 H-NMR (D2O) of 4.7b (n=11) 115 Figure 4.10 1H-NMR of r8 dansyl 116 Figure 4.11 Dansyl ethanol calibration curve in water 117 Figure 4.12 Dansyl ethanol calibration curve in octanol 117 Figure 4.13 Partitioning of 4.6b into the octanol layer 117 Figure 4.14 Concentration dependence of cellular uptake into Jurkat - 1 119 Figure 4.15 Concentration dependence of cellular uptake into Jurkat - 2 120 Figure 4.16 Flow cytometry determined cellular uptake of oligocarbonates - 1 121 Figure 4.17 Flow cytometry determined cellular uptake of oligocarbonates - 2 121 Figure 4.18 Flow cytometry determined cellular uptake of oligocarbonates - 3 122 Figure 4.19 Uptake into Jurkat cells 123 Figure 4.20 1H-NMR MTC-quinine 128 Figure 4.21 1H-NMR of quinine-containing polymers 130 Figure 4.22 1H-NMR of taxol-PMTC 131 Figure 4.23 1H-NMR of the products of the reaction of 4.11 and 4.12 131 Figure 5.1 Plot of allyl methyl carbonate vs. time 138
xviii -1 Figure 5.2 Plot of kobs (M•h ) vs. Ru concentration 139
Figure 5.3 Plot of [allyl methyl carbonate] versus hours in CD3OD 142 Figure 5.4 Plots of [allyl methyl carbonate] versus hours 145 Figure 5.5 IR spectra of PS-supported materials 150
Figure 5.6 Slow approach to equilibrium of Ru complex and D2O 151
Figure 5.7 Slow approach to equilibrium of Ru complex and CD3OD 152 Figure 5.8 1H-NMR spectrum of partially converted allyl methyl carbonate 153 Figure 5.9 Plot of [allyl methyl ether] versus hours 153 Figure 6.1 ESI-MS of the crude polymerization material 162
Figure 6.2 Chain extension of PDHF with L-lactide 163 Figure 6.3 1H-NMR spectra of 2-butene-1,4-diol and PDHF 164 Figure 6.4 1H-1H COSY of PDHF 168 Figure 6.5 ESI of PDHF 169 Figure 6.6 ESI of the supernatant from the polymerization reaction 169 Figure 6.7 13C-NMR spectra of poly(2,5-DHF) and 2-cis-butene-1,4-diol 170 Figure 6.8 13C-NMR spectra of PDHF acquired with power-gated decoupling and gated decoupling 170 Figure 6.9 1H-NMR spectra of allyl alcohol, allyl ether, and a solution containing 6.1 and cis-2-buten-1,4-diol which is partially converted to cis-3-(allyloxy)prop-2-en-1-ol. 171 Figure 6.10 1H-NMR spectrum of 6.3 172 Figure 7.1 X-band EPR signal observed upon the exposure of a THF solution of 7.4 and 18-crown-6 to a K metal mirror in vacuo 180 Figure 7.2 X-band EPR spectra and computer generated simulations 181 Figure 7.3 Destructive decomposition pathways 182 Figure 7.4 Degenerate LUMOs of benzene substituted 183 Figure 7.5 Coupling constants of selected radicals 184 Figure 7.6 Proposed decomposition pathway 184 Figure 7.7 Apparatus used for the reduction of NHCs or their HCl salts 187
xix LIST OF SCHEMES
Scheme 1.1 Coordination-insertion Mechanism for Metal Catalyzed ROP 4 Scheme 1.2 Electrophilic Monomer Activation Mechanism for ROP 5 Scheme 1.3 Nucleophilic Monomer Activation Mechanism for ROP 5 Scheme 1.4 Initiator/Chain-End Activation Mechanism for ROP 6 Scheme 1.5 Bifunctional Activation Using Hydrogen Bonding for ROP 6 Scheme 1.6 ROP of Pivalolactone with Pyridine Initiators 10 Scheme 1.7 DMAP-Catalyzed ROP of LA and lac-OCA to produce PLA 10 Scheme 1.8 Proposed nucleophilic mechanism for ROP of Lactide with DMAP 11 Scheme 1.9 Proposed general-base mechanism for ROP of Lactide with DMAP 11 Scheme 1.10 Zwitterionic Ring-Opening Polymerization of Lactide by IMes 13 Scheme 1.11 Zwitterionic Polymerization of EO 14 Scheme 1.12 NHC-catalyzed poly-Benzoin Condensation 15 Scheme 1.13 Mechanisms of GTP of Acrylic Monomers 16
Scheme 1.14 ROP of TMOSC with Me2IPr 16 Scheme 1.15 Alcohol-activation Mechanism for the BEMP-catalyzed ROP of VL 17
Scheme 1.16 Mechanism for the ROP of Ethylene Oxide by t-BuP4 18 Scheme 1.17 MTBD Reversibly Associates with Benzyl Alcohol but Does Not React with Vinyl Acetate 18 Scheme 1.18 Binding Constants of 1.1a with Selected Monomers 20 Scheme 1.19 ROP of VL by the Dual Activation of Monomer and Initiator by 1.1a and MTBD 20 Scheme 1.20 TBD-catalyzed Acyl-transfer Reaction for Alcohols and Amines 22 Scheme 1.21 Nucleophilic Mechanism for the TBD-catalyzed ROP of LA 22 Scheme 1.22 TBD-catalyzed ROP of LA by the Hydrogen Bonding Mechanism. 23 Scheme 1.23 Synthesis of Guanidinylated Oligocarbonate Molecular Transporters 24 Scheme 1.24 Use of Dual-Function Initiators to generate poly(N,N- dimethylacrylamide)-block-PLA by Tandem NMP-ROP, and poly(vinyl pyridine)-block-PLA by Tandem RAFT-ROP 25 Scheme 1.25 ROP of -lactones using SIMes 27
xx Scheme 1.26 Reversible Activation and Deactivation of SIMes 27 Scheme 1.27 Reversible Activation of Triazolylidene Carbenes 28 Scheme 1.28 Organocatalysts possess the triad of versatility, convenience and functional group tolerance and yield well-defined polymers of precise architecture structures for a specific function 29
Scheme 2.1 Polymerization of LA initiated with PEO-(NH2)2 and PEO-(NH2)4 forming H-shaped and super-H-shaped polymers respectively 47 Scheme 4.1 Synthesis of molecular transporter 96 Scheme 4.2 Targeted Synthesis of a taxol-MoTr 103 Scheme 4.3 Synthesis of Two Oligomers Bearing Quinine Moieties 104 Scheme 4.4 Attachment of a Highly Fluorescent via a “Click” Reaction. 105 Scheme 5.1 Synthesis of Kitamura’s Catalyst 136 Scheme 5.2 Synthesis of the Ru complex 137 Scheme 5.3 Proposed mechanism for catalytic hydrolysis of allyl methyl carbonate in water by RuIV-allyl complex 139 Scheme 5.4 Synthesis of the polystyrene supported Ru complex 144 Scheme 6.1 Conceptual polymerization of butene-1,4-diol 159 Scheme 6.2 Regioselectivity of the polymerization catalyst 165 Scheme 6.3 Polymerization Mechanism 166 Scheme 7.1 The Reduction of Triaz to Yield the Free Carbene 179 Scheme 7.2 Reaction Diagram of the Reduction Products Select NHCs 179
xxi SYMBOLS AND ABBREVIATIONS
[X]eff Effective concentration of X AIBN Azobisisobutyronitrile ATP Adenosine triphosphate aX Hyperfine coupling constant of nucleus X (gauss, G) BEMP 2-tert-butylimino-2-diethylamino-1,3-dimethyl- perhydro-1,2,3-diazaphosphorine Bis-MPA 2,2-bis(methylol)propionic acid CL -caprolactone Cp cyclopentadienyl Cp* Pentamethylcyclopentadienyl DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DCM dichloromethane DHF dihydrofuran DLA D-lactide DMAP 4-(dimethylamino)pyridine DME 1,2-dimethoxyethane DMF Dimethylformamide DMSO Dimethylsulfoxide DP Degree of polymerization (repeat units/initiator) DTT Dithiothreitol EC50 Half maximal effective concentration EDG Electron donating group EO Ethylene oxide EPR Electron Paramagnetic Resonance ESI-MS Electrospray ionization – mass spectrometry FBS Fetal bovine serum GC-MS Gas chromatography – mass spectrometry GR Guanidinium rich GTP Group transfer polymerization Hartrees 1 hartree = 627.509 kcal/mol HPLC High performance liquid chromatography IAd 1,3-bis(diadamantyl)imidazol-2-ylidene IdiMe 1,3-bis(-2,6-dimethylphenyl)imidazol-2-ylidene IMe 1,3-bis(dimethyl)imidazol-2-ylidene IMes 1,3-bis(-2,4,6-trimethylphenyl)imidazol-2-ylidene IPr 1,3-bis(diisopropyl)imidazol-2-ylidene IR Infrared (spectroscopy or spectrum) ItBu 1,3-bis(di-tert-butyl)imidazol-2-ylidene LA Lactide Lac-OCA 5-methyl-1,3-dioxolane-2,4-dione LLA L-lactide LUMO Lowest unoccupied molecular orbital M/I Monomer to initiator ratio
xxii MALDI-TOF Matrix-assisted laser desorption/ionization – time of flight (mass spectrometry) MDR Multi-drug resistant Me2IMe 1,3,4,5-tetramethylimidazol-2-ylidene Me2IPr 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene MeOTf Methyl trifluoromethanesulfonate Mn Number average molecular weight MoTr(s) Molecular transporter(s) MTBD N-methyl-1,5,7-triazabicyclo[4.4.0]dec-1-ene MTC(-OH); (-OR) 5-methyl-2-oxo-1,3-dioxane-5-carboxylic acid; -R ester MTT 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide Mw Weight average molecular weight NHC N-heterocyclic carbene NMP Nitroxide mediated polymerization NMR Nuclear Magnetic Resonance P1-t-Bu N’-tert-butyl-N,N,N’,N’,N”,N”- hexamethylphosphorimidic triamide PBS Phosphate buffered saline PCL Poly(-caprolactone) PDHF Poly(dihydrofuran) PDI Polydispersity index PEG Poly(ethylene glycol) PEO Poly(ethylene oxide) PET Poly(ethyleneterephthalate) Ph2IMes 1,3-bis(-2,4,6-trimethylphenyl)-4,5-diphenylimidazol- 2-ylidene Poly(2,5-DHF) Poly(2,5-dihydrofuran) PPY pyrrolidinopyridine PS polystyrene PTHF Poly(tetrahydrofuran) r8 (R)-octa-arginine RAFT Reversible addition-fragmentation chain transfer polymerization RI Refractive index ROP Ring-opening polymerization RPMI Roswell Park Memorial Institute SIMes 1,3-dimesitylimidazolin-2-ylidene TBD 1,5,7-triazabicyclo[4.4.0]dec-1-ene TBO 1,4,6- triazabicyclo[3.3.0]oct-4-ene t-BuP4 Schwesinger base TGA Thermogravimetric analysis THF Tetrahydrofuran TMC Trimethylene carbonate
xxiii TMOSC 2,2,5,5-tetramethyl-1-oxa-2,5-disilacyclopentane TOF Turnover frequency (s-1) Triaz Triazol-5-ylidene TU Thiourea (generic) VL -valerolactone SR Muon Spin Resonance Electron spin density
xxiv CHAPTER 1
Organocatalysis: Opportunities and Challenges for Polymer Synthesis
Reprinted in part with permission from Macromolecules, 2010, 43, 2093 Copyright 2010 by the American Chemical Society
1 1.1 Introduction Organocatalysis can be traced back at least a century to the enantioselective synthesis of quinine alkaloids.1,2 Many enzymatic reactions are mediated by precise arrays of organic functional groups, and much of the early work was inspired by an effort to understand and mimic the remarkable rates and selectivities of enzymatic catalysis.2-7 The application of chiral organocatalysts for enantioselective synthesis has,1,8,9 particularly in the last decade, spawned an impressive array of new catalysts, processes and mechanistic insights.10-30 While the focus of most recent research in organocatalysis has concentrated on enantioselective synthesis of small molecules, organocatalysis offers a number of opportunities in polymer synthesis and was among the earliest methods of catalyzing the synthesis of polyesters.31 In the following perspective we attempt to highlight the opportunities and challenges in the use of organic molecules as catalysts or initiators for polymerization reactions. The ring-opening polymerization of cyclic monomers will be used as a representative polymerization process to illustrate some of the features of organic catalysts and initiators and to compare them to metal-based approaches. Polymerization can occur by one of two general enchainment processes: step growth or chain growth.32 Ring-opening polymerization is an example of a chain-growth process where repeated addition of the monomer to the chain-end leads to an increase in the molecular weight. The thermodynamics of ring-opening polymerization (ROP) is driven by the enthalpy of ring opening; the kinetics and selectivity of the ring- opening process is strongly influenced by the nature of the reactive chain ends, the monomers, and the presence of catalysts. Catalysis plays a critical role not only in enhancing the rate of chemical reactions, but in controlling the selectivity of the reaction of interest relative to other competing reactions. For fine chemical synthesis, the premium placed on high regio-, chemo-, and stereoselectivities (particularly enantioselectivities) can compensate for modest rates and turnover numbers.29 For polymerization catalysis, rate and turnover number come to the fore, as catalyst residues are often left in the final material due to the difficulty or added cost of separating these impurities from the resultant material.
2 Furthermore, the selectivities critical for the ideal polymer synthesis include those that enable control over the molecular weight, molecular weight distribution, the nature and number of polymer endgroups, the architecture, stereochemistry and topology of the macromolecule (linear, branched, cyclic, degree of crosslinking), and the functionality and sequence of monomers in the chain.33 Indeed, the number of discrete catalytic steps in a ring-opening polymerization that must occur with the correct relative rates to yield a well-controlled reaction is impressive. Under conditions where the rates of initiation and propagation are higher than termination and inter and intra chain reactions, exquisite control over the molecular weight and molecular weight distribution is possible.32 “Living” polymerization reactions are those where termination is absent, enabling control over the molecular weight by control of the monomer/initiator ratio ([M]ₒ/[I]ₒ) and monomer conversion. For living polymerizations where the rate of initiation is faster or comparable to propagation and all other competing reactions are minimized (inter and intra-chain reactions, etc.), very narrow distributions of molecular weights are possible, and a linear relationship between molecular weight and monomer conversion allows for precise tailoring of the molecular weight. Deviations from the living behavior can be attributed to slow initiation or side reactions such as chain transfer and termination reactions,32 and these processes typically result in the broadening of molecular weight distribution (described by the polydispersity index PDI = Mw/Mn, the weight average molecular weight and number average molecular weight, respectively).34-38 If we consider the ring-opening polymerization of lactones as a representative example, a variety of general strategies can be envisioned for enhancing the rate and selectivity of enchainment either catalytically or stoichiometrically. For these reactions, the lactone monomer is electrophilic and the initiator/chain-end is typically a nucleophile such as an alcohol. The rates of these reactions can be increased by activating the monomer, activating the initiator/chain end, or activating both simultaneously.
3 Metal alkoxides represent the most widely used set of catalysts for the ROP of cyclic esters, and they typically operate by a ‘coordination-insertion’ mechanism (Scheme 1.1).39-47 A typical mechanism for these reactions involves activation of the alcohol initiator (or chain-end) by formation of metal-alkoxide. Depending on the Lewis-acidity of the metal (or the availability of open coordination sites), the metal alkoxide can also activate the monomer by binding to the carbonyl (Scheme 1.1). 39-44 In cases where transesterifications of the propagating metal alkoxide are slow, the metal alkoxide functions as an initiator (not a catalyst); however, if the metal alkoxide reacts with alcohols to regenerate a new metal alkoxide, then chain formation is catalytic in M-OR. A wide range of metal complexes, most commonly alkoxides, have been developed using metals such as Al, Mg, Zn, Ca, Sn, Fe, Y, Sm, Lu, Ti, Zr (Turnover frequencies = TOF ~ 0.01~0.1 s-1 for metal alkoxide complexes).39-42 The applications of polyesters in packaging,48 biomedical44 and microelectronic49 applications have motivated efforts to develop more biocompatible metal catalysts50,51 or metal-free organic catalysts.24
O [M] [M] X O [M] O O RO O O [M] O R O [M] O H R RO O R O n H X O R H O H O RO n Scheme 1.1. Coordination-insertion Mechanism for Metal Catalyzed ROP
Organic catalysts and initiators will typically operate by different mechanisms of enchainment than metal alkoxides;24 this diversity of mechanistic pathways has provided new opportunities for the control of polymerization reactions with organic catalysts. In the following, we outline some general strategies for enhancing the rate and selectivity of ring-opening polymerization by the activation of the monomer, activation of the initiator/chain-ends or by cooperative dual activation.
4 1.2 Electrophilic Monomer Activation Electrophiles can activate the monomer toward enchainment. In the case of lactones, activation of the carbonyl by an electrophile facilitates nucleophilic attack by the initiating or propagating alcohol (Scheme 1.2). This has been achieved by protic acids (catalytic) or methylating agents (stoichiometric).
E X E O O H E-X E O X O E-X O O RO O O X ROH R O H H O RO O
E O O O H H O RO O O X RO n E-X Scheme 1.2. Electrophilic Monomer Activation Mechanism for ROP
1.3 Nucleophilic Monomer Activation Nucleophiles can activate the monomer by direct attack on the monomer to generate more reactive chain-carrying intermediates. Protonation of the zwitterionic alkoxide by the initiating or propagating alcohol followed by acylation of the incipient alkoxide leads to the formation of a ring-opened alcohol that can propagate by repeated attack on the activated monomer. This mechanism has been postulated for a variety of nucleophiles including pyridines, imidazoles, phosphines, and N- heterocyclic carbenes.
O O O Nu ROH O Nu Nu OR H O + Nu RO O O OH
Scheme 1.3. Nucleophilic Monomer Activation Mechanism for ROP
5 1.4 Initiator or Chain-end Activation by a General Base Rather than activating the monomer, the initiator or active chain-ends can be activated by a variety of mechanisms. In classical anionic polymerization, the initiator or chain-end is activated by deprotonation to generate an alkoxide,52,53 which is reactive enough to mediate ring-opening even with non-coordinating counterions.54 Attack of the alkoxide at the carbonyl carbon of the monomer is followed by acyl- oxygen bond scission. This forms an ester end-group and an active alcoholate species which reacts with the monomer for further propagation. The high reactivity of alkali metal alkoxides often leads to competitive transesterification. Milder general bases can also activate the initiator or chain-end via H- bonding. By hydrogen bonding to the alcohol, a general base can increase the nucleophilicity of the initiating or propagating alcohol to facilitate nucleophilic attack on the lactone monomer (Scheme 1.4). 55-57
O B B H O O O O R B H R O O H H B + H OR O RO O B O O R Scheme 1.4. Initiator/Chain-End Activation Mechanism for ROP
1.5 Bifunctional Activation of Monomer and Initiator/Chain-End Dual activation of both the monomer and the chain-end is a very effective strategy for enhancing the rate and selectivity of ring-opening polymerization; many of the metal alkoxide catalysts are proposed to operate in this way (Scheme 1.1). The combination of an electrophile to activate the monomer and a general base to activate the initiator/chain-end can activate both partners to effect ring-opening (Scheme 1.5).
B E E B E H O O O R B H O R O O E H H O RO O B O O O R
Scheme 1.5. Bifunctional Activation Using Hydrogen Bonding for ROP
6
The strategies described in Schemes 1.2-1.5 are not mutually exclusive and different catalysts and initiators can operate by a combination of mechanisms; for ring-opening reactions catalyzed or mediated by organic molecules, one or more of these mechanisms may be operative. In the discussion below, we highlight situations where this diversity of mechanistic pathways can provide new opportunities for enhancing the rate of polymerization and influencing the selectivity to generate polymer architectures that are difficult to access by metal-mediated processes. The following discussion will focus primarily on the ring-opening of lactones, but other monomers and processes will also be discussed. Representative organic catalysts and initiators are shown in Figure 1.1.
1.6 Organic Acids The simplest method to effect ROP is by the employment of a strong organic acid.32,58 The polymerization is initiated by the protonation of the monomer and subsequent ring-opening by reaction with a nucleophile, such as an alcohol (initiator). The polymerization propagates by the terminal hydroxyl group of the polymer chain acting as the nucleophile towards the protonated monomer. The use of a catalyst that is free from the propagating polymer (whereas metal alkoxides remain attached to the propagating species) represents a fundamental advantage of this strategy: less than one catalyst per monomer chain.32,59 Stoichiometric activation of the monomer can be achieved with strong methylating agents such as methyl trifluoromethanesulfonate (MeOTf);58 this strategy has proven particularly effective for the ring-opening of 1,3- oxazolin-2-enes32 but for lactones, this procedure requires further optimization to control the molecular weights.
7 N N N N N N N N N H N TBD DBU MTBD DMAP
CF3 CF3 S S rac F3C N N F3C N N H H H H N N 1.1a 1.1 1.1b
N N N N N N P P N P N P N t-Bu N N N N N N N N
Me2IMe BEMP P1-t-Bu P2-t-Bu
N N N N N N
Me IPr IMes SIMes 2
R N N R N N N N N
IMe : R = CH3 IPr : R = CH(CH3)2 ItBu : R = C(CH3)3 Triaz Ph2IMes IAd : R = adamantyl O OH O O O OH HO O HO OH N N N OH R N P N PNP N lactic acid OH citric acid N N N NH2 O OH O OH P amino acids S N N HO O N OH O OH t-BuP p-toluenesulfonic acid 4 tartaric acid Figure 1.1. Representative Organic Catalysts and Initiators
8 Acid catalyzed ROP has a distinct advantage in its simplicity and the wide range of acids available, but as with many cationic processes, the selectivity for propagation relative to other chain-termination or transfer reactions is dictated by the reactivity of the protonated monomer.60 In the presence of an alcohol initiator,
HCl·Et2O polymerizes -valerolactone (VL) and -caprolactone (CL) with controlled 61 molecular weights (Mn ~ 3000) and low polydispersity index (< 1.20). Higher molecular weights could be achieved by increasing the initial monomer 62 concentration. Amino acids (L-alanine, L-leucine, L-phenylalanine, L-proline) have also been used as catalysts for the ROP of CL in the absence of initiators. 1H NMR spectroscopy and titration of carboxyl end group showed that the polymerization was initiated by the amino group of the amino acid.63 Other organic acids such as tartaric acid, lactic acid, citric acid and fumaric acid have been used as catalysts for CL and VL polymerization in the presence of alcohol and carbohydrate initiators.64 The acid catalyzed polymerization of LA with trifluoromethanesulfonic acid (HOTf)58 was faster and more highly controlled in the presence of a protic (alcohol) initiator.65 The low molecular weights, slow rates and high catalyst loadings associated with organic acids is compensated by the operational simplicity of this approach and the 60,65 observation that the polymerization of L-lactide was highly stereospecific.
1.7 Pyridine Bases and Nucleophiles Pyridines are moderate bases and good nucleophiles;66 they have been shown to act as nucleophilic initiators in the zwitterionic ring-opening polymerization of 32,67 - pivalolactone. Linear chains having one pyridinium ion and one CO2 ion as end groups were observed as products, and no cyclic polymers were observed in the MALDI-TOF mass spectra, which suggests that pyridine functions as a nucleophilic initiator by ring-opening the lactone to generate a zwitterionic carboxylate, which propagates by an anionic mechanism (Scheme 1.6).
9 CO2 R N R N O O n R = H, CH3, N(CH3)2 O O
CO O CO O CO CO O CO O CO H + ArOH 2 2 R N n-1 R N n-1 - ArO Scheme 1.6. ROP of Pivalolactone with Pyridine Initiators
The more nucleophilic 4-(dimethylamino)pyridine (DMAP) and 4- pyrrolidinopyridine (PPY), were shown to be very effective for the ROP of lactide (LA) in solution and in the melt.54,68-72 In solution, DMAP loadings on the order of initiator concentration produced PLA up to degree of polymerization (DP) = 100 with PDI < 1.13 in days. PPY was shown to effect the ROP of LA only in the melt and significantly slower than DMAP (20 h vs 20 m).69 DMAP was also shown to be effective for the ROP of substituted lactides and lactide equivalents (Scheme 1.7).73-76 DMAP was originally proposed to react via a nucleophilic monomer activation mechanism (Schemes 1.3 and 1.8),69,77 although subsequent computational studies strongly suggest that an alcohol activation mechanism (either concerted or stepwise) may be operative in the DMAP-catalyzed ROP of lactide (Schemes 1.4 and 1.9).78
O
O O CO2 O O ROH O H RO O O O n Me N N O 2 O DMAP O Scheme 1.7. DMAP-Catalyzed ROP of LA and lac-OCA to produce PLA
10 O
O N N O O O O R O N N ROH N N O OH O O O O OR ROH O O OH N N O Scheme 1.8. Proposed nucleophilic mechanism for ROP of Lactide with DMAP
This mechanistic competition between nucleophilic and general-base mechanisms is a recurrent theme for nucleophilic/basic organic catalysts. Calculations suggest that both pathways are energetically accessible55-57,78 and predict the H- bonded pathway to be lower in energy than the nucleophilic mechanism in the gas phase or in polar aprotic solvents. However, in cases where alcohol initiators are absent (Schemes 1.6 and 1.10) or at low concentration (high monomer/initiator ratio), nucleophilic mechanisms can compete.
N N
N O N N H H H O O N N R R N O O R O H O O O O O O O O O O O ROH O O Scheme 1.9. Proposed general-base mechanism for ROP of Lactide with DMAP
Despite its low monomer scope and slow reaction times, DMAP marked two fundamental advances in the development of polymerization catalysts: 1) these catalysts show a high selectivity for transesterification of the monomer (propagation) relative to the open chain esters of the polymer (chain-shuttling) and 2) these catalysts are compatible with a range of different initiators and co-catalysts. DMAP does not catalyze the transesterification of esters with secondary alcohols, which mitigates transesterification of the PLA backbone by the propagating alcohol endgroups, resulting in narrow polydispersities.68 This idea would be developed further, in much more widely applicable catalysts, later see section 1.11. Also, DMAP was used in the successful ROP of LA from a metalloinitiator when Al(OiPr)3 and Sn(Oct)2 failed to
11 produce the desired polymer.79 This trait, broadly denoted herein as compatibility with functionality, was also to become a defining attribute of organocatalysts, see section 1.12.
1.8 Phosphine and Carbene Bases and Nucleophiles
Phosphines, such as P(Bu)3, PPheMe2, PPh2Me, PPh3, catalyze the ROP of lactide in the presence of an alcohol initiator.80 The substitution on the phosphine controls the reactivity of the catalyst such that the alkyl-substituted phosphines are more active (they are more basic and more nucleophilic) than the aryl-containing ones. Polymerizations are effective at high temperatures (135°C) and in bulk (on the order -1 of 0.01 s for P(Bu)3), which shows the potential for phosphine catalysts to be used in industrial processes.80 N-heterocyclic carbenes (NHCs) are another class of potent neutral bases and nucleophiles.18,81-84 They are widely used in place of phosphines in organometallic complexes.85-88 Early work by Breslow,6 Wanzlick89 Sheehan,90 and Stetter91 demonstrated that they are also potent organocatalysts.18,24 In 2002, the groups of Nolan,92 Hedrick and Waymouth93 demonstrated that NHC’s were potent organocatalysts for transesterification reactions, studies which led to their investigation as catalysts for the ring-opening polymerization of lactones.94,95 The ROP of lactide by the NHC IMes is considerably faster than that catalyzed by DMAP. The polymerization of lactide is extremely rapid (TOF ~ 18 s-1), well- controlled and exhibits features of a living polymerization.24,94-96 A variety of cyclic monomers can be polymerized with NHC’s, including lactones,94-97 cyclic carbonates,98,99 cyclic siloxanes,100,101 acrylates,102-104 dialdehydes,105 and epoxides.106 Both a nucleophilic mechanism (Scheme 1.3)94,95 and a H-bonding alcohol activation mechanism (Scheme 1.4)57 have been proposed for the NHC-catalyzed transesterifications92,93,107 and ring-opening polymerizations of lactide. Theoretical calculations predicted that the H-bond alcohol activation mechanism has a lower barrier than the nucleophilic mechanism.57 Mechanistic studies to test for the viability of the nucleophilic mechanism demonstrated that in the absence of alcohol initiators,
12 the carbene IMes could mediate the zwitterionic ring-opening polymerization of lactide to generate cyclic polylactides (Scheme 1.10).108 These studies strongly support that a nucleophilic activation of the monomer by NHC’s is viable; moreover these studies provided a new strategy to generate well-defined cyclic polyesters. Kinetic and mechanistic investigations108b indicate that the NHC acts as a catalyst/initiator; due to a slower rate of initiation relative to propagation only a small fraction (approx. 30-50%) of the carbenes are converted to active zwitterions which propagate rapidly and extrude the carbene to generate cyclic macrolactones.
O R O O O O N R O O O O N O N R O O O kp O N O O R O 2n Z kc O Z 1 O n k i O N N OO R R N N O n-1 R R O O NN O O O C O n IMes O Scheme 1.10. Zwitterionic Ring-Opening Polymerization of Lactide by IMes
In the presence of alcohol initiators, it is likely that the NHC-catalyzed ROP operates by a combination of both mechanisms, particularly at high monomer/initiator ratios. The carbenes are active for the polymerization of a variety of lactones and the rates and selectivities depend sensitively on both the nature of the carbene and the lactone monomer;24,95 for example, the aryl-substituted carbene IMes is very active for lactide, but much less active for CL. For CL, the more basic and less sterically 97 hindered carbenes Me2IMe and Me2IPr are more effective than IMes. The ring-opening polymerization of ethylene oxide can also be catalyzed by NHC’s such as IPr (Scheme 1.11).106 Under the conditions described (DMSO, 50oC), linear PEO was exclusively obtained, unlike the zwitterionic polymerization of LA
13 described above. A nucleophilic mechanism to generate a zwitterionic imidazol-2- ylidinium alkoxide was proposed. The appropriate choice of the terminating agent gave a variety of ,-difunctionalized PEOs,106 and dendrimer-like PEOs could be obtained by zwitterionic polymerization of EO followed by slow, semi-continuous addition of glycidol and propylene oxide (sequentially or randomly) during the arborization of the PEO chain ends.110
O H O n-1 Nu N O N N NuH O O n N O N N n-1 Nu = N3. OH, Bz
Scheme 1.11. Zwitterionic Polymerization of EO.
In addition to chain-growth ring-opening polymerizations, carbenes are effective for the step-growth polymerization of diesters and diols93 and the depolymerization of polyesters, including poly(ethyleneterephthalate) (PET).93,111 Poly(glycolide) and PCL, biodegradable and commodity polymers, were synthesized by polycondensation of ethyl glycolate and ethyl 6-hydroxyhexanoate, respectively, using IMe generated in situ.93 Carbenes are known to catalyze the benzoin and formoin condensation reactions.18 This reactivity has been exploited for the step-growth polymerization of dialdehydes to obtain polybenzoin polymers.105 Various carbenes, such as IPr, ItBu, IAd and Triaz, were used in the step-growth polymerization of terephthaldehyde to produce poly(1,4-phenylene-1-oxo-2-hydroxyethylene) under mild conditions (THF + DMSO solution, 40oC) (Scheme 1.12).105 Optimization of reaction conditions to achieve higher molecular weights, minimize possible cyclic byproducts and expanding the catalyst and monomer scope are some of the challenges of step-growth polymerization.
14 R' R' O O X X O R R NN N N R NNR R R H X HO H n R' O OH HO OH
O n O n CHO CHO
R' X O O O R NNR H O O OH HO H O m O O H HO H n n+m OH OHC O polybenzoin HO m
CHO Scheme 1.12. NHC-catalyzed poly-Benzoin Condensation
Group transfer polymerization (GTP) employs silyl ketene acetals as initiators in the presence of either nucleophilic or Lewis acid catalysts for controlled 112 102 103,104 polymerization of acrylic monomers. NHCs such as Me2IPr, IPr and ItBu were found to be effective neutral nucleophilic catalysts for the GTP of methacrylates and acrylates. Methylmethacrylate and t-butyl acrylate were successfully polymerized in a controlled manner showing living characteristics enabling synthesis of block copolymers. Such high degree of control was proposed to come from the modulation of the concentration of the propagating enolates via reversible activation/deactivation equilibrium involving dormant bis(enolate) siliconates in the case of the Me2IPr (dissociative mechanism in Scheme 1.13).102 Later, for the case of IPr and ItBu, the associative mechanism (an initiator activation mechanism) was proposed. The first- order dependence of the initial polymerization rate on the initiator concentration and the absence of enolate type species was offered to support the associative mechanism.104
15 Associative
R N N R N N R N N R R R O O O SiMe3 n R' O R = tBu, iPr O OSiMe3 O O R' R' R' OSiMe3 R' = H, Me O OSiMe3 O n O O O O H N H N N O N O Si N N Si O
O OSiMe3
O
H O Si O N O O N Si
Dissociative Scheme 1.13. Mechanisms of Group Transfer Polymerization of Acrylic Monomers
NHCs were also shown to catalyze the ring-opening polymerization of carbosiloxanes in the presence of initiating alcohols.100,101 The ROP of 2,2,5,5- tetramethyl-1-oxa-2,5-disilacyclopentane (TMOSC) with Me2IPr was proposed to occur by an alcohol activation mechanism where the strongly basic NHC activates the alcohol toward nucleophilic attack by H-bonding (Scheme 1.14), but a nucleophilic mechanism is also possible. The ROP reaction of TMOSC with 1 mol% Me2IPr is extremely fast (99 % conv. in 1 min or 1.65 s-1) and yields poly(carbosiloxane) with a molecular weight Mn = 10,200 g/mol and a polydispersity of Mw/Mn = 1.19. The aryl substituted carbene IMes is slower, (80 % after 30 min or 0.044 s-1), but provides a 100 similar degree of control (Mw/Mn = 1.14).
R' Si N O Si Si NN N H NN R' R' Si H O O R' R' ROH R' O Si O Si H R O R R' = iPr R
Scheme 1.14. ROP of TMOSC with Me2IPr.
16 1.9 Strong Neutral Bases: Phosphazenes
The phosphazene bases P1-t-Bu, P2-t-Bu, t-BuP4 and BEMP (Figure 1.2) developed by Schwesinger and Schlemper113,114 are potent neutral bases in aprotic MeCN + DMSO + 115 solvents. ( pKa P1-t-BuH = 27.6, pKa t-BuP4H = 32) These bases are effective catalyst for the ring-opening polymerization of lactones in the presence of MeCN + 116 alcohol initiators. Both BEMP ( pKa BEMPH = 27.6) and P1-t-Bu are active for the polymerization of LA and VL, producing comparable polymers, but BEMP offered enhanced rates (1 day versus several days). The ROP of CL was exceedingly slow (> 10 d for 14% conversion). The BEMP-catalyzed ROP of L-LA was shown to 116 evolve Mn linearly with time and exhibited excellent chain end control, which is consistent with a living polymerization. Based on experimental evidence, an alcohol activated mechanism was proposed whereby the catalyst activates the alcohol toward nucleophilic attack on the monomer, Scheme 1.15. BEMP is inert towards polymer except at high conversion when broadening of PDI occurs due to transesterification of the polymer backbone.116
N N N N N N N N N P P P N P N N N P N N N N N N N H H H O H H O O O O O O O O O O O O R R O R O R R Scheme 1.15. Alcohol-activation Mechanism for the BEMP-catalyzed ROP of VL
These bases have been shown to be effective for the polymerization of siloxanes. The catalyst P1-t-Bu offered improvement in reaction time over alkali metal alkoxide alternatives, as is typical of softer cations, but retained the broad PDIs.114 A similar catalyst, t-BuP4, was shown to be effective for the ROP of ethylene oxide in the presence of acidic initiators (a phenol or benzyl cyanide); in these cases, it was proposed that the phosphazene deprotonates the initator which concomitantly attacks monomer and produces short oligomers (Mw ~3,000) of narrow PDI < 1.09, Scheme 1.16.117 Similar reports were made for propylene oxide monomer.118
17
t-BuP4H O O OH O H n O t-BuP4 H
Scheme 1.16. Mechanism for the ROP of Ethylene Oxide by t-BuP4
1.10 Nitrogen Bases The guanidines and amidines N-methyl-1,5,7-triazabicyclododecene (MTBD, + 115 MeCN + pKa MTBDH = 25.5) and diazabicycloundecene (DBU, pKa DBUH = 24.3)115, have similar basicities. Both MTBD119 and DBU120 are effective for the polymerization of LA, producing polymers of up to DP = 500 with narrow PDI < 1.1 in less than 1 hour (TOF ~ 0.05 s-1). As with the phosphazenes, transesterification of the polymer backbone and accompanying broadening of PDI occurs at high conversion.120 An alcohol activated mechanism was proposed for the MTBD or DBU catalyzed polymerization of LA.120 In such a mechanism, MTBD would activate the initiating alcohol but be inert towards the monomer, and this was shown to be the case in some model reactions. MTBD was shown to associate strongly with benzyl -1 -1 alcohol: Keq=14±2 M at 298 K, H° = -3.82±0.24 kcal mol , S° = -7.17±0.24 cal mol-1, and neither MTBD nor DBU are potent enough nucleophiles to be acylated by vinyl acetate, Scheme 1.17.121-124 While the alcohol activation executed by MTBD and DBU is sufficient for the ROP of LA,100 DBU produced poly(ethylene oxide) in very poor yield;117 neither catalyst is active for the polymerization of BL, VL or CL at up to 20 mol% catalyst loading.120
N K = 14 M-1 N N N N CH N N N N 3 H O H CH3 H O O Ph O Ph Scheme 1.17. MTBD Reversibly Associates with Benzyl Alcohol but Does Not React with Vinyl Acetate
18 1.11 Bifunctional Activation The previous sections have highlighted the selective activation of monomer or initiator, but the dual implementation of an electrophile and nucleophile should allow for the simultaneous activation of both; the attenuation of the strength of each interaction required to effect the transformation can lead to higher selectivities. Catalytic reactions using weak electrophilic interactions (hydrogen bonding) to activate the substrate have been demonstrated for small molecule transformations;21,125 this motif is also a powerful strategy for ring-opening polymerization. For lactones, bifunctional activiation of the monomer by an electrophile and the initiator by a nucleophile has been shown to facilitate the ROP of esters. Both unimolecular and bimolecular catalysts have been employed.
1.11.1 Thiourea / Amines A variety of ureas and thioureas activate carbonyl substrates7,21 in a fashion similar to the hydrogen-bonding motifs in enzyme active sites.7,21,125-130 One particular example combined the hydrogen bonding capabilities of a thiourea (TU) H- bond donor and an amine base in a discrete catalyst, 1.1 (Figure 1.1).127-131 In the
ROP of LA, the thiourea 1.1 produced PLA of narrow PDI (<1.08) whose Mn is dictated by [M]ₒ/[I]ₒ and evolves linearly with time (TOF ~ 0.8 h-1). The thiourea and amine need not be linked; a combination of the thiourea 1.1a and the tertiary amine 1.1b was also active.132 Catalytic activity was modulated by changing the architecture of the thiourea; 1.1a was the most effective of the thioureas tested,121 but amido- indoles can also be used.133 Catalytic activity was significantly augmented when stronger bases are substituted for 1.1b. Whereas DBU and MTBD alone or any TU-tertiary amine combination are only active for the ROP of LA, TU/MTBD and TU/DBU were shown to be active for the ROP of VL, CL, MTC and TMC.99,120,134 The combination of MTBD or DBU and 1.1a (5mol% each) produced PVL with predictable molecular -1 weights up to DP ~200 (TOFMTBD/1.1a ~TOFDBU/1.1a ~ 5 h ). However, MTBD/TU and DBU/TU, required days (TOFs ~ 0.13 h-1) to reach 80% conversion in the ROP of
19 CL.120 The DBU/TU system demonstrated a higher TOF for the ring-opening -1 polymerization of the cyclic carbonates MTC-OR (Figure 1.2, TOFMTC-OBn ~ 19 h ) than for TMC.99 The proposed bifunctional mechanism of action, Scheme 1.19, was supported by observed interactions between benzyl alcohol and MTBD (Scheme 1.17) and between 1.1a and various monomers, Scheme 1.18.120,121,132
CF3 S O N F3C S Keq H + XO N F C N N H 3 O O H H n F3C X n -1 VL: n=1, X=CH2, Keq= 39±5 M -1 CL: n=2, X=CH2, Keq= 42±5 M -1 TMC: n=1, X=O, Keq= 45 M Scheme 1.18. Binding Constants of 1.1a with Selected Monomers
CF3 CF3 CF3 S S S F3C N N H H F3C N N H H F3C N N N H H O N N N O N O O H N N N N O R R OH O OH R O Scheme 1.19. ROP of VL by the Dual Activation of Monomer and Initiator by 1.1a and MTBD
While the activities of the TU/amine (TU/A) catalysts are lower than that of the carbenes, these catalysts are remarkably tolerant and selective, leading to polymers of very narrow polydispersities. Transesterification of the polymer chain was shown to be minimal; when fully converted reaction solutions were allowed to sit for days in the presence of the catalyst, the polydispersities did not increase.120,121,132 The origin of this high selectivity was investigated by NMR experiments measuring the binding constants of lactones and esters with the thioureas. Cyclic lactones and cyclic carbonates bind to the thiourea 1.1a (Scheme 1.18). In contrast, the open chain-ester
20 ethyl acetate exhibited no measurable binding affinity for 1.1a under similar conditions.120 The higher H-bond basicity of s-cis esters of lactones/cyclic carbonates135 relative to that of the acyclic s-trans esters of the polymer chain is the likely origin of this high selectivity for ring-opening relative to transesterification of the chain. This high selectivity is a defining attribute of the TU/A organic catalysts. The selectivity of these catalysts, coupled with their broad substrate tolerance, has created new opportunities for generating highly functionalized polymers with well- defined molecular weights (vide infra).
1.11.2 1,5,7-triazabicyclododecene (TBD) + The bicyclic guanidine TBD has a slightly higher basicity (pKa TBDH = 26.0)115 than its substituted analog MTBD (Figure 1.1, Scheme 1.17) but its activity in ring-opening polymerization reactions is considerably higher. The ROP of lactide requires approx. 30 minutes in the presence of 0.5mol% MTBD (an average turnover frequency of TOF ~0.002 s-1), whereas TBD is much more active. At 0.1 mol% TBD, the ROP of lactide is complete in 1 min (TOF ~81 s-1).120 Similarly, the ring-opening polymerization of the cyclic carbonate TMC (Figure 1.2) requires 6 days (TOF <0.1 h-1) for MTBD, but in the presence of TBD complete conversion is achieved in 15 min (TOF ~0.1s-1).134 This commercially available catalyst has been shown to be active in the ROP of LA, VL, CL, MTC, TMC, and carbosiloxanes with rates that exceed any other organocatalyst or, in some cases, any other catalyst known. For all monomers tested, TBD exhibited characteristics of a living polymerization, but its higher transesterification activity results in the broadening of the molecular weight distribution upon full conversion for all monomers except carbosiloxanes.99,119,120,134 This is in marked contrast to the NHC catalyzed ROP of carbosiloxanes that, while faster, broaden the PDI by transetherification. The methylated analog, MTBD, was inactive in the ROP of carbosiloxanes.100 The much higher reactivity of TBD relative to DBU and MTBD stimulated mechanistic and theoretical studies to illuminate the origin of the higher rates. Treatment to TBD with vinyl acetate led to the rapid formation of acyl-TBD which
21 subsequently reacted with benzyl alcohol to regenerate TBD and the ester (Scheme 1.20A).119 This led to the proposal that TBD might function as a bifunctional nucleophilic catalyst for tranesterification (Scheme 1.21). A nucleophilic mechanism was also proposed for the TBD catalyzed formation of amides from esters (Scheme 1.20B);136 kinetic studies provide strong support for a nucleophilic mechanism involving an acyl-TBD intermediate.137
N N N N X N N N N N N H R H H A H N N + + O O O O H O O R O O X X= O or NH H
O O H B + N R H R' R' H R N + O O N H
N N H Scheme 1.20. TBD-catalyzed Acyl-transfer Reaction for Alcohols and Amines
N N N H N N N N O H N N O O H O O O O O O O O R OH O
N N N N H N N O H O O O R O OH O O R O H O Scheme 1.21. Nucleophilic Mechanism for the TBD-catalyzed ROP of LA
22 Theoretical studies55,56 implied that while a nucleophilic mechanism for TBD catalyzed ring-opening polymerization is feasible, a H-bonding mechanism(Scheme 1.22) exhibited a lower calculated barrier for transesterification. Binding of the alcohol to TBD100 simultaneously activates the alcohol and creates an incipient guanidinium ion, which can function as a H-bond donor to the lactone carbonyl (analogous to a thiourea). The unique structural and electronic features of TBD enable it to catalyze transacylation reactions by a variety of mechanisms137 with high rates and selectivities. Because TBD is commercially available, it is also operationally quite simple and convenient.
N N
N N N N H R N N H H H N HO O O O O R N N R O H N H O O O O O O O O R O OH O O O O Scheme 1.22. TBD-catalyzed ROP of LA by the Hydrogen Bonding Mechanism
1.12 Chemoselectivity, Substrate Tolerance While achieving fast rates and high turnover numbers is highly desirable, the selectivity and tolerance of the catalyst to other functional groups is also important. For example, DMAP catalysts are effective for the ring-opening of lactones75,76 or O- carboxyanhydrides73-76 functionalized with pendant esters. The TU/A catalyst systems are particularly chemoselective and tolerant to a wide variety of functionalized monomers. The high selectivity of the TU/A catalysts for tranesterification of cyclic lactones and carbonates relative to open-chain s-cis esters has created new opportunities for generating highly functionalized polylactones and polycarbonates with a diverse range of functionalities. For example, a wide range of functionalized carbonates (MTC-OR) can be prepared from 2,2-bis(methylol)propionic acid (bis- MPA);99,138,139 these functionalized carbonates are readily polymerized or
23 copolymerized in the presence of the TU/A catalysts to generate a family of functionalized carbonates with a range of pendant functional groups (Figure 1.2).99
OMTC Cl O O
OO OO NBoc S N S R = H R O O O O OMe StBu O MTC-OH MTC-OR
NHBoc (CF2)5CF3
Figure 1.2. A Selection of the MTC-OR Monomers Accessible from MTC-OH
The ring-opening polymerization can be initiated from a wide variety of functional groups including alcohols, thiols, primary amines and silanols.99,121,134 These highly controlled polymerizations led to a new synthesis of well-defined guanidinylated oligocarbonates that were shown to act as molecular transporters140 that traverse cell membranes (Scheme 1.23).141
O O H O O O n 6 85% OH O O HN HN 1) TU/DBU CH2Cl2 O S O O O a: n=8 O S O b: n=11 c: n=22 + O O 2) CF3CO2H NH nTFA
N H2N NH2 N O N O O N N O H H Scheme 1.23. Synthesis of Guanidinylated Oligocarbonate Molecular Transporters
The high chemoselectivity of the TU/A catalysts also creates additional opportunities for tandem polymerizations from multifunctional initiators. Hydroxy- functionalized nitroxides or dithioesters can be used as initiators for tandem free- radical and ring-opening polymerization reactions (Scheme 1.24).134 This is one
24 strategy for the synthesis of complex, multifunctional polymer architectures that is enabled by the high functional group tolerance of the TU catalysts.
N N O NMP O O HO O N ROP N O O O HO O N H O O
O S S S S S S O H OH OH O O RAFT ROP O N N N N N
Scheme 1.24. Use of Dual-Function Initiators to generate (Upper) poly(N,N- dimethylacrylamide)-block-PLA by Tandem NMP-ROP, and (Lower) poly(vinyl pyridine)-block-PLA by Tandem RAFT-ROP
1.13 Architectural Control The mechanistic diversity of organocatalytic polymerization reactions has created new opportunities and strategies to control the architecture of macromolecules. The zwitterionic polymerization with NHC’s to make cyclic polymers (Scheme 1.10)108,109,142 and the step-growth benzoin condensations (Scheme 1.12)105 are two examples; below we highlight several other examples where the selectivity of organic catalysts has provided new strategies for macromolecular design. Stereoselectivity is critically important in fine-chemical synthesis; it is also very important in polymerization catalysis as the relative stereochemistry of stereogenic centers along the chain influences the physical properties of the polymer.143,144 The stereoselective polymerization of the chiral monomer lactide has attracted considerable interest and can be carried out with a variety of metal catalysts.143,145-150 Several organic catalysts have been shown to influence the stereoselectivity of enchainment of lactide. The sterically encumbered carbene, Ph2IMes, is very active for the ROP of LA at room temperature (1.58 s-1) producing atactic poly(LA) from rac-LA, however, when the temperature is lowered (-40 to -70°C) highly isotactic (from rac-LA) and heterotactic (from meso-LA) polylactides are generated.151 Similar
25 stereoselectivities were observed with sterically demanding phosphazenes.116,121,152 The stereoselectivity of these polymerization catalysts was proposed to be due to a chain-end control mechanism whereby the growing chain selectively attacks the activated monomer of the same stereochemistry leading to isotactic enchainment. In addition to the stereochemistry, the comonomer sequence is also an important determinant for polymer properties. Due to their different mechanisms of enchainment, organic catalysts exhibit a different chemoselectivity for copolymerization than typical metal alkoxide catalysts.153-159 The catalysts MTBD/TU, DBU/TU and TBD all show a selectivity for monomer wherein the fastest propagating monomer (kLA >> kVL > kCL) is ring-opened to >95% conversion before ring-opening of the second monomer begins.120 While extensive studies on the copolymerization selectivity have not been done, these selectivities imply that block copolymers might be accessible in one step. In the case of the cyclic carbonates MTC-OR, more random copolymers were observed. Accordingly, block and random MTC-OR copolymers could be generated simply by varying reaction conditions.99,160 Unsaturated carbenes such as IMes generate cyclic polyesters or polyamides in the zwitterionic ring-opening of lactide108 or N-carboxyanhydrides.142 The saturated carbene SIMes (Figure 1.1) also generates cyclic polyesters from -lactones,109 but subtle differences in the reactivity between the unsaturated and saturated carbenes81 lead to different mechanisms. Treatment of the saturated carbene SIMes with one equivalent of -butyrolactone generates the novel spirocycle 1.25S. This spirocycle initiates the ring-opening polymerization of -lactones to yield cyclic polyesters.109 A novel mechanism involving reversible collapse of the zwitterionic intermediate to a neutral imidazolidine spirocycles was proposed (Scheme 1.25). 109,142 The polymerization is highly selective due to the generation of small amount of zwitterionic intermediate by the reversible formation of the spiro-macrocycles.
26
Scheme 1.25. ROP of -lactones using SIMes
Organic catalysts can also be used to generate telechelic poly(valerolactone) or poly(THF) that could be cyclized into large cyclic polymers.161,162 These new strategies for generating macromolecules with a cyclic topology offer new opportunities for generating these architectures.163 Alkyl164 and alcohol165 adducts of saturated N-heterocyclic carbenes have been used in the ROP of LA as a convenient method for generating the NHC catalyst in situ. Chloroform and pentafluorobenzene adducts of saturated imidazolinylidene are stable at room temperature but eliminate the carbene at elevated temperature.164,166 These NHCs polymerize LA in the presence of an alcohol initiator at elevated temperatures (65 oC ~ 144 oC). In contrast, alcohol adducts of the saturated carbene SIMes eliminate the alcohol reversibly at room temperature.165 In these adducts, the alcohol initiator is liberated with the carbene; thus the adducts of SIMes act as single component catalyst/initiators for the ROP of LA, Scheme 1.26. The liberation of the alcohol is rapid in solution at room temperature and PLA is obtained within minutes in high yield with narrow polydispersity.
Mes O Mes N N Mes O N CH3O O CH3O H n H N O OH Mes HOAc CH O O O 3 100 n O Mes O M = 16500 g/mol Mes N N Mes N n OH PDI = 1.16 CH O 3 N 89% yield + CH OH n 3 Mes Scheme 1.26. Reversible Activation and Deactivation of SIMes
27
In contrast to the alcohol adducts of the saturated imidazolinylidene carbenes, alcohol adducts of Enders’ triazol-5-ylidene are stable at room temperature and reversibly eliminate the alcohol only at 90°C.167,168 At room temperature in the presence of alcohols, the triazolylidenes are inactive; at 90°C they polymerize lactide to give polymers of narrow polydispersities. This provides a means of regulating the polymerization with temperature: at elevated temperature, polymerization proceeds; at lower temperature the alcohol “clicks”169 back onto the alcohol terminus of the polymer to give the dormant alcohol adduct (Scheme 1.27).168,170 The reversible formation of the active and dormant carbene species is the key factor that contributes to the exceptional control observed in these polymerizations.
Active, high T O
O Ph O N O O N N RO OH Ph Ph n O O
Ph - ROH Ph O N Dormant N O R N OR N O low T H n Ph N H Ph N Ph Ph Scheme 1.27. Reversible Activation of Triazolylidene Carbenes
The triazol-5-ylidene is also tolerant to a variety of initiators and, in conjunction with telechelic macroinitiators, has been used to produce complex architectures such as block copolymers, star copolymers168 and (super-) H-shaped copolymers.171
1.14 Conclusions The activity, selectivity, convenience and diverse reactivity of organic catalysts have expanded the armamentarium of synthetic methods for polymer synthesis.
28 Organic catalysts have proven of broad utility for the generation of an ever-increasing array of polymer architectures that have interesting properties in their own right, or can be programmed to assemble into larger nanostructures of defined size, shape and function (Scheme 1.28).24,60,98,121,160,172-174
Scheme 1.28. Organocatalysts possess the triad of versatility, convenience and functional group tolerance and yield well-defined polymers of precise architecture structures for a specific function.
The application of organocatalysts to polymer synthesis has provided new mechanistic insights, new strategies for enchaining monomers and new families of materials with a range of structure and function that continues to evolve. In the past decade, the development of new families of organic catalysts has led to impressive advances in the catalytic rates (DMAP to TBD) and selectivities (acids to TU/base). Some organocatalysts exhibit rates that compare or exceed those of organometallic catalysts.175 The development of new families of metal catalysts51 and enzymes176 for ring- opening polymerization continues apace;47,50,60 recent advances in metallic, enzymatic
29 and organic catalysts have highlighted the important role of catalyst development for advancing our ability to generate well-defined macromolecules with specific structure and function.177 In this perspective, we have attempted to highlight where investigations of organic catalysts have provided mechanistic insights and strategies for activating monomers and chain-ends to generate new opportunities for macromolecular design. Organic catalysts in many cases are complementary to metal or enzyme catalysts; choosing between a particular catalyst will depend on a variety of factors relevant to the specific challenge at hand. The use of organic catalysts can provide advantages in microelectronic178-181 or biomedical applications where the presence of metal residues in the final material can be deleterious to their end- use.50,141,160,174,182 An additional advantage of organic species that activate monomers or chain-ends catalytically is that they can be used in concentrations lower than that of the polymer chains, further minimizing the amount of catalyst residues in the final material.50,183 The wide substrate tolerance and exquisite selectivity of the thiourea organic catalysts120,121,132 for ring-opening versus transesterification of open chain esters provides a new strategy for precision polymer synthesis. The reversible capping of endgroups with triazolylidene carbenes168,170,171 and the zwitterionic polymerization to generate cyclic polymers108,109,142 are just a few examples of new strategies to complex molecular architectures and topologies. While the foregoing discussion highlights some of the advantages of organocatalytic approaches, challenges remain. Melt polymerizations are industrially attractive and compliant with the tenets of green chemistry.184,185 However, organocatalysts have not been widely investigated in melt polymerizations and this remains an attractive target for future research. Chiral phosphines have been used to great success in small molecule reactions,186 but chiral phosphines for stereoselective polymerization is an area still to be explored. The trifunctional catalyst for ROP may be just around the corner. A trifunctional organocatalyst modeled on serine hydrolases combines electrophilic activation, nucleophilic activation and a nucleophile in a discrete catalyst and exhibits a million-fold enchancement in acyl-transfer rate from
30 vinyl trifluoroacetate to alcohols.187 A polymerization strategy can be envisaged. This system extends concepts in multifunctional activation by precise arrays of functional groups that mimics the behavior of many enzymatic processes. New catalyst families that combine the attributes of organic and metal catalysis146 or that employ new combinations of activation mechanisms (or a new mechanism entirely) will create new opportunities for polymer synthesis. Many cues are evident from Nature; the extraordinary rates, selectivities and exquisite multicomponent catalytic cascades of natural catalysts inspire emulation. The convergence of convenience, functional group tolerance, fast rates and selectivities will continue to drive innovations in polymerization catalysis, and it is our perspective that organocatalysis will continue to play an important role in these developments.
1.15 References (1) Bredig, G.; Fiske, P. S. Biochem. Z. 1912, 46, 7. (2) Langenbeck, W. Die Organische Katalysatoren und ihre Beziehungen zu den Fermenten; 2nd ed.; Springer: Berlin, 1949. (3) Detar, D. F.; Westheimer, F. H. J. Am. Chem. Soc. 1959, 81, 175-178. (4) Hamilton, G. A.; Westheimer, F. H. J. Am. Chem. Soc. 1959, 81, 6332-6333. (5) Westheimer, F. H. Tetrahedron 1995, 51, 3-20. (6) Breslow, R. J. Am. Chem. Soc. 1958, 80, 3719-3726. (7) Zhang, Z. G.; Schreiner, P. R. Chem. Soc. Rev. 2009, 38, 1187-1198. (8) Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615. (9) Eder, U.; Sauer, G.; Weichert, R. Angew. Chem., Int. Ed. 1971, 10, 496-500. (10) Ahrendt, K. A.; Borths, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2000, 122, 4243-4244. (11) Jen, W. S.; Wiener, J. J. M.; MacMillan, D. W. C. J. Am. Chem. Soc. 2000, 122, 9874-9875. (12) Notz, W.; Tanaka, F.; Barbas, C. F. Acc. Chem. Res. 2004, 37, 580-591. (13) Hanessian, S.; Auzzas, L. Acc. Chem. Res. 2008, 41, 1241-1251. (14) Erkkila, A.; Majander, I.; Pihko, P. M. Chem. Rev. 2007, 107, 5416-5470.
31 (15) Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007, 107, 5471-5569. (16) Wurz, R. P. Chem. Rev. 2007, 107, 5570-5595. (17) Gaunt, M. J.; Johansson, C. C. C. Chem. Rev. 2007, 107, 5596-5605. (18) Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606-5655. (19) Hashimoto, T.; Maruoka, K. Chem. Rev. 2007, 107, 5656-5682. (20) Atodiresei, I.; Schiffers, I.; Bolm, C. Chem. Rev. 2007, 107, 5683-5712. (21) Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107, 5713-5743. (22) Akiyama, T. Chem. Rev. 2007, 107, 5744-5758. (23) Davie, E. A. C.; Mennen, S. M.; Xu, Y.; Miller, S. J. Chem. Rev. 2007, 107, 5759-5812. (24) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813-5840. (25) McGarrigle, E. M.; Myers, E. L.; Illa, O.; Shaw, M. A.; Riches, S. L.; Aggarwal, V. K. Chem. Rev. 2007, 107, 5841-5883. (26) List, B. Chem. Rev. 2007, 107, 5413-5415. (27) Berkessel, A.; Gröger, H. Asymmetric Organocatalysis: From Biomimetic Concepts to Applications in Asymmetric Synthesis; Wiley-VCH: Weinheim, 2005. (28) Barbas, C. F. Angew. Chem., Int. Ed. 2008, 47, 42-47. (29) Bertelsen, S.; Jorgensen, K. A. Chem. Soc. Rev. 2009, 38, 2178-2189. (30) MacMillan, D. W. C. Nature 2008, 455, 304-308. (31) Flory, P. J. J. Am. Chem. Soc. 1940, 62, 2261-2264. (32) Odian, G. Principles of Polymerization; 4th ed.; Wiley-Interscience: Hoboken, NJ, 2004. (33) Ober, C. K.; Cheng, S. Z. D.; Hammond, P. T.; Muthukumar, M.; Reichmanis, E.; Wooley, K. L.; Lodge, T. P. Macromolecules 2009, 42, 465-471. (34) Penczek, S.; Duda, A.; Szymanski, R.; Biela, T. Macromol. Symp. 2000, 153, 1. (35) Szymanski, R. Macromol. Theory Simul. 1998, 7, 27.
32 (36) Pepper, D. C. Eur. Polym. J. 1980, 16, 407. (37) Gold, L. J. Chem. Phys. 1958, 28, 91. (38) Beste, L. F.; Hall, H. K. J. Phys. Chem. 1964, 68, 269. (39) Gibson, V. C.; Marshall, E. L. Comprehensive Coord. Chem. II 2004, 9, 1. (40) Kricheldorf, H. R.; Berl, M.; Scharnagl, N. Macromolecules 1988, 21, 286. (41) Degee, P.; Dubois, P.; Jerome, R.; Jacobsen, S.; Fritz, H.-G. Macromol. Symp. 1999, 144, 289. (42) O'Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001, 2215. (43) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147-6176. (44) Albertsson, A. C.; Varma, I. K. Biomacromolecules 2003, 4, 1466-1486. (45) Kowalski, A.; Libiszowski, J.; Majerska, K.; Duda, A.; Penczek, S. Polymer 2007, 48, 3952-3960. (46) Penczek, S.; Cypryk, M.; Duda, A.; Kubisa, P.; Slomkowski, S. Prog. Polym. Sci. 2007, 32, 247-282. (47) Dove, A. P. Chem. Commun. 2008, 6446-6470. (48) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000, 12, 1841- 1846. (49) Hedrick, J. L.; Magbitang, T.; Connor, E. F.; Glauser, T.; Volksen, W.; Hawker, C. J.; Lee, V. Y.; Miller, R. D. Chem.Eur. J. 2002, 8, 3308-3310. (50) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polymer Reviews 2008, 48, 11- 63. (51) Kricheldorf, H. R. Chem. Rev. 2009, 109, 5579-5594. (52) Jedlinski, Z.; Walach, W.; Kurcok, P.; Adamus, G. Makromol. Chem. 1991, 192, 2051. (53) Kricheldorf, H. R.; Boettcher, C. Makromol. Chem. 1993, 194, 1665. (54) Kricheldorf, H. R. Angew. Chem., Int. Ed. 2006, 45, 5752-5784. (55) Simon, L.; Goodman, J. M. J. Org. Chem. 2007, 72, 9656-9662.
33 (56) Chuma, A.; Horn, H. W.; Swope, W. C.; Pratt, R. C.; Zhang, L.; Lohmeijer, B. G. G.; Wade, C. G.; Waymouth, R. M.; Hedrick, J. L.; Rice, J. E. J. Am. Chem. Soc. 2008, 130, 6749-6754. (57) Lai, C.-L.; Lee, H. M.; Hu, C.-H. Tetrahedron Lett. 2005, 46, 6265-6270. (58) Kricheldorf, H. R.; Dunsing, R. Makromol. Chem. Phys. 1986, 187, 1611- 1625. (59) Saegusa, T. Angew. Chem. Int. Ed. 1977, 16, 826-835. (60) Bourissou, D.; Moebs-Sanchez, S.; Martin-Vaca, B. Comptes Rendus Chimie 2007, 10, 775-794. (61) Shibasaki, Y.; Sanada, H.; Yokoi, M.; Sanda, F.; Endo, T. Macromolecules 2000, 33, 4316. (62) Lou, X. D.; Detrembleur, C.; Jerome, R. Macromolecules 2002, 35, 1190- 1194. (63) Liu, J. Y.; Liu, L. J. Macromolecules 2004, 37, 2674-2678. (64) (a) Persson, P. V.; Schroder, J.; Wickholm, K.; Hedenstrom, E.; Iversen, T. Macromolecules 2004, 37, 5889-5893. (b) Casas, J.; Persson, P. V.; Iversen, T.; Cordova, A. Adv. Synth. Catal. 2004, 346, 1087-1091. (c) Cordova, A.; Hafren, J. Nordic Pulp Paper Res. J. 2005, 20, 477-479. (65) Bourissou, D.; Martin-Vaca, B.; Dumitrescu, A.; Graullier, M.; Lacombe, F. Macromolecules 2005, 38, 9993-9998. (66) Murugan, R.; Scriven, E. F. V. Aldrichim. Acta 2003, 36, 21-27. (67) Kricheldorf, H. R.; Garaleh, M.; Schwarz, G. J. Macromol. Sci., Pure Appl. Chem. 2005, 42A, 139-148. (68) Nederberg, F.; Connor, E. F.; Glausser, T.; Hedrick, J. L. Chem. Commun. 2001, 2066-2067. (69) Nederberg, F.; Connor, E. F.; Moller, M.; Glauser, T.; Hedrick, J. L. Angew. Chem., Int. Ed. 2001, 40, 2712-2715. (70) Kricheldorf, H. R.; Lomadze, N.; Schwarz, G. Macromolecules 2008, 41, 7812-7816.
34 (71) Kricheldorf, H. R.; Lomadze, N.; Schwarz, G. Macromolecules 2007, 40, 4859-4864. (72) Kricheldorf, H. R.; Von Lossow, C.; Schwarz, G. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4680-4695. (73) Thillaye Du Boullay, O.; Marchal, E.; Martin-Vaca, B.; Cossio, F. P.; Bourissou, D. J. Am. Chem. Soc. 2006, 128, 16442-16444. (74) Thillaye du Boullay, O.; Bonduelle, C.; Martin-Vaca, B.; Bourissou, D. Chem. Commun. 2008, 1786-1788. (75) Trimaille, T.; Gurny, R.; Moeller, M. Chimia 2005, 59, 348-350. (76) Trimaille, T.; Moeller, M.; Gurny, R. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4379. (77) du Boullay, O. T.; Marchal, E.; Martin-Vaca, B.; Cossio, F. P.; Bourissou, D. J. Am. Chem. Soc. 2006, 128, 16442-16443. (78) Bonduelle, C.; Martin-Vaca, B.; Cossio, F. P.; Bourissou, D. Chem. Eur. J. 2008, 14, 5304-5312. (79) Johnson, R. M.; Fraser, C. L. Biomacromolecules 2004, 5, 580-588. (80) Myers, M.; Connor, E. F.; Glauser, T.; Mock, A.; Nyce, G.; Hedrick, J. L. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 844-851. (81) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39-91. (82) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. 1997, 36, 2163-2187. (83) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913-921. (84) Alder, R. W.; Allen, P. R.; Williams, S. J. J. Chem. Soc., Chem. Commun. 1995, 1267-1268. (85) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290-1309. (86) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953-957. (87) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674-2678. (88) Weskamp, T.; Schattenmann, W. C.; Spielger, M.; Herrmann, W. A. Angew. Chem. Int. Ed. 1998, 37, 2490-2493.
35 (89) Wanzlick, H. W. Angew. Chem., Int. Ed. 1962, 1, 75-80. (90) Sheehan, J. C.; Hunneman, D. H. J. Am. Chem. Soc. 1966, 88, 3666-3667. (91) Stetter, H. Angew. Chem., Int. Ed. 1976, 15, 639-647. (92) Grasa, G. A.; Kissling, R. M.; Nolan, S. P. Org. Lett. 2002, 4, 3583-3586. (93) Nyce, G. W.; Lamboy, J. A.; Connor, E. F.; Waymouth, R. M.; Hedrick, J. L. Org. Lett. 2002, 4, 3587-3590. (94) Connor, E. F.; Nyce, G. W.; Myers, M.; Mock, A.; Hedrick, J. L. J. Am. Chem. Soc. 2002, 124, 914-915. (95) Nyce, G. W.; Glauser, T.; Connor, E. F.; Mock, A.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2003, 125, 3046-3056. (96) Dove, A. P.; Pratt, R. C.; Lohmeijer, B. G. G.; Culkin, D. A.; Hagberg, E. C.; Nyce, G. W.; Waymouth, R. M.; Hedrick, J. L. Polymer 2006, 47, 4018-4020. (97) Kamber, N. E.; Jeong, W.; Gonzalez, S.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2009, 42, 1634-1639. (98) Nederberg, F.; Trang, V.; Pratt, R. C.; Mason, A. F.; Frank, C. W.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2007, 8, 3294-3297. (99) Pratt, R.; Nederberg, F.; Waymouth, R. M.; Hedrick, J. L. Chem. Commun. 2008, 114-116. (100) Lohmeijer, B. G.; Dubois, G.; Leibfarth, F.; Pratt, R. C.; Nederberg, F.; Nelson, A.; Waymouth, R. M.; Wade, C.; Hedrick, J. L. Org. Lett. 2006, 8, 4683-4687. (101) Rodriguez, M.; Marrot, S.; Kato, T.; Sterin, S.; Fleury, E.; Baceiredo, A. J. Organomet. Chem. 2007, 692, 705-708. (102) Scholten, M. D.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2008, 41, 7399-7404. (103) Raynaud, J.; Ciolino, A.; Baceiredo, A.; Destarac, M.; Bonnette, F.; Kato, T.; Gnanou, Y.; Taton, D. Angew. Chem. Int. Ed. 2008, 47, 5390-5393. (104) Raynaud, J.; Gnanou, Y.; Taton, D. Macromolecules 2009, 42, 5996-6005. (105) Pinaud, J.; Vijayakrishna, K.; Taton, D.; Gnanou, Y. Macromolecules 2009, 42, 4932-4936.
36 (106) Raynaud, J.; Absalon, C.; Gnanou, Y.; Taton, D. J. Am. Chem. Soc. 2009, 131, 3201-3209. (107) Movassaghi, M.; Schmidt, M. A. Org. Lett. 2005, 7, 2453-2456. (108) (a) Culkin, D. A.; Jeong, W.; Csihony, S.; Gomez, E. D.; Balsara, N. P.; Hedrick, J. L.; Waymouth, R. M. Angew. Chem., Int. Ed. 2007, 46, 2627-2630. (b) Jeong, W.; Shin, E. J.; Culkin, D. A.; Hedrick, J. L.; Waymouth, R. M. J. Am. Chem. Soc. 2009, 131, 4884-4891. (109) Jeong, W.; Hedrick, J. L.; Waymouth, R. M. J. Am. Chem. Soc. 2007, 129, 8414-8415. (110) Feng, H.; Dong, C. M. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 5353- 5361. (111) Hedrick, J. L.; Nyce, G. W.; Waymouth, R. M., U. S. Patent, 7,053,221 B2, 2006. (112) Webster, O. W. Adv. Polym. Sci. 2004, 167, 1-34. (113) Schwesinger, R.; Schlemper, H. Angew. Chem., Int. Ed. 1987, 26, 1167. (114) Molenberg, A.; Moller, M. Macromol. Rapid Commun. 1995, 16, 449. (115) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019-1028. (116) Zhang, L.; Nederberg, F.; Pratt, R. C.; Waymouth, R. M.; Hedrick, J. L.; Wade, C. G. Macromolecules 2007, 40, 4154-4158. (117) Schlaad, H.; Kukula, H.; Rudloff, J.; Below, I. Macromolecules 2001, 34, 4302-4304. (118) Rexin, O.; Mulhaupt, R. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 864. (119) Pratt, R. C.; Lohmeijer, B. G.; Long, D. A.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2006, 128, 4556-4557. (120) Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.; Long, D. A.; Dove, A. P.; Nederberg, F.; Choi, J.; Wade, C.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 8574-8583.
37 (121) Pratt, R. C.; Lohmeijer, B. G. G.; Long, D. A.; Lundberg, P. N. P.; Dove, A. P.; Li, H.; Wade, C. G.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 7863-7871. (122) Bensa, D.; Rodriguez, J. Synth. Commun. 2004, 34, 1515-1519. (123) Schuchardt, U.; Vargas, R. M.; Gelbard, G. J. J. Mol. Catal. A: Chem. 1995, 99, 65-70. (124) Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.; Liotta, C. L. J. Org. Chem. 2005, 70, 5335-5338. (125) Schreiner, P. R. Chem. Soc. Rev. 2003, 32, 289-296. (126) Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 4901-4902. (127) Hoashi, Y.; Okino, T.; Takemoto, Y. Angew. Chem., Int. Ed. 2005, 44, 4032- 4036. (128) Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am. Chem. Soc. 2003, 125, 12672- 12674. (129) Okino, T.; Nakamura, S.; Furukawa, T.; Takemoto, Y. Org. Lett. 2004, 6, 625- 629. (130) Berkessel, A.; Cleemann, F.; Mukherjee, S.; Muller, T. N.; Lex, J. Angew. Chem., Int. Ed. 2005, 44, 807-811. (131) Zhu, R. X.; Wang, R. X.; Zhang, D. J.; Liu, C. B. Aust. J. Chem. 2009, 62, 157-164. (132) Dove, A. P.; Pratt, R. C.; Lohmeijer, B. G. G.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2005, 127, 13798. (133) Koeller, S.; Kadota, J.; Deffieux, A.; Peruch, F.; Massip, S.; Leger, J. M.; Desvergne, J. P.; Bibal, B. J. Am. Chem. Soc. 2009, 131, 15088-15060. (134) Nederberg, F.; Lohmeijer, B. G. G.; Leibfarth, F.; Pratt, R. C.; Choi, J.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2007, 8, 153-160. (135) Laurence, C.; Brameld, K. A.; Graton, J.; Le Questel, J. Y.; Renault, E. J. Med. Chem. 2009, 52, 4073-4086. (136) Sabot, C.; Kumar, K. A.; Meunier, S.; Mioskowski, C. Tetrahedron Lett. 2007, 48, 3863-3866.
38 (137) Kiesewetter, M. K.; Scholten, M. D.; Kirn, N.; Weber, R. L.; Hedrick, J. L.; Waymouth, R. M. J. Org. Chem. 2009, 74, 9490-9496. (138) Al-Azemi, T. F.; Bisht, K. S. Macromolecules 1999, 32, 6536-6540. (139) Al-Azemi, T. F.; Harmon, J. P.; Bisht, K. S. Biomacromolecules 2000, 1, 493- 500. (140) Wender, P. A.; Galliher, W. C.; Goun, E. A.; Jones, L. R.; Pillow, T. H. Advanced Drug Delivery Reviews 2008, 60, 452-472. (141) Cooley, C. B.; Trantow, B. M.; Nederberg, F.; Kiesewetter, M. K.; Hedrick, J. L.; Waymouth, R. M.; Wender, P. A. J. Am. Chem. Soc. 2009, 131, 16401- 16403 (142) Guo, L.; Zhang, D. J. Am. Chem. Soc. 2009, 131, 18072 (143) Coates, G. W. Dalton 2002, 467-475. (144) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. 1995, 34, 1143-1170. (145) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229-3238. (146) Jensen, T. R.; Breyfogle, L. E.; Hillmyer, M. A.; Tolman, W. B. Chem. Commun. 2004, 2504-2508. (147) Thakur, K. A. M.; Kean, R. T.; Zell, M. T.; Padden, B. E.; Munson, E. J. Chem. Commun. 1998, 1913-1914. (148) Zell, M. T.; Padden, B. E.; Paterick, A. J.; Hillmyer, M. A.; Kean, R. T.; Thakur, K. A. M.; Munson, E. J. J. Am. Chem. Soc. 1998, 120, 12672-12673. (149) Chisholm, M. H.; Patmore, N. J.; Zhou, Z. P. Chem. Commun. 2005, 127-129. (150) Chisholm, M. H.; Iyer, S. S.; McCollum, D. G.; Pagel, M.; Werner-Zwanziger, U. Macromolecules 1999, 32, 963-973. (151) Dove, A. P.; Li, H.; Pratt, R. C.; Lohmeijer, B. G.; Culkin, D. A.; Waymouth, R. M.; Hedrick, J. L. Chem. Commun. 2006, 2881-2885. (152) Zhang, L.; Nederberg, F.; Messman, J. M.; Pratt, R. C.; Hedrick, J. L.; Wade, C. G. J. Am. Chem. Soc. 2007, 129, 12610-12611. (153) Bero, M.; Kasperczyk, J. Macromol. Chem. Phys. 1996, 197, 3251-3258.
39 (154) Parrish, B.; Quansah, J. K.; Emrick, T. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1983-1990. (155) Stassin, F.; Jerome, R. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2777- 2789. (156) Wahlberg, J.; Persson, P. V.; Olsson, T.; Hedenstrom, E.; Iversen, T. Biomacromolecules 2003, 4, 1068-1071. (157) Kurcok, P.; Penczek, J.; Franek, J.; Jedlinski, Z. Macromolecules 2002, 25, 2285-2289. (158) Mosnacek, J.; Duda, A.; Libiszowski, J.; Penczek, S. Macromolecules 2005, 38, 2027-2029. (159) Kricheldorf, H. R.; Bornhorst, K.; Hachmann-Thiessen, H. Macromolecules 2005, 38, 5017-5024. (160) Fukushima, K.; Pratt, R. C.; Nederberg, F.; Tan, J. P. K.; Yang, Y. Y.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2008, 9, 3051-3056. (161) Misaka, H.; Kakuchi, R.; Zhang, C.; Sakai, R.; Satoh, T.; Kakuchi, T. Macromolecules 2009, 42, 5091-5096. (162) Tezuka, Y.; Ohtsuka, T.; Adachi, K.; Komiya, R.; Ohno, N.; Okui, N. Macromol. Rapid. Commun. 2008, 29, 1237-1241. (163) Semlyen, J. A. In Cyclic Polymers; 2nd ed.; Semlyen, J. A., Ed.; Kluwer: Dordrecht, 2000, p 790. (164) Nyce, G. W.; Csihony, S.; Waymouth, R. M.; Hedrick, J. L. Chem.Eur. J. 2004, 10, 4073. (165) Csihony, S.; Culkin, D. A.; Sentman, A. C.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2005, 127, 9079-9084. (166) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T. L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546-2558. (167) Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.; Melder, J. P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. 1995, 34, 1021.
40 (168) Coulembier, O.; Dove, A. P.; Pratt, R. C.; Sentman, A. C.; Culkin, D. A.; Mespouille, L.; Dubois, P.; Waymouth, R. M.; Hedrick, J. L. Angew. Chem., Int. Ed. 2005, 44, 4964-4968. (169) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004-2021. (170) Coulembier, O.; Lohmeijer, B. G. G.; Dove, A. P.; Pratt, R. C.; Mespouille, L.; Culkin, D. A.; Benight, S. J.; Dubois, P.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 5617-5628. (171) Coulembier, O.; Kiesewetter, M. K.; Mason, A.; Dubois, P.; Hedrick, J. L.; Waymouth, R. M. Angew. Chem., Int. Ed. 2007, 46, 4719-4721. (172) Kim, S. H.; Tan, J. P. K.; Nederberg, F.; Fukushima, K.; Yang, Y. Y.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2008, 42, 25-29. (173) Kim, S. H.; Nederberg, F.; Zhang, L.; Wade, C. G.; Waymouth, R. M.; Hedrick, J. L. Nano Lett. 2007, 8, 294-301. (174) Nederberg, F.; Appel, E.; Tan, J. P. K.; Kim, S. H.; Fukushima, K.; Sly, J.; Miller, R. D.; Waymouth, R. M.; Yang, Y. Y.; Hedrick, J. L. Biomacromolecules 2009, 10, 1460-1468. (175) McLaughlin, M.; Garcia Rubio, S.; Muthyala, R.; Antunes, O. A. C.; Tilstam, U.; Zlota, A.; Yadav, G. D.; Laird, T. Organic Process Research & Development 2006, 10, 687-702. (176) Kobayashi, S.; Makino, A. Chem. Rev. 2009, 109, 5288-5353. (177) Percec, V. Chem. Rev. 2009, 109, 4961-4962. (178) Hermans, T. M.; Choi, J.; Lohmeijer, B. G. G.; Dubois, G.; Pratt, R. C.; Kim, H. C.; Waymouth, R. M.; Hedrick, J. L. Angew. Chem., Int. Ed. 2006, 45, 6648-6652. (179) Choi, J.; Hermans, T. M.; Lohmeijer, B. G. G.; Pratt, R. C.; Dubois, G.; Frommer, J.; Waymouth, R. M.; Hedrick, J. L. Nano Lett. 2006, 6, 1761-1764. (180) Kim, S. H.; Park, O. H.; Nederberg, F.; Topuria, T.; Krupp, L. E.; Kim, H. C.; Waymouth, R. M.; Hedrick, J. L. Small 2008, 4, 2162-2165.
41 (181) Kim, S. H.; Nederberg, F.; Zhang, L.; Wade, C. G.; Waymouth, R. M.; Hedrick, J. L. Nano Lett. 2008, 8, 294-301. (182) Tan, J. P. K.; Kim, S. H.; Nederberg, F.; Appel, E. A.; Waymouth, R. M.; Zhang, Y.; Hedrick, J. L.; Yang, Y. Y. Small 2009, 5, 1504-1507. (183) Williams, C. K.; Hillmyer, M. A. Polymer Reviews 2008, 48, 1-10. (184) Horvath, I. T.; Anastas, P. T. Chem. Rev. 2007, 107, 2169-2173. (185) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686-694. (186) Methot, J. L.; Roush, W. R. Adv. Synth. Catal. 2004, 346, 1035. (187) Ema, T.; Tanida, D.; Matsukawa, T.; Sakai, T. Chem. Commun. 2008, 957- 959.
42 CHAPTER 2
A Distinctive Organocatalytic Approach to Complex Macromolecular Architectures
Reprinted in part with permission from Angew. Chem. Int. Ed. 2007, 46, 4719. Copyright 2007 by Wiley Interscience
43 2.1 Introduction The assembly of precisely aggregated macromolecular assemblies has largely been the domain of biological systems due to the exquisite architectural and compositional makeup of natural macromolecules. Modern synthetic methods are beginning to challenge Nature's monopoly on the creation of well-defined macromolecules of defined architecture and composition.1-3 Living polymerization methods enable the synthesis of a wide variety of block copolymers of different structure and topology.3 In the case of block copolymers, the molecular architecture or topology of the chain has a pronounced effect on the morphology and interfacial activity. For example, ABC triblock copolymers, dendritic-linear hybrid copolymers, radial star-shaped copolymers, comb, tadpole-shaped and linear-nanoparticle copolymers all manifest unique morphologies as a result of their distinctive architectures. 4-9 While the synthesis of linear block copolymers is facile with several methods, the introduction of branch points at specific loci is more challenging and requires multiple steps.3 H-shaped homopolymers, first reported by Roovers et al.,10 exhibit unique rheological behavior. Many variations of this architecture (super-H, - shaped, graft, off-centered graft, etc.) have been prepared with anionic methods,3 typically from styrene, isoprene and butadiene monomers. More recently, controlled radical polymerization and ring-opening metathesis methods have expanded the comonomer classes that can be enchained into specifically branched copolymer architectures, but the construction of branched structures requires multiple steps.11-14 In this contribution, we report an expedient approach to a H-shaped and super H- shaped polymers by ring-opening polymerization from telechelic diamine or polyamine macroinitiators. New catalysts beget new patterns of reactivity for the enchainment of monomers to structurally well defined macromolecules. Organocatalysts complement transition metal catalysts due to their different mechanisms for effecting bond constructions.15 Our research has focused on organocatalytic ring-opening polymerization (ROP) of cyclic esters, primarily motivated to avoid metal contaminants in polymers for microelectronic and biomedical applications.16-24 Recent
44 studies have shown that these catalysts enable the construction of novel polymer architectures.17, 19 We have reported several classes of ROP organocatalysts, including N-heterocyclic carbenes,18 bifunctional thiourea-amines,21, 25 and amidine or guanidine superbases,16, 26 with user-selectable degrees of activity and selectivity. In this contribution, we report a simple approach to a H-shaped and super H-shaped architectures enabled by the unique reactivity of the commercially available 1,3,4- triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (2.1).27 Primary amines were found to function as bifunctional initiators for ROP in the presence of 2.1 to promote polymerization of two chains, enabling the facile introduction of branch points in block copolymers. This is in marked contrast to conventional organometallic promoters where only one chain is initiated from an amine, generating an amide end- group.28, 29
2.2 Results and Discussion The triazole carbene 2.1 is an efficient catalyst for the ROP of lactide in the presence of alcoholic initiators at 50 -90 °C.20, 23 Elevated temperatures are required for 2.1 due to a competitive O-H insertion reactions of 2.1 with terminal alcohols of the growing chains that lead to dormant alcohol adducts that are reactivated reversibly at elevated temperatures (Figure 2.1).23, 27 As analogous N-H insertion reactions are known for secondary amines,30, 31 we investigated the use of primary amines as initiators for lactide polymerization with 2.1.
Ph Ph N N N R N ER + E Ph N H Ph N H Ph Ph E = O, NR Figure 2.1. Reversible E-H insertion reactions of 2.1
The polymerization of rac-lactide (LA) initiated by 4-pyrene-methylamine as an initiator and 2.1 as catalyst in deuterated benzene (C6D6) was carried out at an
45 initial monomer-to-initiator ratio ([M]0/[I]0) of 100 at 90°C. After 15 hours (Conv. = 72.6%), the reaction was cooled to room temperature and directly quenched with a few drops of carbon disulfide. The polymer was precipitated twice from cold methanol and dried under vacuum until constant weight to give polylactide with a Mn = 10600 1 g/mol and Mw/Mn = 1.1. Analysis of the H NMR spectrum, MALDI-TOF mass spectra, and GPC with a UV detector clearly show the presence of the 4-pyrene- methylamino endgroup. Surprisingly, integration of the methine endgroups vs. the pyrene aromatic signals revealed approx. two endgroups per pyrene, suggesting that lactide polymerization was initiated from both N-H groups of the initiator (i.e., both the amine and resulting amide functionality). This is surprising, as it suggests that the N-heterocyclic carbene 2.1 can initiate lactide polymerization both from primary amines and from amides. To establish the competence of amides as initiators in the presence of 2.1, we investigated the polymerization of rac-lactide(LA) initiated from the cyclic amide ε- caprolactam (CLa) ([LA]0/[CLa]0 = 10). After 22 hrs in C6D6 at 90C, the reaction was quenched with acetic acid and the resulting polymer analyzed by 1H, 13C-NMR and electrospray ionization mass spectrometry (ESI-MS). The ESI-MS yields a series of peaks corresponding to CLa-(LA)n with a parent ion at M/z = 1121.3 (M/z = 1 13 1122.03 g/mol for CLa-(LA)7) and the H- and C-NMR spectra clearly show the presence of the CLa endgroup. The observation that amides can serve as initiators for the polymerization of lactide with 2.1 is intriguing as initiation from both N-H bonds of primary amines provides a facile means of generating branched polymers in a single step.
46 H2N NH2 O N O n N
H2N Ph O NH2 N N m O 363 K O H Ph N H O Ph O H2N O n NH2 y O O y
O Ph O O O O N N m O 363 K O O O H Ph N H O Ph O O O N N y O O y O O y O O O O O O O N O n N H O N PEO N O O H O O y H O O y O O O O O O H O N N O O H y O y y y H O O
O O O O
O O y H H
Scheme 2.1. Polymerization of LA initiated with (left) PEO-(NH2)2 and (right) PEO-
(NH2)4 forming H-shaped and super-H-shaped polymers respectively
To test the latter hypothesis, the polymerization of rac-lactide (LA) was carried out with poly(ethylene glycol) bis(3-aminopropyl) (PEO-(NH2)2) as initiator and 2.1 as catalyst ([M]0/[I]0 = 100, [2.1]0/ [I]0 = 4). in C6D6 at 90C (Scheme 2.1). Analysis of aliquots of the reaction mixture revealed a linear dependence of molecular weight Mn versus conversion. Plots of ln(([LA]0-[LA]eq)/([LA]t-[LA]eq)) vs time are linear with zero intercepts (see 2.4 Experimental Section), indicating a first order dependence on LA concentration and an absence of termination. The linear nature of the plot, in conjunction with the narrow polydispersities (Mw/Mn < 1.1), suggests that the polymerization of LA from the amino-adduct of 2.1 is living, as previously described for LA ROP from the alcohol-adduct of 2.1.20, 23 At 71 hrs, the reaction had reached 85% conversion, and a 1H-NMR spectrum (see see 2.4 Experimental Section) of the purified polymer taken after quenching with CS2 revealed the formation of the
[poly(lactide)]2-poly(ethylene glycol)-[poly(lactide)]2 (PLA2-PEO-PLA2) triblock polymer: DPPLA = 48 (by comparison of the PLA and PEO resonances at 5.14 ppm and 3.61 ppm, respectively). Integration of the methine end group (4.34 ppm) versus
47 the PEO segment confirmed the presence of 4.0 end groups, indicated that polymerization had occurred off all four N-H bonds of the initiator. To confirm the initiation from all four N-H groups of the telechelic initiator
PEO-(NH2)2, we compared the kinetics of ROP from PEO-(NH2)2 to that of PEO-
(OH)2 in the presence of 2.1. The kinetics are consistent with the rate law: -d[LA]/dt = kobs[LA] for kobs = k1[PEO][ 2.1], where [PEO] = PEO-(OH)2 or PEO-(NH2)2. These studies reveal that the rate constant for monomer consumption (k1) is approximately two times higher when LA is polymerized from PEO-(NH2)2 (k1-NH2) compared to when LA is polymerized from PEO-(OH)2 (k1-OH), k1-NH2/ k1-OH = 1.97, consistent with the doubling of the number of propagating –OH endgroups when polymerization is initiated from the amine initiators (Figure 2.2). In addition, polymerization of lactide from a 1:1 mixture of PEO-(OH)2 and PEO-(NH2)2 at 90°C with 2.1 in C6D6 yielded a mixture of diblock copolymers of Mn = 20,000 g/mol and 9000 g/ mol (GPC vs. polystyrene), consistent with the generation of a mixture of an H-shaped block polymer of approximately twice the molar mass as the linear block copolymer.
4500
4000
3500 y = 797.79x
3000
2500
2000
1500 y = 407.53x
ln(([LA]o)/([LA]t))/[PEO][triazole] 1000
500
0 01234567 tim e (hrs)
Figure 2.2. Comparison of rates of LA polymerization with (upper) PEO-(NH2)2 and
(lower) PEO-(OH)2 catalyzed by 2.1
48 This strategy also enables a facile synthesis of super H-shaped copolymers32 from a telechelic tetramino-functionalized PEO oligomers (Scheme 2.1). Tetra-amine- functionalized PEO was synthesized by cyanoethylation of PEO-(NH2)2 followed by nitrile reduction using BH3·THF. Polymerization of LA initiated from poly(ethylene glycol) octa(3-aminopropyl) (PEO-(NH2)4) with eight equivalents of 2.1 and 100 equivalents of LA ([LA]0 = 1 M) in C6D6 was carried out at 90°C until near complete conversion after 16 hours, (Scheme 2.1). 1H-NMR analysis confirms the polymerization off each N-H bond (integration of the end groups versus the PEO segment suggests 8.15 end groups) and DP= 93 (integration of PLA versus PEO fragment).
2.3 Conclusion These results indicate that 2.1 with any of a host of commercially available amino-functionalized macroinitiators provide a general procedure for the generation a variety of precisely branched block copolymer architectures. In addition, these results suggest a novel means of functionalizing polyamides or proteins. Further studies are underway to investigate the interfacial and self-assembly properties of these hydrophilic / hydrophobic branched copolymer architectures.
2.4 Experimental Section
2.4.1 General Considerations rac-Lactide was obtained from Purac and used without further purification (water content < 0.02 %). Bis-hydroxyl terminated Poly(ethylene glycol)s (PEO-
(OH)2, Mn = 3400) were purchased from Aldrich and dried under vacuum at 60°C overnight prior to three azeotropic distillations from toluene. Amino-terminated PEGs were prepared by cyanoethylation of PEG-(OH)2 or PEG-(NH2)2 followed by reduction. 1,3,4-triphenyl-4,5-dihydro-1H-1,2-triazol-5-ylidene (from Acros) was purified by exposure to high vacuum at 90°C overnight. Benzene-d6, purchased from Aldrich, was dried over Na/benzophenone and degassed by standard procedures.
49 2.4.2 Cyanoethylation of hydroxy-terminated PEG
(NCCH2CH2O(CH2CH2O)CH2CH2CN)
Hydroxy-terminated PEG (Mn = 3400, 10.0 g, 2.94 mmol) was dissolved in excess acrylonitrile (25 mL) in a round-bottom flask. The clear solution was cooled to 0 °C in an ice water bath, and a small amount of sodium hydride (60% dispersion in mineral oil, 15 mg, 0.4 mmol) was added to start the reaction. After 15 minutes at 0 °C, the pale yellow reaction mixture was quenched with a few drops of concentrated hydrochloric acid. Excess acrylonitrile was removed under vacuum and the product was dissolved in CH2Cl2. Insoluble poly(acrylonitrile) was removed by filtration through a plug of Celite, giving a clear, colorless solution. Solvent removal gave the 1 nitrile-terminated PEG as a white solid (8.90 g, 86%). H NMR (CDCl3, 400 MHz): δ
3.70 (t, 4H, J = 6.4 Hz, OCH2CH2CN), 3.63 (br s, PEG CH2), 2.60 (t, 4H, J = 6.4 Hz, 13 OCH2CH2CN). C NMR (CDCl3, 100 MHz): δ 117.9 (OCH2CH2CN), 70.5 (PEG
CH2), 65.9 (OCH2CH2CN), 18.8 (OCH2CH2CN).
2.4.3 Cyanoethylation of amine-terminated PEG
(NCCH2CH2)2NCH2CH2CH2O(CH2CH2O)CH2CH2CH2N(CH2CH2CN)2
Amine-terminated PEG (Mn = 3510, 5.0 g, 1.42 mmol) was dissolved in methanol (20 mL) and excess acrylonitrile (20 mL) to give a clear solution. The flask was sealed and stirred at 60 °C for six hours. After cooling, the solvent was removed under vacuum. The product was dissolved in CH2Cl2, filtered, and dried to give a 1 white solid (4.95 g, 93%). H NMR (CDCl3, 400 MHz): δ 3.63 (br s, PEG CH2), 3.52
(t, 4H, J = 6.0 Hz, OCH2CH2CH2N), 2.81 (t, 8H, J = 6.4 Hz, NCH2CH2CN), 2.61 (t,
4H, J = 6.4 Hz, OCH2CH2CH2N), 2.47 (t, 8H, J = 6.4 Hz, NCH2CH2CN), 1.68 (p, 4H, 13 J = 6.0 Hz, OCH2CH2CH2N). C NMR (CDCl3, 100 MHz): δ 118.6 (NCH2CH2CN),
70.3 (PEG CH2), 67.6 (OCH2CH2CH2N), 49.5 (NCH2CH2CN), 49.2
(OCH2CH2CH2N), 27.2 (OCH2CH2CH2N), 16.6 (NCH2CH2CN).
50 2.4.4 General procedure for nitrile reduction A dry Schlenk flask equipped with a reflux condenser was placed under nitrogen and charged with dry THF (50 mL). A syringe was used to add an excess
(usuall two- to four-fold) of BH3·THF solution (1.0 M in THF). This mixture was cooled to 0 °C in an ice bath. The nitrile-terminated PEG was also dissolved in a minimum amount of dry THF; in most cases heating was required to obtain a homogeneous solution. The PEG solution was added slowly to the borane solution, then allowed to stir for 30 minutes at 0 °C. The reaction was then heated at reflux under nitrogen overnight. After cooling in an ice bath, the flask was opened and methanol was slowly added to react with excess borane (with hydrogen evolution). Concentrated HCl (2 mL) was added, and the mixture was stirred at 0 °C for one hour. The solvent was removed under vacuum, residues were taken up in methanol and dried under vacuum to remove trimethyl borate side products. The resulting polymer was dissolved in aqueous sodium hydroxide (1M solution). The polymer was thoroughly dried under vacuum, dissolved in CH2Cl2, filtered through Celite, and precipitated in ether, giving a fine white powder.
2.4.5 Synthesis of H2NCH2CH2CH2O(CH2CH2O)CH2CH2CH2NH2
Nitrile-terminated PEG (5.00 g, 1.43 mmol) was reacted with excess BH3·THF solution (20 mL, 20 mmol) as described above to give the desired amine-terminated 1 PEG as a white powder (4.30 g, 86%). H NMR (CDCl3, 400 MHz): δ 3.63 (br s, PEG
CH2), 3.56 (t, 4H, J = 6.4 Hz, OCH2CH2CH2NH2), 2.81 (t, 4H, J = 6.8 Hz,
OCH2CH2CH2NH2), 1.74 (p, 4H, J = 6.4 Hz, OCH2CH2CH2NH2), 1.10 (br s, 4H, 13 OCH2CH2CH2NH2). C NMR (CDCl3, 100 MHz): δ 70.5 (PEG CH2), 69.5
(OCH2CH2CH2NH2), 39.6 (OCH2CH2CH2NH2), 33.1 (OCH2CH2CH2NH2).
2.4.6 Synthesis of
(H2NCH2CH2CH2)2NCH2CH2CH2O(CH2CH2O)CH2CH2CH2N(CH2CH2CH2NH2)2 Tetranitrile-terminated PEG (2.50 g, 0.67 mmol) was reacted with excess
BH3·THF solution (20 mL, 20 mmol) as described above to give the desired
51 1 tetraamine-terminated PEG as a white powder (2.10 g, 84%). H NMR (CDCl3, 400
MHz): δ 3.63 (br s, PEG CH2), 3.46 (t, 4H, J = 6.4 Hz, OCH2CH2CH2N), 2.72 (t, 8H,
J = 6.8 Hz, NCH2CH2CH2NH2), 2.44 (t, 4H, J = 6.8 Hz, OCH2CH2CH2N), 2.42 (t, 8H,
J = 6.8 Hz, NCH2CH2CH2NH2), 1.69 (p, 4H, J = 6.4 Hz, OCH2CH2CH2N), 1.58 (p, 13 8H, J = 6.8 Hz, NCH2CH2CH2NH2), 1.15 (br s, NCH2CH2CH2NH2). C NMR
(CDCl3, 100 MHz): δ 70.5(PEG CH2), 69.6 (OCH2CH2CH2N), 51.9
(OCH2CH2CH2N), 50.8 (NCH2CH2CH2NH2), 40.6 (NCH2CH2CH2NH2), 30.4
(OCH2CH2CH2N), 27.1 (NCH2CH2CH2NH2).
2.4.7 General polymerization procedure In a drybox, an NMR tube equipped with a J-Young valve was charged with rac- lactide (29.7 mg, 2.06 x 10-1 mmol), 1,3,4-triphenyl-4,5-dihydro-1H-1,2-triazol-5- -3 -3 ylidene (2.9 mg, 9.76 x 10 mmol) and PEO-(NH2)2 (9.2 mg, 2.40 x 10 mmol) in 0.6 ml of dry benzene-d6. The sealed NMR tube was then heated at 90°C in an oil bath. After 3 days (conv = 84.3%), the reaction was cooled to room temperature and quenched with ~0.5 mL of carbon disulfide and returned to reflux for 1 hr. The polymer was precipitated twice from cold heptane. The resulting polymer, slightly pink in color indicating the presence of CS2-triazole adduct, can be dissolved in CS2 at
90°C and precipitated by returning to room temperature. The red CS2 solution can be decanted leaving behind white polymer. Isolated polymer is dried under high vacuum until constant weight (31.3 mg, 80.5%, PDI = 1.09, Mn = 9,580), and the degree of polymerization was determined by 1H-NMR spectroscopy (DP = 48). 300 MHz 1H-
NMR (CDCl3): 1.41-1.70 (m, -CH3 & -nCH3), 3.62 (s, -n(CH2-CH2-O)-), 4.32 (q, - CH-), 5.07-5.22 (m, -nCH). For polymerizations with the diol, a single catalyst/initiator species was made by stirring PEO-(OH)2 with excess 2.1 for 1 hour in toluene, followed by precipitation from solution with cold heptane and isolation.
The PEO-(2.1)2 adduct is stable at room temperature. rac-Lactide polymerizations with this species proceed identically to the telechelic polyamine species. Failure to dry the PEO-(OH)2 in this manner results in short oligomeric fragments of PLA
(presumably initiated from H2O) mixed with the triblock polymer.
52 2.4.8 ε-caprolactam initiated polymerization of lactide Using the identical polymerization procedure, rac-lactide (45.8 mg, 3.18 x 10-1 mmol), 1,3,4-triphenyl-4,5-dihydro-1H-1,2-triazol-5-ylidene (6.4 mg, 2.15 x 10-2 mmol) and ε-caprolactam (2.8 mg, 2.48 x 10-2 mmol) were heated in 0.6 mL of dry benzene-d6 at 90°C for 22 hrs before quenching with 5 drops of 1M acetic acid. The resulting polymer, after precipitation in cold heptane and isolation, was analyzed by 1 300 MHz H-NMR (CDCl3): CLa moiety: 2.41 (m, -CH2-), 2.18 (m, -CH2-), 1.18-
0.98 (m, -(CH2)3-); PLA moiety: 5.04 (m, -CH-), 4.27 (q, -CH-OH), 1.5 (m, -CH3).
Figure 2.3. MALDI-TOF Mass spectra of poly(rac-Lactide) initated from 4-pyrene-
methylamine ([LA]0 = 1 M, [pyr-NH2]0 = 0.02M, [2.1]0 = 0.02M, 68% conv, Mn =
6,400 (GPC vs PS), Mw/Mn = 1.06, degree of polymerization (a) entire spectra, (b) blowup of spectra. Calculated m/z for n(Lactic acid repeat unit = 72) + pyrene amine + Na = 4791 (n = 63), 4862 (n = 64), 4935 (n = 65), 5007 (n = 66), 5079 (n = 67), 5152 (n = 68).
53 Table 2.1. Polymerization of Lactide from Amine and Alcohol Initiators.
Entries [M]0/[I]0 Polym. Conv. Mn(theory) MnNMR MnGPC Mw/Mn Initiator Time (%) (g.mol-1) (g.mol-1) (g.mol-1) (h) a 1 Pyr-NH2 100 22.5 70 10022 8064 8549 1.07 b 2 PEO(NH2)2 86 71 79 13612 11605 9580 1.09 c 3 PEO(NH2)4 105 18 ~100 19720 18099 19388 1.24 d 4 PEO(OH)2 101 46 75 16902 14058 12874 1.17 5 -Cla e 13 22 85 1984 1984 NA - a b c [LA]0= 0.82 M, [Pyr-NH2]0/[2.1]0 = 1. [LA]0= 0.35 M, [PEO(NH2)2]0/[2.1]0 = 0.25. [LA]0= 1.31 d e M, [PEO(NH2)4]0/[2.1]0 = 0.125. [LA]0= 1.31 M, [PEO(OH)2]0/[2.1]0 = 1. [LA]0= 0.35 M, [- Cla]0/[2.1]0 = 1.
15000 1.5 14000 1.45 13000 y = 100.71x + 5396.7 1.4 2 12000 R = 0.9499 1.35 11000 1.3 10000 1.25 9000 1.2 PDI GPC (g/mol)GPC
n 8000 1.15 M 7000 1.1 6000 1.05 5000 1 0 20406080100 Conv.(%)
Figure 2.4. Chart of Molecular weight (-- GPC vs. Polystyrene) and Mw/Mn (-o-) versus conversion for the polymerization of rac-lactide initiated from PEO-(NH2)2 in -2 C6D6 at 90°C using 2.1 as catalyst. Conditions: [LA]0 = 1M, [PEO]0 = 1.99 x 10 M -2 and [2.1]0 = 3.99 x 10 M.
54
1 Figure 2.5. 300 MHz H-NMR (CDCl3) of PLA2-PEO-PLA2 (Mn = 9,580; Mw/Mn = 1.09). The numbers under the brackets indicate the integration of the designated peaks. See text for peak assignments.
3.5
3
y = 0.6334x 2.5 R2 = 0.9501
2
1.5 ln([LA]o/[LA]t)
1
y = 0.03184x 0.5 R2 = 0.99703
0 01234567 hours Figure 2.6. First-order plots for the rac-lactide polymerization initiated with either
PEO-(NH2)2 (upper) or PEO-(OH)2 (lower) in C6D6 at 90°C using triazolium carbene
2.1 as catalyst. Conditions: PEO-(NH2)2 polymerization: [LA]0 = 1 M, [PEO]0 = 1.99
55 -2 -2 x 10 M and [2.1]0 = 3.99 x 10 M; PEO-(OH)2 polymerization: [LA]0 = 6.25 M, -3 -2 [PEO]0 = 6.25 x 10 M and [2.1]0 = 1.25 x 10 M. As noted in text, k1-NH2/ k1-OH =
1.97, where kobs = k1[PEO][ 2.1], see ref. 6.
16
14 20703
12
10 8926 Intensity 8
6
4 25 30 35 40 Ret. Vol. (ml)
Figure 2.7. Gel Permeation Chromatogram of lactide polymerization carried out with
an equimolar mixture of PEO-(NH2)2 and PEO-(OH)2 in C6D6 at 90°C using -2 triazolium carbene 2.1 as catalyst. Conditions: [LA]0 = 1.16 M, [1]0 = 1.0 x 10 M, -3 PEO-(NH2)2, [PEO-(NH2)2]0 = [PEO-(OH)2]0 = 1.72 x 10 M
2.5 References (1) Hawker, C. J.; Wooley, K. L. Science 2005, 309, 1200-1206. (2) Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel, A.; Voit, B.; Pyun, J.; Fréchet, J. M. J.; Sharpless, K. B.; Fokin, V. V. Angew. Chem. Int. Ed. 2004, 43, 3928-3930. (3) Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H.; Chem. Rev 2001, 101, 3747-3792. (4) Kim, Y.; Pyun, J.; Frechet, J. M. J.; Hawker, C. J.; Frank, C. W. Langmuir 2005, 21, 10444-10458. (5) Magbitang, T.; Lee, V. Y.; Cha, J. N.; Wang, H. L.; Chung, W. R.; Miller, R. D.; Dubois, G.; Volksen, W.; Kim, H. C.; Hedrick, J. L. Angew. Chem., Int. Ed. 2005, 44, 7574-7576.
56 (6) Vanhest, J. C. M.; Delnoye, D. A. P.; Baars, M. W. P. L.; Vangenderen, M. H. P.; Meijer, E. W. Science 1995, 268, 1592-1596. (7) Percec, V.; Ahn, C. H.; Ungar, G.; Yeardley, D. J. P.; Moller, M.; Sheiko, S. S. Nature 1998, 391, 161-164. (8) Cong, Y.; Li, B. Y.; Han, Y. C.; Li, Y. G.; Pan, C. Y. Macromolecules 2005, 38, 9836-9846. (9) Gido, S. P.; Lee, C.; Pochan, D. J.; Pispas, S.; Mays, J. W.; Hadjichristidis, N. Macromolecules 1996, 29, 7022-7028. (10) Roovers, J.; Toporowski, P. M. Macromolecules 1981, 14, 1174-1176. (11) Yu, X. F.; Shi, T. F.; Zhang, G.; An, L. J. Polymer 2006, 47, 1538-1546. (12) Tezuka, Y.; Ohashi, F. Macromol. Rapid. Commun. 2005, 26, 608-610. (13) Li, Y. G.; Shi, P. J.; Pan, C. Y. Macromolecules 2004, 37, 5190-5194. (14) Han, D. H.; Pan, C. Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2794- 2798. (15) Kocovsky, P.; Malkov, A. V. Tetrahedron (Symposium in Print) 2006, 62, 243-244. (16) Pratt, R. C.; Lohmeijer, B. G. G.; Long, D. A.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2006, 128, 4556-4558. (17) Hermans, T. M.; Choi, J.; Lohmeijer, B. G. G.; Dubois, G.; Pratt, R. C.; Kim, H. C.; Waymouth, R. M.; Hedrick, J. L. Angew. Chem., Int. Ed. 2006, 45, 6648-6650. (18) Dove, A. P.; Pratt, R. C.; Lohmeijer, B. G. G.; Culkin, D. A.; Hagberg, E. C.; Nyce, G. W.; Waymouth, R. M.; Hedrick, J. L. Polymer 2006, 47, 4018-4025. (19) Dove, A. P.; Li, H. B.; Pratt, R. C.; Lohmeijer, B. G. G.; Culkin, D. A.; Waymouth, R. M.; Hedrick, J. L. Chem. Commun. 2006, 2881-2883. (20) Coulembier, O.; Lohmeijer, B. G. G.; Dove, A. P.; Pratt, R. C.; Mespouille, L.; Culkin, D. A.; Benight, S. J.; Dubois, P.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 5617-5628. (21) Dove, A. P.; Pratt, R. C.; Lohmeijer, B. G. G.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2005, 127, 13798-13800.
57 (22) Csihony, S.; Culkin, D. A.; Sentman, A. C.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2005, 127, 9079-9081. (23) Coulembier, O.; Dove, A. P.; Pratt, R. C.; Sentman, A. C.; Culkin, D. A.; Mespouille, L.; Dubois, P.; Waymouth, R. M.; Hedrick, J. L. Angew. Chem., Int. Ed. 2005, 44, 4964-4968. (24) Nyce, G. W.; Glauser, T.; Connor, E. F.; Moeck, A.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2003, 125, 3046-3048. (25) Pratt, R. C.; Lohmeijer, B. G. G.; Long, D. A.; Lundberg, P. N. P.; Dove, A. P.; Li, H.; Wade, C.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 7863-7871. (26) Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.; Long, D. A.; Dove, A. P.; Nederberg, F.; Choi, J.; Wade, C.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 8574-8583. (27) Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.; Melder, J. P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1021-1024. (28) Kowalski, A.; Libiszowski, J.; Biela, T.; Cypryk, M.; Duda, A.; Penczek, S. Macromolecules 2005, 38, 8170-8176. (29) Cai, Q.; Zhao, Y. L.; Bei, J. Z.; Xi, F.; Wang, S. G. Biomacromolecules 2003, 4, 828-834. (30) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004, 37, 534-541. (31) Hoffmann, R. W.; Hagenbruch, B.; Smith, D. M. Chem. Ber. 1977, 110, 23- 57. (32) Iatrou, H.; Avgeropoulos, A.; Hadjichristidis, N. Macromolecules 1994, 27, 6232-6233.
58 CHAPTER 3
Cyclic Guanidine Organic Catalysts: What Is Magic About Triazabicyclodecene?
Reprinted in part with permission from J. Org. Chem. 2009, 74, 9490. Copyright 2007 by the American Chemical Society
59 3.1 Introduction Almost a century after its beginnings,1-4 the field of organocatalysis has undergone a resurgence with the development of new classes of highly enantioselective organocatalysts.5-8 Notwithstanding the extraordinary pace of developments in transition metal and organometallic catalysis,9,10 it is clear that organocatalytic reactions have evolved to provide a powerful addition to the armamentarium of methods for chemical synthesis.5-8 Organocatalysis has also proven a powerful strategy for polymer synthesis.11 We have investigated a variety of nucleophilic and basic organic molecules as catalysts for transesterification12,13 and ring-opening polymerization reactions (Figure 3.1).11 The highly basic and nucleophilic N-heterocyclic carbenes are potent organocatalysts for the ring-opening polymerization of lactones, generating polyesters of defined molecular weights in seconds at room temperature.11,14 Mechanistic and theoretical studies indicate that N- heterocyclic carbenes bind readily to alcohols,15,16 activating the alcohol for nucleophilic attack and stabilizing the resultant tetrahedral intermediates.16,17 In the absence of alcohols, the N-heterocyclic carbenes react directly with lactones and mediate the zwitterionic ring-opening polymerization of esters by a nucleophilic mechanism.14,18 This mechanistic duality is common to many acylation reactions catalyzed by amines and nitrogen heterocycles.19,20 Me N N N NtBu P N N N N N NEt2 R' R' H TBD Me MTBD Me BEMP NN R R Me2N NMe2 N Me2N P N P N NHC N tBu DBU Me2N NMe2 P2-tBu Figure 3.1. Nucleophilic and basic organocatalysts for ring-opening polymerization
In addition to the N-heterocyclic carbenes, we have also surveyed a variety of other potent neutral organic bases as catalysts for ring-opening polymerization
60 reactions. Guanidines such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD),21,22 N- methyl-TBD (MTBD), and 1,4,6- triazabicyclo[3.3.0]oct-4-ene (TBO),21 amidines such as 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU)23-25 and phosphazenes such as 2- tert- butylimino- 2- diethylamino- 1,3- dimethyl-perhydro- 1,3,2- diazaphosphorine (BEMP) and 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2Λ5,4Λ5- catenadi- 26 (phosphazene) (P2-t-Bu) are all effective catalysts for the ring-opening polymerization of lactones and cyclic carbonates27-32 (Figure 3.1). TBD is among the most active ring-opening polymerization catalysts that we have investigated to date. The ring-opening polymerization of lactide with 0.1% TBD in THF exhibits a turnover frequency of 80 s-1 at room temperature,23,25 which is comparable to those of the most active metal catalysts reported for ROP of lactide.33-35 These polymerizations are also remarkably well-controlled, yielding polylactide with well-defined molecular weights and narrow polydispersities (Figure 3.2).
OH O O NN Mes Mes O H O + O O O n 0.5 % THF, O O 25 °C, TOF = 18 s-1 20 seconds! 96 %, Mn = 23,000, PDI = 1.09
OH N O O N N O H O + H O O n O O 0.1%, 25°C O 0.1%, 25 C 20 seconds! TOF = 80 s-1 99 %, Mn = 63,000, PDI = 1.11 Figure 3.2. Ring-opening polymerization of lactide with TBD is even faster than with NHC's
TBD is a more active catalyst than MTBD or DBU for lactide polymerization and catalyzes the ring-opening polymerization of -valerolactone and -caprolactone 23,25 + under conditions where MTBD and DBU are inactive. TBD (TBDH , pKa = 26), + MTBD (MTBDH , pKa = 25) and DBU (DBUH+, pKa = 24) have comparable basicities in THF36 to those calculated for the N-aryl substituted N-heterocyclic
61 37-39 carbenes (pKa 27-28). The large differences in activity observed for TBD, MTBD and DBU implies that thermodynamic basicity is not the sole criterion for predicting catalytic activity. Guanidines and amidines are effective catalysts for a variety of organic reactions.22,40-44 These commercially available, easily handled bases have been reported as transesterification catalysts.27,45-48 In water, guanidines and amidines are readily protonated,36 and their biological activity49-51 and much of their reaction chemistry is assumed to proceed via guanidinium or amidinium intermediates.42-44,51,52 However, several studies have shown that guanidines and amidines can act as nucleophiles.22,53,54 We had previously shown that TBD can be acylated by vinyl acetate, implicating that TBD can act as a nucleophile.23 Subsequent reaction of acyl-TBD with benzyl alcohol yielded the ester, leading us to propose a nucleophilic mechanism as a potential pathway for ring-opening by TBD (Figure 3.3).23,25 Subsequent theoretical studies24,55 indicated that a nucleophilic mechanism was feasible, but had a considerably higher barrier than a hydrogen-bond mediated mechanism (Figure 3.4) for transesterification reactions.
O O O Ph OH Ph O N N
N N H N N N H O O N N H Figure 3.3. Model studies demonstrating the acylating ability of TBD
62 N N N N N N N N N H H R O H H H H O O O O O O R O O R O O O O O O Figure 3.4. Hydrogen-bonding mechanism suggested by theoretical studies24,55
The implication that TBD can act as a bifunctional nucleophilic catalyst suggests that it may be able to acylate other nucleophiles. The acylation of amines is of particular interest as the formation of amides from esters is an exceedingly useful reaction typically carried out under forcing conditions with highly basic catalysts.56-67 Maggi had previously demonstrated that TBD catalyzes the formation of ureas from carbonates and primary amines,68,69 and while this work was ongoing, Mioskowski reported the aminolysis of esters with amines to form amides under solvent-free conditions.70 In this article, we report kinetic studies on the acylation of amines by esters which strongly implicate a nucleophilic mechanism for the conversion of esters to amides in the presence of TBD. Studies of the analogous bicyclic guanidine 1,4,6- triazabicyclo[3.3.0]oct-4-ene (TBO)21 revealed it to be a much slower catalyst; mechanistic and theoretical studies provide useful insights on the stereoelectronic properties of TBD that contribute to its remarkable ability to catalyze acylation reactions.
3.2 Results and Discussion The amidation of vinyl acetate with 4 equiv. of n-butylamine with 10 mol % TBD in toluene solution at 25°C affords n-butylacetamide quantitatively in 6 minutes (Figure 3.5). While vinyl esters are known to acylate primary amines in the absence of a catalyst,71,72 these reactions are significantly slower than that observed in the presence of TBD (6 min vs. 24 hours). In the presence of four equivalents of n-
63 butylamine, benzyl acetate converted cleanly (99% conversion, 94.9% isolated yield) to n-butylacetamide in 5 hours at 80°C in toluene in the presence of 10 mol% TBD. Under solvent-free conditions reported by Mioskowski,70 neat benzyl acetate reacts with 1.3 equiv. of butylamine in the presence of 22 mol % TBD to give n- butylacetamide in 89% isolated yield after 2 hours. No special care is needed in the purification of solvents: reactions in reagent grade toluene, THF, DMSO or CH2Cl2 proceeded in quantitative yield, albeit with slightly slower rates than those in dry toluene. The conversion of benzyl acetate to n-butylacetamide did not proceed at an appreciable rate when N-methylTBD (MTBD) or triethylamine were substituted for TBD. O O TBD (10%) H + Bu NH2 Bu + O N tol,tol. 25¡C,25°C, 6 min min H O 99%
O O TBD (10%) + Bu NH2 Bu + PhCH2OH N O Ph tol.tol, 80°C,80¡C, 55 hh H 25 ¡C, 10 h 25°C, 10 h 95% O O TBD (10%) Bu + Bu NH2 + N HO O tol, 80°C, 2.5 h H OH OH 97% retention of configuration
TBD (10%) H O N + Bu NH2 Bu Me tol, 80°C, 20 days O O O O + MeOH 64% Figure 3.5. Catalytic amidation of esters
The amidation of (S)-ethyl lactate is considerably faster than benzyl acetate, generating a 71% yield of n-butyl lactamide within 30 minutes at 80°C. Analysis of the Mosher ester of n-butyl lactamide reveals it to be of high diastereomeric purity (only one isomer observed by 19F NMR) and derived from (S)-n-butyl lactamide,
64 indicating that the amidation of ethyl lactate proceeds with retention of configuration with minimal epimerization. In contrast, amidation of the more acidic and sterically demanding (S)-2-(6-Methoxy-naphthalen-2-yl)-propionic acid methyl ester was considerably slower (20 days, 64% yield) and yielded racemic n-butyl 2-napthyl propranamide. In the latter case, due to the much slower rate, TBD-catalyzed epimerization of either the ester or amide can compete with amidation. Thus, while we had anticipated that the basic nature of TBD might lead to the racemization of acidic esters, it is clear that the extent of racemization depends sensitively on the nature of the ester. Screening experiments, Table 3.1, by 1H NMR revealed that catalytic formation of amides from branched and secondary amines or from branched esters exhibited much slower rates at 25°C in solution ([Ester]o = 0.23 M) than those of benzyl acetate and ethyl lactate. However, under solvent-free conditions, the catalytic amidation of methyl phenylacetate yields the amides in 94% yield after 12 hours at 75°C.70
Table 3.1. Substrate Screening for the TBD Catalyzed Amidation of Esters
entry R1 R2 R3 time (h) temperature conversion (C) (%)b 1 Bz Me n-Bu 10 25 99 (94.9)f 2 vinyl Me n-Bu 0.1 25 99 3 Me Me n-Bu 48 25 97 4 i-Pr Me n-Bu 121 25 57 5 t-Bu Me n-Bu NRd -- -- 6 Me i-Pr n-Bu 120 25 99 7 Me Et n-Bu 75 25 89 8 Me Ph n-Bu 121 25 76
65 9 Me cyclohexyl n-Bu 600 25 89
NH2 12 Bz Me 122 25 99
2-naphthyl 13e Me n-Bu 504 25 65(64)f propionate
Boc NH H 14 Me n-Bu 648 25 84
15 Et ethan-1-ol n-Bu 2.5 25 99 (97)f 1 Bz Me n-Bu 5 80 99 2 Me Me n-Bu 15 80 99 3 i-Pr Me n-Bu 21 80 57 4 t-Bu Me n-Bu NRd -- -- 5 Me i-Pr n-Bu 24 80 99 6 Me Et n-Bu 31 80 99 7 Me Ph n-Bu 75 80 96 8 Et ethan-1-ol n-Bu <0.5 80 99 a b Reaction Conditions: Amine (0.92M), ester (0.23M) and TBD (0.023M) in toluene-d8 at 25C. Conversion based on 1H NMR analysis of crude reaction mixture. c Yield determined by integration versus an internal standard, TBD. d No reaction observed after 96 h. e see experimental for naproxen rxn conditions f Isolated yield.
3.2.1 Kinetics and Mechanism Theoretical studies implicate that a hydrogen-bonded mechanism has a lower barrier than a nucleophilic acylation mechanism for transesterification reactions (Figure 3.4).24,55 For amine nucleophiles, an H-bonded mechanism analogous to Figure 3.4 is less likely and motivated us to investigate the chemical and kinetic competence of a nucleophilic mechanism for amine acylation. To this end, kinetic investigations of the reaction of benzyl acetate with n- 1 butylamine were studied by H NMR in the presence of TBD. In toluene-d8 at 298K under pseudo first order conditions, the rate of disappearance of benzyl acetate is first order in benzyl acetate, first order in TBD, first order in amine and inverse first order in benzyl alcohol, yielding a rate law described by eq. (1),
66
d[Ester] [Ester][RNH ][TBD] k 2 (1) dt obs [ROH] where [Ester], [RNH2], [TBD], and [ROH] equal the concentrations of benzyl acetate, -3 -1 -1 n-butylamine, TBD, and benzyl alcohol, respectively; and kobs =1.9 ± 0.1 x 10 M s . This rate law can be accommodated by the mechanism shown in Figure 3.6 involving the reversible formation of an acyl-TBD intermediate, followed by irreversible trapping of acyl-TBD with butylamine to generate n-butylacetamide and TBD.
N N N N k k N N O 1 2 H N O Ph O + + O N N k-1 Bu NH2 H Ph OH Bu N H Figure 3.6. Proposed mechanism for formation of n-butylacetamide from benzyl acetate and butylamine.
Application of the steady-state assumption to the mechanism described in Figure 3.6 yields the rate law in eq (2): d[Ester] k k [Ester][RNH ][TBD] 1 2 2 (2) dt k1[ROH ] k2[RNH 2 ] This rate equation would be consistent with the experimental rate law under conditions where k-1[ROH] >> k2[RNH2], for which:
k1k2 kobs . (3) k1 In an effort to demonstrate the validity of this assumption, we measured the individual rate constants for some of the discrete steps of the proposed mechanism in Figure 3.6. For these studies, acyl-TBD was generated in situ from the reaction of TBD and vinyl acetate (Figure 3.7).23 When 10 equivalents of butylamine were added to the in situ-generated acyl-TBD intermediate, we observed a first-order decay in
67 1 -3 -1 -1 acyl-TBD concentration by H NMR with a rate constant of k2 = 0.9 ± 0.2 x 10 M s (298K). The reaction of acyl-TBD with excess benzyl alcohol exhibited first-order -2 -1 -1 kinetics and provided an estimate for k-1 = 1.86 ± 0.39 x 10 M s (298K). The greater than tenfold difference between these two rate constants supports our approximation used to derive equation 3, and is in accordance with an acyl transfer mechanism defined by a steady-state concentration of acyl-TBD.
O + TBD Bu NH Bu 2 N O H k2 N O N k N N H N N -1 H O + TBD O O Ph OH Ph O
Figure 3.7. Generation of Acyl-TBD and acylation with amines and alcohols
Given the kinetic parameters reported above, equation 3 was employed to -2 -1 -1 estimate a value for k1 equal to 3.7 ± 0.5 x 10 M s under the catalytic conditions
used to determine kobs. Independent measurement of k1 from the reaction of benzyl acetate with TBD was unsuccessful, as a rapid approach to equilibrium did not permit accurate measurement of the forward rate. The equilibrium constant for the formation
of the acyl-TBD intermediate in the absence of amine was determined Keq = 1.6 ± 0.2 x 10-4 (303K). Analysis of the temperature dependence of this equilibrium constant yielded the following thermodynamic data: ∆H = 18.22 ± 0.01 kJ/mol, ∆S = -1.7 ± 0.3 J/mol. These data describe an endergonic reaction between benzyl acetate and TBD, and are consistent with the inverse first order dependence of the catalytic rate on benzyl alcohol concentration.
3.2.2 Effect of Catalyst Structure The modest rates for the amidation of sterically demanding substrates with TBD motivated us to investigate less sterically hindered bicyclic guanidines as
68 catalysts. We prepared 1,4,6-triazabicyclo[3.3.0]oct-4-ene (TBO),21,73 analogs of which had been shown to be effective catalysts for enantioselective Strecker reactions.40 We anticipated that the larger N-C-N angle of TBO22 and the lower basicity of TBO relative to TBD74 might facilitate reactions with more sterically hindered substrates and potentially mitigate the racemization of α-substituted esters. However, the catalytic activity of TBO for the amidation of benzyl acetate with butylamine was considerably slower than that of TBD. The amidation of benzyl acetate with 10 equivalents of butylamine in toluene with 10 mol % TBO did not proceed at a measurable rate at 25 °C, whereas under comparable conditions TBD catalyzed the amidation of benzyl acetate to butylacetamide in 90% yield after 100 minutes. At higher temperatures (70 °C), TBO catalyzed the formation of butylacetamide, but in only 20% conversion after 100 minutes. Kinetic analysis revealed that the observed rate constant for amidation of benzyl acetate by TBO was -4 -1 -1 kobs(TBO, 343K) = 2.3 ± 0.4 x 10 M s , whereas that for TBD at 298K was -3 -1 -1 kobs(TBD, 298K) = 1.9 ± 0.1 x 10 M s . Mechanistic studies were carried out to illuminate the origin of the lower rates observed with TBO relative to TBD. In toluene solution at room temperature, TBD reacts quantitatively with vinyl acetate within minutes to generate acyl-TBD. In contrast the acylation of TBO with vinyl acetate requires over 16 hours to generate N- acyl-TBO. Isolation of this intermediate provides support for the nucleophilic75 attack of TBO on vinyl acetate to generate the acylated guanidine, but the slower rate implies that TBO is a less potent nucleophile than TBD. This was supported by experiments with benzyl acetate. Treatment of TBD with one equivalent of benzyl acetate led to a very rapid reaction to generate an equilibrium mixture of acyl-TBD, benzyl alcohol, and TBD. In contrast, the reaction between TBO and benzyl acetate was much slower, even at elevated temperature. An equimolar mixture of TBO and benzyl acetate (both 0.13 M in toluene) slowly converted to acyl-TBO and benzyl alcohol but even after 100 hours at 343K equilibrium was not established. Likewise, acyl-TBO reacted slowly with benzyl alcohol to generate benzyl acetate but this mixture did not reach equilibrium even after 16 hours at 343K. These data imply that TBO is much less
69 reactive towards esters than TBD, and acyl-TBO is much less reactive towards alcohols than acyl-TBD. This was confirmed with kinetic studies for the reaction of acyl-TBO with excess butylamine. Under pseudo-first order conditions, the rate of disappearance of acyl-TBO followed first order kinetics at 343K, yielding a rate constant for acylation -4 -1 -1 of butylamine, k2(ATBO, 343K) = 2.6 ± 0.1 x 10 M s . This rate constant is lower -3 -1 -1 than that measured for TBD k2(ATBD, 298K) = 0.9 ±0.2 x 10 M s . These data show that the slower rates observed for TBO are a consequence both of the lower nucleophilicity of TBO relative to TBD as well as the slower rate of transacylation of the acylguanidine intermediate. We turned to computer simulations to elucidate the structural differences between TBD, TBO and the acylated guanidines, acyl-TBD and acyl-TBO. Coles has previously compared the coordination chemistry of TBD and TBO and has carried out DFT calculations on the geometries and frontier molecular orbitals of these bicyclic guanidines.76 On the basis of natural bond order analysis, Coles observed a higher electron density on the imine nitrogen of TBD, relative to that of TBO, consistent with our observations of the higher reactivity of TBD toward vinyl and benzyl acetate. The structures of acyl-TBD and acyl-TBO were geometry optimized at the B3LYP/6-31G* level using Spartan ’02. Analysis of the calculated structures of the two acylated guanidines is revealing. For acyl-TBO, the acyl is coplanar with the guanidine moiety (C1-C2-N3-C4 dihedral angle of 3°) which is consistent with a planar amide. In contrast, the calculated C1-C2-N3-C4 dihedral angle of acyl-TBD is approximately 15°, indicative of a twisted amide, Figure 3.8. This deviation from amide planarity is also manifested in the loss of carbonyl-guanidine conjugation
(TBD: C2-N3 = 1.403Å, TBO C2-N3 = 1.393Å) and loss of conjugation within the guanidine moiety (TBD: (C4-N)max–(C4-N)min = 0.036Å, TBO: (C4-N)max–(C4-N)min = 0.007Å). The twisted amide calculated for acyl-TBD might explain its enhanced reactivity towards amine and alcohol nucleophiles. In particular, the much slower rate at which the planar amide of acyl-TBO reacts with amines relative to the twisted
70 amide of acyl-TBD suggests that a planar amide acyl-guanidine intermediate is too stable for effective catalysis.77 The same DFT protocol was used to optimize the geometries of TBD and TBO76 so that an isodesmic acyl-exchange reaction could be calculated, eq. 4. The acyl exchange from acyl-TBD to acyl-TBO is predicted to be E° = -7.2 kcal/mol, indicating that the structural differences calculated for acyl-TBD and acyl-TBO are also manifested in a greater thermodynamic stability for acyl-TBO.
N N N N + + 6 6 N 4 N 3 (4) N 4 N 3 N N E° = - 7.2 kcal/mol N N 5 2 H (calc) H 5 2 O 1 O 1
ATBD ATBO Figure 3.8. B3LYP/6-31G* calculated geometries of ATBD and ATBO
3.2.3 Lactide Polymerization The relative reactivity of TBD and TBO for amidation of esters is also reflected in their relative reactivity for the ring-opening polymerization (ROP) of lactide (Table 3.2). The ROP of lactide occurs within minutes in the presence of TBD and an alcohol initiator.23,25 In contrast, TBO demonstrates much lower catalytic activity. ROP of lactide with 2 mol % TBO proceeded to 16 % conversion after 1 hour at room temperature in CH2Cl2. To achieve complete conversion over a reasonable time period, catalyst loading was increased to 4 mol %. Under these conditions nearly quantitative monomer conversion was observed after 4 hours for various targeted degrees of polymerization (Table 3.2) but the molecular weight distributions were broad, Mw/Mn = 1.8-2.0. The molecular weights were close to that predicted by the monomer to initiator ratio for target DP = 100 and 200, but were
71 lower than predicted for DP = 400. Transesterification of the formed polymer chain could account for this lack of control, as the relative rate of polymerization would be reduced due to the lower nucleophilicity of TBO.
Table 3.2. Polymerization of L-lactide (L-LA) with TBO a Entry Time [M]0/[I]0 Conversion Mn Mn PDI b c (hours) (%) predicted (GPC) 1 1.0 100 92 13,200 15,800 1.84 2 2.5 200 82 23,600 18,900 1.90 3 2.5 400 95 54,700 19,500 2.02
Conditions: [M]0 = 1 M [TBO]0 = 0.04 M in CH2Cl2 at room temperature. [I]0 = 11, 5.3, 2.4 mM for entries, 1, 2, and 3, respectively (a) monomer conversion as determined by 1H NMR using integration of methyl proton resonances of L-LA and the formed polymer. (b) PLA
molecular weight predicted based on ([M]0/[I]0)*144(g/mol)*conversion(%). (c) GPC calibrated to polystyrene in THF.
3.3 Conclusion The bicyclic guanidine TBD is a potent transacylation catalyst, mediating both the transesterification of esters and the formation of amides from esters. Mechanistic and theoretical studies reveal that TBD can act both as a bifunctional general base/H- bond donor and as a nucleophile for transacylation reactions. For transesterification reactions, a general base mechanism is predicted where H-bonding of the alcohol to TBD simultaneously activates the alcohol toward nucleophilic attack and generates a guanidinium species that stabilizes the tetrahedral intermediate. For amidation reactions, kinetic studies implicate a nucleophilic acylation mechanism where TBD reacts reversibly with the ester to generate a 'twisted amide' acyl-TBD intermediate, which subsequently acylates the amine. Comparative investigations of the bicyclic guanidine TBO provide further insights on the unusual geometric features of TBD that render it such an effective acylation catalyst. TBD is both more basic and more nucleophilic than TBO, facilitating the general base/H-bonding transesterification reactions and nucleophilic acylation pathways. In addition, the acyl-guanidine
72 intermediate generated from TBD cannot adopt a planar amide structure, rendering it more active for subsequent acylation by amines. The unique structural and stereoelectronic features of TBD contribute to its remarkable catalytic activity for transesterification and ring-opening polymerization reactions. More generally, these studies provide further insights on the chemical and biological role of bicyclic guanidine motifs in a number of natural products.49
3.4 Experimental Section
3.4.1 General Considerations All syntheses and kinetic studies were performed using standard glovebox and Schlenk techniques, unless stated otherwise. All chemicals were ordered from Aldrich and used as received unless stated otherwise. Toluene-d8 was distilled from potassium/benzophenone prior to use. Benzyl alcohol was dissolved in THF, stirred overnight over CaH2, filtered, and recovered by evaporation of the solvent before use. 1,4,6-triazabicyclo[3.3.0]oct-4-ene (TBO),21,73TBO and rac-N-butyl-2-hydroxy- propanamide78 were prepared as described in the literature. For kinetic studies of acyl transfer, conversion data were acquired by 300 MHz 1H NMR, using the integration of the acyl methyl resonance on benzyl acetate and that of the formed acetamide versus an internal standard (anisole).
3.4.2 Procedure for Kinetic Experiments
To a vial containing 0.8 mL of toluene-d8 was added 15.0 mg (0.11 mmol) TBD, 0.1 mL (0.074 g, 1.0 mmol) n-butylamine, and 0.1 mL (0.11 g, 1.0 mmol) benzyl alcohol. This solution was transferred to a J-Young NMR tube, and the reaction was initiated with addition of benzyl actetate (15.4 μL, 16.2 mg, 0.11 mmol). At this point the NMR tube was sealed, removed from the glove box, and assayed for conversion by 1H NMR. Disappearance of benzyl acetate and appearance of n- butylacetamide were monitored by integration of the benzylic methylene and acyl methyl resonances, respectively.
73
3.4.2.1 Kinetic and Thermodynamic Data
kobs vs. [Alcohol]
0.0003
0.00025
0.0002 y = 0.00032x R2 = 0.99189 0.00015
kobs (1/sec) kobs 0.0001
0.00005
0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1/[Alcohol] (1/M)
-1 Figure 3.9. Determining kobs(TBD) from kobs vs [alcohol] . [amine]o = 1.28 M;
[TBD]o= 0.128 M; [alcohol]o= given in toluene-d8; where slope = kobs [TBD]o[amine]o
kobs vs. [Amine]
0.0012
0.001
0.0008
0.0006 y = 0.000974x 2
kobs (1/sec) R = 0.989175 0.0004
0.0002
0 0 0.2 0.4 0.6 0.8 1 1.2 [Amine] (M)
Figure 3.10. Determining k2(TBD) from kobs vs. [amine]o. [amine]o = given;
[acylTBD]o = 0.045 M in toluene-d8, where slope = k2 [amine]o
74 kobs vs. [TBD]^m y = 0.4585x R2 = 0.9492 0.08
0.07
0.06
0.05
0.04 kobs 0.03 kobs vs. [TBD]^1
0.02
0.01
0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 [TBD]^m
Figure 3.11. kobs vs. [TBD]o demonstrates first order in TBD. [ROH]o = [amine]o =
1.276M, [ester]o = 0.1276M in toluene-d8
1st Order
0 01234567 -0.2
-0.4
-0.6 y = -0.2024x R2 = 0.9863
-0.8 ln(1-conv)
-1
-1.2
-1.4 Time (min.)
Figure 3.12. Determining k-1(TBD) from first order plots of ln[ROH]/[ROH]o vs time, [acylTBD]o= 0.02 M in toluene-d8. The slope = -k-1, and an average over several
[ROH]o was taken.
75 [ROH] vs time
0.14 0.12 0.1 0.08 0.06 0.04
[benzyl alcohol] 0.02 0 02468101214 min
Figure 3.13. One of the [ROH] vs time used to construct Figure 3.9.
-7
y = -2191.9x - 1.6615 R2 = 0.92
-8 room temp, K= 0.00016 ln K
-9
0.0024 0.0026 0.0028 0.0030 0.0032 0.0034 0.0036 0.0038 1/T (1/K)
Figure 3.14. Temperature dependent equilibrium between benzyl acetate/TBD and
benzyl alcohol/acyl-TBD. [ester]o = 10•[TBD]o = 0.867M in toluene-d8, ∆H = 18.22±0.010 kJ/mol, ∆S = -1.662±0.316 J/mol
76 0.05
0 0 20406080100120140 -0.05
-0.1
-0.15 y = -0.002376x + 0.013741 2 ln(1-conv) R = 0.989617 -0.2
-0.25
-0.3 min
Figure 3.15. Determining kobs(TBO) from ln([benzylacetate]/[ benzylacetate]o) vs
time. [amine]o = 1.26 M; [TBO]o= 0.135 M in toluene-d8; where slope = -kobs
[TBO]o[amine]o
0.5
0 0 20 40 60 80 100 120 140 -0.5
-1
ln(1-conv) -1.5 y = -0.0196x + 0.1527 -2 R2 = 0.9918
-2.5 min
Figure 3.16. Determining k2(TBO) from ln([acylTBO]/[ acylTBO]o) vs time.
[amine]o = 1.26; [acylTBD]o = 0.135 M in toluene-d8, where slope = -k2[amine]o
3.4.3 Typical Substrate Screening Experiment
In a dry box under N2 atmosphere, TBD (3.2 mg, 0.023 mmol) and n- butylamine (67.2 mg, 90.8 L, 0.92 mmol) were dissolved in 0.5 mL toluene-d8 and transferred to a J-Young NMR tube. To initiate the reaction, benzyl acetate (34.5 mg, 32.7 L, 0.23 mmol) was added to the NMR tube by micropipette.
77
Figure 3.17. 300 MHz 1H-NMR spectra of the TBD-catalyzed reaction of benzyl
acetate with n-butylamine in toluene-d8 t minutes after the addition of butylamine. The peaks marked with * are due to TBD, with are due to benzyl acetate, are due to butylamine, are due to benzyl alcohol and with red arrows are due to n- butylacetamide. The inset shows the evolution of the reaction with time.
3.4.4 Synthesis of n-butylacetamide (solution) Into an NMR tube in a dry box was loaded 0.0015 g (0.01 mmol) TBD, 0.45
mL toluene-d8, 0.043 mL (0.44 mmol) n-butylamine and 0.0155 mL (0.11 mmol) benzyl acetate. Reaction progress was monitored by 1H NMR. The crude reaction mixture was poured into water and extracted with 3x8 mL diethyl ether, dried with
MgSO4 and solvent was removed under reduced pressure, yielding 12 mg (94.9%). Characterization matched the literature.79
78 3.4.5 Synthesis of n-butylacetamide (neat) Into a vial in a dry box was loaded 67.3 mg (0.48 mmol) TBD, 0.3237 g (2.17 mmol) benzyl acetate, and 0.2099 mL (2.88 mmol) n-butylamine. After 2 hours, the crude reaction mixture was poured into water and extracted with 3x8 mL diethyl ether, dried with MgSO4 and solvent was removed under reduced pressure, yielding 0.221 g (89%). Characterization matched the literature.79
3.4.6 Synthesis of (S)-n-butyl-2-hydroxypropanamide Into an NMR tube in a dry box was loaded 0.0015 g (0.01 mmol) TBD, 0.7 mL toluene-d8, 0.05 mL (0.5 mmol) n-butylamine and 0.011 mL (0.1 mmol) ethyl (S)-(-)- lactate. Reaction progress was monitored by 1H NMR. The crude reaction mixture was poured into water and extracted with 3x8 mL diethyl ether, organics dried with
MgSO4, filtered and solvent removed under reduced pressure. The residue was shaken with pentane and the solution decanted off the product, yielding 14 mg (97%). 1H
NMR (CDCl3, 300 MHz, 25 °C) = 0.86 (t 3H), 1.16-1.49 (m 7H), 2.21 (s 1H), 3.21 (q 2H), 4.16 (q 1H).78
3.4.7 Synthesis of the Mosher Ester n-butyl-2-hydroxypropanamide (0.003 g, 0.021 mmol, 1 eq.) and (R)-(−)-α- methoxy-α-(trifluoromethyl)phenylacetyl chloride (0.0039 mL, 0.022 mmol, 1.01 eq.) were mixed with carbon tetrachloride (5 drops) and dry pyridine (1 drop) and stirred in a closed vial for 20 h. Reaction mixture was extracted with diethyl ether and water and washed twice with ether. After washing with dilute hydrochloric acid and saturated sodium carbonate solution, and drying with MgSO4 the mixture was filtered and solvent evaporated under reduced pressure. NMR spectra were taken without further purification.
3.4.8 Synthesis of (S)-naproxen methyl ester In a three neck round bottom flask equipped with a reflux condenser, 0.94 g
(4.09 mmol) (S)-naproxen was stirred in 20 mL methanol under N2. The flask was
79 cooled to 0°C and 0.049 mL (0.68 mmol) thionyl chloride was added dropwise via syringe. The mixture was warmed to room temperature over an hour and refluxed for 6 h. Volatiles were removed under high vacuum Yield 0.60 g, 60.5 %. 1H NMR
(CDCl3, 300 MHz, 25 °C) = 1.51 (d 3H), 3.59 (s 3H), 3.79 (q 1H), 3.84 (s 3H), 7.02- 24.5 7.66 (m 6H) []D = +79.65° (c = 26 mg/ 100mL; CH2Cl2), []D,Lit = +76.9 (c = 20 80 mg/ 100mL; CDCl3).
3.4.9 Synthesis of rac-naproxen methyl ester Racemic naproxen methyl ester was made by dissolving 0.2 g (0.819 mmol) (S)-naproxen methyl ester in 10 mL methanol with 0.02 g (0.164 mmol) 5- diazabicyclo[4.3.0]non-5-ene and refluxing overnight. The methanol solution was diluted with 50 mL water and extracted 3x25 mL methylene chloride, dried over
MgSO4 and solvent removed under reduced pressure, yielding 0.196 mg (97.9%). 23.7 []D = +0.0018° (c = 26 mg/ 1mL; CH2Cl2).
3.4.10 Synthesis of (S)-naproxen butyl amide In a dry box, an NMR tube was loaded with 1.6 mg (0.012 mmol) TBD, 1 mL
toluene-d8, 0.046 mL (0.46 mmol) n-butylamine and 28 mg (0.115 mmol) (S)- naproxen methyl ester. Reaction conversion was monitored by 1H NMR. After several days, the reaction had reached 65% conversion and was quenched by pouring into water. The water was extracted with 3x8mL diethyl ether, organics dried with
MgSO4 and solvent removed under reduced pressure. The residue was purified by silica gel column chromatography using ethyl acetate: hexanes (1:2). Yield: 21 mg 1 (0.073 mmol, 63.7%) H NMR (CDCl3, 300 MHz, 25 °C) δ = 0.77 (t 3H), 1.06-1.37 (m 4H), 1.53 (d 2H), 3.11 (q 2H), 3.61 (q 1H), 3.86 (s 3H),5.23 (s 1H), 7.05-7.69 (m 24.5 6H) []D = +1.11 (c = 20.9 mg/1 mL; CH2Cl2) HPLC; 99:10 Heptane / 2- 81 Propanol; flow rate = 0.8 mL/min; t1 = 12.3 min, t2 = 14.4 min.
80 3.4.11 Synthesis of 1,4,6-triazabicyclo[3.3.0]-oct-4-ene (TBO) Synthesized by the method of Cotton, et al.73 With stirring at room temperature under nitrogen atmosphere, xylenes (300 mL), diethylenetriamine (20.6 g, 21.7 mL, 0.2 mol), and carbon disulfide (15.2 g, 12.0 mL, 0.2 mol) were added to a three-necked flask. A white precipitate formed immediately and the suspension was heated to reflux. Evolution of H2S from the reaction exhaust was monitored using filter paper soaked in a methanolic suspension of lead(II) acetate. After 10 days of reflux under nitrogen, GC/MS analysis confirmed quantitative conversion to the target compound. Upon cooling to room temperature a white solid crystallized from solution, and the supernatant was decanted. The solid was washed with 2 x 50 mL portions of acetone and pentane, respectively, and dried under vacuum overnight. 1 (8.65 g, 39 %). H NMR 400 MHz (CDCl3) = 6.02 (br s, 1H), 3.79 (t, 2H, J = 7.0 13 Hz), 3.05 (t, 2H, J = 7.0 Hz). C NMR 100 MHz (CDCl3) = 171.18, 52.62, 49.38. LRMS (m/z): 112.1 (positive ion, M+H).
81
P PM 6.8 6.8 6.4 6.4 6.0 6.0 5.6 5.6 5.2 5.2 4.8 4.8 4.4 4.4 4.0 4.0
2.000
2.015 3.6 3.6 3.2 3.2 2.8 2.8
2.799
2.036 2.4 2.4
1.998 1 Figure 3.18. 400 MHz H NMR spectrum of acyl-TBO in CDCl3
82 30.0 30.0 40.0 50.0 60.0 70.0 70.0 80.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 PM
P 13 Figure 3.19. 100 MHz C NMR spectrum of acyl-TBO in toluene-d8.
83 3.4.12 Synthesis of 1-(2,3,5,6-tetrahydro-1H-imidazo[1,2-a]-imidazol-1- yl)ethanone (acyl-TBO) With stirring at room temperature in a drybox, THF (5 mL), TBO (0.115 g, 1.03 mmol), and vinyl acetate (0.1 mL, 0.09 g, 1.1 mmol) were added to a 20 mL glass vial. The solution was stirred for approximately 16 hours, at which point solvent and all volatiles were removed under vacuum, yielding a slightly off-white solid (0.143 g, 1 0.93 mmol, 90.3 %). H NMR 400 MHz (toluene-d8) = 3.77 (t, 2H, J = 7.8 Hz), 3.62 (t, 2H, J = 7.0 Hz), 2.51 (s, 3H), 2.45 (t, 2H, J = 7.2 Hz), 2.14 (t, 2H, J = 7.0 Hz). 13 C NMR 100 MHz (d8-toluene) = 168.54, 59.55, 50.76, 48.43, 44.34, 23.08. LRMS (m/z): 154.2 (positive ion, M+H). Elemental analysis: calcd: C = 54.89 %, H = 7.24 %, N = 27.43 %. Found: C = 54.71 % H = 7.23 % N = 27.20 %.
3.4.13 Polymerization of L-LA Using TBO catalyst L-LA (300 mg, 2.1 mmol) and TBO (10.0 mg, 0.1 mmol) were dissolved in
CH2Cl2 (2 mL). To initiate the polymerization benzyl alcohol (2.2 μL, 0.02 mmol) was added. The polymerization was quenched after 1 h by addition of excess benzoic acid 1 (~20 mg, 0.16 mmol), and solvent was removed under vacuum. H-NMR (CDCl3): δ = 8.15-7.45 (5H), 5.30-5.13 (m, ~200H), 4.4 (t, 2H), 1.67-1.43 (br d); GPC (RI -1 detection, polystyrene calibration): Mn = 15,800 g mol , PDI = 1.84.
3.4.14 Computational Details Structures were built in the Spartan ’02 software package (Windows version, Wavefunction Inc., Irvine, CA) and geometry optimized directly from the structure as drawn. An “equilibrium geometry” calculation (in the gas phase) at the “ground state” with “density functional” at the B3LYP/6-31G* level of theory, without enforcing symmetry, was performed on each structure. Final, optimized, coordinates and energies are given below.
TBD: -438.8240398 hartrees
Coordinates (Angstroms) ATOM X Y Z 1 H -1.282304 -2.168556 0.265972
84 2 C -1.319915 -1.163419 -0.173160 3 C -2.418024 1.071767 -0.147267 4 C -2.492255 -0.363114 0.387051 5 H -2.739018 1.089336 -1.201981 6 H -2.427236 -0.356270 1.482174 7 H -1.433737 -1.292425 -1.264773 8 H -3.133390 1.707527 0.391737 9 H -3.440756 -0.840888 0.113446 10 N -0.064453 -0.490055 0.145651 11 C 1.142496 -1.243806 -0.183752 12 H 1.232515 -1.383437 -1.277493 13 H 1.042393 -2.245196 0.253927 14 C 2.395050 -0.552147 0.348030 15 H 3.287348 -1.082370 -0.003124 16 H 2.392273 -0.570872 1.443647 17 C 2.405079 0.895792 -0.131726 18 H 2.537381 0.921731 -1.229030 19 H 3.242643 1.447118 0.309544 20 N 1.164791 1.523688 0.300066 21 H 1.077402 2.516842 0.120072 22 C -0.070271 0.907130 0.089469 23 N -1.098010 1.671624 -0.038479 Point Group: c1 Number of degrees of freedom: 63
Acyl-TBD: -591.4776079 hartrees
Coordinates (Angstroms) ATOM X Y Z 1 C -2.300469 0.337905 0.430541 2 H -3.093193 0.246347 -0.316843 3 H -2.743221 0.041186 1.387656 4 C -1.771120 1.771568 0.478470 5 H -1.354437 2.006558 1.464838 6 H -2.596916 2.467943 0.294165 7 C -0.664910 1.935492 -0.559917 8 H -0.260709 2.952659 -0.549580 9 H -1.052933 1.747786 -1.574197 10 N 0.413382 1.019983 -0.215664 11 C 0.133200 -0.293489 0.123324 12 N -1.245416 -0.630126 0.056016 13 C 1.802425 1.464834 -0.282334 14 H 1.836709 2.528426 -0.016546 15 H 2.186183 1.377859 -1.311753 16 C 2.666202 0.640359 0.671479 17 H 2.410001 0.897902 1.706552
85 18 H 3.724530 0.882687 0.520989 19 C 2.397355 -0.848848 0.434493 20 H 2.909191 -1.449275 1.196195 21 H 2.827588 -1.155290 -0.532963 22 N 0.982794 -1.192608 0.469040 23 C -1.726407 -1.915056 -0.224013 24 O -2.917325 -2.144127 -0.054330 25 C -0.805130 -2.971893 -0.798315 26 H -0.103552 -2.565740 -1.530519 27 H -1.446559 -3.725838 -1.260446 28 H -0.207264 -3.427203 -0.006338 Point Group: c1 Number of degrees of freedom: 78
TBO: -360.183104 1hartrees
Coordinates (Angstroms) ATOM X Y Z 1 H -1.900046 -1.590023 0.630623 2 C -1.456111 -0.824610 -0.013264 3 C -2.198359 0.547129 0.044223 4 H -2.946729 0.644001 -0.749713 5 H -1.415076 -1.215907 -1.045077 6 H -2.721900 0.664085 1.004354 7 N -0.122795 -0.425241 0.440424 8 C 1.120537 -1.024257 -0.033610 9 H 1.036410 -1.339321 -1.089086 10 H 1.416231 -1.894542 0.560380 11 C 2.121703 0.155887 0.118483 12 H 2.845036 0.184538 -0.703168 13 H 2.680009 0.081772 1.059906 14 N 1.255408 1.342602 0.127227 15 C -0.061621 0.947367 0.142011 16 N -1.150131 1.579668 -0.091845 17 H 1.497435 2.166853 -0.401868 Point Group: c1 Number of degrees of freedom: 45
Acyl-TBO: -512.8505944 hartrees
Coordinates (Angstroms) ATOM X Y Z 1 C 1.672412 1.864296 0.133099 2 H 1.882072 2.289272 1.124640 3 H 1.998230 2.593594 -0.614750 4 C 2.379037 0.483810 -0.034527 5 H 3.267008 0.368410 0.593522
86 6 H 2.662282 0.297182 -1.084527 7 N 1.294570 -0.408043 0.376978 8 C 0.130057 0.328365 0.146142 9 N 0.221985 1.597332 0.001549 10 C 1.075561 -1.772400 -0.088323 11 H 1.669233 -2.500356 0.472151 12 H 1.316281 -1.877411 -1.160254 13 N -0.939761 -0.555100 0.060740 14 C -0.439108 -1.939086 0.162017 15 H -0.646775 -2.340693 1.160107 16 H -0.937134 -2.573840 -0.572208 17 C -2.304713 -0.297278 -0.042720 18 O -3.085894 -1.236055 -0.065334 19 C -2.729715 1.150161 -0.127546 20 H -3.817122 1.174230 -0.211610 21 H -2.265778 1.646269 -0.984959 22 H -2.402725 1.707340 0.755811 Point Group: c1 Number of degrees of freedom: 60
3.5 References (1) Bredig, G.; Fiske, P. S. Biochemische Zeitschrift 1912, 46, 7-23. (2) Langenbeck, W. Die Organische Katalysatoren und ihre Beziehungen zu den Fermenten; 2nd ed.; Springer: Berlin, 1949. (3) Eder, U.; Sauer, G.; Weichert, R. Angew. Chem. Int. Ed. 1971, 10, 496-497. (4) Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615-1621. (5) For a recent overview see "Organocatalysis" special issue: List, B., Ed, Chem. Rev., 2007, 107, p. 5413-5883. (6) Bertelsen, S.; Jorgensen, K. A. Chem. Soc. Rev. 2009, 38, 2178-2189. (7) Barbas, C. F. Angew. Chem. Int. Ed. 2008, 47, 42-47. (8) MacMillan, D. W. C. Nature 2008, 455, 304-308. (9) Cornils, B.; Herrmann, W. A.; Editors Applied Homogeneous Catalysis with Organometallic Compounds VCH: Weinheim, 2002; Vol. 1, 2. (10) Thomas, J. M.; Raja, R.; Lewis, D. W. Angew. Chem., Int. Ed. 2005, 44, 6456- 6482. (11) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813-5840.
87 (12) Nyce, G. W.; Lamboy, J. A.; Connor, E. F.; Waymouth, R. M.; Hedrick, J. L. Org. Lett. 2002, 4, 3587-3590. (13) Grasa, G.; Kissling, R. M.; Nolan, S. P. Org. Lett. 2002, 4, 3583-3586. (14) Jeong, W.; Shin, E. J.; Culkin, D. A.; Hedrick, J. L.; Waymouth, R. M. J. Am. Chem. Soc. 2009, 131, 4884-4891. (15) Cowan, J. A.; Clyburne, J. A. C.; Davidson, M. G.; Harris, R. L. W.; Howard, J. A. K.; Kupper, P.; Leech, M. A.; Richards, S. P. Angew. Chem. Int. Ed. 2002, 41, 1432-1434. (16) Movassaghi, M.; Schmidt, M. A. Org. Lett. 2005, 7, 2453-2456. (17) Lai, C. L.; Lee, H. M.; Hu, C. H. Tetrahedron Lett. 2005, 46, 6265-6270. (18) Culkin, D. A.; Jeong, W. H.; Csihony, S.; Gomez, E. D.; Balsara, N. R.; Hedrick, J. L.; Waymouth, R. M. Angew. Chem. Int. Ed. 2007, 46, 2627-2630. (19) Spivey, A. C.; Arseniyadis, S. Angewandte Chemie - International Edition 2004, 43, 5436-5441. (20) Allemann, C.; Gordillo, R.; Clemente, F. R.; Cheong, P. H. Y.; Houk, K. N. Acc. Chem. Res. 2004, 37, 558-569. (21) In this work, the von Baeyer nomenclature for the bicyclic guanidines used. The IUPAC name for TBD is 1,3,4,6,7,8-Hexahydroimidazo[1,2-a]pyrimidine (H-hpp) and for TBO the IUPAC name is 2,3,5,6-tetrahydro-1H-imidazo[1,2- a]imidazole (H-tbo).. (22) Coles, M. P. Chem. Commun. 2009, 3659-3676. (23) Pratt, R. C.; Lohmeijer, B. G. G.; Long, D. A.; Waymouth, R. M.; Hedrick, J. L. J. Am. Chem. Soc. 2006, 128, 4556. (24) Chuma, A.; Horn, H. W.; Swope, W. C.; Pratt, R. C.; Zhang, L.; Lohmeijer, B. G. G.; Wade, C. G.; Waymouth, R. M.; Hedrick, J. L.; Rice, J. E. J. Am. Chem. Soc. 2008, 130, 6749-6754. (25) Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.; Long, D. A.; Dove, A. P.; Nederberg, F.; Choi, J.; Wade, C.; Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 8574-8583.
88 (26) Schwesinger, R.; Schlemper, H.; Hasenfratz, C.; Willaredt, J.; Dambacher, T.; Breuer, T.; Ottaway, C.; Fletschinger, M.; Boele, J.; Fritz, H.; Putzas, D.; Rotter, H. W.; Bordwell, F. G.; Satish, A. V.; Ji, G. Z.; Peters, E. M.; Peters, K.; vonSchnering, H. G.; Walz, L. Liebigs Annalen 1996, 1055-1081. (27) Le Perchec, P.; Baudry, R.; Alvarez, F., US Patent, 6,646,103 B1, 2003. (28) Zhang, L.; Nederberg, F.; Pratt, R. C.; Waymouth, R. M.; Hedrick, J. L.; Wade, C. G. Macromolecules 2007, 40, 4154-4158. (29) Zhang, L.; Nederberg, F.; Messman, J. M.; Pratt, R. C.; Hedrick, J. L.; Wade, C. G. J. Am. Chem. Soc. 2007, 129, 12610-12611. (30) Nederberg, F.; Trang, V.; Pratt, R. C.; Mason, A. F.; Frank, C. W.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2007, 8, 3294-3297. (31) Nederberg, F.; Lohmeijer, B. G. G.; Leibfarth, F.; Pratt, R. C.; Choi, J.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2007, 8, 153-160. (32) Jing, F.; Hillmyer, M. A. J. Am. Chem. Soc. 2008, 130, 13826-13827. (33) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229-3238. (34) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V. G.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125, 11350-11359. (35) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Chem. Commun. 2003, 48-49. (36) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019-1028. (37) Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J. Am. Chem. Soc. 2004, 126, 4366-4374. (38) Kim, Y. J.; Streitwieser, A. J. Am. Chem. Soc. 2002, 124, 5757-5761. (39) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717- 8724. (40) Corey, E. J.; Grogan, M. J. Org. Lett. 1999, 1, 157-160. (41) Simoni, D.; Rossi, M.; Rondanin, R.; Mazzali, A.; Baruchello, R.; Malagutti, C.; Roberti, M.; Invidiata, F. P. Org. Lett. 2000, 2, 3765-3768. (42) Ishikawa, T.; Kumamoto, T. Synthesis-Stuttgart 2006, 737-752.
89 (43) Leow, D.; Tan, C. H. Chemistry-an Asian Journal 2009, 4, 488-507. (44) Uyeda, C.; Jacobsen, E. N. J. Am. Chem. Soc. 2008, 130, 9228-9229. (45) Green, M. J., Eur. Patent Appl., 110629, 1986. (46) Gelbard, G.; Vielfaure-Joly, F. Tetrahedron Lett. 1998, 39, 2743-2746. (47) Kantam, M. L.; Sreekanth, P. Catal. Lett. 2001, 77, 241-243. (48) Schuchardt, U.; Vargas, R. M.; Gelbard, G. J. J. Mol. Catal. A: Chem. 1995, 99, 65-70. (49) Berlinck, R. G. S.; Burtoloso, A. C. B.; Kossuga, M. H. Natural Product Reports 2008, 25, 919-954. (50) O'Brien, P. J.; Lassila, J. K.; Fenn, T. D.; Zalatan, J. G.; Herschlag, D. Biochemistry 2008, 47, 7663-7672. (51) Perreault, D. M.; Cabell, L. A.; Anslyn, E. V. Biorg. Med. Chem. 1997, 5, 1209-1220. (52) Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107, 5713-5743. (53) Ghosh, N. Synlett. 2004, 3, 574-575. (54) Baidya, M.; Mayr, H. Chem. Commun. 2008, 1792-1794. (55) Simon, L.; Goodman, J. M. The Journal of Organic Chemistry 2007, 72, 9656- 9662. (56) Waghoo, G.; Jayaram, R. V.; Joshi, M. V. Synth. Commun. 1999, 29, 513-515. (57) Huang, P.; Zheng, X.; Deng, X. Tetrahedron Lett. 2001, 42, 9039-9041. (58) Smith, L. A.; Wang, W.; Burnell-Curty, C. Synlett. 1993, 850-852. (59) Houghton, R. P.; Williams, C. S. Tetrahedron Lett. 1967, 40, 3929-3931. (60) Lui, W.; Xu, D. D.; Repic, O.; Blacklock, T. J. Tetrahedron Lett. 2001, 42, 2439-2441. (61) Riviere-Baudet, M.; Morere, A.; Dias, M. Tetrahedron Lett. 1992, 33, 6453- 6456. (62) Sim, T. B.; Yoon, N. M. Synlett. 1994, 10, 827-829. (63) Han, C.; Lee, J. P.; Lobkovsky, E.; Porco, J. A. J. Am. Chem. Soc. 2005, 127, 10039-10044.
90 (64) Sidler, D. R.; Lovelace, T. C.; McNamara, J. M.; Reider, P. J. J. Org. Chem. 1994, 59, 1231-1233. (65) Feo, R. J. D.; Strickler, P. D. J. Org. Chem. 1963, 28, 2915-2917. (66) Wang, J.; Rosingana, M.; Discordia, R. P.; Soundararajan, N.; Polniaszek, R. Synlett. 2001, 9, 1485-1488. (67) Stephenson, N. A.; Zhu, J.; Gellman, S. H.; Stahl, S. S. J. Am. Chem. Soc. 2009, 131, 10003-10008. (68) Carloni, S.; De Vos, D. E.; Jacobs, P. A.; Maggi, R.; Sartori, G.; Sartorio, R. J. Catal. 2002, 205, 199-204. (69) Ballini, R.; Fiorini, D.; Maggi, R.; Righi, P.; Sartori, G.; Sartorio, R. Green Chemistry 2003, 5, 396-398. (70) Sabot, C.; Kumar, K. A.; Meunier, S.; Mioskowski, C. Tetrahedron Lett. 2007, 48, 3863-3866. (71) Kabouche, Z.; Bruneau, C.; Dixneuf, P. H. Tetrahedron Lett. 1991, 32, 5359- 5362. (72) Kita, Y.; Maeda, H.; Omori, K.; Okuno, T.; Tamura, Y. Journal of the Chemical Society-Perkin Transactions 1 1993, 2999-3005. (73) Cotton, F. A.; Murillo, C. A.; Wang, X.; Wilkinson, C. C. Inorg. Chem. 2006, 45, 5493-5500. (74) McKay, A. F. K.; Kreling, M.-E. Can. J. Chem. 1962, 40, 1160-1163. (75) McKay, A. F. K.; Kreling, M.-E.; Paris, G. Y.; Braun, R. O.; Whittingham, D. J. Can. J. Chem. 1957, 35, 843-849. (76) Khalaf, M. S.; Coles, M. P.; Hitchcock, P. B. Dalton Transactions 2008, 4288- 4295. (77) Kozuch, S.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 3355-3365. (78) Shih, C.; Waldron, N.; Zentner, G. M. J. Controlled Release 1996, 38, 69-73. (79) Steffel, L. R.; Cashman, T. J.; Reutershan, M. H.; Linton, B. R. J. Am. Chem. Soc. 2007, 129, 12956-12957. (80) Tsuchihashi, G.; Mitamura, S.; Kitajima, K.; Kobayashi, K. Tetrahedron Lett. 1982, 23, 5427-5430.
91 (81) Pirkle, W. H.; Welch, C. J. J. Liq. Chromatogr. 1991, 14, 3387-3396.
92 CHAPTER 4
Oligocarbonate Molecular Transporters: Oligomerization-Based Syntheses and Cell- Penetrating Studies
Reprinted in part with permission from J. Am. Chem. Soc. 2009, 131, 16401. Copyright 2009 by the American Chemical Society
93 4.1 Introduction New strategies, devices and agents that enable or enhance the passage of drugs or probes across biological barriers are required to address a range of major challenges in chemotherapy, imaging, diagnostics, and mechanistic chemical biology.1 In 2000, the Wender lab reported that the cellular uptake of the Tat49-57 peptide could be mimicked by homooligomers of arginine.2 Uptake was shown to be a function of the number and array of guanidinium groups, observations that led to the design and synthesis of the first guanidinium-rich (GR) peptoids,2 GR-spaced peptides,3 GR- oligocarbamates4 and GR-dendrimeric molecular transporters (MoTrs).5 Noteworthy subsequent studies from several groups showed that a variety of other scaffolds, including beta-peptides, carbohydrates, heterocycles, and peptide nucleic acids, upon perguanidinylation, exhibit cell-penetrating activity.6 GR MoTrs have been shown to carry a variety of cargos into cells, including small molecules, probes, metals, peptides, proteins, siRNA, morpholino-RNAs, and DNA plasmids.7 Activatable MoTrs have been reported for targeted therapy and imaging,8 a releasable octaarginine-drug conjugate has been shown to overcome P-glycoprotein-mediated resistance in animal models of cancer,9 and a drug-heptaarginine conjugate has been advanced to phase II human clinical trials.10 While octaarginine MoTrs have been made on scale under good manufacturing practice conditions and a step-saving segment doubling approach has been introduced,11 the length and associated costs of these syntheses preclude some anticipated applications. A solid phase synthesis of octaarginine requires ≥16 steps, while the segment doubling approach involves 9 steps.11 Reported herein is a new family of oligocarbonate GR MoTrs that can be flexibly and efficiently assembled in a one-step organocatalytic ring opening oligomerization process that also allows for concomitant probe (or drug) attachment and control over transporter length. The Waymouth and Hedrick groups have previously shown that a metal-free, organocatalytic ring-opening polymerization (ROP)12 of cyclic carbonates13 initiated by a variety of nucleophiles, including alcohols, amines and thiols, provides narrowly dispersed polymers of predictable molecular weights and end-group fidelity.14 We
94 reasoned that if cyclic carbonates incorporating a guanidinium side chain could be used in this process, and if the initiator could be a drug or probe, then one-step assembly of oligocarbonate MoTr-drug or -probe conjugates could be realized. Significantly, unlike solid or solution phase syntheses of oligomeric MoTrs in which step count increases with transporter length, this controlled catalytic oligomerization strategy would provide access to various lengths in one step simply through adjustment of the initiator-monomer ratio.15 Moreover, the metal-free nature of the catalysts and low catalyst loadings (typically 5%) are anticipated to avoid the cytotoxicity associated with catalyst residues.
4.2 Results and Discussion The new guanidine-protected monomer 4.3 was prepared by coupling the cyclic carbonate 4.1 and 1,3-di-Boc-2-(2-hydroxyethyl) guanidine 4.2. It is noteworthy that alcohol 4.4 does not initiate oligomerization of monomer 4.3 in the absence of catalyst. However, when the alcohol-tagged dansyl fluorophore initiator 4.4a or protected sulfur alcohol 4.4b (Scheme 4.1) is mixed with monomer 4.3 in the presence of the bifunctional thiourea/amine catalyst TU/DBU,16 ring opening oligomerization readily occurs. This catalyst exhibits exquisite selectivity for ring-opening oligomerization; no transesterification is observed. This exquisite control stems from the high selectivity of this catalyst combination towards the strained cyclic carbonate of the monomer relative to the acyclic carbonate and ester moieties of the oligomers.14,16 Moreover, oligomers of various lengths are generated by simply controlling the monomer-to-initiator ratio ([M]o/[I]o). Oligomers 4.5a-e exhibit well defined molecular weights and narrow polydispersities (Mn = 3,800, 5,200, 10,000,
3,900, 5,100 g/mol; Mw/Mn = 1.16, 1.11, 1.15, 1.16, 1.16, respectively). With a 5 mol
% catalyst loading ([M]0 = 1M), full conversion is reached in 1.25 h at room temperature. The process is highly reproducible over the range of scales studied (50mg to 2.5g). 1H NMR spectroscopy showed that each oligomer was end-labeled with the initiator, and the overlay of the GPC traces from the RI and UV detectors confirms quantitative initiation and predictable molecular weights. Removal of the Boc groups
95 by simple exposure of 4.5a-e to TFA gave oligocarbonate MoTr conjugates 4.6a-e in high yields from 4.3.
Scheme 4.1. Synthesis of molecular transporter
The new MoTr conjugates 4.6 incorporate a backbone scaffold (carbonate) and side chain spacing (1,7) previously unexplored in cell uptake studies.3 A distinguishing feature of these molecular transporters is their stability profile; while they are stable for months as solids at room temperature or in buffer (PBS) at 4°C, they degrade under physiologically relevant conditions (Hepes buffered saline, pH 7.4) with a half-life of ~8 h at 37°C. This affords excellent shelf stability, but also the novel ability to degrade after cellular uptake. Additionally, the MoTrs are non-toxic at concentrations required for uptake analysis (5 min incubation, EC50 4.6a=160M; 4.6b=48M). Like analogous oligoarginines, these transporters are highly water-
96 soluble, but as shown for 4.6a and 4.6b, they readily partition into octanol when treated with sodium laurate (1.2 equiv. per charge).17 The ability of GR oligocarbonate MoTrs 4.6a-c to enter cells was initially determined by flow cytometry with Jurkat cells that had been incubated for 5 min at 23ºC with the dansylated oligomers, washed with PBS to remove the remaining oligomers, and resuspended in PBS for analysis (Figure 4.1). The uptake of 4.6a-c was compared to that of a dansylated octaarginine derivative (r8) as a positive control and the dansyl initiator 4.4a as a negative control using the same 5 min pulse strategy. The dansyl probe initiator 4.4a alone does not enter Jurkat cells. In striking contrast, dansyl-oligocarbonate conjugates 4.6a and 4.6b exhibited rapid and concentration-dependent uptake similar to that of the dansylated r8 positive control. The extended oligomer 4.6c showed uptake but also cell-cell adhesion behavior and was excluded from further analysis. The significant increase in uptake observed for 4.6b relative to 4.6a at higher concentrations is consistent with the increase in uptake observed for MoTrs with increasing guanidinium content (up to n=15)18 and provides further evidence that the GR-oligocarbonates are functionally analogous to oligoarginines.
97 450
400 r8 350 4.6a 4.6b 300 4.4a +r8 K 250 +4.6a K +4.6b K 200 +4.4a K
150 Mean Fluorescence Mean 100
50
0 0 5 10 15 20 25 30 Concentration (μM)
Figure 4.1. Flow cytometry determined cellular uptake of oligocarbonates 4.6a and 4.6b, dansylated-r8, and dansyl initiator 4.4 in either PBS or high [K+] PBS. Jurkat cells were incubated with the various transporters or positive and negative controls for 5 minutes at 23ºC. Cell viability was >80% as determined by propidium iodide analysis.
Not unlike the behavior of other GR MoTrs, the uptake of 4.6a and 4.6b was drastically decreased when cells were incubated with modified PBS in which all sodium ions were replaced with potassium ions (Figure 4.1), a protocol used to decrease the voltage potential across the cell membrane.17 Additionally, incubating 19 cells with NaN3, conditions known to interfere with ATP dependent processes, led to a decrease in uptake. Finally, decreased uptake (18-37%) was observed with cells incubated at 4°C, suggesting a mixed mechanistic pathway in which endocytosis could play a role.20
98 In addition to flow cytometry studies, fluorescence microscopy using a two- photon excitation method established that both 4.6a and 4.6b were internalized into Jurkat cells upon incubation for 5 minutes at 23ºC (Figure 4.2).
Figure 4.2. Fluorescence microscopy images showing internalization of 4.6b throughout various layers (0.9 m wide) of a Jurkat cell (5 min incubation, 25 M at 23ºC). Panels A, G, L and O show a series of z-cuts through the cell as illustrated in the diagram at top left.
To further probe the ability of the oligocarbonate MoTrs to function as delivery vectors, experiments examining the delivery of the bioluminescent small molecule luciferin were conducted. In this recently introduced assay,21 the ability of a conjugate to enter cells and release its luciferin cargo is measured by the light emitted when luciferin is converted by luciferase to oxyluciferin and a photon of detectable light. Only free luciferin is measured and the analysis is independent of the mechanism(s) of entry, providing a real-time measure of drug/probe availability. A new strategy to access thiol-terminated oligomers 4.6d and 4.6e (Scheme 4.1) enabled the facile synthesis of disulfide-releasable luciferin conjugates 4.7a and 4.7b (Figure 4.3). The ability of 4.7a and 4.7b to deliver luciferin into HepG2 cells expressing click beetle luciferase was analyzed with a cooled charge-coupled device camera (photon count). Importantly, alkylated luciferin is not a substrate for the luciferase enzyme,22
99 and all light observed is therefore derived from the intracellular release and turnover of free luciferin.
O H HO2C N S O O O O O S S n O O S N O
4.7 NH a: n=8 b: n=11 H2N NH2 TFA luciferin - releasable linker - transporter
glutathione glutathione-S-S-transporter
HO2C N S OH luciferase HO2C O O hv N S SH S N O O S N O N S OH O S S N
Figure 4.3. Assay for measurement of intracellular luciferin delivery
Figure 4.4 shows the uptake and delivery of luciferin for 4.7a, 4.7b, an analogous D-cysteine-r8 conjugate,21 and luciferin alone in Ringers (140 mM NaCl, 5 mM KCl, 10 mM HEPES, 10 mM D-glucose, 2 mM MgCl2, and 2 mM CaCl2) and high [K+] Ringers (70 mM NaCl, 75 mM KCl, 10 mM HEPES, 10 mM D-glucose, 2 mM MgCl2, and 2 mM CaCl2) solutions, imaging buffers which contain a variety of ions and glucose to maintain healthy cells during longer imaging times. Following a 5 min incubation of the luciferase-expressing cells with transporter, both oligocarbonate MoTr conjugates 4.7a and 4.7b continuously release free luciferin over a period of about one hour. This behavior is in contrast to the r8 control, which exhibits much faster release kinetics both in cells (over about 20 minutes, Figure 4.4), and when treated with DTT in buffer, as observed by analytical HPLC analysis (see 4.5 Experimental Section). The ability of these MoTrs to release cargo over time offers several advantages, including the potential to avoid bolus effects associated with
100 administration of a free drug alone. The oligocarbonate MoTrs are able to deliver free luciferin in a concentration-dependent manner that is inhibited by high [K+] conditions associated with a diminished membrane potential. Free luciferin alone, while marginally cell permeable, exhibits negligible light output after a 5 min incubation. Taken together, these data demonstrate that the novel oligocarbonate MoTrs are able to not only penetrate the cell membranes of multiple cell types, but also efficiently deliver and release small molecule cargos where they are available for turnover by intracellular targets.
3.50E+06
3.00E+06 cysr8 4.7a 4.7b 2.50E+06 Luc +cysr8 K +4.7a K 2.00E+06 +4.7b K +Luc K
1.50E+06 Flux (photons/sec) Flux
1.00E+06
5.00E+05
0.00E+00 0 10203040506070 Time (min) Figure 4.4. Observed bioluminescence from HepG2 cells expressing click beetle luciferase following a 5 minute incubation with 25M 4.7a, 4.7b, cysr8 luciferin derivative, or luciferin alone in either Ringers or [K+] Ringers solutions.
4.3 Conclusion An expedient one-step, metal-free oligomerization route to a new family of MoTrs is described. This strategy enables the direct conjugation of probes and, by analogy, drug moieties as part of the oligomerization process. The monomers could thus be used as “kit” reagents for transporter-conjugate synthesis. Importantly, these
101 oligocarbonate MoTrs show low cytotoxicity and exhibit uptake comparable to or better than that of the parent oligoarginines as determined by flow cytometry and fluorescence microscopy. In addition, their ability to intracellularly deliver and release the bioluminescent small molecule probe luciferin was demonstrated, confirming the intracellular availability of the free cargo to interact with its target enzyme. The facile cellular uptake exhibited by these new MoTrs, the ease with which short to long oligomers (and presumably mixed oligomers) can be prepared, and their ability to degrade after uptake offer many advantages for drug/probe delivery, particularly for biological and macromolecular cargos.
4.4 Preliminary Results for Future Directions Having successfully initiated from a drug surrogate and delivered the resulting probe-MoTr conjugates (4.6 and 4.7) to cells and released free probe, the labs of Hedrick, Waymouth and Wender are interested in advancing the method to a therapeutic system. Initial research into this broad goal has progressed along two avenues: initiating the oligomerization reaction directly from a drug and determining the release kinetics of a therapeutic moiety conjugated to the MoTr via an ester or carbonate linkage. A one step oligomerization from a drug would be beneficial in the commercialization of a MoTr-drug construct.11 Taxol is an attractive target for direct oligomer initiation; its poor water solubility (~0.4 g/mL)27 and marked ineffectiveness (vis-à-vis other chemotherapeutics) against readily available MDR (multi-drug resistant) cell lines facilitates the evaluation of MoTr constructs bearing taxol moieties as a strategy for overcoming MDR.9 In previous studies, taxol-r8 constructs have been shown to be effective for the treatment of taxol resistant cell lines,9 but the synthetic cost of r8 and low drug density per transporter make a commercialized system costly.11 Taxol contains three alcohols (two 2° alcohols and one 3° alcohol) that could serve as initiators, but one alcohol (the C2’ alcohol) is significantly more reactive than the others, Scheme 4.2.
102
Scheme 4.2. Targeted Synthesis of a taxol-MoTr (4.8) with 4.3 from taxol at the C2’ position
The treatment of taxol with 4.3 and TU/sparteine (5 mol%) in anhydrous DCM effected a polymerization reaction (the use of the TU/DBU combination degrades taxol, as determined by 1H-NMR). The molecular weight of the resulting polymer, however, was not predictable by [M]ₒ/[I]ₒ, Table 4.1.
Table 4.1. Oligomerization from Taxol with 4.3a b b c [M]ₒ/[I]ₒ Reaction time Conversion DP Mn (h) (%) (g/mol) 8 5 --- 7 3700 8 5 38 4 2700 8 15 ~100 15 5400 8 6 --- 4 2300 a) For reaction conditions, see Experimental. b) determined by 1H-NMR c) determined by GPC after dialysis (no GPCs were taken before dialysis)
The TU/DBU catalyzed oligomerization of MTC-Bn from quinine exhibits a broad PDI, 4.9a in Scheme 4.3 (the same is true of cholesterol), which is consistent with quinine exhibiting a rate of propagation that is no longer (vis-à-vis 4.6) trivial relative to the rate of initiation.28 However, when quinine is incorporated into the monomer backbone to make 4.9b, the oligomerization reaction proceeds to DP ~
103 [M]ₒ/[I]ₒ with narrow PDI. In the future, the Waymouth/Hedrick labs are interested in synthesizing the taxol containing polymer by means of a monomer of taxol, 4.10 (MTC-taxol). This strategy will allow for the incorporation of many drugs per transporter chain.
Scheme 4.3. Synthesis of Two Oligomers Bearing Quinine Moieties, 4.9a and 4.9b
The lability of the MTC backbone towards amines provides another means of incorporating functionality into 4.6 or 4.7. We observed that amines react with closed MTC carbonates to form the ring opened species,30 constituting a “click” reaction.31 To test the viability of the guanidiniums of a modified version of 4.6, Scheme 4.4, as a click handle, highly fluorescent monomer, 4.11, was prepared by known methods.14 When this monomer is mixed with a guanidine-containing polymer, 4.12, the previously silent UV channel of the GPC trace of the polymer becomes active indicating the conjugation of 4.11 to the polymer, Scheme 4.4. This same strategy could be employed with 4.10 to attach taxol, or another drug, to a preformed polymer chain.
104
Scheme 4.4. Attachment of the Highly Fluorescent 4.11 to 4.12 via a “Click” Reaction.
Probes linked to 4.7 via a labile disulfide linker have a fast release from the MoTr once they have entered cells, but we are interested in exploring the release profile of a probe or drug attached to the MoTr via a hydrolyzable ester or carbonate linkage. We have previously established the half life of the 4.6 as 8 hours in buffered saline solution, as determined by disappearance of polymer signal in an HPLC assay (see Results and Discussion section). We chose to explore the degradation release kinetics of quinine from the polymer 4.9b. Quinine is a particularly attractive probe because of its low cost, fluorescence32 and biological activity,33 and when in concentrated solution (>1 mM) quinine is known to self quench.34 The effective concentration of quinine on 4.9b is above the self quenching concentration, yet when the ester linkages appending quinine to the polymer backbone are hydrolyzed free quinine will be released into solution causing a solution fluorescence signal that is proportional to free quinine. In pH 7 water, 15 M quinine•H2SO4 has a fluorescence intensity 370 times that of 1 M 4.9b(n=16) •16 H2SO4 under the same conditions.
105 Refluxing 4.9b with base causes rapid hydrolysis and dramatic increase in fluorescence signal in a matter of minutes, indicating the viability of the method, see Experimental Section. The experiments to determine the degradation release kinetics of quinine from 4.9b, and co-polymers (both block and random) with 4.3 were underway as of the writing of this document.
4.5 Experimental Section
4.5.1 General Considerations All chemicals were purchased from Aldrich and used as received unless stated otherwise. 1-(3,5-Bis-trifluoromethyl-phenyl)-3-cyclohexyl-thiourea (TU)23 and octaarginine (r8)24 were prepared according to literature procedures. Dansyl aminocaproic acid NHS ester and propidium iodide were obtained from Invitrogen.
Methylene chloride was stirred over CaH2 overnight, degassed by three freeze-pump- thaw cycles and vacuum transferred into a flame-dried bomb. Gel permeation chromatography (GPC) was performed in tetrahydrofuran (THF) at a flow rate of 1.0 mL/min on a Waters chromatograph equipped with four 5 μm Waters columns (300 mm x 7.7 mm) connected in series. A Viscotek S3580 refractive index detector, VE3210 UV/vis detector and Viscotek GPCmax autosampler were employed. The system was calibrated using monodisperse polystyrene standards (Polymer Laboratories). Reverse-phase high performance liquid chromatography (RP-HPLC) was performed with a Varian ProStar 210/215 HPLC using a preparative column (Alltec Alltima C18, 250 x 22 mm). The products were eluted utilizing a solvent gradient (solvent A = 0.1% TFA/H2O; solvent B = 0.1% TFA/CH3CN). NMR spectra were recorded on Varian INOVA 500 MHz and Varian Mercury 400 MHz magnetic resonance spectrometers.
106 4.5.2 Synthesis of 5-(dimethylamino)-N-(2-hydroxyethyl)naphthalene-1- sulfonamide, 4.4a OH HN O S O
N
Under nitrogen, dansyl chloride (5.05g, 18.72mmol) was placed in a dry 250mL round bottom flask equipped with a stir bar. After dry methylene chloride (50mL) was added via syringe, the flask was attached to an addition funnel and the system was cooled to 0°C. Ethanolamine (1.25g, 1.24mL, 20.59mmol), triethylamine (2.27g, 3.13mL, 22.46 mmol), and 75mL of dry methylene chloride were loaded into the addition funnel, and the solution was added dropwise with stirring over 30 min. The solution was stirred for an additional 30 min before the ice bath was removed, the solution allowed to reach ambient temperature and left to stir for an additional 14 hours. The product was isolated using flash chromatography initially eluting with methylene chloride before gradually increasing the polarity to 5% methanol in methylene chloride. Following removal of the solvent, a yellow oil was obtained that 1 solidified upon standing. Yield 5.0g (83%). H-NMR (CDCl3) δ: 8.6-7.2 (m, 6H,
ArH), 5.45 (t, 1H, -NH), 3.62 (m, 2H, -CH2OH), 3.07 (m, 2H, -NHCH2-), 2.90 (s, 6H,
(-CH3)2), 2.25 (bs, 1H, -OH).
4.5.3 2-(tritylthio)ethanol
OH S
107 Mercaptoethanol (1 mL, 14.2 mmol) was added to a solution of trityl chloride (4.317 g, 15.5 mmol) in 10 mL THF (ACS grade). The flask was equipped with a reflux condenser and heated at reflux for 3 h. The volatiles were removed under high vacuum yielding an off white solid which, when washed with about 50 mL ethyl acetate/hexanes (1:2), yielded a pure white powder. Characterization matched the literature.25
4.5.4 MTC-ethylguanidine-BOC
O O O H N O N O O HN O O O 5-Methyl-2-oxo-[1,3]dioxane-5-carboxylic acid (MTC-OOH) (1.26g, 7.9mmol) was initially converted to MTC-Cl using standard procedures with oxalylchloride.14 In a dry 250mL round bottom flask equipped with a stir bar, the formed intermediate was dissolved in 75mL of dry methylene chloride. Under nitrogen flow, an addition funnel was attached into which 1,3-di-boc-2-(2- hydroxyethyl)guanidine (2.0g, 5.59mmol), pyridine (0.55g, 0.56mL, 6.92mmol), and 30mL of dry methylene chloride was charged. The flask was cooled to 0°C, and the solution was added dropwise over 30 min. The formed solution was stirred for an additional 30 min before the ice bath was removed, and the solution stirred for an additional 4 hours under nitrogen. The crude product was placed directly onto a silica gel column, and the product separated by eluting with 100% ethyl acetate. The product fractions were removed and the solvent evaporated to yield the product as white 1 crystals. Yield 2.70g (92%). H-NMR (CDCl3) δ: 11.5 (s, 1H, NH), 8.65 (t, 1H, NH),
4.70 (d, 2H, CH2), 4.35 (t, 2H, CH2), 4.23 (d, 2H, CH2), 3.75 (q, 2H, CH2), 1.55 (s,
18H, CH3), 1.45 (s, 3H, CH3). HR-MS-ESI: m/z calculated for C19H31N3O9+Na 468.45 found 468.1952.
108 4.5.5 Synthesis of Oligomers: Dansyl 4.5a-c, Trityl 4.5d-e
O O O R H n HN O O O O S O S NH R=
N NH N O O O O dansyl initiator trityl initiator
Representative Example 4.5b. In a glove box with N2 atmosphere using flame dried glassware TU (21mg, 56mol), 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) (8.5mg, 56mol), and initiator (see Table 4.2) were charged in a 20mL glass vial equipped with a stir bar. A small volume of methylene chloride was added, and the formed solution stirred for 10 minutes. MTC-guanidine-boc 4.3 (0.5g, 1.12mmol) dissolved in enough additional DCM for a final concentration of 1M monomer was added to the catalyst/initiator solution, and the resulting solution kept stirring for 1.25 hours (conversion studied by 1H NMR analysis). Benzoic acid (15mg, 120μmol) was added to quench the catalyst. The crude reaction solution was transferred into a dialysis bag (1,000 g/mol cut off), and the solution dialyzed against methanol for 48 hours, the methanol solution was changed after 24 hours. The remaining solvent was evaporated yielding 4.5b (0.425g) as an off white solid. Alternatively direct addition of TFA in DCM (see next procedure) to the crude reaction mixture allowed for direct conversion to the PMTC-Guanidines in 85% yield. 1H-NMR of boc-oligomers
(CDCl3) δ: For oligomer (same for trityl and dansyl): 11.49 (s, 11H, NH), 8.66 (t,
11H, NH), 4.30 (m, 65H, polyMTC-CH2), 3.75 (m, 22H, polyMTC-CH2), 1.51 (s,
200H, CH3-boc), 1.28 (s, 3H, polyMTC-CH3); For DP11 dansyl 4.5b: 8.32 (d, 1H, ArH), 8.26 (d, 1H, ArH), 7.58 (m, 2H, ArH), 7.34 (m, 1H, ArH), 7.23 (d, 1H, ArH),
4.14 (t, 2H, CH2), 3.22 (q, 2H, CH2), 2.92 (s, 6H, CH3); For DP11 trityl 4.5e: 7.4 (d,
6H, ArH), 7.3 (d, 6H, ArH), 7.25 (t, 3H, ArH), 3.82 (t, 2H, CH2), 2.55, 2H, CH2).
109 Table 4.2: Synthesis and Characterization of PMTC-guanidine-boc oligomers a Initiator Initiator amount n MnNMR MnGPC PDI Yield 4.5a Dansyl 41.5 mg, 141 μmol 8 3,854 3,029 1.16 85% 4.5b Dansyl 33 mg, 112 mol 11 5,189 3,160 1.11 85% 4.5c Dansyl 15.0 mg, 51 μmol 22 10,084 4,692 1.15 85% 4.5d Trityl 44.9 mg, 140 μmol 8 3,880 4,235 1.16 78% 4.5e Trityl 36.0 mg, 112 mol 11 5,215 3,342 1.16 78%
a) PDI (polydispersity index) = Mw/Mn
RI-detector UV-detector Si g nal stren g th
Ret. Vol. (mL)
Figure 4.5. GPC overlay of RI and UV detector signals of 4.5b PMTC-guanidine-boc, PDI~1.11
110
Figure 4.6. 1H NMR spectrum of 4.5b with integrations displayed below selected resonances. Degree of polymerization was determined by integration of the end group resonances (i.e. b and e for the dansyl-initiated polymer) versus the polymer backbone g and g’.
4.5.6 Synthesis of PMTC-guanidines Dansyl 4.6a-c, Trityl 4.6d-e:
O O O HN R H n O S O O O O S R= H NH N from trityl initiated H2N NH2 oligomers dansyl initiator TFA PMTC-guanidine-boc (0.23g, 46mol) was charged in a 100mL round bottom flask equipped with a stir bar and dissolved in 18mL of methylene chloride. Trifluoroacetic acid (TFA) (2mL) was added and the flask sealed and left under stirring at ambient
111 temperature for 18 hours. For oligomers containing the trityl- group, 200 L triisopropylsilane was added as a cation scavenger. Nitrogen gas was bubbled through the solution for 30 minutes and the remaining solvent evaporated by rotational evaporation yielding (0.20g, 85%) of 4.6b as a slightly yellow waxy solid. Further purification (if needed for cell uptake analysis) was carried out by reverse-phase
HPLC (H2O:CH3CN, 5-60% gradient). The product containing fractions were 1 collected and lyophilized to yield H-NMR (DMSO-d6) δ: [from dansyl: 8.40 (d, 1H,
ArH), 8.20 (m, 2H, ArH), 7.55 (m, 2H, ArH), 3.92 (t, 2H, CH2), 2.99 (q, 2H, CH2),
2.79 (s, 6H, CH3)] 7.90 (bs, 11H, polyMTC-NH), 7.25 (bs, 44H, polyMTC-NH), 7.18
(m, 44H, polyMTC-CH2), 4.02 (m, 22H, polyMTC-CH2), 3.37 (m, 22H, polyMTC- + CH2), 1.10 (s, 33H, polyMTC-CH3). MALDI MS analysis: [M ] calculated for Dansyl-PMTC-guanidine 4.6a (n=8, no TFA), 2264.29, found, 2256.203. 4.6b (n=11, no TFA) calculated, 2991.13, found, 2995.
11 10 9 8 7 6 5 4 3 2 1
11 10 9 8 7 6 5 4 3 2 1
11 10 9 8 7 ppm 6 5 4 3 2 1
1 Figure 4.7. Stacked H-NMR of MTC-guanidine-boc (top, CDCl3), PMTC-
guanidine-boc (mid, CDCl3), and PMTC-guanidine (below, DMSO-d6).
112 For oligomers containing the trityl group, the deprotected PMTC-guanidine thiol 4.6d- e could be purified similarly to the dansyl moieties (above) or taken on crude to the next reaction (luciferin coupling). Verification of deprotection was observed by MALDI MS analysis: [M+] calculated for HS-PMTC-guanidine oligomer 4.6d (n=8, no TFA), 2048.06, found 2042. 4.6e (n=11, no TFA) calculated, 2786.77, found 2780.
4.5.7 Synthesis of Luciferin Oligomers 4.7a and 4.7b
To crude HS-PMTC-guan 4.6d (after solvent removal via stream of nitrogen and high vacuum, 0.012873 mmol) was added luciferin carbonate aldrithiol (4.0 mg, 0.007668 mmol, for preparation and characterization see reference 5) in 400 L of DMF which had been deoxygenated by 6 freeze-pump-thaw cycles. The reaction stirred at rt for 12 hours then acetonitrile (0.5 mL) and deionized water (1.5 mL) were added, and the reaction was purified by reverse-phase HPLC (H2O:CH3CN, 10-80% gradient). The product containing fractions were collected and lyophilized to yield luciferin oligomer 4.7a (7.5 mg, 29% over two steps) as a white/yellow amorphous 1 solid; Prep RP-HPLC (H2O:CH3CN) > 95% purity. H NMR (400 MHz, D2O): δ = 8.01 (d, J = 8.8 Hz, 1 H), 7.92 (d, J = 2.0 Hz, 1 H), 7.46 (d, J = 8.8 Hz, 1 H), 5.33 (t, J = 8.8 Hz, 1 H), 4.37-4.22 (m, 42 H), 3.84-3.83 (m, 2H), 3.76-3.66 (m, 4H), 3.47 (bs, 14 H), 2.94 (m, 2H), 2.79-2.72 (m, 4H), 1.78 (bs, 4H), 1.20 (s, 24H) ppm. MALDI + MS: [M] calculated for C90H148N26O46S4 (no TFA), 2458.54; found, 2452.9. Luciferin oligomer 4.7b was accessed in a similar fashion (5 mg, 24% over two steps) 1 as a white/yellow amorphous solid; Prep RP-HPLC (H2O:CH3CN) > 95% purity. H
NMR (400 MHz, D2O): δ = 8.14 (d, J = 8.8 Hz, 1 H), 7.95 (d, J = 2.0 Hz, 1 H), 7.48 (d, J = 8.8 Hz, 1 H), 5.35 (t, J = 8.8 Hz, 1 H), 4.40-4.29 (m, 80 H), 3.91-3.86 (m, 2 H), 3.77-3.69 (m, 4 H), 3.69 (bs, 24 H), 2.98 (m, 2 H), 2.77-2.75 (m, 4 H), 1.81 (bs, 4 H),
113 + 1.23 (s, 33 H) ppm. MALDI MS: [M] calculated for C117H196N35O61S4 (no TFA), 3197.25; found, 3190.135.
ppm 1 Figure 4.8. H-NMR (D2O) of 4.7a (n=8)
4.5.8 Synthesis of Dansyl-r8
TFA H N 2 NH2 NH
O O H H O N N O S N NH2 H O 7
NH TFA H2N NH2 N Octaarginine (25 mg, 0.01834 mmol) was added to a small conical vial with stirbar and deionized water, then lyophilized. Dansyl aminocaproic acid NHS ester (10 mg, 0.021668 mmol, Invitrogen) was dissolved in 100 L DMF and added to the
114 conical vial. Diisopropylethylamine (2.8 L, 0.16251 mmol) was added and the reaction stirred at rt for 22.4 hours, when DMF was blown off with a stream of nitrogen. Acetonitrile (0.5 mL) and deionized water (1.5 mL) were added, followed by trifluoroacetic acid (1.7 L, 20 equivalents), and the reaction was purified by reverse-phase HPLC (H2O:CH3CN, 5-60% gradient). The product containing fractions were collected and lyophilized to yield dansyl r8 (25 mg, 90%) as a 1 white/yellow amorphous solid; Prep RP-HPLC (H2O:CH3CN) > 95% purity. H
NMR (400 MHz, D2O): δ = 8.652 (d, J = 8.70 Hz, 2 H), 8,424 (d, J = 8.74, 1 H), 8.306 (d, J = 6.98, 2 H), 7.93 (d, J = 7.71, 1 H), 7.85-7.80 (m, 3 H), 4.33-4.18 (m, 8 H), 3.354 (s, 6 H), 3.18-3.11 (m, 14 H), 2.896 (t, J = 6.61, 2 H), 2.116 (t, J = 7.17, 3 H), 1.84-1.58 (m, 29 H), 1.38-1.29 (m, 4 H), 1.137 (d, J = 7.39, 3 H) ppm. MALDI + MS: [M] calculated for C66H129N35O11S (no TFA), 1620.04; found, 1615.365.
ppm 1 Figure 4.9. H-NMR (D2O) of 4.7b (n=11)
115 ppm 1 Figure 4.10. H-NMR of r8 dansyl, D2O.
4.5.9 Octanol-Water Partitioning For the partitioning experiments calibration curves of dansyl ethanol in water and octanol respectively were initially made (see below). The UV-vis spectra were recorded using an Agilent 8453 spectrophotometer at λ=335nm. From the calibration curves, ideal polymer concentrations were made which allowed absorbance between 1.0-1.5 instrumentation absorbance units (AU), this corresponded to initial polymer concentration of ~ 0.1mM (in water). After the polymer was dissolved in water (1mL), octanol (1mL) was added and the UV-spectra recorded in both the water and octanol layer. Sodium laurate (1.2eq to the total guanidine concentration) was added, the vial gently shaken, and the UV spectra recorded in both the water and octanol layer. Following partitioning the water and octanol layers were separated after which the aqueous phase was lyophilized and its contents analyzed with 1H-NMR.
116 1.6 Concentration (M) Absorbance (AU) y = 14940x + 0.0018 1.4 R2 = 0.9998 1.2 0.0001 1.5011 1 0.00005 0.73687 0.8 0.6 0.00001 0.15813 0.4 Absorbance (AU) 0.2 0.000001 0.016527 0 0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012 Concentration (M)
Figure 4.11. Dansyl ethanol calibration curve in water
1 . 8 y = 1 6 4 8 1 x + 0 .0089
1 . 6 R 2 = 0 . 9 9 9 7 [M] Absorbance (AU) 1 . 4 1 . 2 0.0001 1.6526 1 0.00005 0.83922 (AU) 0 . 8 0 . 6 0.00001 0.18729 0 . 4 0 . 2 0.000001 0.0097547 0 0 2 E- 0 5 4 E - 0 5 6 E - 05 8E-05 0.0001 0.0001 Concentra tion (M)
Figure 4.12. Dansyl ethanol calibration curve in octanol
Figure 4.13. Picture of the total partitioning of 4.6b (n = 11 dansyl functionalized poly(MTC-guanidine)) into the octanol layer (top) following the addition of sodium laurate and UV excitation.
117 4.5.10 Cellular Uptake Assays by Flow Cytometry Dansyl-tagged oligomers and octaarginine control were brought up in pH 7.2 PBS buffer at 5μM, 12.5μM, and 25μM concentrations. Jurkat cells grown in 10% FBS in RPMI media 1640 (+ glutamine) were used for cellular uptake experiments. Cells were plated on a microtiter plate at 3.0 x 106 cells/mL with 200L/well. The plate was centrifuged (1300 rpm for 3 min), media removed, and cells resuspended in PBS buffer twice. Compounds 4.6(a-c) and r8 control at varying concentrations were incubated with the cells for 5 minutes at 23°C. The microtiter plates were centrifuged and the cells were isolated, washed with PBS and resuspended in PBS containing propidium iodide (3 g/mL, 0.01%). The cells were analyzed using a fluorescent flow cytometer (Vantoo, Stanford University) equipped with a UV laser for excitation of the dansyl fluorophore, and cells stained with propidium iodide were excluded from the analysis. The data presented are the mean fluorescent signals for the 20,000 cells analyzed.
4.5.11 Cellular Uptake Assay at 4°C or in the Presence of Sodium Azide Uptake assays at 4°C were performed as described above except that solutions were precooled at 4°C and cells were incubated on ice. Uptake assays in the presence of sodium azide were performed as described above with the exception that cells used were preincubated for 30 min with 0.5% sodium azide in 2% FBS/PBS buffer before the addition of fluorescently labeled oligomers and cells were washed with 0.5% sodium azide in PBS buffer.
4.5.12 Cellular Uptake Assay in the Presence of High Potassium [K+] Buffer A high potassium PBS buffer was prepared by mixing 136.9 mmol KCl, 1.5 mmol KH2PO4, and 8.3 mmol K2HPO4*7 H2O and titrated to pH = 7.2. Stock solutions of all oligomers were made in the high potassium PBS buffers. Uptake assays were performed as described above with the exception that the cells were washed twice with the high potassium buffer. The cells were then exposed to the
118 oligomer in high [K+] buffer, washed with the same buffer, and finally resuspended in that buffer for analysis.
1400
1200
1000
800 r8 7.6b 7.6c 600 4.4a
400 Mean Fluorescence 200
0 0 5 10 15 20 25 30 Concentration (uM)
Figure 4.14. Concentration dependence of cellular uptake into Jurkat Cells in pH 7.2 PBS, incubated at 5 minutes with r8 dansyl, oligomers 4.6b or 4.6c, or the dansyl alcohol initiator 4.4 at 23°C.
119 2500
2000
r8 1500 4.6b 1 4.6b 2 4.4a r8 K+ 4.6b1K+ 1000 4.6b2K+ 4.4a K+ Mean Fluorescence Fluorescence Mean
500
0 0 5 10 15 20 25 30 Concentration (uM)
Figure 4.15. Concentration dependence of cellular uptake into Jurkat Cells in pH 7.2 PBS or K+ PBS, incubated at 5 minutes with r8 dansyl, oligomers 4.6b in two separate batches, 1 or 2, or the dansyl alcohol initiator 4.4 at 23°C.
120 450
400
350
300
250
200
150
100 Mean Fluorescence Fluorescence Mean 50
0 4.6b 4.6a r 4.4a 4.6b 4C 4.6a 4C r8 4.4a 4C 8 4C Sample
Figure 4.16. Flow cytometry determined cellular uptake of oligocarbonates 4.6a, 4.6b, r8 dansyl, and dansyl alcohol initiator 4.4 in PBS. Jurkat cells were incubated with the various transporters for 5 minutes at either 23°C or 4°C.
450
350
250
150 Mean Fluorescence Fluorescence Mean
50
0 4.6 4.6 r 4.4 4.6b N3 4.6a N3 r8 4.4a N3 b a 8 a Sample N3
Figure 4.17. Flow cytometry determined cellular uptake of oligocarbonates 4.6a, 4.6b, r8 dansyl, and dansyl alcohol initiator 4.4 in PBS. Jurkat cells were incubated with the various transporters for 5 minutes at 23°C, in the presence and absence of
sodium azide (NaN3).
121 450
400
350
300
250 25 uM 200
150
Mean Fluorescence Fluorescence Mean 100
50
0 4.6b 4.6a r8 4.4a 4.6bK+ 4.6aK+ r8 K+ 4.4aK+ Sample
Figure 4.18. Flow cytometry determined cellular uptake of oligocarbonates 4.6a, 4.6b, r8 dansyl, and dansyl alcohol 4.4 in PBS or [K+] PBS. Jurkat cells were incubated with the various transporters for 5 minutes at 23°C.
4.5.13 Fluorescence Microscopy Studies Cells were washed, incubated with oligomers, and suspended for analysis as described above, with the exception that cells were not stained with propidium iodide before analysis. Analysis was performed on a Zeiss LSM 510 with Ti:Sapphire laser for 2-photon excitation to excite the dansyl fluorophore.
122
Figure 4.19. Uptake of 4.6b into Jurkat cells, 5 min incubation, 25 M at 23°C visualized by fluorescence microscopy using a two-photon excitation. Panels A-O show varying Z-cuts through the cell (from top to bottom) with 0.9 m resolution and 0.7 m between cuts. Panel P shows the bright field image of the same cell.
123 4.5.14 Cellular Assays for Luciferin Release from Conjugates 4.7a and 4.7b A hepatocellular carcinoma cell line stably transfected with click beetle luciferase, Hep-G2, was plated at 25,000 cells per well in a 96 well, flat bottomed plate 24 hours prior to the assay. The cells were washed once with 100 L of Ringers solution (140 mM NaCl, 5 mM KCl, 10 mM HEPES, 10 mM D-glucose, 2 mM
MgCl2, and 2 mM CaCl2) or [K+] Ringers (70 mM NaCl, 75 mM KCl, 10 mM
HEPES, 10 mM D-glucose, 2 mM MgCl2, and 2 mM CaCl2), then incubated with varying concentrations of either the potassium salt of luciferin (Xenogen Corp., Alameda, CA) r8-luciferin carbonate 7,5 or luciferin oligomers 4.7a or 4.7b, in triplicate, for 5 minutes in Ringers or [K+] Ringers. The cells were washed, resuspended with the appropriate buffer, and the resultant luminescence was measured using a charge coupled device camera and Living Image Software (IVIS29, Xenogen, Corp., Alameda, CA).
4.5.15 Assays Measuring the Hydrolytic Stabilities of the Dansylated Conjugates 4.6a-c: Each of the conjugates was dissolved in 190 L HEPES buffered saline (HBS) pH = 7.4 (1 mM) in 1.5 mL microfuge tubes and incubated at 37°C with 10 L of a solution of 10 mg 1-naphthalenemethanol in 24 mL of methanol, which served as an internal standard. At appropriate time points 20 L of the solution was removed and analyzed by RP-HPLC. The percent decomposition was calculated from the integrated peak areas of the conjugate, the internal standard, and the various decomposition products.
4.5.16 MTT Toxicity Assays Jurkat cells grown in 10% FBS in RPMI media 1640 (+ glutamine) were plated at 5-10 x 104 cells/mL on a 96-well microtiter plate in the same media. In a second 96-well plate, compound was serially diluted in triplicate over 20 wells in columns 2- 11 and rows 1-3 and 5-7; typical dilutions spanned the concentration range of 400µM to 20nM. Columns 1 and 12 contained no cells and no compound, respectively. Rows
124 4 and 8 contained a serial dilution of colchicine as internal control; colchicine concentrations were generally much lower than compound concentrations in order to obtain an EC50 (half-maximal effective concentration). Compounds were added to the plate containing cells and the cells were incubated at 37˚C for 24 h or 5 min, at which point the plate was centrifuged, compound removed, and the cells resuspended in fresh media. The cells were incubated for an additional 48 h (72 h assay total), centrifuged, media removed, and 150µL of 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) solution (5mg/mL media) was added to each well. Cells were incubated with MTT for 3 hours at 37˚C, 150μL solubilizing solution (10% triton X- 100, 90% 0.1N HCl in isopropanol) was added to each well, and colorimetry data obtained on a plate reader. Numerical data from plate reader was standardized using values from columns 1 and 12 and curve fitting was performed using Graph Pad Prism software to obtain an EC50 for each compound.
Table 4.3. Toxicity and Stability of Oligoguanidines 4.6a-c
Compound EC50 (μM) EC50 (μM) Stability (t1/2, hours) Jurkat cells, Jurkat cells, 5 24 hour min 95% HBS pH incubation incubation 7.4, 5% MeoH, 37ºC
Dansyl-DP8 4.6a 17.85 ± 1.07 160.3 ± 27.8 8.3 ± 1.4
Dansyl-DP11 4.6b 8.53 ± 0.75 48.0 ± 6.4 8.10 ± 0.14
Dansyl-DP22 4.6c 2.32 ± 0.63 ~4 15.9 ± 0.4
Dansyl initiator 4.4 311 -- --
125 4.5.17 Assays Measuring the Hydrolytic Stabilities and Luciferin Release of the Luciferin Conjugates 4.7a-b Each of the conjugates (0.5 mg) were dissolved in 200 L Hepes Buffered Saline (HBS) pH 7.4 (1 mM) in 1.5 mL microfuge tubes and incubated at 37°C with 15 L of a solution of 10 mg p-methoxyphenol in 10 mL of methanol, which served as an internal standard. At appropriate time points 20 L of the solution was removed and analyzed by RP-HPLC. The percent decomposition was calculated from the integrated peak areas of the conjugate, the internal standard, and the various decomposition products. For release studies with DTT (dithiothreatol), 180 L of HBS pH 7.4 was added, followed by 20 L of a 100 mM DTT solution in HBS (giving 10 mM total DTT in solution). At appropriate time points 20 L of the solution was removed and analyzed by RP-HPLC. The percent decomposition was calculated from the integrated peak areas of the disulfide reduced luciferin intermediate, the internal standard, and the appearance of free luciferin.
Table 4.4. Stability and Release of Luciferin oligomers 4.7a-b Release (min) Stability (h) Compound HBS pH 7.4, 37°C, HBS pH 7.4, 37°C 10 mM DTT Luciferin-DP8 4.7a 4.5 8.6 Luciferin-DP8 4.7b 5.4 8.7 Luciferin-Cys-r8 33a 2.6 (3a) aLiterature value26
4.5.18 Synthesis of MTC-pyrene, 4.11
To a flame dried flask under N2 flow containing 4.1 (0.0073 mol), which was prepared in situ according to established methods (ref. 14), and 25 mL anhydrous THF was added a solution of pyrene butanol (2.0 g, 0.0073 mol), pyridine (0.76 mL, 0.0095 mol) in 25 mL anhydrous THF dropwise over 30 minutes at 0°C. The reaction was stirred and allowed to warm to room temperature overnight. The contents of the flask
126 were filtered, and the solvent removed under reduced pressure. The solid was dissolved in ~9 mL of DCM and loaded onto a silica gel column and eluted with ethyl acetate/hexanes (2:1) yielding pure 4.11, yield: 1.939g (64%). 1H-NMR (300 MHz,
CDCl3) : 8.32-7.84 (m, 9H), 4.66 (d, 2H), 4.28 (t, 2H), 4.17 (d, 2H), 3.42 (t, 2H), 13 2.02-1.79 (m, 4H), 1.28 (s, 3H). C-NMR (100 MHz; CDCl3) : 171.32, 147.76, 136.17, 131.64, 131.08, 130.17, 128.81, 127.74, 127.63, 127.53, 126.97, 126.15, 125.33, 125.23, 125.09, 125.05, 123.42, 76.99, 73.20, 66.30, 40.36, 33.16, 28.60,
28.06, 17.75. ESI-MS: Experimental: 439.12 m/z (parent); Theoretical: C26H24O5 + Na+ : 439.15 g/mol (parent)
4.5.19 Synthesis of MTC-quinine
To a flame dried flask under N2 flow containing 4.1 (0.0028 mol), which was prepared in situ according to established methods (ref. 14), and 20 mL anhydrous THF was added a solution of quinine (1.0 g, 0.0031 mol), triethyl amine (0.86 mL, 0.0062 mol) in 25 mL anhydrous THF dropwise over 30 minutes at 0°C. The reaction was stirred and allowed to warm to room temperature overnight. The contents of the flask were filtered, and the solvent removed under reduced pressure. The solid was washed with ~50 mL of cold methanol under vacuum filtration yielding a pure white powder: 13 0.39 g (30%). C-NMR (125 MHz; CDCl3) : 170.83, 158.38, 147.76, 146.39, 145.13, 141.81, 132.27, 127.29, 122.27, 120.18, 118.84, 115.03, 101.34, 101.31, 73.24, 72.97, 59.59, 59.57, 56.62, 55.94, 42.56, 40.61, 39.81, 28.03, 27.52, 25.41, 17.57. ESI-MS: Experimental: 467.18 m/z (parent), 468.17 m/z (25%), 469.11 m/z + (10%); Theoretical: C26H30N2O6 + H : 467.22 g/mol (parent), 468.22 (30%), 469.22 (5%).
127 8 7 6 5 4 3 2 1 Chemical Shift (ppm) 1 Figure 4.20. H-NMR (500 MHz, CDCl3) MTC-quinine
4.5.20 Synthesis of PMTC-quinine, 4.9b
In a glove box with N2 atmosphere using flame dried glassware TU (7.9 mg, 22 mol), 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) (3.3 mg, 22 mol), and benzyl alcohol (1.8 L, 17.2 mol) were charged in a 20mL glass vial equipped with a stir bar. A small volume of methylene chloride was added, and the formed solution was stirred for 10 minutes. MTC-quinine (0.2 g, 0.43 mmol) dissolved in enough additional DCM for a final concentration of 0.37 M monomer was added to the catalyst/initiator solution, and the resulting solution was kept stirring for 0.75 hours (73% conversion by 1H-NMR analysis). Benzoic acid (15 mg, 120 μmol) was added to quench the catalyst. The crude reaction solution was transferred into a dialysis bag (1,000 g/mol cut off), and the solution dialyzed against methanol for 48 hours, the methanol solution was changed after 24 hours. The remaining solvent was evaporated yielding 4.9b (0.05 g) as an off white solid. GPC: Mn=2,800; Mw=3,800; PDI = 1.212.
128 4.5.21 Initiation of Oligomerization from Quinine, 4.9a
In a glove box with N2 atmosphere using flame dried glassware TU (8.0 mg, 22 mol), 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) (4.4 mg, 29 mol), quinine (5.2 mg, 0.016 mmol) were charged in a 20mL glass vial equipped with a stir bar. A small volume of methylene chloride was added, and the formed solution stirred for 10 minutes. MTC-benzyl (0.1002 g, 0.40 mmol) dissolved in enough additional DCM for a final concentration of 1M monomer was added to the catalyst/initiator solution, and the resulting solution kept stirring for 1.25. Benzoic acid (15 mg, 120 μmol) was added to quench the catalyst. The solvent was evaporated yielding 4.9a as an off white solid. GPC: Mn=3,900; Mw=6,700; PDI = 1.721.
4.5.22 Initiation of Oligomerization from Taxol, 4.8
In a representative polymerization, in a glove box with N2 atmosphere using flame dried glassware TU (4.1 mg, 11 mol), sparteine (4.9 mg, 21 mol), taxol (21.9 mg, 0.026 mmol) were charged in a 20mL glass vial equipped with a stir bar. A small volume of methylene chloride was added, and the formed solution stirred for 10 minutes. 4.3 (0.0961 g, 0.216 mmol) dissolved in enough additional DCM for a final concentration of 1M monomer was added to the catalyst/initiator solution, and the resulting solution kept stirring for 1.25. Benzoic acid (15 mg, 120 μmol) was added to quench the catalyst. The crude reaction solution was transferred into a dialysis bag (1,000 g/mol cut off) that had been rinsed with copious amounts of DI water and soaked in 500 mL DI water for 10 minutes, and the solution dialyzed against methanol for 48 hours, the methanol solution was changed after 24 hours. The solvent was evaporated yielding 4.8 as an off white solid. GPC: Mn=2,700; Mw=3,000; PDI = 1.113.
129 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Chemical Shift (ppm)
9 8 7 6 5 4 3 2 1 Chemical Shift (ppm) 1 Figure 4.21. H-NMR (500 MHz, CDCl3) of (upper) 4.9b and (lower) 4.9a
130 11 10 9 8 7 6 5 4 3 2 1 Chemical Shift (ppm) 1 Figure 4.22. H-NMR (300 MHz, CDCl3) of 4.8a
4.5.23 MTC/amine “Click” Reaction, 4.11 + 4.12 To a vial containing 4.12 (40 mg, 0.009 mmol; PDI=1.146) was added 2 mL methanol and 4.11 (3.4 mg, 0.0082 mmol) which was stirred for 2 h. The solution was treated with DCM (~2 mL) which precipitated a white oily solid. The supernatant was decanted and the residue washed with DCM (~2 mL). The residue was taken to dryness under high vacuum yielding a white sticky solid. GPC: overlapping RI and
UV signals indicate Mn=250; Mw=275; PDI=1.10.
8.8 8.4 8.0 7.6 7.2 6.8 6.4 6.0 5.6 5.2 4.8ppm 4.4 4.0 3.6 3.2 2.8 2.4 2.0 1.6 1.2 0.8 0.4 1 Figure 4.23. H-NMR (500 MHz, CD3OD) of the products of the reaction of 4.11 and 4.12
131 4.6 References (1) a) Wender, P.A.; Galliher, W.C.; Goun, E.A.; Jones, L.R.; Pillow, T.H. Adv. Drug Del. Rev. 2008, 60, 452-472. b) Snyder, E.L.; Dowdy, S.F. Expert Opin. Drug Del. 2005, 2, 43-51. c) Langel, U.; Cell-Penetrating Peptides: Processes and Applications; CRC Press: Boca Raton, FL, 2002. (2) Wender, P. A.; Mitchell, D. J.; Pattabiraman, K.; Pelkey, E. T.; Steinman, L.; Rothbard, J. B. Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 13003-13008. (3) Rothbard, J.B.; Kreider, E.; VanDeusen, C.L.; Wright, L.; Wylie, B.L.; Wender, P.A. J. Med. Chem. 2002, 45, 3612-3618. (4) Wender, P. A.; Rothbard, J. B.; Jessop, T. C.; Kreider, E. L.; Wylie, B. L. J. Am. Chem. Soc. 2002, 124, 13382-13383. (5) Wender, P. A.; Kreider, E.; Pelkey, E. T.; Rothbard, J.; VanDeusen, C. L. Org. Lett. 2005, 7, 4815-4818. (6) For a review and lead references on GR transporters from the groups of Torchilin, Prochiantz, Langel, Futaki, Vives, Wender, Dowdy, Piwnica- Worms, Lebleu, Seebach, Gellman, Goodman, Tor, Chung, Kiso, Mendoza and others see: Adv. Drug Delivery Rev. 2008, 60, 452. For further lead references see a) Hamilton, S.K.; Harth, E. ACS Nano 2009, 3, 402-410. b) Geisler, I.; Chmielewski, J. J. Chem. Biol. Drug Des. 2009, 73, 39-45. c) Seow, W.Y.; Yang, Y-Y. Adv. Mater. 2009, 21, 86-90. d) Daniels, D.S.; Schepartz, A. J. Am. Chem. Soc. 2007, 129, 14578. (7) For a recent review on arginine-rich peptides and their many cargos see: Tung, C.H.; Weissleder, R. Adv. Drug Delivery Rev. 2003, 55, 281-294. (8) a) Goun, E.A.; Shinde, R.; Dehnert, K.W.; Adams-Bond, A.; Wender, P.A.; Contag, C.H.; Franc, B.L. Bioconjugate Chem. 2005, 17, 787-796. b) Jiang, T.; Olson, E.S.; Nguyen, Q.T.; Roy, M.; Jennings, P.A.; Tsien, R.Y. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17867-17872. (9) Dubikovskaya, E.A.; Thorne, S.H.; Pillow, T.H.; Contag, C.H.; Wender, P.A. Proc. Natl. Acad. Sci. USA 2008, 105, 12128.
132 (10) Rothbard, J.; Garlington, S.; Lin, Q.; Kirschberg, T.; Kreider, E.; McGrane, P.; Wender, P.; Khavari, P. Nat. Med. 2000, 6, 1253. (11) Wender, P.A.; Jessop, T.C.; Pattabiraman, K.; Pelkey, E.T.; VanDeusen, C.L. Org. Lett. 2001, 3, 3229-3232. (12) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813-5840. (13) Rokicki, G. Prog. Polym. Sci. 2000, 25, 259-342. (b) Al-Azemi, T. F.; Bisht, K. S. Macromolecules 1999, 32, 6536-6540. (14) (a) Pratt, R.C.; Nederberg, F.; Waymouth, R.M.; Hedrick, J.L. Chem. Comm. 2008, 114-116. (b) Nederberg, F.; Lohmeijer, B. G. G.; Leibfarth, F.; Pratt, R. C.; Choi, J.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2007, 8, 153-160. (15) Guanidinylated oligomers have been generated by ring-opening metathesis to provide cell or artificial membrane transporters with hydrocarbon backbones. See Kolonko, E.M.; Kiessling, L.L. J. Am. Chem. Soc. 2008, 130, 5626-5627; Kolonko, E.M.; Pontrello, J.K.; Mangold, S.H.; Kiessling, L.L. J. Am. Chem. Soc. 2009, 131, 7327-7333; Gabriel, G. J.; Madkour, A. E.; Dabkowski, J. M.; Nelson, C. F.; Nusslein, K.; Tew, G. N. Biomacromolecules 2008, 9, 2980- 2983; and Hennig, A.; Gabriel, G. J.; Tew, G. N.; Matile, S. J. Am. Chem. Soc. 2008, 130, 10338-10344. (16) (a) Pratt, R.C.; Lohmeijer, B.G.G.; Long, D.A.; Lundberg, P.N.P.; Dove, A.P.; Li, H.; Wade, C.G.; Waymouth, R.M.; Hedrick, J.L. Macromolecules, 2006, 39, 7863–7871. (b) Dove, A. P., Pratt, R.C.; Lohmeijer, B.G.G.; Waymouth, R.M.; Hedrick, J.L, J. Am. Chem. Soc. 2005, 127, 13798-13799. (17) Rothbard, J.B.; Jessop, T.C; Lewis, R.S.; Murray, B.A.; and Wender, P.A. J. Am. Chem. Soc. 2004, 126, 9506-9507. (18) Mitchell, D.J.; Kim, D.T.; Steinman, L.; Fathman, C.G.; Rothbard, J.B. J. Peptide Res., 2000, 56, 318-325. (19) Sandvig, K.; Olsnes, S. J. Biol. Chem. 1982, 257, 7504-7513.
133 (20) Silhol, M.; Tyagi, M.; Giacca, M.; Lebleu, B.; Vives, E. Eur. J. Biochem. 2002, 269, 494. (21) Jones, L.R.; Goun, E.A.; Shinde, R.; Rothbard, J.B., Contag, C.H., Wender, P.A. J. Am. Chem. Soc. 2006, 128, 6526. (22) Denburg, J.L.; Lee, R.T.; McElroy, W.D. Arch Biochem Biophys 1969, 134, 381. (23) Pratt, R.C.; Lohmeijer, B.G.G.; Long, D.A.; Lundberg, P.N.P.; Dove, A.P.; Li, H.; Wade, C.G.; Waymouth, R.M.; Hedrick, J.L. Macromolecules, 2006, 39, 7863–7871 (24) Wender, P.A.; Jessop, T.C.; Pattabiraman, K.; Pekley, E.T.; VanDeusen, C.L. Org. Lett. 2001, 3, 3229-3232. (25) Maltese, M. J. Org. Chem. 2001, 66, 7615-7625. (26) Jones, L.R.; Goun, E.A.; Shinde, R.; Rothbard, J.B.; Contag, C.H.; Wender, P.A. J. Am. Chem. Soc. 2006, 128, 6526-6527. (27) Richheimer, S. L.; Tinnermeier, D. M.; Timmons, D. W. Anal. Chem. 1992, 64, 2323-2326. (28) (a) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2010, 43, 2093-2107. (b) Odian, G. Principles of Polymerization, 4th ed.;Wiley-Interscience: Hoboken, NJ, 2004. (29) Kiesewetter, M. K.; Hedrick, J. L.; Waymouth, R. M. Unpublished results. (30) Nederberg, F.; Appel, E.; Tan, J. P. K.; Kim, S. H.; Fukushima, K.; Sly, J.; Miller, R. D.; Waymouth, R. M.; Yang, Y. Y.; Hedrick, J. L. Biomacromolecules 2009, 10, 1460–1468. (31) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. Int. Ed. 2001, 40, 2004–2021. (32) Eaton, D. F. Pure and Appl. Chem. 1988, 60, 1107-1114. (33) White, N. J. et al. Lancet, 2005, 366, 717-725. (34) Pant, D.; Tripathi, U. C.; Joshi, G. C.; Tripathi, H. B.; Pant, D. D. J. Photochem. Photobio. A 1990, 51, 313-315.
134 CHAPTER 5
Kinetics of an Air and Water Stable Ruthenium(IV) Catalyst for the Deprotection of Allyl Alcohols
135 5.1 Introduction The rich reaction chemistry of transition metal allyl complexes has spawned a diverse array of catalytic reactions for bond forming and cleaving reactions.1-4 Catalytic nucleophilic allylic substitution is a powerful synthetic method for the selective formation of carbon-carbon and carbon heteroatom bonds.1-3 Allylic ethers, esters, carbonates and carbamates are also versatile protecting groups due to their high reactivity with many transition metals.1,5-9 Several metals are known to cleave allyl esters/ethers in aprotic solvents,6,7,10 and Ru complexes, including commercially 11,12 available [CpRu(CH3CN)3]PF6, are particularly attractive due to their tolerance to many functional groups.13-18 Kitamura reported Ru complexes derived from
[CpRu(CH3CN)3]PF6 and quinaldic acid that are particularly effective for deallylation reactions that operate in protic solvents and feature an internal base, which obviates the need for additives.5,10,19-21 Complex 5.1 exhibits a wide substrate tolerance in alcoholic solvents10 and has been shown to reversibly allylate a variety biological molecules, including peptides and nucleic acids.5,20,22 As part of a program on the synthesis of water-soluble bioactive polymers,23 we sought a facile method for the deprotection of allylic carbonates or carbamates in aqueous solution. Herein, we report our investigations on the kinetics and mechanism of the hydrolysis of allyl carbonates in water in the presence of both homogeneous and supported analogs of complex 5.1. We also compare the rates of hydrolysis and methanolysis of allyl methyl carbonate in both aqueous and methanol solutions.
O OH PF N PF6 6 OH Ru + Ru NCMe N acetone MeCN NCMe O O 5.1 Scheme 5.1. Synthesis of Kitamura’s Catalyst, 5.1.
5.2 Results and Discussion Quinaldic acid and pyridine carboxylic acids have proven to be effective ligands for Ru-catalyzed allylation reactions and allyl alcohol/carbonate
136 deprotection.5,10,19-21 We sought a readily functionalized derivative of quinaldic acid that might enable facile anchoring of Ru complexes to inorganic24 or organic supports. Kynurenic acid (4-hydroxyquinoline-2-carboxylic acid), a metabolite of tryptophan,25,26 provided an attractive and readily available synthon. The kynurenic acid allyl ester can be readily alkylated with a variety of functional groups to generate a series of substituted kynurenic acid derivatives. The kynurenic allyl ester, 5.2, reacts IV with [CpRu(NCCH3)3]PF6 in dry acetone to yield the orange-yellow Ru allyl, 5.4, which precipitates from concentrated solution (Scheme 5.2).19,21,27
Scheme 5.2. Synthesis of the Ru complex 5.4.
The Ru allyl complex 5.1, derived from quinaldic acid, has been shown to be an active catalyst for the deallylation of allyl carbonates10 and carbamates20 in methanolic solutions under argon (eq 1). For these reactions, it was proposed that the RuIV allyl species is the resting state and that attack of methanol on the Ru allyl is rate- limiting.5,10,19 To assess the activity of the analogous Ru allyl complex 5.4 in aqueous solution, we carried out the kinetics of deallylation of allyl methyl carbonate in D2O, eq 1.
(1)
137 In the absence of Ru, allyl methyl carbonate is stable in water for more than a week at room temperature. In the presence of the Ru complex 5.4 (0.6 - 10 mol%), allyl methyl carbonate hydrolyzes to allyl alcohol, methanol and carbon dioxide. In the solid-state, complex 5.4 is stable in air for months without the loss of activity. Kinetic studies carried out in air at room temperature revealed that the hydrolysis of allyl methyl carbonate was zero order in carbonate, Figure 5.1, and first order in Ru to give a rate law:
-d[carbonate]/dt = R = kobs (2)
-3 -1 where kobs = k’[5.4] = 1.25 (± 0.05) x 10 M h (20 °C), Figure 5.2.
(a)
(b)
(c) (d)
Figure 5.1. Plot of allyl methyl carbonate [C] vs. time. Conditions: [5.4]ₒ=0.5 mM in
D2O with a) [C]ₒ = 0.105 M; b) [C]ₒ = 0.054 M; c) [C]ₒ = 0.025 M; d) [C]ₒ = 0.013 M
138
-1 Figure 5.2. Plot of kobs (M•h ) vs. Ru concentration [5.4]0. Conditions: [allyl methyl
carbonate]ₒ = 0.10 M in D2O.
The mechanism proposed for the deprotection of allyl carbonates in alcohol solvents was proposed to involve the nucleophilic attack of the alcohol on the RuIV allyl species as a key step.5,10,20 An analogous mechanism for the aqueous hydrolysis of allyl methyl carbonate is shown in Scheme 5.3. This mechanism assumes that attack of water on the RuIV allyl generates allyl alcohol and a solvated RuII species which subsequently reacts with allyl methyl carbonate to regenerate the RuIV allyl species 5.4.
Scheme 5.3. Proposed mechanism for catalytic hydrolysis of allyl methyl carbonate IV in water by Ru -allyl complex 5.4. (PF6 anion omitted for clarity)
139 A rate law for the mechanism in Scheme 5.3 can be derived if a low steady- state concentration of the RuII intermediate and the reversible formation of 5.4 from allyl alcohol is assumed (eqs 3 and 4).19,27
(3)
(4)
With these assumptions, the rate of disappearance of allyl methyl carbonate can be derived (eq 5): d[C] k k [5.4][H O][C] R 1 2 2 (5) dt k1[A] k2 [C] where [C] = concentration of allyl methyl carbonate, [A] = concentration of allyl alcohol, and [5.4] = concentration of complex 5.4. This rate equation would be consistent with the experimental rate law under conditions where the rate of formation of 5.4 from the reaction of allyl methyl carbonate with RuII is faster than the formation II 19,27 of 5.4 from the reaction of Ru with allyl alcohol (k2[C] >> k-1[A]), for which:
Rate = k1[H2O][5.4]; where k’ = k1[H2O] (6)
These results imply that the attack of water on the RuIV allyl complex 5.4 is rate limiting, even in aqueous solution ([H2O] ~ 55.5 M; [D2O] ~ 55.5 M, 20 °C) and that the RuIV allyl complex 5.4 is the resting state during the catalytic reaction (i.e.
[Ru]T ~ [5.4]0) under these conditions. Analysis of the kinetics of the catalytic
140 hydrolysis of allyl methyl carbonate reaction in a 1:1 mixture of H2O: D2O solvent revealed a kinetic isotope effect of kH/kD = 2.5, which is consistent with proposals by Kitamura5,19,27 that nucleophilic attack on the RuIV allyl is coupled to deprotonation of the nucleophile (H2O in the present case, ROH in previous examples), presumably by the Ru carboxylate (Scheme 5.3). The rate law derived in eq (5) assumes the reversible formation of 5.4 from allyl alcohol and the RuII, but an equally valid rate law could be derived without this assumption (ie. k-1[A] = 0). As CpRu complexes ligated by pyridine carboxylic acids or quinaldic acids have been shown to catalyze allylic substitutions of allyl alcohols,19,27-31 we sought independent kinetic evidence for the equilibrium represented in eq (3). To this end, we treated the Ru complex 5.4 with D2O in acetone-d6, and measured both the equilibrium constant and the rate of approach to equilibrium at 20 °C (see Experimental Section).32 Under these conditions, the II equilibrium constant (eq. 3) was determined to be Keq = [Ru ][A]/[5.4][D2O] = 0.0075 -1 -1 -1 -1 ± 0.0008 with k1= 0.036 ± 0.011 M h and k-1= 4.80 ± 1.53 M h . For comparison, the kinetics of the catalytic methanolysis of allyl methyl carbonate with 5.4 were examined in CD3OD. When carried out in air at room temperature, the methanolysis of allyl methyl carbonate with 5.4 only proceeds to 40% conversion under these conditions. Kinetic studies revealed that the evolution of [allyl methyl carbonate] with time is linear to approx. 25% conversion; at longer times, the rate decays rapidly. Similar rates and conversions are observed in the presence and absence of 5 equivalents of allyl methyl ether, implicating that product inhibition by allyl methyl ether is not the cause of decaying rate. However, if the methanolysis of allyl methyl carbonate with 5.4 is carried out under argon, the reaction proceeds to full conversion in less than 90 minutes with a rate that is zero order in allyl carbonate, -2 -1 kobs(methanol) = 6.6 (± 0.2) x 10 M h , Figure 5.3. While we haven't unambiguously determined the cause of the low conversions observed for the methanolysis in air, we propose that oxidative decomposition of the catalyst is a likely cause, due to the higher conversions observed under argon. We propose that the different behavior observed in water and methanol is due to the higher
141 solubility of oxygen in methanol relative to water: O2/water (298 K, 1 atm) = -5 -4 33-35 IV 2.3x10 ; O2/CH3OH (298 K, 1 atm) = 4.2x10 . As the Ru compounds 5.4 and 5.1 are shelf stable for months in air, it is likely that oxidation of the RuII species is competitive with catalysis in methanol, whereas in water the lower solubility of O2 leads to longer catalysts lifetimes. If borne out in further studies, this hypothesis highlights another potential advantage of water as reaction solvent.
0.12 0.1 0.08 [C] 0.06 0.04 0.02 0 0 0.2 0.4 0.6 0.8 1 1.2 hours
Figure 5.3. Plot of [allyl methyl carbonate] ([C]) versus hours in CD3OD.
Conditions: under argon, 1 mM 5.4, 100 mM allyl methyl carbonate, 0.3 mL CD3OD.
A comparison of the rates of hydrolysis of allyl methyl carbonate in water (under air) vs. the methanolysis (under argon) reveals that the deallylation of allyl methyl carbonate is considerably faster in methanol. If we assume that k' k1[ROH ]
(for R = H, or CH3) and that the concentration of [D2O] = 55.5 M and [CD3OH] = 24.6
M, then estimates for k1 reveal that the rate constant for nucleophilic attack by -1 -1 methanol is approx. 60 times that of water (k1(methanol) = 2.68 ± 0.10 M h ; -1 -1 (k1(water) = 0.045 ± 0.002 M h ). These estimates imply that methanol is considerably more reactive than water toward the RuIV allyl. Further insights were obtained from measurements of the equilibration of the RuIV allyl 5.4 and methanol in acetone. The slow approach to equilibrium of the Ru 32 complex 5.4 treated with methanol in acetone-d6, eq 7, was examined at 20 °C.
142 Under these conditions, the equilibrium constant (eq 7) was determined to be II Keq(methanol) = [Ru ][AE]/[5.4][CH3OH] = 0.0828 ± 0.0083 with k1’= 0.86 x ± 0.26 -1 -1 -1 -1 M h and k-1’= 10.4 ± 3.3 M h (where [AE] = concentration of allyl methyl ether). These results confirm the higher rate of attack of methanol on the RuIV allyl 5.4. In either methanol or water, 5.4 is the predominant species at equilibrium, but the equilibrium constant is higher in the case of methanol than for water in acetone solution.
(7)
Studies of the catalytic hydrolysis of allyl methyl carbonate suggest 5.4 is air/water compatible. To facilitate removal of the catalyst from the products, we generated a solid-supported version24 of the Ru catalyst. Solid supported catalysts facilitate the removal and recycling of metal residues,36 and several heterogeneous catalysts for (de)allylation have been reported.37-39 Indeed, among other solid supported allylation catalysts,40,41 a heterogeneous version of 5.1 supported on silica was developed by Kitamura and coworkers.24 The supported catalyst provided comparable yields and rates to the homogeneous analogues24 and exhibited no leaching from the solid support after nine reaction cycles in methanol solvent under argon, but only one instance of water being used as a co-solvent was reported.24 The amino-functionalized ligand 5.3 was coupled to a chloromethyl polystyrene resin (1% crosslinked, 200-400 mesh, 3.5-4.5 mmol Cl-/g) to generate the polystyrene-supported ligand PS-5.3. Microanalysis of PS-5.3 provided an estimate of 1.164 mmol/g for the amount of ligand loaded onto the polystyrene. Treatment of
PS-5.3 with excess [CpRu(CH3CN)3]PF6 for 1.5 hours in acetone generated the polystyrene supported complex 5.5, Scheme 5.4.
143
Scheme 5.4. Synthesis of the polystyrene supported Ru complex 5.5.
The supported catalyst 5.5 is competent for the hydrolysis of allyl methyl carbonate in aqueous solution. Conducting the hydrolysis of allyl methyl carbonate under identical conditions with the supported Ru complex 5.5 and homogeneous analogue 5.4 allowed for the direct comparison of the rates, enabling an estimate of the Ru loading on the PS bead.42,43 When 0.9 mg of 5.5 was used for the hydrolysis of allyl methyl carbonate in 0.32 mL D2O, the rate constant, kobs, from the zero order plot of [allyl methyl carbonate] vs time of this reaction was compared to kobs vs [5.4] plot
(Figure 5.2) to determine an effective concentration of ruthenium, [Ru]eff = 1.847 ± 0.047 mM, and a catalyst loading of 0.66 ± 0.15 mmol/g of active RuIV on the 5.5 bead. Combustion analysis of 5.5 provided an independent estimate of the Ru content in 5.5, [Ru] = 0.48 ± 0.04 mmol/g, which is within error of the loading determined by kinetic analysis. After 4 hours of reaction time, the supported catalyst was isolated from the reaction solution by filtering, washed with allyl alcohol and acetone, removed of volatiles under high vacuum and subjected to identical catalytic conditions. For the second run, the rate was lower and the calculated catalyst loading on 5.5 was 0.53 ± 0.06 mmol/g. This process was repeated and on the third run the rate was even lower, yielding an estimate of 0.31 ± 0.03 mmol/g, Figure 5.4.
144
Figure 5.4. Plots of [allyl methyl carbonate] versus hours for the deallylation of allyl
methyl carbonate in D2O catalyzed by 5.5, recycled over successive runs.
The lower rates observed on subsequent runs imply that Ru is leaching from the polystyrene support. As a further test for leaching, the supernatant from the washings was assessed for catalytic activity;43 these experiments revealed that the supernatant was catalytically active albeit at a much slower rate than 5.5 (approximately 20 times slower). We could not detect any free kynurenic acid ligand in the supernatant by 1H-NMR or ESI-MS, implying that in water, Ru is extracted off of the bead, presumably by solvolysis of the kynurenic ligand, leaving PS-5.3 intact. While similar leaching phenomenon have been observed with other PS-supported Ru catalysts,44,45 this behavior is in contrast to that reported by Kitamura for a silica- supported version of 5.1 that did not exhibit loss of catalytic activity over nine recycling cycles under an argon atmosphere.24 To test whether the presence of air might contribute to the higher leaching, we repeated the catalyst recycling experiments with 5.5 in water under argon. For these experiments, leaching was attenuated but still evident with each catalyst cycle giving catalyst loadings of: 0.84 ± 0.08 mmol/g,46 0.66 ± 0.15 mmol/g, and 0.31 ± 0.03 mmol/g over three successive runs. The higher degree of leaching observed for the polystyrene catalyst 5.5 in water, relative to the silica-supported analog24 could be due to the nature of the solvent (water vs. methanol), the nature of the modified ligands (kynurenic acid vs. amide- substituted pyridine carboxylic acid), or the support itself. Arenes are known to
145 displace coordinated ligands on RuII species.47 The phenyl-rich environment of the PS-bead could labilize the Ru complexes; further studies are ongoing to test these hypotheses.
5.3 Conclusions Cyclopentadienyl ruthenium complexes of a modified natural product, kynurenic acid, was found to be an effective catalyst for the deallylation of carbonates in water. A variety of substituted kynurenic acid ligands is available using simple procedures and commercially available materials. In the presence of air, catalyst 5.4 exhibits increased stability in water versus methanol; although methanol is a better nucleophile towards 5.4. A kynurenic acid ligand bearing a pendant amine allows for the attachment of the catalyst to a solid PS support. However, catalyst 5.5, a heterogeneous version of 5.4, was observed to leach from the polystyrene support upon several catalyst recycling experiments, suggesting that the stability of supported versions of these homogeneous catalysts depends sensitively on the nature of the support and reaction conditions.
5.4 Experimental Section
5.4.1 General considerations All materials were purchased from Aldrich and used as received unless stated otherwise. Tris(acetonitrile)cyclopentadienylruthenium(II) hexafluorophosphate was purchased from Strem and used as received. All preparations of ruthenium(IV) compounds were carried out in an inert atmosphere using standard glove box or Schlenk techniques but stored in atmosphere in closed vials. Combustion analyses were conducted on a Perkin-Elmer TGA 7 Thermogravimetric Analyzer.
5.4.2 Synthesis of kynurenic acid allyl ester Allyl bromide (1.4 mL, 16.6 mmol) was added in a single aliquot to a vigorously stirred mixture of kynurenic acid (2.0 g, 10.6 mmol) and KHCO3 (1.7 g,
146 17.4 mmol) in 60 mL DMF. The single neck round bottom flask was equipped with reflux condenser and heated to 40°C for 15h. Reaction was quenched with water (~60 mL) and extracted with 3x ethyl acetate. Combined organics were washed with 5% aqueous NaCl, dried with MgSO4, filtered and removed of volatiles. Material was taken on without further purification. Yield: 1.575g (65%). 1H-NMR (300 MHz,
CDCl3): 8.38 (1H, d, 8.37 Hz); 8.05 (1H, bs); 7.70 (1H, t, 7.75); 7.49 (1H, d, 8.40); 7.41 (1H, t, 7.75); 7.05 (1H, s); 6.4 (1H, m); 5.44 (2H, m); 4.93 (2H, d, 4.93). 13C-
NMR (125 MHz, CDCl3) 162.9; 136.6; 130.6; 133.4; 103.8; 126.7; 126.5; 126.4; 124.8; 120.2; 118.4; 112.0; 67.8.
5.4.3 Synthesis of 5.2 (R=CH3) Methyl iodide (0.155 mL, 2.5 mmol) was added in a single aliquot to a vigorously stirred mixture of kynurenic acid allyl ester (0.505 g, 2.2 mmol) and
Cs2CO3 (0.803 g, 2.5 mmol) in 130 mL acetone. The single neck round bottom flask was fitted with a reflux condenser and heated to reflux for 15h. The reaction was quenched with brine (~70 mL) and extracted with 3x ethyl acetate. The combined organics were dried with MgSO4, filtered and removed of volatiles. Material was taken on without further purification. Yield: 0.301g (56%). 1H-NMR (300 MHz, acetone-d6): 8.22 (1H, d, 8.27 Hz); 8.12 (1H, d, 8.56 Hz); 7.84 (1H, t, 7.66 Hz); 7.68 (1H, t, 7.66 Hz); 7.59 (1H, s); 6.17 (1H, m); 5.44 (2H, m); 4.95 (2H, d, 5.83 Hz); 13 4.20 (3H, s). C-NMR (125 MHz, acetone-d6) 163.4; 149.9; 148.7; 132.9; 130.7; 130.2; 127.8; 122.2; 121.9; 118.2; 118.0; 100.3; 66.1; 56.1.
5.4.4 Synthesis of 5.4
5.2 (0.21 g, 0.86 mmol) and [CpRu(NCCH3)3]PF6 (0.316 g, 0.73 mmol) were added to a flame-dried Schlenk flask with a stir bar and removed from the glove box to a Schlenk line. Under N2, 10 mL extra-dry acetone (Aldrich) was added via syringe. The red solution produced a yellow precipitate over 25 min. The stirring was stopped and, under N2 flow, the supernatant was removed from the precipitate with a pipette and discarded. The precipitate was washed with approximately 1 mL extra-dry
147 acetone, and the precipitate was removed of volatiles under high vacuum yielding pure 1 4: 0.218g (54%). H-NMR (300 MHz, acetone-d6): 8.43 (1H, d, 8.74 Hz); 8.10 (2H, m); 7.91 (1H, t, 7.12); 7.59 (1H, s); 4.87 (2H, m); 4.70 (1H, d, 10.46 Hz); 4.45 (1H, m); 4.40 (1H, m); 4.35 (1H, s). Elemental analysis for C19H18F6NO3PRu calcd: 41.16% C, 3.27% H, 2.53 % N; found: 41.26% C, 3.08% H, 2.53 % N.
5.4.5 Synthesis of 5.3 2-bromoethylamine•HBr (0.21 g, 1.02 mmol) was added in a single aliquot to a vigorously stirred mixture of kynurenic acid allyl ester (0.22 g, 0.87 mmol) and
Cs2CO3 (0.8 g, 2.5 mmol) in 65 mL acetone. The single neck round bottom flask was fitted with a reflux condenser and heated to reflux for 15h. The reaction was quenched with brine (~70 mL) and extracted with 3x ethyl acetate. The combined organics were dried with MgSO4, filtered, removed of volatiles and taken into acetone (material is self-labile when neat). Material was taken on without further purification. 1 Yield: 0.120g (46%). H-NMR (300 MHz, acetone-d6): 8.30 (1H, d, 8.4 Hz); 7.86 (1H, d, 8.4 Hz); 7.53 (1H, t, 7.8 Hz); 7.27 (1H, t, 7.8 Hz); 7.06 (1H, s); 6.04 (1H, m); 5.41 (1H, d, 17.4 Hz); 5.22 (1H, d, 10.7); 4.79 (2H, d, 5.6); 4.36 (2H, t, 7.8); 3.59 (2H, 13 t, 7.8 Hz); 3.48 (2H, bs). C-NMR (125 MHz, acetone-d6) 163.0; 132.3; 132.1; 126.8; 125.3; 124.2; 120.9; 118.5; 110.3; 66.8; 64.6; 54.9; 53.0; 40.6. IR (diamond anvil cell) cm-1: 1708 (s, C=O); 1607 (m, 1°NH); 1564 (m, C-C aromatic); 1095 (s, C- O ester); 760 (s, 1°NH).
5.4.6 Synthesis of PS-5.3 5.3 (1.23 g, 4.5 mmol) and Merrifield’s peptide resin, 1% crosslinked, 200-400 mesh (0.132 g, 0.49 mmol) were added to a round bottom flask with a stir bar.
Acetone (100 mL) and Cs2CO3 (0.169 g, 0.52 mmol) were added, and the reaction mixture was stirred for 19 h. The crude reaction mixture was filtered and the filtrate was washed with copious quantities of acetone. IR (diamond anvil cell) cm-1: 1708 (s, C=O); 1564 (m, C-C aromatic); 1095 (s, C-O ester); 760 (s, 2°NH) (see Figure 5.5).
148 5.4.7 Synthesis of 5.5
PS-5.3 (0.141 g, 0.52 mmol) and [CpRu(NCCH3)3]PF6 (0.5363 g, 1.24 mmol) were added to a flame-dried Schlenk flask with a stir bar and removed from the glove box to a Schlenk line. Under N2, 15 mL extra-dry acetone (Aldrich) was added via syringe, and the reaction mixture was stirred for 1.5 h. The stirring was stopped and, under N2 flow, the supernatant was removed from the precipitate with a pipette and discarded. The precipitate was washed with copious extra-dry acetone and removed of volatiles under high vacuum. IR (diamond anvil cell) cm-1: 1587 (m); 1334 (m); 833 (s) (see Figure 5.5). Combustion analysis of 5.5. Samples were loaded on to the analytical balance of the TGA under air flow and subjected to an annealing program: 1 min hold at 50°C, temperature ramp to 100°C, 2 min hold at 100°C, temperature ramp to 900°C, 1 min hold at 900°C, temperature ramp to 50°C; all temperature ramps were 50°C/min. Initial sample masses were taken after the 100°C temperature hold and final masses at the end of the program. A sample of Merrifield’s peptide resin subjected to the annealing program resulted in complete loss of mass. After subjecting 5.4 (1.2567 mg, 0.00227 mmol) to the annealing program, a black residue remained (0.3341 mg) that provided a molecular weight of the fully oxidized material (0.3341 mg/0.00227 mmol = 147.3 g/mol; RuO2 is 133 g/mol). In a representative annealing, 5.5 (1.7490 mg) was reduced to a black residue (0.1202 mg) which was taken to be 147.3 g/mol in molecular weight, yielding a Ru loading on 5.5 of 0.467 mmol/g. Triplicate measurements yield: 0.478 ± 0.034 mmol/g.
5.4.8 Kinetic analysis to give the Ru loading on 5.5 In a typical experiment, 5.5 (0.9 mg) was loaded directly into an NMR tube along with D2O (0.32 mL) and allyl methyl carbonate (3.6 L, 0.032 mmol), the tube was shaken to mix and reaction progress was monitored by 1H-NMR against an internal standard (acetone). From a (zero order) plot of ‘[allyl methyl carbonate] versus hours’ (Figure 5.1), the slope was taken to be kobs, and this value was divided by k’ (the slope of the ‘kobs versus [Ru]’ plot generated from the solution catalysis, Figure 5.2) to determine a [Ru] in solution, 1.847 mM. Accounting for the solution
149 volume and mass of 5.5 in the given experiment gives the loading of 5.5, 0.657 mmol/g of active RuIV on 5.5. The error of Ru loading was propagated from the rate constant errors.
A
B
C
4000 3500 3000 2500 2000 1500 1000 cm-1 Figure 5.5. IR spectra of A) IR spectrum of Merrifield’s peptide resin, 1% crosslinked B) 5.3, and C) PS-5.3.
5.4.9 Determining Equilibrium Into an NMR tube under argon was added 5.4 (2.6 mg, 0.0047 mmol), 1.0 mL acetone-d6, and D2O (8.6 L, 0.4773 mmol), in order. (For methanol the amounts were: 5.4 (2.7 mg, 0.0049 mmol), 0.5 mL acetone-d6, and CD3OD (2.0 L, 0.049 mmol) added in the given order). Tube was shaken to mix and reaction progress was monitored by 1H-NMR (see Figure 5.8). Concentrations of allyl alcohol and 5.4 were determined by integration against an internal standard. The concentration of RuII was determined from mass balance: [RuII] = [5.4]ₒ – [5.4]. The reported rate and equilibrium constant errors were propagated from the error of the NMR integrations, which were taken to be ±10%.
150
(8)
[A] = [A]e + t 9
t ln 1 k1 t C (10) t (1 K ) IV -1 II where = [Ru ]e + [ROH]e + K ([Ru ]e + [allyl alcohol/ether]e)
-4 0 20406080100 -5
-6
-7 y = -0.0497x - 6.6136 -8 R2 = 0.9946 -9
ln(dt/(1-K-1)+alpha)) -10
-11
-12 h
Figure 5.6. Slow approach to equilibrium of the reaction shown in eq 8, R=H, in acetone-d6. Thermodynamic data extracted from plot using eqs 8-10: Keq = 0.0075;
G°=2.90 kcal/mol; [5.4]e=0.0047M; [RuII]e=0.0027M; [allyl alcohol]e=0.0027M; -3 -1 -1 -3 -1 -1 [D2O]e= 0.48M, = 1.1898. k1=0.0116x10 M s and k-1=1.54 x10 M s
151
Figure 5.7. Slow approach to equilibrium of 5.4 and CD3OD in acetone-d6 (eq 8,
R=Me). Thermodynamic data was extracted using eqs 8-10: Keq = 0.0828; G°=1.45 II kcal/mol; [5.4]e=0.0042M; [Ru ]e=0.0057M; [allyl methyl ether]e=0.0057M; -3 -1 -1 -3 -1 -1 [CD3OD]e= 0.0933M, = 0.2341. k1=0.238x10 M s and k-1=2.87 x10 M s
5.4.2 Kinetic Data Typical kinetic experiment. 5.4 (1.8 mg, 0.0032 mmol) was stirred with 1.1 mL D2O (or other solvent) to yield a 2.95 mM stock solution. To an NMR tube, 0.22 mL of stock solution was diluted with 0.43 mL D2O to give a final concentration of 4 (0.65 mol, 0.001 M) to which was added allyl methyl carbonate (7.35 L, 0.065 mmol, 0.100 M) via syringe. Tube was capped and shaken to mix. Reaction was monitored by 1H-NMR (see Figure 5.8), and concentrations were determined versus an internal standard (acetone). The reported rate constant errors were propagated from the error of the NMR integrations, which were taken to be ±10%. When reactions were conducted under argon, the solvent was sparged with a submerged jet of gas for 2 min, and the reaction vessel was purged out before and after reagent addition.
152 HOD *
● * ■ * ■ ■ *
5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 Figure 5.8. 1H-NMR spectrum of partially converted allyl methyl carbonate. The resonances marked with * are due to allyl methyl carbonate, with ● are due to methanol and with ■ are due to allyl alcohol.
0.09
0.08
0.07
0.06
0.05
0.04 [allyl methyl carbonate]
0.03
0.02 012345 hours
Figure 5.9. Plot of [allyl methyl ether] versus hours. Initial reaction conditions, all
reactions under air in methanol-d4: (▲) [5.4]ₒ=1.8 mM, [allyl methyl carbonate]ₒ= 100 mM, [allyl methyl ether]ₒ= 0.0 M; (X) [5.4]ₒ=1.8 mM, [allyl methyl carbonate]ₒ= 49 mM, [allyl methyl ether]ₒ= 0.0 M;
(♦) [5.4]ₒ=3.6 mM, [allyl methyl carbonate]o= 51mM, [allyl methyl ether]o= 0.274 M;
(■) [5.4]ₒ=3.6 mM, [allyl methyl carbonate]o= 51mM, [allyl methyl ether]o= 0.0 M;
153 5.5 References (1) Trost, B. M.; Van Vranken, D. L. Chem. Rev. 1996, 96, 395-422. (2) Trost, B. M.; Crawley, M. L. Chem. Rev. 2003, 103, 2921-2944. (3) Tietze, L. F.; Ila, H.; Bell, H. P. Chem. Rev. 2004, 104, 3453-3516. (4) Consiglio, G.; Waymouth, R. M. Chem. Rev. 1989, 89, 257-276. (5) Tanaka, S.; Saburi, H.; Ishibashi, Y.; Kitamura, M. Org. Lett. 2004, 6, 1873- 1875. (6) Greene, T. W.; Wuts, P. G. M.; Eds. Protective Groups in Organic Synthesis; 3rd ed.; John Wiley & Sons: New York, 2000. (7) Tsuji, J.; Mandai, T. Synthesis 1996, 1-24. (8) Trost, B. M.; Fullerton, T. J. J. Am. Chem. Soc. 1973, 95, 292-294. (9) Tsuji, J.; Takahashi, H.; Morikawa, M. Tetrahedron Lett. 1965, 6, 4387-4388. (10) Tanaka, S.; Hajime, S.; Murase, T.; Ishibashi, Y.; Kitamura, M. J. Organomet. Chem. 2007, 692, 295-298. (11) Trost, B. M.; Older, C. M. Organometallics 2002, 21, 2544-2546. (12) Trost, B. M.; Fraisse, P. L.; Ball, Z. T. Angew. Chem. Int. Ed. 2002, 41, 1059- 1061. (13) Morisaki, Y.; Kondo, T.; Mitsudo, T.-a. Organometallics 1999, 18, 4742-4746. (14) Hermatschweiler, R.; Ferandez, I.; Breher, F.; Pregosin, P. S.; Veiros, L. F.; Calhorda, M. J. Angew. Chem. Int. Ed. 2005, 44, 4397-4400. (15) Fernandez, I.; Hermatschweiler, R.; Breher, F.; Pregosin, P. S.; Veiros, L. F.; Calhorda, M. J. Angew. Chem. Int. Ed. 2006, 45, 6386-6391. (16) Achard, M.; Derrien, N.; Zhang, H.-J.; Demerseman, B.; Bruneau, C. Org. Lett. 2008, 11, 185-188. (17) Gruber, S.; Zaitsev, A. B.; Wörle, M.; Pregosin, P. S.; Veiros, L. F. Organometallics 2008, 27, 3796-3805. (18) Bruneau, C.; Renaud, J.-L.; Demerseman, B. Chem. Eur. J. 2006, 12, 5178- 5187. (19) Saburi, H.; Tanaka, S.; Kitamura, M. Angew. Chem. Int. Ed. 2005, 44, 1730- 1732.
154 (20) Tanaka, S.; Saburi, H.; Murase, T.; Yoshimura, M.; Kitamura, M. J. Org. Chem. 2006, 71, 4682-4684. (21) Tanake, S.; Saburi, H.; Kitamura, M. Adv. Synth. Catal. 2006, 348, 375-378. (22) Tanaka, S.; Hirakawa, T.; Oishi, K.; Hayakawa, Y.; Kitamura, M. Tetrahedron Lett. 2007, 48, 7320-7322. (23) Cooley, C. B.; Trantow, B. M.; Nederberg, F.; Kiesewetter, M. K.; Hedrick, J. L.; Waymouth, R. M.; Wender, P. A. J. Am. Chem. Soc. 2009, 131, 16401- 16403. (24) Hirakawa, T.; Tanaka, S.; Usuki, N.; Kanzaki, H.; Kishimoto, M.; Kitamura, M. Eur. J. Org. Chem. 2009, 789-792. (25) Erhardt, S.; Schwieler, L.; Nilsson, L.; Linderholm, K.; Engberg, G. Physiology & Behavior 2007, 92, 203-209. (26) Barth, M. C.; Ahluwalia, N.; Anderson, T. J. T.; Hardy, G. J.; Sinha, S.; Alvarez-Cardona, J. A.; Pruitt, I. E.; Rhee, E. P.; Colvin, R. A.; Gerszten, R. E. J. Biol. Chem. 2009, 284, 19189-19195. (27) Tanaka, S. J.; Saburi, H.; Hirakawa, T.; Seki, T.; Kitamura, M. Chem. Lett. 2009, 38, 188-189. (28) Tanaka, S.; Seki, T.; Kitamura, M. Angew. Chem. Int. Ed. 2009, 48, 8948- 8951. (29) Sundararaju, B.; Achard, M.; Demerseman, B.; Toupet, L.; Sharma, G. V. M.; Bruneau, C. Angew. Chem. Int. Ed. 2010, 49, 2782-2785. (30) Gruber, S.; Pregosin, P. S. Adv. Synth. Catal. 2009, 351, 3235-3242. (31) Zaitsev, A. B.; Caldwell, H. F.; Pregosin, P. S.; Veiros, L. F. Chem. Eur. J. 2009, 15, 6468-6477. (32) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms; 2nd ed.; McGraw-Hill: New York, 2002. (33) Fischer, K.; Wilken, M. J. Chem. Thermo. 2001, 33, 1285-1308. (34) Battino, R.; Rettich, T. R.; Tominaga, T. J. Phys. Chem. Ref. Data 1983, 12, 163-178. (35) Steinhoff, B. A.; Stahl, S. S. J. Am. Chem. Soc. 2006, 128, 4348-4355.
155 (36) Bergbreiter, D. E.; Tian, J. H.; Hongfa, C. Chem. Rev. 2009, 109, 530-582. (37) Miura, H.; Wada, K.; Hosokawa, S.; Sai, M.; Kondo, T.; Inoue, M. Chem. Commun. 2009, 4112-4114. (38) Li, G.; Zhao, G. Org. Lett. 2006, 8, 633-636. (39) McNamara, C. A.; Dixon, M. J.; Bradley, M. Chem. Rev. 2002, 102, 3275- 3300. (40) Uozumi, Y.; Danjo, H.; Hayashi, T. J. Org. Chem. 1999, 64, 3384-3388. (41) Akiyama, R.; Kobayashi, S. Angew. Chem. Int. Ed. Engl. 2001, 40, 34693471. (42) Demerseman, B.; Renaud, J. L.; Toupet, L.; Hubert, C.; Bruneau, C. Eur. J. Inorg. Chem. 2006, 1371-1380. (43) Phan, T. S.; Sluys, M. V. D.; Jones, C. M. Adv. Synth. Catal. 2006, 348, 609- 679. (44) Nguyen, S. T.; Grubbs, R. H. J. Organomet. Chem. 1995, 497, 195-200. (45) Nieczypor, P.; Buchowicz, W.; Meester, W. J. N.; Rutjes, F. P. J. T.; Mol, J. C. Tetrahedron Lett. 2001, 42, 7103. (46) The initial Ru loading estimated from the rates is higher under argon atmosphere than under air. . (47) Fairchild, R. M.; Holman, K. T. Organometallics 2008, 27, 1823-1833.
156 CHAPTER 6
Poly(2,5-dihydrofuran) from 2-cis-butene-1,4-diol and a Ruthenium Allylation Catalyst
157 6.1 Introduction The ever shrinking dichotomy of organic and polymer synthesis has engendered macromolecular syntheses increasingly reminiscent of their small molecule congeners.1 Ongoing programs with the common theme of organocatalysis for polymer synthesis have adopted the tools of sub-nanometer synthesis for the generation of polymeric constructs.2-4 In the realm of transition metal catalysis, the powerful C-C bond forming Tsuji-Trost reaction5 has been adopted for polymer synthesis.6 Polymer constructs are available via a one monomer method, where each allylic monomer possesses a nucleophilic and an electrophilic end,7 or a two monomer method where a bis-nucleophile is combined with an allylic monomer that is telechelic in leaving group.8 The Pd-allyl-containing reaction mechanism produces primarily linear enchainments,6 but the recent entry of Ir-allyl polymerization catalysts combined with similar synthons as the Pd-allyl reaction have provided access to primarily branched products.9 The successful implementation of the Pd- and Ir-allyls as tools in polymerization chemistry has not been paralleled in the rich chemistry of Ru-allyls and their ability to form C-heteroatom bonds. As was previously noted, the ability of cyclopentadienyl ruthenium (CpRu) complexes of quinaldic acid to activate allyl alcohol to form RuIV-allyls10, 11 is remarkable,12 and a general approach to RuIV-allyl cations was demonstrated.12 The allylation of alcohols via their nucleophilic attack on a RuIV-allyl begs the possibility of implementing allylic alcohols as both the electrophile and nucleophile for ether formation via RuIV-allyl intermediates. For example, a CpRuIV-allyl catalyzed condensation of 2-butene-1,4-diol, which would serve as both electrophile and nucleophile in this paradigm, would generate homo- poly(2,5-dihydrofuran) (poly(2,5-DHF)) if the catalytic hydrolysis of allyl ethers were slower than their formation, Scheme 6.1. While butenediols have been used in condensation reactions to make urethanes or polyesters,13-15 we do not know of an example of the homo-condensation of 2-butene-1,4-diol to make poly(2,5-DHF).
158
Scheme 6.1. Conceptual polymerization of butene-1,4-diol
Polytetrahydrofuran (PTHF) is a commodity telechelic polymer produced on the order of 200,000 tons annually.16 Telechelic polymers are valuable building blocks for the construction of higher molecular weight materials.17 One such class of materials is ABA triblock copolymers generated by grafting or growing polymers from the telechelic endgroup functionality.17-20 Such copolymers have been used in applications such as adhesives, biomedical and elastomeric materials.21 The treatment of THF with a Lewis acid results in the polymerization to PTHF.16 Large scale commercial processes will produce PTHF of approximately 800-1,000 g/mol from air/water free THF, and the post-polymerization formation of telechelic hydroxyl end groups allows for the conjugation of PTHF into commercial products.16
(1)
(2)
(3)
(4)
The treatment of 1,2-dihydrofuran (1,2-DHF) with BF3 produces poly(1,2- DHF) homopolymer not of the ring opened product but one where the repeat unit is an
159 22, 23 intact THF ring, eq 2. The addition of BF3 to 2,5-dihydrofuran (2,5-DHF) and a co-monomer (THF, epichlorohydrin, or propylene oxide) produces random co- polymers of the ring opened 2,5-DHF, eq 3.24 It was observed that the rate of polymerization is inversely proportional to the concentration of 2,5-DHF, which was taken as an indication that homo-poly(2,5-dihydrofuran) (poly(2,5-DHF)) could not be made.24 As far as we know, poly(2,5-DHF) has never been made. Despite their structural similarities, PTHF and poly(2,5-DHF) require different synthetic strategies. Poly(2,5-DHF) should be accessible via the promised bifunctional nature of 2-butene- 1,4-diol in Ru-allyl chemistry. Kinetic investigations25 on 6.1 (see eq 5), a modified version of a RuIV (de)allylation catalyst first reported by Kitamura,10, 26, 27 demonstrated that it possesses both high activity and tolerance to air and water.25 We have shown that 6.1 is effective for the deallylation of allyl carbonates in either aqueous or methanol solution, and kinetic studies suggest a mechanism by which the catalyst is operative.25 In the case of a water nucleophile, the reversible rate determining nucleophilic attack of water on 6.1 generates a solvated RuII species, eq 5, and the back reaction dominates II the forward reaction (i.e. Keq<1). The rapid reaction of the Ru with allyl methyl carbonate regenerates the RuIV-allyl, liberating methanol and carbon dioxide.25 The equilibrium step (eq 5) lies further to the RuII product in the case of methanol versus water, which suggests that ether products are favored at least in the condensation of methanol and allyl alcohol.
160
(5)
(6)
6.2 Results and Discussion As a preliminary test of the paradigm outlined in Scheme 6.1, 6.1 was treated with one equiv. of cis-2-butene-1,4-diol resulting in the formation of 3-(allyloxy)prop- 2-en-1-ol as determined by 1H-NMR, see 6.4 Experimental Section. This suggests that allylic alcohols are competent nucleophiles for 6.1. The molecular weight of a 50/50 solution of acetone/cis-2-butene-1,4-diol and 6.1 (0.10 mol% to diol) is observed to increase with time, as determined by gel permeation chromatography (GPC). A molecular weight plateau is reached at 48 hours (determined by GPC) which corresponds to 80% conversion from monomer (1H-NMR). Evaporation of the reaction solvent yields a mixture of products that can be shown by electrospray ionization mass spectrometry (ESI-MS) to be composed of materials with masses + consistent with a mixture of cyclic (ESI: (C4H6O)n, n=3-10; for n=3+ H , m/z =210.82 vs 211.12 g/mol) and telechelic, linear polymer chains (see 6.4.2 General polymerization experiment) spaced by the mass of a buteneol monomer unit, Figure 6.1. Dissolution of the crude material in dichloromethane (DCM) followed by treatment with hexanes causes the oiling out of an orange material (color is presumably due to Ru products) from a turbid hexanes supernatant. Analysis of the evaporated oil indicates the presence of telechelic polymer, 6.2, (GPC: Mn=770, PDI + =2.97; ESI: HO(C4H6O)nH, n=5-27; for n=16 + Na , m/z =1161.59 vs 1161.64 g/mol), while the concentrated supernatant contains primarily cyclic oligomers by ESI (see 6.4
161 Experimental Section). These cycles are the unsaturated versions of long-ago reported 5n-Crown-n series of crown ethers.28-30 Taken together, these results suggest that 6.1 is an effective catalyst for the polymerization of cis-2-butene-1,4-diol to make poly(2,5-DHF), eq 7. Treatment of cis-2-butene-1,4-diol with BF3•(OC6H6)2 or trifluoroacetic acid (TFA) does not result in the generation of polymer.
(7)
Figure 6.1. ESI-MS of the crude polymerization material. The signals marked with the ▲are due to linear, telechelic oligomers + H+; the ■ to linear, telechelic oligomers + Na+; and * to cyclic oligomers + H+.
If 6.2 is telechelic, the symmetrical chain extension should be possible from both ends of the polymer yielding a telechelic ABA triblock co-polymer. The GPC trace of the polymer, 6.3, resulting from the addition of L-lactide (L-LA) (0.3004 g; 2.09 mmol) to a DCM solution of 1,5,7-triazabicyclo[4.4.0]dodec-1-ene (TBD)
162 (0.0021 mmol, 0.57 mM) and 6.2 (1.7 mg; 0.0027 mmol) suggests the chain extension of 6.2 by a polymer of narrowly dispersed molecular weight (vis-à-vis 6.2), Figure 6.2. In the 1H-NMR of 6.3, the integrations of the end group resonances of the poly(lactide) (PLA) segments versus the polymer backbone indicate the presence of 1.92 endgroups per chain 1H-NMR, see 6.4 Experimental Section. This suggests that 6.3, as well as the macroinitiator 6.2, is a telechelic diol.
O
O O HO H + O 6.2 6 O
N DCM 30 min. N N H TBD
O O O HO O O O O O O OH O x 6O yO 6.3 x+y=240 two end groups Figure 6.2. Chain extension of 6.2 with L-lactide to yield 6.3
The regiochemistry of polymerization reactions are often evident by the information locked in the repeat units of the polymer. Dialysis of the crude 6.2 against methanol (1,000 g/mol molecular weight cut off) leaves only the highest molecular weight material, 6.2b. The 1H-NMR of 6.2b is consistent with the expected symmetrical homoallylation product, Figure 6.3. In addition to the major resonances associated with the buteneol repeat units in 6.2b, there are smaller resonances in the 1H-NMR spectrum (1/16th the intensity) indicative of a vinyl moiety. This would be expected were the RuIV-allyl attacked by the alcoholic nucleophile at the more substituted position, producing a 1,2-addition (branched) product rather than the dominant 1,4-addition (linear) product. This assignment is corroborated by 13C-NMR spectra using fully coupled and decoupled acquisitions. The cis-/trans- isomerization was replicated in a small molecule study by altering the reaction conditions to a lower
163 concentration of cis-2-butene-1,4-diol in aqueous solvent (see 6.4 Experimental Section), which prevents the formation of oligomers.
6:94
Figure 6.3. 1H-NMR spectra of (upper) 2-butene-1,4-diol and (lower) 6.2b, where the insets are 10x enlargements of the spectrum below.
The high selectivity of 6.1 for linear enchainment of cis-2-butene-1,4-diol became interesting to us particularly in light of the behavior of similar catalysts. There is a great deal of interest in the reactivity of pentamethylcyclopentadienyl Ru (Cp*Ru) complexes (including those ligated by quinaldic acid) because of the high selectivity these catalysts typically exhibit for branched isomers: branched/linear (B/L) ratios typically >>1.12, 31-34 Complexes of CpRu typically exhibit low regioselectivity in favor of the linear isomer, 31,35 but CpRuIV-allyls have been used for enantioselective ring closing allylations that obviate the regioselectivity issue.11 To this end, we sought to examine the source of the high regioselectivity of 6.1. A mixture of 6.1 (0.0065 mmol; 0.0036 M) and 2-cis-butene-1,4-diol (10 L, 0.122 mmol) in CD3OD is observed to generate butenyl methyl ethers in a B/L ratio 11/89, similar to the polymerization reaction. The branched products are approximately 2/3 bisalkylated and 1/3 mono-alkylated. The linear products are primarily mono- alkylated. The analogous reaction where cis-2-pentenol is the allyl source is observed to generate pentenyl methyl ethers in a similar B/L ratio 19/81, Scheme 6.2. These
164 observations suggest the regioselectivity of 6.1 is not specific to allyl substitution nor to the identity of the nucleophile.
Scheme 6.2. Regioselectivity of 6.1
We propose a mechanism of polymerization that proceeds through a RuIV- allyl/RuII-solvent cycle, Scheme 6.3. Analogous to the small molecule study,25, 26 a solvated RuII intermediate coordinates with a polymer chain end or monomer to liberate water and generate the RuIV-allyl, 6.4. We have previously shown that nucleophilic displacement of the allyl is coupled to a deprotonation step for the incoming nucleophile, presumably from the internal carboxylate.25 Accordingly, isomerization of the RuIV-allyl prior to the nucleophilic attack by a monomer/polymer provides a Curtin-Hammett distribution of products from 6.4, resulting in chain extension and regeneration of the RuII-solvent species. Alternatively, the -end of a IV Ru -allylic oligomer, 6.4 (R=(CH2CH=CHCH2O)nH), can attack its own allyl to generate a cycle, Scheme 6.3. It is not clear whether the RuIV is preferentially associated with monomer or growing polymer, but the presence of cyclic oligomers suggests that the RuIV must be bound to the growing polymer some of the time. We were not able to isolate the putative intermediate, 6.4.
165 OR RO OH -H O 2 N Ru O O 6.4b Ru N Ru Solv. O N N Ru OH O O O O OR OR O O O O O 6.4c 6.4a
RO OH O N Ru O RO OH HO OH O OR O RO O OH k ~ 16(k +k ) >>k 6.4d b c d a Scheme 6.3. Polymerization Mechanism (PF6 anion omitted for clarity)
If the polymerization reaction is accurately described by eq 7, removal of water should result in higher conversions. When a solution of cis-2-butene-1,4-diol (4 mL; 0.049 mol) and 6.1 (15.4 mg; 0.028 mmol) in a mixture of ethyl ether (2 mL) and dichloromethane (4 mL) was refluxed beneath a condenser column packed with 4Å molecular sieves, the solution reached 90% conversion in monomer after 48 h and the molecular weight of the polymer was 2,300 g/mol (GPC). The polymer exhibited a B/L ratio of 17/83. Changing the solvent has been previously observed to alter B/L ratios.12 These results suggest that higher molecular weight polymer can be obtained by implementing drying conditions, but that care must be taken to segregate the drying agent and catalyst.35b
6.3 Conclusions The suite of M-allyl catalysts that are adaptable to polymer synthesis was expanded to Ru wherein 2-cis-butene-1,4-diol was employed as a nucleophile and electrophile to make poly(2,5-DHF) via a RuIV-allyl mechanism. The CpRu-allyl catalyst was found to be extremely regioselective for the linear product, and higher molecular weight poly(2,5-DHF) can be accessed by implementing drying conditions. The saturated version of this polymer, PTHF, is a commodity material that is produced under air/water free conditions with post-polymerization modification.16 For four decades it was thought that the polymer which is different from PTHF only by one H2
166 per repeat unit was inaccessible,24 but the application of old catalysts to old problems begets new mechanisms and yields new materials.
6.4 Experimental Section
6.4.1 General Considerations All materials were purchased from Aldrich and used as received unless stated otherwise. Dialysis bags were purchased from SpectraPor. The catalyst 6.1 was prepared as previously described.25 The ROP experiments were performed in a glove box under nitrogen atmosphere. Gel permeation chromatography (GPC) was performed in tetrahydrofuran (THF) at a flow rate of 1.0 mL/min on a Waters chromatograph equipped with four 5 μm Waters columns (300 mm x 7.7 mm) connected in series. A Viscotek S3580 refractive index detector, Visotek VE3210 UV/vis detector and Viscotek GPCmax autosampler were employed. The system was calibrated using monodisperse polystyrene standards (Polymer Laboratories). Electrospray Ionization (ESI) Mass Spectra were collected on a ThermoFinnigan LCQ ion trap mass spectrometer operated in positive ion electrospray.
6.4.2 General polymerization experiment Catalyst 6.1 (16.3 mg, 0.029 mmol) was weighed directly into a glass vial with a tightly fitting cap. To the vial was added reagent grade acetone (2 mL) and a stir bar, and the mixture was stirred until homogeneous. In a single aliquot, 2-cis-butene-1,4- diol was added (2 mL, 24.4 mmol), and the vial was tightly capped and left stirring for 48 hours. Acetonitrile was added (1 mL) to quench the reaction, and the reaction was removed of volatiles under high vacuum. The crude polymer was taken up in reagent grade dichloromethane (~2 mL), and the addition of wash hexanes (~5 mL) oils out 6.2 which was isolated from the turbid supernatant by pipette. Crude 6.2: ESI: + HO(C4H6O)nH, n=3-20; for n=7 + Na , m/z =531.44 vs 531.28 g/mol (see Figure 6.5).
GPC: Mn=415; PDI=2.21. Yield: 1.715 g, 80%
167 6.2 was loaded into a dialysis bag (1,000 g/mol molecular weight cut off) with approximately 10x volume of dichloromethane. The bag was closed and submerged in a stirred vessel of methanol. The methanol was changed once after 5 h and the contents of the bag were taken to dryness after 19 h. 6.2b: 1H-NMR (500 MHz, acetone-d6) 5.80 (d, 1H); 5.70 (s, 32H); 5.31 (m, 2H); 4.08 (s, 64H); 4.00 (m, 1H); 13 3.46 (m, 2H). C-NMR (100 MHz, CDCl3) 135.9; 133.0; 129.6; 128.2; 118.8; 79.8;
73.3; 70.5; 66.2; 65.8; 65.7; 64.6; 58.8 (see Figure 6.8). GPC: Mn=4,987; PDI= 1.303. Yield: 21 mg, 4%. The supernatant (cyclic oligomer fraction) was removed of volatiles and analyzed: 1H-
NMR (400 MHz, CDCl3) 5.80 (d, 1H); 5.70 (s, 32H); 5.31 (m, 2H); 4.08 (s, 64H); 13 4.00 (m, 1H); 3.46 (m, 2H). C-NMR (100 MHz, CDCl3) 129.6; 66.2. ESI: + (C4H6O)n, n=3-10; for n=3+ H , m/z =210.87 vs 211.12 g/mol (see Figure 6.6).
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2 PPM (F2) 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 PPM (F1) 1 1 Figure 6.4. H- H COSY (500 MHz, acetone-d6) of 6.2b
168 531.44 6000000 461.45 5500000
5000000 601.42 4500000
4000000
3500000 671.42
3000000
Intensity 391.41 741.44 2500000
2000000 811.43 1500000 321.36 881.49 951.52 1000000 1021.50 1091.56 500000 1092.59 1231.53 251.29 1441.84 0 200 400 600 800 1000 1200 1400 + Figure 6.5. ESI of 6.2: HO(C4H6O)nH, n=3-20; for n=7 + Na , m/z =531.44 vs 531.28 g/mol
303.20 100
90 280.89 80 210.87 70
60 373.26
50 443.29 40 545.23 Relative Abundance 30 615.32 690.82 20 717.15 843.06 992.73 10 1087.08 1224.31 1434.68 1635.89 1764.76 1931.29 0 200 400 600 800 1000 1200 1400 1600 1800 2000 m/z Figure 6.6. ESI of the supernatant from the polymerization reaction. (C4H6O)n, n=3- 10; for n=3+ H+, m/z =210.87 vs 211.12 g/mol, the Na+ adducts are visible at +23 m/z
6.4.3 2-trans-1,4-butenediol 6.1 (1 mg, 0.0018 mmol) was weighed directly into an NMR tube to which was added D2O (0.5 mL) and 3 drops of acetone-d6 (reference) and 2-cis-1,4- butenediol (0.1 mL, 1.2 mmol). The tube was shaken to mix, and the isomerization 1 occurred over several hours yielding the trans- product: H-NMR (400 MHz, D2O)
169 13 5.74 (t, 2H); 4.21 (d, 4H). C-NMR (100 MHz, D2O) 131.59; 58.37 (c.f. Figure 6.7)
13 Figure 6.7. C-NMR (D2O, acetone internal standard) spectrum of poly(2,5-DHF),
6.2b; (lower, a=CH; b=CH2) and 2-cis-butene-1,4-diol (upper, c=CH; b=CH2)
13 Figure 6.8. (upper) C-NMR (100 MHz, CDCl3) spectrum of 6.2b acquired with 13 power-gated decoupling. (lower) C-NMR (100 MHz, CDCl3) spectrum of 6.2b acquired with gated decoupling
170
6.4.6 Reaction of 6.2 with 2-cis-butene-1,4-diol
To an NMR tube containing 6.1 (1.6 mg; 0.0029 mmol) in acetone-d6 (0.4 mL) was added 2-cis-butene-1,4-diol (0.2 uL, 0.0029 mmol). The tube was shaken to mix, and the reaction was monitored by 1H-NMR. Characterization matched the literature.36
■ ■ ●
● ▲ ▲ ▲ ▲ ▲ ▲ ● ▲ ●
1 Figure 6.9. (upper) H-NMR (500 MHz, acetone-d6) spectrum of allyl alcohol 1 (Aldrich). (middle) H-NMR (500 MHz, acetone-d6) spectrum of allyl ether (Aldrich). 1 (bottom) H-NMR (500 MHz, acetone-d6) spectrum of a solution containing 6.1 (●) and cis-2-buten-1,4-diol (■) partially converted to cis-3-(allyloxy)prop-2-en-1-ol (▲).
6.4.7 Synthesis of 6.3 (ROP of LA from the macroinitiator 6.2) To a vial containing a stir bar, 6.2 (2.0 mg; 0.0048 mmol) and TBD (0.0021 mmol) in dichloromethane (DCM) (~2 mL) was added L-lactide (0.3004 g; 2.1 mmol) in DCM in the balance of volume required to yield a final DCM volume of 3.54 mL. The vial was sealed and stirred for 30 min. during which time the reaction had reached 55% conversion to polymer (1H-NMR). The reaction was quenched by the addition of
171 10 mg (0.082 mmol) benzoic acid, and the addition of hexanes to the crude solution 1 precipitated a crystalline solid. H-NMR (400 MHz, CDCl3) d 5.8-5-6 (m, 12H); 5.2
(q, 480H); 4.35 (q, 1.92 H); 4.1-4.0 (m, 24H). GPC: Mn=16,900; PDI= 1.222. Yield: 150 mg, 50%.
4.38 4.36 4.34 4.32
6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.8 ppm 12 480 1.92 24 1 Figure 6.10. H-NMR (400 MHz, CDCl3) spectrum of 6.3. The integrations of the resonances are given below the signals.
6.4.8 2-cis-butene-1,4-diol Polymerization Experiment with Drying A round bottom flask loaded with a stir bar 6.1 (15.4 mg; 0.028 mmol), 2-cis- 1,4-butene-1,4-diol (4 mL; 0.049 mol), 4 mL DCM and 2 mL ethyl ether was equipped with a reflux condenser packed with 4Å molecular sieves. The solution was stirred at reflux for 48 h. Acetonitrile was added (1 mL) to quench the reaction. The reaction was cooled, and conversion was assessed by 1H-NMR. The crude material was placed under high vacuum until constant weight. 1H-NMR (300 MHz, acetone- d6) 5.80 (d, 1H); 5.70 (s, 10H); 5.31 (m, 2H); 4.08 (s, 20H); 4.00 (m, 1H); 3.46 (m,
2H). GPC: Mn=2,300; PDI= 2.639. Yield: 2.99 g, 80%.
172
6.4.8 Allylation of CD3OD with 2-cis-butene-1,4-diol To an NMR tube containing 6.1 (0.6 mg; 0.0011 mmol) was added 0.3 mL
CD3OD and 2-cis-butene-1,4-diol (10 L; 0.122 mmol). The tube was shaken to mix, and the reaction was monitored by 1H-NMR. The reaction proceeded to 56% conversion in 24 hours. The B/L ratio could be measured by integration of the vinyl signal at 5.3 ppm and the methylene signal at 4.05 ppm. Characterization of the branched (both bis- and mono- alkylated products) and linear products matched the literature.37,38,39
6.4.8 Allylation of CD3OD with 2-cis-pentene-1-ol To an NMR tube containing 6.1 (0.6 mg; 0.0011 mmol) was added 0.3 mL
CD3OD and 2-cis-pentene-1-ol (12 L; 0.119 mmol). The tube was shaken to mix, and the reaction was monitored by 1H-NMR. The reaction proceeded to 44% conversion in 24 hours. The B/L ratio could be measured by integration of the vinyl signal at 5.23 ppm and the methylene signal at 4.02 ppm. NMR characterization of the branched and linear products matched the literature.40,41
6.5 References (1) Hawker, C. J.; Wooley, K. L. Science 2005, 309, 1200-1205. (2) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2010, 43, 2093. (3) Iha, R. K.; Wooley, K. L.; Nystrom, A. M.; Burke, D. J.; Kade, M. J.; Hawker, C. J. Chem. Rev. 2009, 109 (11), 5620-5686. (4) Jensen, T. R.; Breyfogle, L. E.; Hillmyer, M. A.; Tolman, W. B. Chem. Commun. 2004, (21), 2504-2505. (5) Trost, B. M.; Van Vranken, D. L. Chem. Rev. 1996, 96 (1), 395-422. (6) Nomura, N.; Tsurugi, K.; Okada, M. J. Am. Chem. Soc. 1999, 121 (31), 7268- 7269. (7) Nomura, N.; Tsurugi, K.; Okada, M. Angew. Chem. Int. Ed. 2001, 40 (10), 1932-1935.
173 (8) Nomura, N.; Yoshida, N.; Tsurugi, K.; Aoi, K. Macromolecules 2003, 36 (9), 3007-3009. (9) Nomura, N.; Komiyama, S.; Kasugai, H.; Saba, M. J. Am. Chem. Soc. 2008, 130 (3), 812-814. (10) Tanaka, S.; Saburi, H.; Ishibashi, Y.; Kitamura, M. Organic Letters 2004, 6 (11), 1873-1875. (11) Tanaka, S.; Seki, T.; Kitamura, M. Angew. Chem. Int. Ed. 2009, 48 (47), 8948- 8951. (12) Zhang, H. J.; Demerseman, B.; Toupet, L.; Xi, Z.; Bruneau, C. Adv. Synth. Catal. 2008, 350 (10), 1601-1609. (13) Marvel, C. S.; Young, C. H. J. Am. Chem. Soc. 1951, 73 (3), 1066-1069. (14) Pourjavadi, A.; Rezai, N.; Zohuriaan-M., M. J. J. Appl. Poly. Sci. 1998, 68, 173-183. (15) Vlad, S.; Oprea, S.; Stanciu, A.; Ciobanu, C.; Bulacovschi, V. E. Poly. J. 2000, 36, 1495-1501. (16) Zeitsch, K. J., The Chemistry and Technology of Furfural and its Many By- products. Elsevier Science B.V.: Amsterdam, The Netherlands, 2000; p 358. (17) Goethals, E. J., Telechelic Polymers: Synthesis and Applications. CRC Press: Boca Raton, 1989; p 403. (18) Phillips, J. P.; Deng, X.; Stephen, R. R.; Fortenberry, E. L.; Todd, M. L.; McClusky, D. M.; Stevenson, S.; Misra, R.; Morgan, S.; Long, T. E. Polymer 2007, 48 (23), 6773-6781. (19) Young, A. M.; Ho, S. M. J. Controlled Rel. 2008, 127, 162-172. (20) Pitet, L. M.; Hillmyer, M. A. Macromolecules 2009, 42 (11), 3674-3680. (21) Thomas, R. M.; Grubbs, R. H. Macromolecules 2010, 43 (8), 3705-3709. (22) Sanda, F.; Matsumoto, M. Macromolecules 1995, 28 (20), 6911-6914. (23) Sanda, F.; Matsumoto, M. J. Appl. Poly. Sci. 1996, 59, 295-299. (24) Minoura, Y.; Mitoh, M. Die Makromolekulare Chemie 1968, 119, 104-112. (25) Kiesewetter, M. K.; Waymouth, R. M. see Chapter 5.
174 (26) Tanaka, S.; Saburi, H.; Murase, T.; Yoshimura, M.; Kitamura, M. J. Org. Chem. 2006, 71 (12), 4682-4684. (27) Kitamura, M.; Tanaka, S.; Yoshimura, M. J. Org. Chem. 2002, 67 (14), 4975- 4977. (28) McKenna, J. M.; Wu, T. K.; Pruckmayr, G. Macromolecules 1977, 10 (4), 877-879. (29) Pruckmayr, G.; Wu, T. K. Macromolecules 1978, 11 (1), 265-270. (30) Thu, C. T.; Bastelberger, T.; Hocker, H. J. Mol. Catal. 1985, 28, 279-292. (31) Bruneau, C.; Renaud, J. L.; Demerseman, B. Chem. Eur. J. 2006, 12 (20), 5178-5187. (32) Gruber, S.; Zaitsev, A. B.; Worle, M.; Pregosin, P. S.; Veiros, L. F. Organometallics 2009, 28 (12), 3437-3448. (33) Achard, M.; Derrien, N.; Zhang, H.-J.; Demerseman, B.; Bruneau, C. Org. Lett. 2008, 11 (1), 185-188. (34) Sundararaju, B.; Achard, M.; Demerseman, B.; Toupet, L.; Sharma, G. V. M.; Bruneau, C. Angew. Chem. Int. Ed. 2010, 49 (15), 2782-2785. (35) Trost, B. M.; Fraisse, P. L.; Ball, Z. T. Angew. Chem. Int. Ed. 2002, 41 (6), 1059-1061. (35b) Attempting to remove the water to favor products by in situ chemical drying
(MgSO4, NaSO4, or 4Ǻ molecular sieves) did not increase the molecular weight of the polymer. (36) Ooi, T.; Hokke, Y.; Tayame, E.; Marouka, K. Tetrahedron 2001, 57 (1), 135- 144. (37) Lu, K.; Huang, M.; Xiang, Z.; Liu, Y.; Chen, J.; Yang, Z. Org. Lett. 2006, 8 (6), 1193-1196. (38) Baltes, H.; Steckhan, E.; Schafer, H. J. Chem. Ber. 1978, 111, 1294-1314. (39) Kumaraswamy, G.; Sadaiah, K.; Ramakrishna, D. S.; Police, N.; Sridhar, B.; Bharatam, J. Chem. Commun. 2008, (42), 5324-5326. (40) Lodge, E. P.; Heathcock, C. H. J. Am. Chem. Soc. 1987, 109, 3353-3361. (41) Alexakis, A.; Normant, J. F.; Villieras, J. J. Mol. Catal. 1975, 1 (1), 43-58.
175 CHAPTER 7
Alkali Metal Reductions of N-Heterocyclic Carbenes and Their HCl Salts
176 7.1 Introduction The use of N-heterocyclic carbenes (NHCs) experienced a renaissance with the work of Arduengo in 1991.1 Their chemistry has recently been the subject of a thematic issue of Chemical Reviews 2 and has been the subject of several independent reviews.3-5 NHCs are used in almost every conceivable arena of chemistry from transition metal ligands to organocatalysts5,6, where they are employed in our lab as potent transesterification catalysts for polymerization.7 NHCs are renowned for their potent nucleophilicity and strong -donation.3 However, the dearth of empirical information on the electron distribution of NHCs is surprising, especially given the debate over their aromaticity8 and -accepting abilities.9 Indeed, to our knowledge, only one EPR spectrum has been recorded of an NHC parent anion radical (i.e. not complexed to metal).10 In that case, the ‘odd’ electron was localized away from the carbene carbon (see hyperfine coupling constants of triazole carbene below), which suggests that the five-membered ring is too high in energy to accept an electron.2, 10 The electron distributions of NHC anion radicals that do not possess a phenyl moiety in the backbone into which the added electron can diffuse would be of interest.
The large amount of interest in the exact nature of the NHC-M bond has spurred many theoretical and experimental studies.3 Several studies to gauge the bond donating abilities of NHCs have been performed.11-13 The anion radicals of complexes of several imidazolylidene-type NHCs or their heavy atom congeners (at the carbene carbon) as well as the gas phase anion radicals of chlorocarbenes have been studied to probe their electrochemical activity and ability to participate in electron transfer
177 processes.14-17 However, many of these studies remain computational in nature due to the elusive NHC anion radical, and the reluctance of the imidazolylidene moiety to accept an electron is a common theme.18, 19 Taking the nature of the imidazolylidene congener of [6]annulene or cyclopentadienyl anion as a cue, the -acidity of NHCs would seem to be minimal for the aromatic8 NHCs. The radical anion of a parent imidazolylidene-type NHC has not been observed, but if these species are like [6]annulene (as they are predicted to be),8 their radical anions would not be stable in most solvents at room temperature. The unmistakable seven line pattern of the benzene anion radical, generated in THF with K, is observable at 170 K but unobtainable at room temperature.20 Only with the addition of crown ether (18-crown-6) is the radical stable at room temperature.21 The signal intensity of organic ion radicals has been shown to be a function of the concentration of alkali metal solvating agent.22 18-crown-6 also renders observable the [8]annulene anion radical in THF, which undergoes disproportionation to [8]annulene dianion and neutral molecules in the absence of crown ether.22, 23 The electrochemical or chemical reduction of imidazolium salts (7.1) is a means of preparatively producing the free NHCs.24-26 In the case of 7.1, the purported initial reduction product is the neutral radical of the imidazolium, 7.3a.24, 25 This species has not been characterized by EPR spectroscopy due to its rapid elimination of . H to form the free carbene, 7.4.24 Electrochemical investigation of the transformation suggests an irreverisible one electron reduction of 7.1 and no further reduction signals due to the reduction of 7.4.24 The five membered ring moiety is proposed to be aromatic8 (4n+2 electrons), and the smaller ring (vis-à-vis benzene) would suggest a higher reduction potential than the parent aromatic annulene.20, 27 However, it was our reasoning that the strong reducing conditions of alkali metal in THF with crown ether might sufficiently stabilize 7.3a and 7.4 to allow for the empirical characterization of the electron distribution in the NHC radical anions and NHC-salt radicals. The muonium addition product of 7.4 has been characterized by Muon Spin Resonance (SR) which suggests that the muonium resides on the imidazolium ring, preferentially on the carbenic carbon.28
178
Scheme 7.1. The Reduction of 7.1 to Yield the Free Carbene
7.2 Results and Discussion
7.2.1 Reductions of NHCs and their HCl Salts
Scheme 7.2. Reaction Diagram of the Reduction Products of 7.1, 7.2, 7.4 and 7.5
The EPR signal of the green solution that results from the exposure of 7.4 or 7.5 (Scheme 7.2) in THF with excess 18-crown-6 in vacuo to a freshly distilled K mirror is shown in Figure 7.1. The three line pattern is well explained by an electron 29 coupling to a single nitrogen with aN = 40 G. This large hyperfine coupling constant
179 to a single nitrogen is ~2/3 the theoretical maximum from the McConnell equation (aN
= Q1Q2(), where Q1 = 30-35 G, Q2= -14 G and 1 is the spin density on the 29, 30 N and 2 and 3 are the spin densities on the adjacent atoms), and cannot be explained in terms of the expected radical, 7.6. The signal is attributed to a breakdown product, 7A, see 7.2.2 Decomposition Products. The preparative scale synthesis of 7.4 with excess K suggests25 that 18-crown-6 facilitates the electron transfer to 7.4 or 7.5 at room temperature as with [6]annulene, but the added electron is extremely destabilizing to 7.4 or 7.5.20, 27 Further exposure of the solution to the K mirror results in the EPR spectra of species attributed to the breakdown products of the solvent system.31, 32 The 1H-NMR spectra of the fully reduced solutions (to the solvated electron) do not indicate the presence of any resonances attributable to 7.6, nor do the solutions that were quenched with D2O possess the D2O adducts/Birch reduction products of 7.4 (or 7.5) by GC-MS, which corroborates a degradation phenomenon.
3440 3460 3480 3500 3520 3540 3560 3580 Gauss Figure 7.1. X-band EPR signal observed upon the exposure of a THF solution of 7.4 and 18-crown-6 to a K metal mirror in vacuo.
Since the exposure of 7.4 to reducing conditions in the presence of 18-crown-6 gives access to different products than in the absence of crown ether, we were interested in determining the effect of crown ether, if any, on the reduction of 7.1 (and 7.2). The exposure of 7.1 or 7.2 in THF with excess 18-crown-6 to a K metal mirror yields at least two species that are observable by EPR. One of these species (7B) is well simulated by 6 protons with aH= 6.83 G and the other (7C) by 3Hs at aH = 6.80 G
180 and 2Hs at aH = 6.15 G. There is no measurable g-shift between the two species. When 7.1 is reduced, the two species occur in a ratio of 1:1 (7B:7C). Surprisingly, the K metal reduction of 7.2 under identical conditions yields the same spectra albeit in different relative intensities, 7B:7C = 2:1, Figure 7.2. There are no electron distribution assignments conceivable to us that would account for 7B and 7C being assigned as 7.3, see section 7.2.2 Decomposition Products.
A) * * * * B) * *
C)
D)
3490 3500 3510 3520 3530 Gauss Figure 7.2. (A) X-band EPR spectrum of the K metal reduction products of 7.1; (B) Computer generated simulation of two species, 7B and 7C, in a ratio of 1:1; (C) X- band EPR spectrum of the K metal reduction products 7.2; (D) Computer generated simulation of two species, 7B and 7C, in a ratio of 2:1. The species 7B and 7C are simulated using the parameters given in the text. The resonances marked with (*) are due to 7C and those with (■) to 7B.
However, the inconsistency that develops as a result of the reduction products of 7.4 and 7.1 (which leads to 7.4 under reducing conditions)25 being different must be due to the presence of 18-crown-6. In the preparative synthesis of 7.4 from 7.1, we propose that Lewis acidic 7.4 is stabilized by coordination to K+ analogously to the solvation of the cation by THF or DME in the absence of 18-crown-6.33, 34 In the
181 presence of crown ether, 7.4 is no longer stabilized by coordination to the K+ which is being sequestered by the stronger interaction of K+ with the crown ether. To the . extent that the carbene character evolving concomitant with the loss of H in the 7.3 to 7.4 (or 7.5) transformation is being stabilized by K+, the presence of 18-crown-6 will attenuate that interaction resulting in destabilization of that transition state, Figure 7.3. Further, the presence of the K+ encapsulated in the crown ether will hinder the intermolecular loss of H2 (ion pairs or covalent-type interactions involving alkali metal in the presence of crown ether involves the entire M+/18-crown-6 complex)35, 36. In sum or separately, these processes allow 7.3 to access decomposition pathways that lead away from 7.4, see 7.2.2 Decomposition Products.37 Also, the ability of crown ether to attenuate the interaction of an alkali metal cation and an imidazolylidene has been previously observed to induce decomposition.38 The exhaustive reduction (to solvated electron) of the solutions that provided the EPR spectra in Figure 7.2 and 1 subsequent quenching with D2O indicates no trace of 7.1 or 7.2 by H-NMR.
O O O O O K+ O O O O O O O R R O H N O O N N N K+ R H O O N H R' O O N N 7.4 R=CH3 (IMes) N O O 7.5 R=H (IdiMe) K+ O O O R R + NN
H 7B R' R' N N + O K O
R R R H R H N N N N
7.4 R=CH (IMes) 7.3 3 7.5 R=H (IdiMe) Figure 7.3. The addition of 18-crown-6 disfavors the path from 7.3 to 7.4 or 7.5 causing destructive decomposition pathways.
182 7.2.2 Decomposition Products The features of the EPR spectra permit tentative assignment of the decomposition products. The normal relative distributions of spin densities of phenyl moieties substituted with electron donating groups are at odds with the phenyl substitution patterns of 7.1 and 7.2 (the presence of a group of 3 protons is, of course, indicative of a methyl group), Figure 7.4.30 Further, the wide spectral window 30 (spectral width > Q, aH = Q) indicates that 7B and 7C are radicals and not radicals.30, 39 The ability of 7.3 to eliminate a radical to form a stable, neutral species is well established,24-26 and the presence of 18-crown-6 is reasonably expected to destabilize the pathway leading to 7.4 or 7.5 (see 7.2.1 Reductions of NHCs and their HCl Salts) allowing other pathways to dominate. Species 7.3 could reasonably . eliminate a aryl radical to form a stable aryl imidazole if the H elimination pathway
Figure 7.4. The degenerate LUMOs of benzene substituted with an electron pushing group where the dashed lines represent nodes. were disfavored. This aryl radical is assigned as 7B. The phenyl () radical40 has been observed as has the p-tolyl radical41. In these molecules, the spin densities rapidly decrease in magnitude away from the radical center, and the p-methyl of the p- tolyl radical is not observed,39, 41 Figure 7.5, which would account for the identical
EPR spectra produced by 7B when R=H or CH3. The coupling constants ortho- to the radical center are 17 G and 18 G for the phenyl and p-tolyl radicals respectively,39 and one would expect a coupling constant of the protons of a methyl group in those positions to be approximately one third those values, as observed in 7B, Figure 7.6. The identical g-shift of 7C (to 7B) suggests a species that is structurally similar to 7B (another neutral radical of similar structure).30 However, the assignment of a structure does not seem possible without evoking reduction/rearrangement products of 7B.
183 18.2 G 17.4 G 6.25 G 2.04 G p-tolyl phenyl Figure 7.5. Coupling constants of selected s radicals.39
Coupling constants similar magnitude to those observed upon the reduction of 7.4 or 7.5 are often seen in nitroxide type radicals where the electron is considered to be shared evenly between N and O atoms.39 In order to increase the magnitude of the 42 coupling constant from a nitroxide-type radical to aN = 40G, as observed for 7A, the spin from the ‘odd’ electron must be more localized on the N atom. The proposed homolysis of the N-C(aryl) bond in 7.3 does not occur in this system (leading to 7B) presumably because the stable imidazolium cannot be easily formed from the free NHC (as opposed to the 7.3 decomposition, see Figure 7.6). The 4N+3 electron 7.6 could, however, decompose along a retro-[3+2] cycloaddition43 pathway followed by a proton migration to form a nitrogen centered radical whose unpaired electron is predominately N in character, Figure 7.6. Any hyperfine coupling from the aryl moieties would be hidden in the linewidth.42
Figure 7.6. Proposed decomposition pathway leading to 7A and 7B.
184 The extreme instability of 7.4 and 7.5 upon reduction is counter to the observation of a stable triazole anion radical.10 We propose that the presence of the phenyl ring on the triazole backbone is the source of that molecule’s marked stability (vis-à-vis 7.4 or 7.5). In the imidazolylidenes, the nitrogen atoms and phenyl groups are mutually destabilizing for single electron addition, and decomposition is the result upon reduction. Taken together, these observations suggest that the N’s, which stabilize the carbene carbon, are the source of the instability of NHC single electron reduction products. Triazole can obviate this instability due to the phenyl moiety in the backbone away from the electron pushing nitrogen atoms.
7.3 Conclusion Much in the same manner as the prototypical aromatic system [6]annulene, the exposure of 7.4 or 7.5 to K metal in THF does not produce a reduced species at room temperature except in the presence of crown ether. The imidazol-2-ylidenes (7.4 and 7.5) are destabilized by the addition of an electron resulting in decomposition to a paramagnetic species whose unpaired electron is predominately localized on a single N. These results suggest that the imidazol-2-ylidenes are aromatic8 and would not readily accept an electron through a backbonding interaction from a metal center.9 Likewise, the putative intermediate, 7.3, in the reductive formation of free carbene from 7.1 or 7.2 is destabilized in the presence of 18-crown-6, preventing the formation of free carbene and leading to putative intramolecular decomposition products. Certainly, an experimental or theoretical treatment of the electron donating/accepting abilities of NHCs requires the explicit consideration of solvent effects to give meaningful data.
7.4 Experimental Section
7.4.1 General Considerations Tetrahydrofuran was dried over Na/benzophenone and distilled onto sodium- potassium eutectic under high vacuum. 18-crown-6 was purchased from Aldrich and stored under high active vacuum for several days before use. 7.1 was purchased from
185 Strem Chemicals and stored in a glove box under N2. 7.2 was prepared from standard procedures, and characterization matched the literature.44 Potassium in mineral oil was purchased from Aldrich.
7.4.2 Example Reduction Experiment and Quenching
A glass capillary with fragile ends containing 7.1 (2.2 mg, 0.0065 mmol) was loaded into the custom apparatus shown in Figure 7.7 along with 18-crown-6 (5 mg, 0.019 mmol). A small amount of potassium was washed with hexanes and loaded into the apparatus at #2 (numbers in Figure 7.7). The apparatus was evacuated under high active vacuum by attaching to a vacuum line at #3. The apparatus was sealed shut at #4, and a K metal mirror was formed at #5 by distilling the potassium through #6. The arm #2 was sealed from the apparatus at #6. THF (1.5 mL) from sodium-potassium eutectic was distilled into the apparatus at #7, and the apparatus was sealed from the vacuum line at #7. The apparatus was gently agitated to dissolve the 18-crown-6 and then shaken vigorously to break the capillary. The apparatus was rotated to allow the solution to come into contact with the K mirror, and the solution was transferred to the attached EPR tube which was inserted directly into the EPR cavity to record the spectra. The apparatus could be removed from the cavity to allow the solution to be re-exposed to the metal mirror and subsequent EPR spectra recorded. Quenching reduction products: The end of the EPR tube was scored and snapped off when submerged in water. The contents of the apparatus were extracted with chloroform, and the concentrated fractions were analyzed by 1H-NMR and GC-MS.
186
Figure 7.7. Apparatus used for the reduction of NHCs or their HCl salts.
7.5 References (1) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113 (1), 361-363. (2) Arduengo, A. J.; Bertrand, G. Chem. Rev. 2009, 109 (8), 3209-3210. (3) Herrmann, W. A. Angew. Chem. Int. Ed. 2002, 41, 1290-1309. (4) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 1999, 100 (1), 39-92. (5) Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107 (12), 5606-5655. (6) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107 (12), 5813-5840. (7) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2010, 43 (5), 2093-2107.
187 (8) Heinemann, C.; Muller, T.; Apeloig, Y.; Schwarz, H. J. Am. Chem. Soc. 1996, 118 (8), 2023-2038. (9) Jacobsen, H.; Correa, A.; Costabile, C.; Cavallo, L. J. Organomet. Chem. 2006, 691, 4350-4358. (10) Enders, D.; Breuer, K.; Raabe, G.; Simonet, J.; Ghanimi, A.; Stegmann, H. B.; Teles, J. H. Tetrahedron Lett. 1997, 38 (16), 2833-2836. (11) Gusev, D. G. Organometallics 2009, 28 (22), 6458-6461. (12) Furstner, A.; Alcarazo, M.; Krause, H.; Lehmann, C. W. J. Am. Chem. Soc. 2007, 129 (42), 12676-12677. (13) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. Organometallics 2009, 28 (18), 5395-5404. (14) Tumanskii, B.; Sheberla, D.; Molev, G.; Apeloig, Y. Angew. Chem. Int. Ed. 2007, 46 (39), 7408-7411. (15) Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.; LacoÌ‚te, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D. P. J. Am. Chem. Soc. 2009, 131 (31), 11256-11262. (16) Walton, J. C.; Brahmi, M. M.; Fensterbank, L.; Lacote, E.; Malacria, M.; Chu, Q.; Ueng, S.-H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc. 2010, 132 (7), 2350-2358. (17) Villano, S. M.; Eyet, N.; Lineberger, W. C.; Bierbaum, V. M. J. Am. Chem. Soc. 2008, 130 (23), 7214-7215. (18) Pause, L.; Robert, M.; Heinicke, J.; Kuhl, O. J. Chem. Soc., Perkin Trans. 2 2001, 1383-1388. (19) Penka, E. F.; Schläpfer, C. W.; Atanasov, M.; Albrecht, M.; Daul, C. J. Organomet. Chem. 2007, 692 (26), 5709-5716. (20) Lawler, R. G.; Bolton, J. R.; Fraenkel, G. K.; Brown, T. H. J. Am. Chem. Soc. 1964, 86, 520-521. (21) Komarynsky, M. A.; Weissman, S. I. J. Am. Chem. Soc. 1975, 97 (6), 1589- 1590. (22) Stevenson, C. D.; Concepcion, J. G. J. Phys. Chem. 1972, 76 (15), 2176-2178.
188 (23) Fray, G. I.; Saxton, R. G., The chemistry of cyclo-octatetraene and its derivatives. Cambridge University Press: New York, 1978. (24) Gorodetsky, B.; Ramnial, T.; Branda, N. R.; Clyburne, J. A. C. Chem. Commun. 2004, 1972-1973. (25) Canal, J. P.; Ramnial, T.; Dickie, D. A.; Clyburne, J. A. C. Chem. Commun. 2006, 1809-1818. (26) Mery, D.; Aranzaes, J. R.; Astruc, D. J. Am. Chem. Soc. 2006, 128 (17), 5602- 5603. (27) Stevenson, C. D.; Morgan, G. J. Org. Chem. 1998, 63 (22), 7694-7697. (28) McKenzie, I.; Brodovitch, J.-C.; Percival, P. W.; Ramnial, T.; Clyburne, J. A. C. J. Am. Chem. Soc. 2003, 125 (38), 11565-11570.
(29) Other large aN's have been observed, see: a) Kaupp, M.; Arbuznikov, A.V.; Heselmann, A.; Gorling, A. J. Chem. Phys. 2010, 132, 184107. and b) L.-D., L.; Wang, F.; Yang, D.-S.; Wu, L.-M. Res. Chem. Intermed. 2004, 30, 269- 278. (30) Carrington, A.; McLachlan, A. D., Introduction of Magnetic Resonance. Harper and Row: Evanston, 1967. (31) The heavy reduction of solutions containing crown ethers is known to generate ethylene and methylene radicals as breakdown products. The EPR spectra of said radicals exhibit multiple g-values: a) Grobelny, Z.; Stolarzewicz, A.; Morejko-Buz, B.; Bartsch, R. A.; Yamato, K.; Fernandez, F. A.; Maercker, A. J. Org. Chem. 2002, 67, 7807-7812. (32) Maguire, M. M.; Bernheim, R. A. J. Mag. Res. 1971, 4 (2), 167-174. (33) Carey, F. A.; Sundberg, R. J., Advanced Organic Chemistry Part A: Structure and Mechanisms. 3rd ed.; Plenum Press: New York, 1990; p 802. (34) Liotta, C. L.; Harris, H. P. J. Am. Chem. Soc. 1974, 96 (7), 2250-2252. (35) Eastman, M. P.; Ramirez, D. A.; Jaeger, C. D.; Watts, M. T. J. Phys. Chem. 1976, 80 (2), 182-186. (36) Peters, S. J.; Turk, M. R.; Kiesewetter, M. K.; Reiter, R. C.; Stevenson, C. D. J. Am. Chem. Soc. 2003, 125, 11212-11213.
189 . (37) The loss of H from 7.3 to produce the free carbene (7.4 or 7.5) is reminiscent of the heterogeneous reaction of water with K at the metal surface, and the low concentrations of the EPR experiment would bias the intermolecular loss of hydrogen gas towards an intramolecular decomposition process. (38) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326 (5952), 556-559. (39) Gerson, F.; Huber, W., Electron Spin Resonance Spectroscopy of Organic Radicals. Wiley-VCH: Germany, 2003. (40) Zemel, H.; Fessenden, R. W. J. Phys. Chem. 1975, 79, 1419-1427. (41) Bargigelletti, F.; Poggi, G.; Breccia, A. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1198-1201. (42) See representative iminoxyl radicals on pg 204 of Gerson, F.; Huber, W., Electron Spin Resonance Spectroscopy of Organic Radicals. Wiley-VCH: Germany, 2003. (43) Tidwell, T. T., Ketenes. 2nd ed.; Wiley: New Yord, 2006. (44) Leuthäußer, S.; Schmidts, V.; Thiele, C.; Plenio, H. Chem. Eur. J. 2008, 14 (18), 5465-5481.
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