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Applications of Boronic Acids in Organic Synthesis

Applications of Boronic Acids in Organic Synthesis

Applications of Boronic in Organic Synthesis

A dissertation presented by Pavel Starkov

in partial fulfilment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY at UNIVERSITY COLLEGE LONDON

Department of Christopher Ingold Laboratories University College London 20 Gordon Street WC1H 0AJ London Declaration

This dissertation is the result of my own work. Where information has been derived from other sources it has been clearly indicated so and acknowledged accordingly.

/Pavel Starkov/

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Abstract

This thesis describes progress on the application of boronic acids and borate as catalysts and reagents in synthetic organic synthesis, focusing on two areas: one-pot enolate formation/aldol reactions and bond formation.

Chapter 1 introduces the reader to boronic acids and derivatives thereof, their methods of preparation and their use in synthetic organic chemistry as reactants, reagents and catalysts.

Chapter 2 covers current chemical methods and cellular alternatives for amide bond formation. Here, we also discuss our use of reagents for the activation of carboxylic acids as well as .

Chapter 3 introduces a new concept in catalytic aldol reactions, i.e. an alternative strategy to access boron enolates in situ. The work covers successful demonstration of the feasibility of such an approach on an intramolecular system. A novel variation of aerobic Chan–Evans– Lam coupling, an intramolecular coupling of an aliphatic with a boronic using catalytic copper, is also introduced

Chapter 4 builds on our observations on gold and especially that in relation to electrophilic halogenations.

Chapter 5 contains full details of the experimental procedures.

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Contents

Declaration ii Abstract iii Contents iv Abbreviations vi Acknowledgements vii

1 Boronic Acids in Organic Synthesis 1 1.1 Introduction 2 1.2 Preparation 5 1.2.1 Arylboronic Acids 5 1.2.2 Other Boronic Acids 10 1.3 Boronic Acids as Reactants 12 1.3.1 Transition Metal Catalysed Reactions 12 1.3.2 Chan–Evans–Lam Coupling 15 1.3.3 Converting Boronic Acids 18 1.4 Boronic Acids as Reagents and Catalysts 18 1.4.1 Activation of Carboxylic Acids 21 1.5 Summary 27

2 Development of Boron Based Reagents and Catalysts for 30 Activation of Carboxylic Acids and Amides 2.1 Amide Bond Formation: An Overview 31 2.1.1 Methods for Amide Bond Formation 33 2.1.1.2 Activation of Carboxylic Acids 34 2.1.1.3 Alternative Methods 37 2.1.1.4 Catalytic Methods 38 2.1.1.5 Emerging Methods 38 2.1.2 Amide Bond Formation in Nature 44 2.1.2.1 Ribosomal Peptide Bond Formation 46 2.1.2.2 Nonribosomal Peptide Synthetases 47 2.1.2.3 Acyl Transferases 48 2.1.2.4 Lipases 50

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2.2 Results and Discussion 53 2.2.1 Introduction 53 2.2.2 Aims and Objectives 55 2.2.3 Synthesis of Boronic Acids 57 2.2.3.1 Synthesis of (1-Hydroxy-1H-benzo[d][1,2,3]triazol-7-yl) 57 2.2.3.2 Synthesis of “Sulfur-Armed” Boronic Acid 63 2.2.4 Evaluation of Boronic Acids and Borates for Catalytic Amide Bond Formation 64 2.2.5 Borates as a Novel Class of Coupling Reagents for Amide Bond Formation 74 2.2.6 Tris(2,2,2-trifluoroethyl) Borate as a Reagent for the Activation of Primary Amides 78 2.2.7 Mechanistic Considerations 80 2.2.8 Conclusions and Outlook 82

3 Gold-Catalysed Boron Enolate Formation 87 3.1 Background 89 3.2 Aims and Objectives 95 3.3 Results and Discussions 97 3.3.1 Gold-Catalysed Boron Enolate Formation 97 3.3.2 One-Pot Boron Enolate Formation/Aldol Reaction 100 3.3.3 Elaboration of Aldol Products: Oxidation, Suzuki, and Chan–Evans–Lam 112 3.3.4 Miscellaneous 116 3.4 Summary and Outlook 118

4 Observations on the Role of Cationic Gold and Brønsted Acids in Electrophilic Halogenation 122 4.1 Results and Discussion 123 4.2 Summary and Outlook 133

5 Experimental 134 5.1 General 135 5.2 Procedures for Chapter 2 136 5.2.1 Synthesis of Boron and Silicone Based Reagents 136 5.2.2 Direct Carboxamidation 147 5.2.3 Transamidations of Primary Amides 154

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5.3 Procedures for Chapter 3 156 5.3.1 Synthesis of ortho-Alkynylphenylboronic Acids 156 5.3.2 Boron Enolate Formation 161 5.3.3 One-Pot Boron Enolate Formation/Aldol Reaction 164 5.3.4 Aldol/Oxidation 165 5.3.5 Aldol/Suzuki–Miyaura Coupling 168 5.3.6 Aldol/Intramolecular Chan–Evans–Lam Coupling 170 5.3.7 Aldol/Protodeboronation 172 5.4 Procedures for Chapter 4 173

References 175 Appendix 196

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Abbreviations

General ACS American Chemical Society aq aqueous bp cat catalytic conc concentrated conv conversion DFT density functional theory DMG directed metalation group DoE design of experiments ee enantiomeric excess EI electron ionisation equiv equivalent ESI electrospray ionisation EWG electron withdrawing group h hour(s) HMBC heteronuclear multiple bond connectivity HMQC heteronuclear multiple quantum connectivity HRMS high resolution IR infrared spectrometry J coupling constant LA Lewis acid lit literature value LUMO lowest unoccupied orbital m meta M+ parent molecular min minute(s) mp MS mass spectrometry MS molecular sieves MW microwave NMR nuclear magnetic resonance

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NRPS nonribosomal peptide synthetase o ortho p para PNA peptide nucleic acid ppm part(s) per million ref reference rds rate determining step RT room temperature sat saturated tRNA transport ribonucleic acid quant quantitative UV ultraviolet

Reagents, and acac acetylacetonate AIBN 2,2’-azobis(isobutyronitrile) BINAP 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl. bmim 1-butyl-3-methylimidazolium BOP (benzotriazol-1-yloxy)tris(dimethylamino) hexafluorophosphate BPO benzoyl peroxide. BQ 1,4-benzoquinone CDI carbonyldiimidazole cod 1,5-cyclooctadiene CuMeSal copper(I) 3-methylsalicylate CuTC copper(I) thiophen-2-carboxylate CYC cyanuric chloride dba dtby di-tert-butylbipyridine DCC dicyclohexylcarbodiimide DCE 1,2-dichloroethane DCB o-dichlorobenzene DCM dichloromethane DHA dihydroxyacetone DHAP dihydroxyacetone DIC diisopropylcarbodiimide DIPEA N,N-diisopropylethylamine DMAD dimethyl acetylenedicarboxylate

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DMAP 4-(N,N-dimethylamino) DMSO dimethylsulfoxide DO dioxane dppe 1,1-bis(diphenylphosphino)ethane dppf 1,1'-bis(diphenylphosphino)ferrocene dppm 1,1-bis(diphenylphosphino)methane dppp 1,3-bis(diphenylphosphino)propane dtby di-tert-butylbipyridine DTNO di-tert-butyl nitroxide EDCl [3-(dimethylamino)propyl]ethylcarbodiimide hydrochloride HATU O-(7-azobenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate HBTU 1-[bis(dimethylamino)methylene]-1H-benzotriazolium hexafluorophosphate HEH Hantzsch HOAt 1-hydroxy-7-azabenzo[d][1,2,3]triazole HOBt 1-hydroxybenzo[d][1,2,3]triazole HOI N-hydroxyindolin-2-one Im imidazole IMes 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene iPP2BH di(isopropylprenyl)borane LHMDS lithium hexamethyldisilazide LTMP lithium 2,2,6,6-tetramethylpiperidide lut lutidine MIDA N-methyliminodiacetic acid MCPBA m-chloroperoxybenzoic acid MTBE methyl tert-butyl PE petroleum ether (boiling range 60–80 °C) phen 1,10-phenanthroline PhMe Pro proline PTSA p-toluenesulfonic acid PyBOP (benzotriazol-1-yloxy)tris(pyrrolidinophosphonium) hexafluorophosphate nbd norbornadiene NBP N-butyl-2-pyrrolidinone NBS N-bromosuccinimide NHC N-heterocyclic NMO N-methylmorpholine-N-oxide NMP N-methyl-2-pyrrolidinone

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PFP pentafluorophenyl PNO pyridine N-oxide PNP p-nitrophenol SIPr N,N'-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol)-2-ylidene TCP 2,4,6-trichlorophenyl TBD 1,5,7- triazabicyclo[4.4.0]dec-5-ene TEA triethylamine TEMPO 2,2,6,6-tetramethylpiperidine-1-oxyl TFA trifluoroacetic acid THF tetrahydrofuran

Substituents Ac acetyl All allyl An anisyl, 4-methoxyphenyl Ar Bn benzyl Boc tert-butoxycarbonyl Bu n-butyl iBu isobutyl sBu sec-butyl tBu tert-butyl Bz benzoyl cat catecholate Cbz benzyloxycarbonyl Cp cyclopentadienyl Cy cyclohexyl Cyp cyclopentyl dan derivative of 1,8-diaminonaphthalene Et ethyl Fur furanyl Hal halide Me methyl Mes mesityl, 2,4,6-trimethylphenyl MOM methoxymethyl neop neopentandiolate OTf

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PFP pentafluorophenyl Ph phenyl pin pinacolato iPr isopropyl pza 2-pyrazol-5-ylanilinyl Sia siamyl, sec-isoamyl, TCP 2,4,5-trichlorophenyl Tf triflyl, trifluoromethanesulfonyl Tol tolyl Tr trityl, triphenylmethyl Ts tosyl, p-toluenesulfonyl

xi

Acknowledgements

I would like to thank Dr Tom Sheppard, my PhD thesis supervisor, for his time, patience and extensive advice.

Also, Dr Abil Aliev and Dr Lisa Harris for help with NMR and MS, respectively.

I thank friends that from time to time have encouraged and/or questioned me, and were always there with at least a helpful suggestion or a glass of wine, a pint of beer or a shot: Mikk, Anton, Nadja, Uno, Cindy and Armando, Sonya, Olya, John, Lena, Selene, Albin, Boaz, Lynsey, Karina, Jon, Sasha, James, Victoria, Lizzie. Also, the guys in the Sheppard lab (Oz, Fil, Martin, Sam, Matt and the summer students) and the Motherwell group (Matt, Josie, Helen, Sandra, Chi, Yumi). Last and definitely not least, my family.

Financial support from EPSRC (EP/E052789/1), Estonian Ministry of Education and Research and Archimedes Foundation is acknowledged.

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On päris kindel: jalge alla jääb sul tuge liiga vähe, kui sa kõik tõkked teelt ei talla ja mööda enesest ei lähe. Kui suur on korraga su isu! Hing, ära ohus karda hukku, vaid senisest end lahti kisu ja keera vanad uksed lukku! Sind ümbritsevad jäised tuuled, ööst kerkib tühi mägiahel. Sa aimad sügavust ja kuuled metsloomi kaljuseinte vahel. Kui sa nüüd minna julgeks! Sillaks su ees siis kuristikud kaanduks, hall kivi raskeid vilju pillaks ja kiskjad alandlikult taanduks.

Betti Alver. "Ekstaas"

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Chapter 1

Chapter 1

1 Boronic Acids and Other Boron-Centred Reagents in Organic Synthesis

1.1 Introduction

The first boronic acid, ethylboronic acid, was discovered back in 1860,[1] but it took a long time for boronic acids to become widely applied in either industrial or academic settings. The seminal work by Negishi[2] and Suzuki[3] on -catalysed arylation of aryl halides[4] with lithium alkynyl(tributyl)borate 1 and alkenylboronic acid esters 2 (Scheme 1) led to a substantial increase in interest towards boronic acids and organoboron compounds in general.

Scheme 1. The first two examples[2,3] of Suzuki–Miyaura coupling. Sia = siamyl, sec-isoamyl, cat = catecholate.[4]

Structurally, boronic acids [RB(OH)2] contain one carbon–boron bond along with two hydroxyl groups on boron (Figure 1).[5,6] The carbon can be greatly varied to include aryl, alkenyl, and alkynyl moieties. Boronic acids often undergo trimerisation to 7 and water. Other boron based compounds include triorganoboranes [R3B], borinic acids [R2BOH], and borate esters [B(OR)3], the derivatives of [B(OH)3].

On condensation of boronic acids with , diamines, diacids and hydroxyacids corresponding boronic acid esters/amides are formed (Figure 2). This conversion is often used to increase the stability and modify the reactivity of the acids (Scheme 2). For instance, Suginome demonstrated the use of differentially protected 1--1,2-diboronic acid derivatives (Scheme 2a).[7] The same group reported 2-pyrazol-5-ylaniline as a protecting/ortho-directing group for ruthenium-catalysed C–H activation/silylation of boronic

2

Chapter 1

Figure 1. (a) Organoboron compounds: triorganoboranes 3, borinic acids 4, boronic acids 5 and borate esters 6. (b) 7 formation from boronic acids.

Figure 2. Boronic acid derivatives. pin = pinacolate, cat = catecholate, neop = neopentandiolate, dan = derivative of dan 1,8-diaminonaphthalene, pza = 2-pyrazol-5-ylaniline, MIDA = N-methyliminodiacetic acid. acids 9 (Scheme 2b).[8] In recent work by Dennis Hall, diaminonaphthalene-protection was crucial to achieve high enantioselectivities in conjugate addition of Grignard reagents to β- boronyl unsaturated (thio)esters 10.[9]

Potassium organofluoroborates[10] are a compelling alternative to boronic acids and esters (Scheme 3a). They show increased stability, greater efficiency in Pd-catalysed cross- couplings, and often do not require additional base and/or . For instance, alkynyltrifluoroborates were recently demonstated to undergo copper-catalysed coupling with amides at ambient temperature in the absence of base.[11] However, due to fluorine’s high electronegativity, nucleophilicity of the carbon adjacent to boron is decreased. To solve this, sodium trihydroxyborates [R(OH)3Na] and cyclic triolborates [RB(O3R’)Li, where triol is 2- (hydroxymethyl)-2-methylpropane-1,3-] were prepared by Cammidge[12] and Miyaura,[13] respectively. For example, a cyclic triolborate of 2-pyridineboronic acid 11, which as the free

3

Chapter 1

Scheme 2. Applications of protected boronic acids. cod = 1,5-cyclooctadiene, dppf = 1,1'- bis(diphenylphosphino)ferrocene, BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl.

Scheme 3. Applications of (a) trifluoroborates,[11] (b) cyclic triolborates[13b] and (c) MIDA boronates.[14]

4

Chapter 1 acid is sensitive to protodeboronation, successfully underwent C–N coupling with morpholine (Scheme 3b).[13]

Martin Burke’s group developed N-methyliminodiacetic acid (MIDA) based protection of boronic acids.[14] This allowed them to carry out transformations that otherwise were incompatible with boronic acids, including selective cross-coupling of haloaryl [14a] and haloalkenyl[14b] MIDA boronates 12 with boronic acids (Scheme 3c). This methodology was extended to the synthesis of natural products[14a-c,e] and used to improve the reactivity profiles of less stable 2-heteroarylboronic acids.[14d]

1.2 Preparation

In the past decade, significant advances have been made in the synthesis of boronic acids. These include Hosomi–Miyaura and Hartwig borylations (discussed below). These advancements allow more straightforward synthesis of functionalised boronic acids. While most of examples mentioned below are for the generation of arylboronic acids many methods may well be extended to alkenyl-, alkynyl- and, to a lesser extent, alkylboronic acids.

1.2.1 Arylboronic Acids

Traditionally, arylboronic acids are prepared by of metalated species such as organolithium[15] and Grignard reagents 13 (Scheme 4).[16] Both of these reagents can be generated in situ by halogen–metal exchange[17] or metal insertion.[18] Where an arene of interest bears a directing metalation group (DMG), organolithiums 14 can be accessed by directed ortho-lithiation.[19]

Scheme 4. Synthesis of arylboronic acids using main group metals via (a) halogen–metal exchange and metal insertion, and (b) directed ortho-lithiation.

5

Chapter 1

Another popular method employs palladium catalysis (Scheme 5). In Hosomi–Miyaura [20] borylation, bis(pinacolato)diboron (B2pin2) is cross-coupled with aryl halides and pseudohalides 15 (Table 1, entries 1–6).[21] The borylation of the latter substrates is often referred to as Miyaura–Masuda borylation. The choice of an appropriate base (e.g. KOAc or PhOK) and a ligand for Pd (usually, dppf 16) plays a crucial role in achieving good conversions. Notably, a few examples employing aryl halides as substrates have also been [22–24] reported. Miyaura used Pd(dba)2 as catalyst source along with trialkylphosphine ligand [22] (Cy3P). Fürstner exploited an NHC ligand 17 and Pd(OAc)2 as catalyst precursor under both thermal and microwave conditions.[23] Finally, Buchwald and co-workers employed dialkylphosphinobiphenyl ligand 18 to give good yields with aryl chlorides at 110 ºC.[24] Switching to a more electron-rich ligand allowed them to conduct Miyaura borylations at ambient temperature.

Scheme 5. Synthesis of arylboronic acids using palladium-catalysed cross-coupling.

Alternatively, cheaper sources of boron such as (HBpin)[25–31] and catecholborane (HBcat)[26] are used instead of bis(pinacolato)diboron (Table 1, entries 7–12). [25b] In these instances a tertiary such as Et3N is required as a base, while reactions conducted in ionic liquids can proceed in under an hour.[30] Murata et al. showed that bisphosphine 19 can serve as an efficient ligand for Pd-catalysed borylation of iodides, bromides and notably, electron-rich chlorides.[28] Furthermore, for both diboron and borane cases, palladium can be effectively substituted with copper.[29,32]

Nickel is yet another transition metal that efficiently catalyses borylation of aryl halides with

HBpin and related compounds. Back in 2000, Tour employed nickel catalysts, Ni(dppe)Cl2 [33] and Ni(dppp)Cl2, to access di- and triboronylarenes in reasonable yields (Scheme 6). In 2007, Mindiola showed that the pincer-liganded boryl–nickel complex 20, (PNP)Ni(Bcat), reacts with bromobenzene to give the corresponding B–C coupled product 21 (Scheme 7).[34] They were, however, unable to establish a catalytic procedure via in situ regeneration of nickel hydride precursor. Later, Percec neopentylglycolborylated aryl halides (X = I, Br) with a corresponding borane (HBneop), which was generated directly from BH3·DMS and neopentyl glycol prior to the coupling step (Scheme 8).[35] The transformation was catalysed by Ni complexes initially reported by Tour in the presence of equimolar amounts of ligand.

6

Chapter 1

Table 1. Palladium and copper-catalysed borylation of aryl halides and pseudohalides with diboron and borane reagents. bmim = 1-butyl-3-methyl-imidazolium, cat = catheholate, DO =dioxane, dba = dibenzylideneacetone, neop = neopentandiolate, pin = pinacolate, TEA = triethylamine.

entry X boron source metal source conditions ref (equiv) and ligand

1 I, Br B2pin2 (1.1) Pd(dppf)Cl2 3 mol% [Pd], 3 eq KOAc, [ 21 a] DO, 80 ºC

2 OTf B2pin2 (1.1) Pd(dppf)Cl2 3 mol% [Pd], 3 mol% ligand, 3 eq [21b] dppf KOAc, DO, 80ºC

3 Cl, Br, I, OTf B2pin2 (1.1) Pd(dba)2 3 mol% [Pd], 3.3-7.2 mol% ligand, [22] Cy3P 1.5 eq KOAc, DO, 80 ºC

4 Cl B2pin2 (1.16) Pd(OAc)2 thermal : 3 mol% [Pd], 6 mol% ligand, [23] SIPr·HCl (17) 2.5 equiv KOAc, THF, rfx; MW: 2× 3 mol% [Pd], 6 mol% ligand, 2.5 equiv KOAc, THF, 110 ºC, 2×10 min

5 Cl B2pin2 (1.2-3.0) Pd2dba3 0.1 –4.0 mol% [Pd], 0.2–8.0 mol% [24] 18a and 18b ligand, 3 equiv KOAc or K3PO4, DO, 110 ºC with 18a or RT with 18b

6 I, Br B2pin2 or CuI 10 mol% [Cu], 13 mol% ligand, 1.5 [32] t B2(neop)2 (1.5) Bu3P KO Bu, THF, RT

7 Br, I, OTf HBpin or Pd(dppf)Cl2 3 mol% [Pd], 3 equiv TEA, DO, 80- [25a,26] HBcat (1.5) 110 ºC

8 I, Br HBpin (7.0) Pd(OAc)2 5 mol% [Pd], 20 mol% ligand, 3 equiv [31] 18c TEA , DO, 80-100 ºC or Pd(dppf)Cl2

9 Cl, Br, I HBpin (2.0) Pd(dba)2 5 mol% [Pd], 5 mol% ligand, 3 equiv [28] 19 TEA, DO, 80-100ºC

10 I, Br HBpin (1.3) Pd(dppf)Cl2 3 mol% [Pd], 3 equiv TEA, [30] [bmim][BF4], 100 ºC

11 Cl, Br, I HBpin (1.5) PdCl2(MeCN)2 0.1–4 mol% [Pd], 0.4–16 mol% [27] 18b ligand, 3 equiv TEA, DO, 110 ºC

12 I HBpin (1.5) CuI 10 mol% [Cu], 1.5 equiv TEA, THF, [29] RT

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Chapter 1

[33] Scheme 6. First examples of Ni-catalysed borylation of aryl halides. dppp = 1,3-bis(diphenyl- phosphino)propane

Scheme 7. Synthesis of the first nickel-boryl complex, B–C coupling on nickel-boryl complex (neat PhBr, [34] 120ºC, 18 h, 68%) and attempted formation with a reducing agent (NaBH4).

Scheme 8. Nickel-catalysed borylations. (a) Borane reagent, neopentylglycolborane, was generated prior to (b) C–B coupling of aryl bromides, chlorides, and pseudo halides.[35–37] DMS =dimethylsulfide.

8

Chapter 1

This methodology was then extended to aryl chlorides[36] and pseudohalides[37] by applying mixed ligands. Addition of metallic zinc led to drastic improvements in the case of mesylates and tosylates.[37]

While in every example above, aryl halides and pseudohalides were utilised as starting materials, it is also possible to use other aryl donors. Metal-free borylation of arylamines 22 [38] with tert-butyl nitrite and B2pin2 was recently developed (Scheme 9). Alternatively, several strategies provide ways to derivatise existing boronic acids.[7, 9,14,39] For example, by silver- mediated ortho-bromination and iodination (Scheme 10)[39] or via cycloaddition of alkynylboronic esters with a diene (Scheme 11).[40]

Scheme 9. Metal-free synthesis of arylboronates.[38] BPO = benzoyl peroxide.

Scheme 10. Silver-mediated halogenations of boronic acids.[39]

Scheme 11. Synthesis of pyrazole boronic esters from sydnones.[40] DCB = o-dichlorobenzene.

A direct approach to access arylboronates from arenes, and via C–H activation/functionalisation is known as Smith–Miyaura–Hartwig borylation.[41] Hartwig led the field by introducing iron, rhenium, tungsten and ruthenium based catalysts for catalytic photoactivated, and later thermal, C–H derivatization of alkanes with B2pin2 (Scheme 12).[42,43] Later, Smith developed a highly selective “solventless” -based process for direct synthesis of arylboronates (Scheme 13).[44] Miyaura used 3 mol% Pd/C (10 mol% loading) to C–H activate/borylate benzylic positions (Scheme 14).[45] Within the recent decade, many other improvements (importantly the introduction of bipyridine ligands)[46] have been

9

Chapter 1

Scheme 12. First thermal examples of C-H activation/borylation.[43]

Scheme 13. Iridium-catalysed solventless direct borylation of arenes.[44]

Scheme 14. Pd-catalysed C-H activation/borylation of benzylic positions.[45] explored to control and enhance both selectivity and reactivity of this transformation. These have also been documented in a thorough review.[41]

1.2.2 Other Boronic Acids

Boronic acids carrying moieties other than aryl groups, are often synthesised in similar ways to those described in the section above. For instance, alkenylboronic acids may be accessed from corresponding alkenyl bromides and iodides by halogen/metal exchange with sBuLi and borylated with trialkylborate.[46] Examples of Hosomi–Miyaura Pd-catalysed borylations include reactions using alkenyl [21c,48,49] and [49] as well as benzyl halides[50] as substrates. Recently, Duñach reported magnesium-catalysed borylation of benzylic halides with pinacolborane (Scheme 15).[51]

10

Chapter 1

Scheme 15. Magnesium-catalysed borylation of benzyl halides.[51]

Alkynylboronic acids are less stable and usually only their corresponding esters can be obtained. They are synthesised by deprotonation and subsequent borylation with borate esters.[52]

Addition of B–H across multiple bonds was first noted by Brown in 1956 (Scheme 16).[53] It represents yet another strategy to access alkenyl and alkylboronic acids. The process with terminal is especially effective and, depending on the choice of conditions, both trans and cis-alkenylboronic acids/esters can be obtained. In case of non-catalysed cis- hydroborylation, bulky dialkylboranes like 9-BBN[54], dicyclohexylborane[55] and [56] di(isopropylprenyl)borane (iPP2BH) are used to avoid multiple addition of borane to the . Subsequent mild oxidation with trimethylamine oxide[57] gives the boronic acid that is usually esterified with a diol to give a more stable alkenylboronate. Often, iPP2BH is used as a milder alternative. In that case, acetaldehyde can be used as an oxidant.[56] Alkenylboronic esters can be accessed directly using HBpin[58] and HBcat[59] to provide the boronic esters [60] [61] under thermal conditions. To accelerate this process, HZrCp2Cl, Cp2Ti(CO)2, [62] [63] [62] Rh(CO)(PPh3)2Cl, Rh(PPh3)3Cl, and CpNi(PPh3)Cl as well many other early and late transition metals were employed.[64]

Scheme 16. Hydroboration of alkenes and alkynes.[64]

Notable is Suginome’s nickel-catalysed addition of alkynylborones across carbon–carbon triple bonds (Scheme 17).[65] This was achieved with pronounced to give predominantly a cis-addition product 23.

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Chapter 1

Scheme 17. Ni-catalysed regioselective cis-addition of alkynylboranes to alkynes.[65]

Alkylboronic acids and esters are used to a lesser extent than their alkenyl and aryl equivalents. This is due to their decreased shelf-stability and the strong tendency of alkylmetal species to undergo β-hydride elimination in the cross-coupling reactions.[66] Nevertheless, such derivatives can be accessed via hydroboration of alkenes,[64] borylation of organolithium and organomagnesium reagents with borate esters,[67] and Pd[68] and Rh[69]- catalysed hydrogenation of alkenylboronic acids.

1.3 Boronic Acids as Reactants

Traditionally, the synthetic value of boronic acid derivatives as reactants is demonstrated by numerous examples of transition-metal catalysed processes. This is due to (a) the relative ease of the step to a variety of metals (Pd, Cu, Hg, Pb, Ir, Co, Zn, Fe, Cu, Au),[5] (b) greater -economy of boronic acids in comparison to other organometallics (c.f. organobismuthanes[70,71] and organostibanes[72]), and (c) their low toxicity and increased stability (c.f. organostibanes and organoplumbanes[71]).

1.3.1 Transition Metal Catalysed Reactions

In cross-coupling reactions, two reactants are combined to make a product. The “coupling portfolio” of boronic acids caters a wide variety of chemistries, and we will briefly discuss most interesting examples below.

Palladium-catalysed Suzuki–Miyaura coupling was one the first major applications of boronic acids and it is perhaps the most widely used today.[4] In this reaction, a boronic acid is coupled with a halide or pseudohalide to give a biaryl. In 2005, Buchwald reported a highly efficient class of phosphorus-based bulky palladium ligands 18 for this transformation, efficient enough to catalyse coupling between unactivated aryl bromides and chlorides with arylboronic acids (Scheme 18)[73]

12

Chapter 1

Scheme 18. Buchwald’s SPhos ligand 18b for the Suzuki–Miyaura reaction.[73]

Another popular transformation involving boronic acids is their conjugative 1,4-addition to enones, developed by Hayashi and Miyaura.[74] For this process, rhodium is often the preferred catalyst. In conjunction with a chiral ligand (e.g. olefin, , or ) products with high enantioselectivities are obtained (Scheme 19).[75]

Scheme 19. (a) First rhodium-catalysed asymmetric conjugate addition of boronic acids to enones[76] and (b) its application in a tandem intramolecular 1,4-addition–aldol cyclisation process.[77] acac = acetylacetonate.

Also of note is the nickel-catalysed arylation of aryl[78] and alkenyl[79] methyl 24 reported by Chatani (Scheme 20).

Scheme 20. Ni-catalysed cross-coupling of (a) aryl and (b) alkenyl methyl ethers with arylboronic esters.[78,79]

Arylboronic acids are useful partners in the Liebeskind–Srogl synthesis (Scheme 21). Notably, unlike Suzuki–Miyaura coupling, this transformation does not require the presence of a base. It has been applied in the of litseaverticillol B[80] and (-)-D-erythro-

13

Chapter 1 sphingosine.[81] The first generation coupling system was catalysed by Pd(0) and mediated by copper(I) via C–S addition to Pd(0), transmetallation of an arylcopper(I) species (generated in situ from the boronic acid) and reductive elimination.[82] The second generation system is based on aerobic copper(I) catalysis, however, use of a sacrificial equivalent of boronic acid and a specifically funtionalised ester is required.[83]

Scheme 21. (a) First[82] and (b) second[83] generation Liebeskind–Srogl coupling reactions. (c) Two natural products, for the synthesis of which, Liebeskind-Srogl coupling was employed.[80,81] CuTC = copper(I) thiophen-2-carboxylate. CuMeSal = copper(I) 3-methylsalicylate.

In the recent years, a number of oxidative coupling reactions were developed, where boronic acids or esters act as aryl donors (Scheme 22). Yu showed palladium(II/IV)-catalysed direct arylation of sp2 and sp3 C–H bonds in simple carboxylic acids.[84] Zhang found that in the ® presence of a gold catalyst and Selectfluor , propargylic acetates not only rearrange to give enones but the intermediate vinyl gold species can be trapped and arylated with arylboronic acids to give 2-arylenones 24 .[85] Later, using analogous conditions but a different gold source, Toste reported three component oxyarylation of alkenes.[86] Recently, Baran demonstrated Ag-catalysed direct arylations of heteroarenes, which proceed at ambient temperature with persulfate as a terminal oxidant.[87]

14

Chapter 1

Scheme 22. Boronic acids in oxidative couplings: (a) Directed Pd-catalysed C–H activation/arylation of t carboxylic acids (10 mol% Pd(OAc)2, 1 equiv PhBneop, 0.5 equiv BQ, 1 equiv Ag2CO3, BuOH, 120 ºC, ® 3 h, 63%); (b) Au-catalysed cross-coupling (5 mol% Ph3PAuCl, 4 equiv ArB(OH)2, 2 equiv Selectfluor ,

MeCN/H2O 20:1, 80 °C, 30 min, 68%); (c) Au-catalysed three component reaction (5 mol% ® dppm(AuBr)2, 2 equiv PhB(OH)2, 2 equiv Selectfluor , MeCN:ROH 9:1, 50 ºC, 14 h, 66%); (d) Ag- catalysed arylation of pyridine (1.5 equiv PhB(OH)2, 20 mol% AgNO3, 3 equiv K2S2O8, 1 equiv TFA, 23 ºC, 12 h, 80%). BQ = 1,4-benzoquinone, dppm = 1,1-bis(diphenylphosphino)methane. TFA = trifluoroacetic acid

1.3.2 Chan–Evans– Lam Coupling

Apart from carbon–carbon bond forming reactions, boronic acids also participate in carbon– heteroatom bond construction, where nitrogen, , and less frequently sulphur,[88] can act undergo arylation. The first examples of such copper-mediated processes (now known as Chan–Evans–Lam or Chan–Lam coupling[89]) where reported back in 1998 in three back-to- back communications (Scheme 23).[90] At that time, palladium ligands were not sophisticated enough to offer a sound alternative for the synthesis of diaryl ethers. Another reason why this reaction became popular was an easy access to N-arylated heterocycles.[89]

15

Chapter 1

Scheme 23. (a) Chan–Evans Lam and (b) proposed mechanism.[91]

This practical procedure is carried out under mild conditions and boronic acids, unlike organobismuthanes, organostannanes, organolead, iodonium salts and organoantimony compounds, are “aryl-economic”, that is they bear only one equivalent of aryl moiety and do not require 'dummy ligands'. In the majority of examples, at least an equimolar quantity of copper salt (preferably, Cu(OAc)2 or its crystallohydrate) and a nitrogen-base/promoter (triethylamine and/or pyridine, and recently, alkynes[92b]) are used. Lam et al. also looked into the possibility of using co-oxidants to establish a catalytic copper protocol (Scheme 24). Pyridine N-oxide, TEMPO and N-methylmorpholine oxide gave the best improvements.[93]

Scheme 24. Co-oxidant screening in Chan–Evans–Lam reaction.[93]

16

Chapter 1

In 2003, Lan et al. found that the reaction can be most effectively carried out in as without any additives or enhancers and using air as the terminal oxidant. Methanol and to some extent are believed to (a) activate the carbon–boron bond towards transmetallation via coordination to boron and also (b) enhance the aerobic oxidation of copper.[90]

Initially, the methodology was developed for arylation of , sulphonamides and . Recently, however, reports on monoalkylation of anilines[94] and alkenylation of [92] have been reported. In addition to that, with potassium alkenyltrifluoroborates, Batey was able to alkenylate amides and [95]and arylate alcohols[96] efficiently.

Chan–Evans–Lam coupling has also found use in the synthesis of macrocyclic structures. Evans and co-workers were the first to exercise such a strategy, delivering macrocyclic diethers 25, the hydroxamic acids of which, represented a new chemical scaffold for inhibitors of matrix metalloproteinases (MMP) (Scheme 25).[97]

Scheme 25. Synthesis of the first macrocyclic diaryl ethers.[97]

The only major limitation in the substrate scope are aliphatic alcohols.[96] In 2010, Merlic was able to demonstrate a few copper-promoted examples, but the reactions were conducted in neat alcohol and often limited to activated (allyl alcohol, 2-chloroethanol and 2- trimethylsilylethanol) or simple (methanol, ethanol) examples.[92a,b]

Work on Chan–Evans–Lam coupling reactions also led to several reports on regioselective 1,2-additions of boronic acid to azo[97] and [98] compounds, aromatic [99] and alkynoates.[100]

17

Chapter 1

1.3.3 Converting Boronic Acids

In addition to various coupling reactions, boronic acids can be readily and efficiently transformed into many other functionalities (Scheme 26, Table 2).[101–120] Taking into consideration, the ease of preparation of boronic acids and importance of structure–activity relationships, in the future, the boronic acid moiety may serve as an important starting point for various divergent transformations. In that regard, the ability to encode a para- boronylsubstituted phenylalanine (Phe) into the is perhaps most exciting.[121]

Scheme 26. Conversion of boronic acids.

1.4 Boron-Based as Reagents and Catalysts

There is only a limited body of work on boron-based systems acting as catalysts and promoters. The first such example was demonstrated by Letsinger in 1963, when 8- quinolineboronic acid enhanced the rate of of some chloroalcohols.[122] In 1979, Nagata reported phenylboronic acids-mediated ortho-α-hydroxyalkylation of phenols by aldehydes (Scheme 27).[123] This condensation was used in the synthesis of the decaline portion of (+)-compactin[124] and hexahydrocannabinoids.[125]

[123] Scheme 27. (a) Boronic acid-medited ortho-α-hydroxyalkylation of phenols and (b) its application.

18

Chapter 1

Table 2. Conversion of boronic acids. aBoronic ester used as substrate. phen = 1,10-phenanthroline

X source metal source entry X conditions ref (equiv) (equiv)

AgNO3 or NH4NO3 1 NO2 – [Ag] or [NH4], 2 equiv TMSCl, [101] (2.2) DCM, RT–50 ºC, 30–72 h

NH3·H2O Cu2O 2 NH2 MeOH, air, RT [102] (5.0) (0.1)

NaN3 CuSO 4 or 3 N3 [103] (1.2–1.5) Cu(OAc)2 (0.1) MeOH, RT–55°C, 24 h

TMSN3 CuCl 4 N3 1.2 equiv TBAF, [ 104 ] (1.2) (0.1) MeOH, reflux

KOH CuSO 5 OH 4 [105] (3.0) (0.1) 0.2 equiv phen, H2O, RT, 1-10 h

SR CuMeSal 6 R = Alk, 2 equiv RB(OH)2, THF, [ 106 ] (0.3) Ar 45–50 °C, 2-18

RS–CN (1.0) Pd(PPh 3)4 (0.03), 7 CN 1.5 equiv RB(OH)2, [ 107 ] R = Alk, Ar CuTC (1.5–3.0) DO, 100 ºC, 12 h

Zn(CN)2 Cu(NO3)2∙H2O 8 CN 1 equiv CsF, MeOH/H2O, [ 108 ] (3.0) (2.0) 80 ºC, 3-6 h

[Rh(OH)(cod)] CO (1 atm), 7-10 mol% dppp, 9* COOH CO 2 2 [109] 2 (0.03–0.05) 3 equiv CsF, DO, 60 °C

i a PrCuCl t 10 COOH CO2 CO2 (1 atm), 1.05 equiv BuOK, [ 110 ] (0.01) THF, reflux, 24 h

Pd(OAc)2 11* COOR CO, ROH CO (1 atm),10–16 mol% PPh3, [111] (0.05–0.08) 1 equiv BQ

12 F CsOSO3F – [112] 1.5 equiv RB(OR’)2, MeCN, reflux

® a Selectfluor AgOTf 13 F (i) 1 equiv NaOH, MeOH; [Ag], 0 ºC; [113] (1.05) (2.0) ® (ii) Selectfluor , 3Å MS, Me2CO, 3 h

Selectfluor® 14 F – [114] (1.0) MeCN, RT, 24 h

CuX 15 Cl, Br 2 – [115] (3.0) MeOH/H2O

NaI 16 I 2 equiv TsNNaCl, NaOH, [116] (excess) THF/H2O, RT 5 min

NBS, NIS 17 Br, I – [117] (1.0–2.0) MeCN, 25–81 °C, 1–24 h

continued on next page

19

Chapter 1

continued from previous page

X source metal source entry X conditions ref (equiv) (equiv)

18 Cl, Br – 5 mol% NaOMe, MeCN, RT [118]

NCS, NBS, NIS CuCl or CuCl 17 Cl, Br, I 2 [119] (1.0) (0.1–1.0) MeCN, 80 °C, 1–24 h

TMSCF3 [Cu(OTf)] 2∙PhH 1.2 equiv phen, 5 equiv KF, 3 equiv [120] 18 CF3 (5.0) (0.6) K3PO4, DMF, 45ºC, 4 h

Nagasaki introduced boronic acids as templates in the Diels–Alder reaction,[126] which Nicolaou later used in the total synthesis of Taxol (Scheme 28).[127] Chiral boron-based Lewis acids, especially acyloxyboranes and oxazaborolidines, are extensively employed in asymmetric reductions and Diels–Alder reactions.[128]

Scheme 28. Use of boronic acid as a template for the Diels–Alder reaction.[127]

Recently, Štefane demonstrated that 1,3-dioxa-BF2 complexes 26 are readily accessible from aryl 3-oxopropanoates and BF3·OEt2. These undergo highly chemoselective addition of organolithium reagents to give 1,3-diketones 27 (Scheme 29).[129]

[129] Scheme 29. Selective addition of organolithium reagents to BF2-chelates of β-ketoesters.

Suginome’s group employed as a nonacidic iminium ion generator for both Mannich and Ugi-type reactions (Scheme 30).[130]

20

Chapter 1

Scheme 30. Trimethyl borate mediated Mannich and Ugi reactions.[130]

During the past decade, several groups have noted that boron-based systems can activate carboxylic acids. One of the most studied applications is direct amide bond formation.[131] In a similar fashion, unsaturated carboxylic acids are activated towards cycloadditions.[132] Moreover, several substoichiometic reagents and catalysts have been devised. We will discuss this mode of activation in more detail below.

1.4.1 Activation of Carboxylic Acids

A number of boron based reagents (Scheme 31) react with carboxylic acids to give acyloxyboronate/acyloxyborane intermediates, which undergo aminolysis to give an amide.[133] In most cases, reaction conditions are mild and neutral, and often no or insignificant epimerisation is observed. However, the main drawbacks of these stoichiometric reagents are low conversions, incompatibility with several functional groups, and the fact that often an excess of either an amine or a is required to achieve good yields.

Scheme 31. Simple boron reagents for stoichiometric amide bond formation.

The first substoichiometric boron reagents for direct carboxamidation were reported in 1996. Hisashi Yamamoto and co-workers demonstrated that electron-deficient arylboronic acids act

21

Chapter 1 as catalysts for direct amide bond formation (Scheme 32).[134] They showed that 3,4,5- trifluorobenzeneboronic acid 28 showed superior activities. While the initial screening gave excellent results, most of the reactions were carried out under harsh conditions. Namely, heating under reflux in toluene (bp 110 C), xylene (bp 140 C), anisole (154 C) and mesitylene (bp 164 C) was required along with concomitant removal of water (4 Å MS in a Soxhlet thimble or azeotropic reflux) and prolonged reaction times (ca 20 h). However, even under these conditions practically no racemisation was observed.

Further work carried out by Yamamoto aimed at identifying better boron-based catalysts (Figure 3)[131c, 135] as well as their use for the synthesis of polyamidic polymers[136] and derivatives.[137] In 2001, they showed that 3,5-bis(perfluorodecyl) 32 was immobile in the fluorous recyclable phase and possessed catalytic activity similar to that of 28 and 30.[138] In 2005, they reported N-alkyl-4-boronopyridinium salts 33 and 34 as thermally stable and reusable catalysts for direct amidation.[135] Interestingly, 4,5,6,7- tetrachlorobenzo[d][1,3,2]dioxaborole 35 was shown to be superior for the direct amidation

Scheme 32. Boronic acid-catalysed direct amide bond formation. (a) Catalytic activity of various arylboronic acids. (b) Efficacy of 1 mol% 28 as a catalyst. In the last example, starting material was >98% ee and 10 mol% of 28 was used.

22

Chapter 1

Figure 3. Arylboronic acids for direct amide bond formation.

Scheme 33. Catalytic activities of boron compounds for direct amidation of cyclohexanecarboxylic acid with benzylamine. of only sterically hindered carboxylic acids (Scheme 33).[135] However, no explanation to account for such a reactivity profile was provided.

While Yamamoto and co-workers were investigating various arylboronic acids, Tang demonstrated that boric acid alone is a sufficient substoichiometric catalyst for direct amide bond formation in some cases.[139] However, extensive heating under reflux in high boiling point solvents was required.

Historically, bringing the reaction temperature below 100 °C for this transformation was challenging. Whiting et al. proposed that having an additional N,N-dialkylaminomethyl in the ortho-position to the boronyl moiety would promote the reaction by abstracting a proton from an amine during the formation of a tetrahedral intermediate. In order to test this hypothesis, they prepared five “Wulff-type”[140] arylboronic acids 38 (Figure 4).[141,142] In the N,N- dimethylaminomethyl variant 39, chelation of nitrogen to boron led to low catalytic activity. In 40, however, the two bulky isopropyl groups on nitrogen prevent N→B chelation. As a result, 40 showed improved activity at lower temperatures (in refluxing fluorobenzene, bp 84

23

Chapter 1

ºC). Additionally, with 40 carboxamidation of a less reactive acids such as benzoic acid was more successful then with other catalysts. However, under higher temperature conditions both boric acid and 30 were superior to 40.[141]

Figure 4. ortho-(N,N-Diisopropyl)methylphenylboronic acids.

Further introduction of electron-deficient groups into the phenyl ring raised the catalytic activity of ortho-(N,N-diisopropyl)methylphenylboronic acids. The yields for the catalysed condensation between benzoic acid and benzylamine in refluxing fluorobenzene using 5 mol% of arylboronic acid decreased in the following row 43> 40 > 41 >> 42 >> thermal.[142] Some interesting observations were reported by Hall et al. in 2008 (Scheme 34).[132b]. Firstly, they found that boronic acid catalysed carboxamide formation can be carried out at ambient temperature (25 °C), but in dilute solutions and in the presence of molecular sieves (both for the sake of effective water removal). Secondly, they identified ortho-iodoboronic acid 47 as a superior catalyst under these conditions. They examined the catalytic efficiency of 45 diverse arylboronic acids but no obvious direct correlation between the reactivity and the substituents’ steric or inductive effects could easily be drawn. For example, the fact that among ortho- halide derivatives ortho-iodophenylboronic acid 47 is the most efficient catalyst, while the fluoro analogue 44 is not, rules out the possibility that inductive effects account solely for the catalytic properties. While is the most bulky in the halogen row, comparison of activities of o-methyl and o-isopropylphenylboronic acids 49 and 50 confirmed that alone could not have explained the catalyst efficiency (Figure 5). The acidity of the boronic acids was also unlikely to provide a rationalisation as pKas of 47 and inactive phenylboronic acid are 9.80 and 9.90, respectively. Since the X-ray crystal structure of 47 showed an angular distortion of the B-C-C bonds (117°, 126°), the authors conclude that “subtle electronic or structural effects may be at play”.

24

Chapter 1

Scheme 34. Boronic acids as catalysts for room temperature direct amidation.[132b]

(a) (b)

Figure 5. (a) Catalytic activities of ortho-alkylboronic acids. Reaction conditions: 10 mol% catalyst, 0.07 M DCM, 25 h. (b) Crystal structure of 47.

The explanation of this reactivity was recently provided by Marcelli.[143] His computational studies indicated that the halogen acts as a Lewis base, promoting hydrogen abstraction from nitrogen in the tetrahedral intermediate 51 (Figure 6). The precise spatial positioning of the halogen in relation to the boron may be the reason why Hall’s catalyst outperforms Wulff- type boronic acids, which might be expected to be superior Lewis acid/Lewis base catalysts. We will return to the mechanistic considerations in Chapter 2.

25

Chapter 1

Figure 6. Iodine-assisted direct carboxamidation. Simplified model using methylamine, acetic acids, ortho-iodophenylboronic acid. patterns are shown in green.

In additional to amidation Hall showed that ortho-iodophenylboronic acid promotes Diels– Alder reactions between α, -unsaturated acids and dienes (Scheme 35) at ambient temperatures.[132b] Formation of a mixed anhydride 52 would lower the LUMO of a dienophile, which would then more readily undergo [4+2] cycloaddition. This concept was essentially borrowed from Yamamoto, who successfully used -THF complex to achieve similar reactivities.[132a] Hall expanded this methodology to include a series of [3+2] dipolar cycloadditions involving , oxides and nitrones.[132c]

Scheme 35. Boronic acid-catalysed Diels-Alder reaction with acrylic acid as a dienophile.

Apart from direct carboxamidation, boron-based catalysts were shown to promote several specific cases of esterfication. In 2004, Houston et al. reported that α-hydroxyacids 53 (but not succinic or benzoic acids) can be converted to the corresponding esters 54 with boric acid as a catalyst in an excess of alcohol (Scheme 36).[144] This was despite the fact that some earlier reports claimed that the use of alcohols as solvents deactivated arylboronic acids by forming alkylesters of the boronic acids. This enhanced reactivity can be explained by the formation of cyclic borates and acyloxyboronates. This was further supported by the fact that while monoesters of maleic and malonic acids 55 and 56, respectively, were obtained under similar conditions, while fumaric acid remained unreacted.[145]

26

Chapter 1

Scheme 36. Esterification of (a) α-hydroxycarboxylic acids,[144] (b) monoesterification of maleic and malonic acids.[145]

Later, Yamamoto showed that 33 was superior to boric acid and gave methyl esters at ambient temperatures.[131c] Other coordinating functionalities on the α-carbon also promoted esterification (Scheme 37).[135] No similar effect was reported for amidation of α- functionalised carboxylic acids.

Scheme 37. Methyl esterification α-functionalised carboxylic acids.

Other boronic acid-catalysed reactions include (a) synthesis of oxalinone and thiazolines from carboxylic acids and 1,2-aminoalcohols and 1,2-aminothiols, respectively;[146] (b) preparation of acyl azides from carboxylic acids;[147] and (c) reduction of carboxylic acids to alcohols.[148]

1.5 Summary

The chemistry of boronic acids has advanced substantially during the past decade.[5] A number of new strategies for the synthesis of a variety of boronic acids have been developed. Several new conditions allow the introduction of boron in the presence of several otherwise

27

Chapter 1 restrictive functionalities. Numerous strategies for the conversion of boronic acids to other useful derivatives (e.g. halides including fluorides, azides, carboxylic esters etc.) have also been introduced. Major progress was made in understanding and controlling the reactivity of boronic acids and esters in transition metal catalysed processes.[149] Furthermore, aerobic catalytic systems (e.g. for Chan–Lam coupling) were devised[142,150] and experimentally studied.[90] The mechanistic details of these transformations significantly contributed to the design of other aerobic systems.[151] As a result of these advances, boronic acids and their surrogates are often used in the total syntheses of complex natural products, including for late stage transformations, an indicator of the maturation of the field (Scheme 38).[14e,152,153]

Furthermore, the use of boronic acids has also been extended to the development of sensors[154] and pharmaceuticals (Figure 7).[155] For example, is a first-in-class inhibitor,[156] while other boron-based substances currently under development may find their niche as anti-infectives (e.g. AN-2690 is a highly potent antifungal agent[157]).

In spite of all these accomplishments, applications of boronic acids and other boron-centred systems as catalysts and reagents are rather underexplored. In the next chapters, we will introduce and discuss new applications of boron-centred reagents and assess their potential in catalysis.

28

Chapter 1

Scheme 38. (a) Miyaura borylation was crucial to Nicolaou’s second generation total synthesis of Diazonamide A.[152] (b) Late stage Ir-catalysed C–H activation/borylation in Sarpong’s total synthesis of Complanadine A.[153] (c) Total synthesis of (–)-Peridinin via iterative cross-coupling strategy.[14e] dtby = di-tert-butylbipyridine

Figure 7. Structures of Bortezomib, a proteasome inhibitor and AN-2690, an antifungal.

29

Chapter 2

Chapter 2

2 Development of Boron Based Reagents and Catalysts for Activation of Carboxylic Acids and Amides

2.1 Amide Bond Formation: An Overview

The amide bond is one of the most fundamental and widely occurring bond types in nature.[158,159] Not only is it the “linking” bond in peptides, related conjugates (e.g. peptide nucleic acids[160]), and polymers, it is also found in a vast number of natural and unnatural compound, e.g. polyketides, cyclopeptides, antibiotics, and pharmaceuticals (Figure 8). In addition, amides are commonly used as protecting and/or directing groups (Scheme 39) and as starting materials for interconversions leading to , acids and amines, to name just a few (Scheme 40).

A recently conducted analysis of the reactions used for the preparation of drug candidates[161] as well as commercially available drugs,[162] indicated a significant use of amide bond formation in industry. Namely, 9.1% of all reactions involved amide formation, and the carboxamide unit was found in 25% of all marketed drugs.[163]

Despite the vast number of methods available to construct an amide bond, most have disadvantages such as high substrate dependence and poor atom[164] and [165] economy. Other commonly encountered problems are substrate racemisation and the toxicity of the reagents used. As a result, amide bond formation was ranked the highest among the reactions that are “already in use but require better reagents” by the ACS Institute Pharmaceuticals Roundtable as well as by other bodies.[166]

While numerous strategies have been developed for the of carboxamides, few of them have any resemblance to the catalytic methods of enzymes/ribozymes employed in in vivo amide and peptide bond formation. This, perhaps, may also be attributed to difficulties in obtaining the crystal structures of the respective ribozymes/protein complexes with transition-state analogues.[167]

31

Chapter 2

Figure 8. A selection of bioactive compounds containing amide bonds. (a) Trapoxin, 57, a fungal natural product, known to inhibit histone deacetylases.[168] (b) Vancomycin, 58, a glucopeptidic antibiotic.[169] (c) Tacrolimus, 59, a polyketide, an immunosuppressive drug.[170] (d) Valsartan, 60, an angiotensin II receptor antagonist, a multibillion-dollar drug.[171]

Scheme 39. Amide-directed chemical transformations. (a) Directed ortho-lithiation.[172] (b) Regioselective oxidation of a benzylic position.[173] (c) Catalytic asymmetric hydroboration of alkenes.[174]

32

Chapter 2

Scheme 40. Synthetic utility of amines: (a) Hofmann rearrangement.[175] (b) Petasis–Tebbe olefination.[176] (c) Myers asymmetric alkylation.[177]

2.1.1 Methods for Amide Bond Formation

Carboxamide formation has been studied extensively over the last century and as a result, there are a number of reviews published in this area.[178] Below, the existing methods will be classified and their main limitations will be outlined. Several emerging alternative and/or catalytic methods will also be covered.

Direct amide formation, which essentially is a simple condensation, is a thermodynamically favoured process with the overall free energy of formation of an amide being negative. However, mixing a carboxylic acid with an amine results in spontaneous ammonium salt 62 formation (Scheme 41, route a).[179] Further condensation to give an amide is then kinetically disfavoured. There are a few examples of thermally driven direct carboxamidations,[180] however, the temperatures required for these processes to occur are often substantially high.

33

Chapter 2

Scheme 41. Strategies for amide bond formation from carboxylic acids and amides via (a) thermal condensation and (b) activation of carboxyl by introducing a good .

2.1.1.2 Activation of Carboxylic Acids

Most synthetic routes make use of pre-formed or in situ generated activated acyl derivatives 63 (Scheme 41, route b).[178d] This helps to prevent ammonium salt formation and provides a good leaving group at the acyl carbon. These methods may be further classified based on the stability of the acyl derivative. Firstly, acyl derivatives that have a long shelf-life can be used including acyl halides and esters of electron-deficient phenols such as 64–66 (Figure 9). Secondly, acyl derivatives that have to be pre-formed before an amine is added (Scheme 42). These include acyl halides (e.g. 67 although a few of acyl halides can often be isolated and stored or are commercially available), a wide selection of mixed anhydrides (e.g. 68) and imidazolium salts 69. In the third case, the activated acyl derivative is formed in situ from a so-called coupling reagent, and this then reacts with an amine present in the reaction mixture. These commonly used reagents include carbodiimides 70–72, phosphonium salts 73–74, uronium/guanidinium salts 75–76 and ammonium salts, e.g. 77 and Mukaiyama‟s reagent 78 (Figure 10).

Figure 9. Phenols used to prepare activated esters with a long shelf-life. PNPOH = p-nitrophenol, PFPOH = pentafluorophenol, TCPOH = 2,4,5-trichlorophenol.

34

Chapter 2

With all of these acyl activation techniques, at least a stoichiometric amount of an activating or coupling reagent is required. In many cases, additional bases, dehydrating agents and promoters are employed. This all contributes to a significant amount of waste and by-products are also often generated.

Scheme 42. Preparation of halides and mixed anhydrides. CDI = 1,1'-carbonyldiimidazole, CYC = cyanuric chloride.

Figure 10. Common coupling reagents.

Another frequent drawback is racemisation at the α-carbon (Scheme 43). For example, acyl chlorides with an α-hydrogen 79 can racemise via ketenes 80. In the C→N peptide synthesis, the oxygen of the carboxamide may intramolecularly attack an activated acyl intermediate 81 to form an oxazolone 82, which would undergo racemisation via formation of a conjugated

35

Chapter 2

anionic/aromatic intermediate 83. When a racemic oxazolone then reacts with an amine, it gives a racemic product 84. Hence, that is why peptides are usually synthesised from the N- terminus (N→C). Moreover, to suppress the racemisation process additives such as 85–87 are used in conjunction with carbodiimides (Figure 11). More “powerful” coupling reagents (e.g. 73–76) already carry a HOBt/HOAt unit for this reason.

Scheme 43. Racemisation of an asymmetric carbon adjacent to a carboxyl group via (a) ketene and (b) oxazolone.

Figure 11. (a) Peptide coupling additives: HOBt,[181] HOAt, [182] HOI.[183] (b) Yield and racemisation [183] during formation of Cbz-Phg-Pro-NH2 (DMF, RT).

The recent analysis of reactions employed in the pharmaceutical industry showed that only a handful of reagents meet the three requirements of sustainability, wide usability and scalability (Figure 12).[166] However, the “ideal” reagents in this study – , CDI and iBuOCOCl – are used in equimolar amounts and generate toxic and corrosive waste products. Thus, they are far from ideal, and the environmental and economic need for the development of sustainable and efficient catalysts for direct amide bond formation is still great.

36

Chapter 2

(a) (b)

Figure 12. Venn diagrams that form the basis for the reagent guide depending on wide utility, scalability and sustainability. (a) Venn diagram for amide formation from acids and amines (only examples that are not prone to racemisation are included). (b) Venn diagram for amide formation from acids and amines (examples that are prone to racemisation are included). Adopted with changes from ref. [166] – Reproduced with permission of the Royal Society of Chemistry.

2.1.1.3 Alternative Methods

Alternatively, amides can be synthesised thermally (pyrolysis of the ammonium salts,[180] in the presence of silica gel[184]), under microwave conditions[185], by amine activation,[186] or biocatalytically.[187] However, these methods are often unreliable as they require harsh conditions and/or have narrow substrate scope.

CDI-mediated amide coupling is a rare example of dual activation. First, an amine reacts with CDI to give an amine/CDI surrogate, that is subsequently methylated to give a stable crystalline carbamoylimidazolium salt 88 (Scheme 44).[188] This salt readily reacts with an acid in the presence of a mild base and proceeds via an in situ formed activated acyl derivative 89.

Scheme 44. CDI-mediated amide synthesis via amide activation.[188]

37

Chapter 2

2.1.1.4 Catalytic Methods

Apart from boron-based systems, which were discussed in Chapter 1, catalytic methods for direct carboxamide formation are limited to only a few examples.

Nomura et al. reported antimony-based catalysts (Scheme 45).[189] In their studies, triphenylantimony oxide 90 reacted with an acid in situ to give triphenylantimony dicarboxylate 91. This organoantimony(V) compound 91 then reacted with an amine to yield a corresponding amide and regenerate Ph3SbO. The initial reactions were carried out with 10 mol% of Ph3SbO and a slight excess of a primary amine in pyridine at elevated temperatures.

Under optimised conditions using P4S10 as a dehydrating agent, they showed that the catalyst was active at 30–60 C for the acylation of dialkylamines and anilines and also the formation [190] of dipeptides. To a certain extent the reaction was taking place even without Ph3SbO. However, the high toxicity of antimony and tetraphosphorus decasulfide limits the employment of this chemistry. Dialkyltin oxide was also shown to be active for lactamisation.[191]

Scheme 45. Catalytic cycle for triphenylantimony oxide catalysed direct amide formation.[189]

In 1988, Mader and Helquist found that titanium(IV) isopropoxide (50 mol%) but not TiCl4 mediated formation of five and six-membered lactams from ω-amino acids in good yields.[192]

2.1.1.5 Emerging Methods

In this section, we will briefly review approaches that exploit unactivated alkyl carboxylic esters,[193–196] carboxylates,[197] aldehydes[198-201], ketones[202], alcohols[203-206] and α-bromo nitroalkanes[207] as acyl precursors. The main disadvantages of these methods are that they have only been demonstrated on a range of simple and often unfunctionalised substrates, or

38

Chapter 2

ones that are, on the contrary, laborious to access. However, most of the examples below represent new strategies for amide formation.

[193] [194] [208] Yamamoto et al. used tris(dimethylamino)borane and Sb(OEt)3 for templated macrolactamisation (macrocyclic amide formation), which was a crucial step in the synthesis of spermine-derived alkaloids (Scheme 46). The boron reagent was effective only for the cyclisation of triamino esters 92 giving a 13-membered lactam 94. This lactamisation proceeded through boron compound 93, which was isolated in a separate experiment (93%) and was stable to hydrolysis (4.5 M HCl/MeOH, reflux, 4 h and glacial AcOH, reflux, 4 h).

To access 17-membered lactams such as 95, Sb(OEt)3 was used as the boron reagent proved to be ineffective. Some activity was also observed with titanium(IV) ethoxide and zirconium(IV) isopropoxide.[194]

Scheme 46. Boron[193] and antimony[194] templated syntheses of macrolactams.

Although the above transformations required stoichiometric use of reagent, they inspired further work into metal-catalysed amidation of esters. In 2005, Porco et al. screened transition metal salts and alkoxides for catalytic ester–amide exchange (Scheme 47). They found that group (IV) alkoxides were the most efficient and that their activity was significantly raised if additives such as HOAt, HOBt, PFPOH and 2-hydroxypyridine were used.[195] Mechanistic studies showed that both an additive and an amine coordinated to the metal centre resulting in the dimeric species 96, which was characterised by X-ray . Further coordination of an ester and nucleophilic attack by the amine gave the desired amide. These ester amidations occurred at RT, 60 ºC or 100 ºC, depending on the substrate pair, and with good functional group tolerance (e.g. OH, NHBoc, ketals).

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Scheme 47. (a) Zirconium(IV) tert-butoxide catalysed ester amidation. (b) Isolated zirconium dimeric species.

Yang and Birman showed that a 1,2,4-triazole anion can act as a substoichiometric catalyst for ester amidation (Scheme 48).[196]

Scheme 48. Catalytic ester amidation with 1,2,4-triazole.[196]

A number of catalytic routes to carboxamides involving N-heterocyclic (NHC) have been reported. In 2005, Movassaghi and Schmidt showed that NHCs promoted amidation of unactivated esters by 1,2-amino alcohols (Scheme 49).[198] In the first step, the alcohol is activated by the virtue of formation of an NHC–alcohol complex 97. Oxygen then attacks the ester to give the tetrahedral intermediate 98 that yields an O-acylated amine 99. Finally, an intramolecular N→O acyl transfer takes place to furnish the corresponding N-acylated alcohol 100.

Further concomitant reports by Rovis and Bode showed that a wide variety of α- functionalised aldehydes underwent NHC-catalysed redox amidation (Scheme 50).[199,200]. The reaction was co-catalysed by HOBt, HOAt, PFPOH, DMAP and imidazole. These small effectively act as acyl acceptors forming either activated esters or acyl pyridinium or imidazolium salt,[209] which then easily undergo amidation.

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Scheme 49. (a) NHC-catalysed amidation of an unactivated ester with a -amino alcohols and (b) the proposed mechanism. IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene.

Scheme 50. (a) NHC-catalysed redox amidation of α-functionalised aldehydes.[199,200] (b) Proposed catalytic cycle for 101a with 1-HOBt as a co-catalyst for acyl transfer.

Later, Bode came up with α‟-hydroxyenones 102 as an alternative for enals 101b (Scheme 51).[202] These substrates can be readily accessed from condensation of an aromatic with 3-hydroxy-3-methyl-2-butanone. Unfortunately, derivatives of aliphatic aldehydes gave poor yields in the NHC-catalysed amidation step.

Scheme 51. NHC-catalysed redox amidation of α’-hydroxyenones.[202]

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In 2006, Yoo and Li described the first system for oxidative amidation, employing aldehydes and amine hydrochloride salts (Scheme 52).[201] The oxidation of the intermediate aminol was achieved in the presence of CuI and AgIO3 as co-catalysts and tert-butyl (TBHP) as the terminal oxidant. However, this process was limited to aromatic aldehydes.

[201] Scheme 52. CuI/AgIO3-catalysed oxidative amination of aldehydes. Cy = cyclohexyl.

In 2007, Milstein reported the first example of direct dehydrative coupling of amines and alcohols that proceeded via aminol and led to liberation of two equivalents of dihydrogen (Scheme 53).[203] This transformation was achieved using a low loading of PNN-type pincer ruthenium complex 103 (0.1 mol%) to yield carboxamides under mild conditions. However, only primary unfunctionalised amines were reported to work.

Scheme 53. Direct amide synthesis of secondary amides from alcohols and amines catalysed by a Milstein catalyst 103 that carries a dearomatised PNN-type pincer ligand.[203] Fur = furanyl.

In 2008, Madsen devised a mixed ligand (carbene and phosphine) Ru-based system (Scheme 54) that showcased a more diverse set of alcohol/amine partners.[204] While aryl chlorides, and secondary/tertiary amines were well tolerated, double bonds were not. Amidations with secondary amines and anilines were also unsuccessful. The phosphine ligand was proposed to stabilise ruthenium-catalyst resting states and not be involved in the catalytic cycle itself.

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Scheme 54. Madsen’s Ru-based system for oxidative amide synthesis from amines and alcohols.[204] cod = 1,5-cyclooctadiene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine, Cyp = cyclopentyl.

Later, Williams et al. came up with another ruthenium-based system that used methyl isopropyl ketone (MIPK) as a dihydrogen acceptor (Scheme 55).[205]

Scheme 55. Oxidative coupling of alcohols and amines using hydrogen transfer conditions.[205] dppb = 1,4-bis(diphenylphosphino)butane, MIPK = methyl isopropyl ketone, Ind = indolyl.

Grützmacher and co-workers reported the first rhodium-based system for this type of transformation.[206] For coupling of primary alcohols with amines, they used

[Rh(trop2N)(PPh3)] along with methylmethacrylate (MMA) as a terminal dihydrogen acceptor (Scheme 56). While only five examples were reported, the reaction proceeded at low temperatures and showed good functional group tolerance.

Scheme 56. Rhodium-catalysed oxidative coupling of alcohols and amines using hydrogen transfer [206] conditions. trop2NH = bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine, MMA = methylmethacrylate.

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Recently, Johnston reported an umpolung-based strategy for amide bond formation, where an α-bromo nitroalkane, a nucleophilic component, serves as a masked and an in situ generated N-iodo amine is an electrophilic component (Scheme 57).[207] To support this approach, an efficient protocol was developed to access α-bromo nitroalkanes using chiral proton catalysis from N-Boc . These substrates are, however, not always readily available and those derived from aliphatic aldehydes are much less stable and their reactivity profile was not reported. The amide coupling reaction requires prolonged reaction times (48 h) and, interestingly, the presence of water. In that regard, water is important for hydrolysis but it may also be possible that this transformation is a metal-catalysed process. For instance, Norrby and Bolm have shown that ppm copper loadings present in water are sufficient enough to catalyse carbon–heteroatom coupling reactions.[210] No sound experimental or computational evidence was provided by Johnston et al. to support their mechanism (nitronate‟s nucleophilic attack at the nitrogen of the N-iodo amine).

Scheme 57. (a) Generation of α-bromo nitroalkane precursor via chiral proton catalysis. (b) Umpolung amide coupling.[207]

2.1.2 Amide Bond Formation in Nature

Amide bond formation within the cell is carried out in a number of ways. These vary depending on the substrate‟s biological function. Long peptidic chains, which form , are synthesised in the ribosomes (Section 2.1.6.1). Nonribosomal peptides, which often have branched and/or cyclic structures and may contain non-proteinogenic amino acids, are synthesised by highly specific nonribosomal peptide synthetases (NRPSs) (Section 2.1.6.2). These multifunctional enzyme complexes are found in fungi and bacteria and are responsible

44

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for making many biologically active and complex compounds. In a cell, a vast number of heteroatom-acylations also take place. These events are carried out by numerous enzymes, which are collectively known as acyltransferases (Section 2.1.6.3).

All of the above-mentioned processes require different sets of co-factors and acyl donors. In addition, the structures of the catalytic sites and, as a consequence, the reaction mechanisms vary considerably. However, in nearly all the cases, a carboxyl moiety of an is initially activated to form an acyl-adenylate or an acyl-phosphate at the expense of high- energy bonds within ATP, and this intermediate may or may not be converted to a (Scheme 58). Observation of an ATP-independent strategy for amide bond formation in the capuramycin-type antibiotic was reported recently (Scheme 59).[211] Funabashi et al. found that in the NRPS cluster, CapS, acts not as a β-lactamase but as an S- adenosylmethionine-dependent carboxyl methyltransferase (i.e. methylates a carboxylic acid to give a methyl ester). CapW then catalyses the intermolecular amidation of the resulting unactivated ester by an unknown mechanism.

Scheme 58. Strategies for carboxylic acid activation and amide bond formation in Nature. ATP = adenosine triphosphate, ADP = adenosine diphosphate, SAM = S-Adenosyl-L-methionine, SAH = S- adenosyl-L-homocysteine.

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Scheme 59. Proposed biosynthetic pathway to A-503083 B.

2.1.2.1 Ribosomal Peptide Bond Formation

Protein synthesis is performed on the ribosome, which consists of three RNA molecules and more than 50 proteins.[167] The active site is 15 Å wide and incorporates only RNA residues. The amine equivalent is delivered to the peptidyl-tRNA site (P site) 106 in the form of an aminoacyl-tRNA (A site), which is a ribose ester 105 (Figure 13). Then substrate 106 assisted nucleophilic attack takes place to give prolonged peptidic chain and the free tRNA.

It is believed that peptide bond formation is in part driven by lowering the transition state‟s entropy via effective positioning of substrates, reacting groups and organisation of water molecules.[212] However, it was also shown by numerous experimental and theoretical studies that the 2‟-hydroxyl group[213] is essential for the reaction to take place. The 2‟-OH functions by providing an organised hydrogen bond network that stabilises the transition state.

Figure 13. (a) Aminoacyl-tRNA 105. (b) Concerted proton shuttle mechanism of peptide bond formation (dashed lines represent the new bonds, which are forming).

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2.1.2.2 Nonribosomal Peptide Synthetases

NRPSs are multiprotein complexes, which consist of a series of repeating enzymes fused together. There are three main domains found in NPRS: the adenylation (A), the peptidyl carrier (PCP) and the condensation (C) domains (Figure 14).[214] In the A domain, an amino acid is activated by adenylation, resulting in an aminoacyl adenylate (Scheme 60). The resulting highly reactive intermediate can then react with the thiol of a phosphopantetheinyl linker (that is attached to a serine residue in the PCP domain) to form a thioester. The free amino group of the newly formed thioester then attacks a thioester linked to another PCP domain. This process takes place in the C domain, where the basic residue (e.g. His195 in structurally related chloramphenicol acetyl transferase (CAT)) acts as a general base deprotonating the ammonium group to give an amino group (not shown), which subsequently attacks the thioester. This attack forms a negatively charged tetrahedral intermediate, which is stabilised by other residues (e.g. Asn355 in VibH, a condensation domain in vibriobactin synthetase, or Ser148 in CAT).

In the further steps a number of additional domains (fused enzymes) catalyse a variety of transformations including epimerisation of the α-carbon of the amino acid (E domain), intramolecular heterocyclisation of serine, cysteine or threonine residues (cyclisation (Cy) domain), N-methylation of the amine (methyltransferase (MT) domain) and others. Finally, the fully assembled peptide is cleaved from the PCP domain by the thioesterase (TE) domain.

The fold of the TE domain belongs to the α,β-hydrolase containing the conventional catalytic triad comprising serine, histidine and aspartic acid residues (see Section 2.1.6.4). The first step of the reaction is the formation of a tetrahedral enzyme-linked intermediate, in which the negative charge is stabilised by an oxyanion hole. The intermediate is decomposed losing the phosphopantetheinyl thiol to give an acyl enzyme intermediate that subsequently undergoes nucleophilic attack by water (giving a linear peptide) or an internal nucleophile (giving a cyclic peptide).

Figure 14. General schematic representation of NRPS, a multienzyme collinear complex.

47

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Scheme 60. General schematic representation of peptide bond formation in a NRPS.

In all, the limitations in structural and biochemical characterisation of NRPSs as well as complications with their genetic engineering have greatly restricted the understanding of the reaction mechanisms underlying these processes.[214]

2.1.2.3 Acyl Transfer

The most common N-acyl transfer process in the cell is the acetyl transfer from acetyl CoA 107 (Figure 15) to the ε-amino group of lysine residues of histones and many other proteins, which is catalysed by histone acetyltransferases (HATs). Based on their structure and homology, the nuclear HATs are divided into three families (Gcn5/PCAF, MYST, and p300/CBP) that are governed by different catalytic mechanisms (Scheme 61).[215] For example, in yeast HAT Esa1 the ping-pong mechanism involves acetyl transfer from CoA to an enzyme nucleophile (cysteine) prior to transfer to the amino group. Site-directed

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mutagenesis showed that Cys307 was crucial for the enzyme activity and the crystal structure of the acylated enzyme intermediate was obtained.[216] However, in yeast HAT Gcn5, which belongs to a different family, the ternary mechanism is employed.[217] The does not form any enzyme intermediate but is directly transferred to the ε-amino group.

Figure 15. Acetyl Coenzyme A (AcCoA).

Scheme 61. Mechanisms of acetyl transfer. (a) Ping-pong mechanism in yeast Esa1. (b) Ternary complex mechanism in yeast Gcn5. “simplified” denotes that protonation/deprotonation steps are missed out.

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Chapter 2

2.1.2.4 Lipases

There is one more biocatalytic approach to carboxamides. It is not observed in nature but has been successfully employed in chemistry.[218]

Lipases catalyse the hydrolysis of esters. They function by forming a tetrahedral intermediate, which is subsequently hydrolysed (Scheme 62). It was envisaged that an analogical route (formation of a tetrahedral intermediate from a carboxylic acid or an ester followed by nucleophilic attack by an amine) could be used for the biocatalytic amide bond formation.[187]

Scheme 62. Schematic mechanism of serine hydrolase biocatalysis. 108: Michaelis complexes; 109: tetrahedral intermediate; 110: acyl-enzyme covalent intermediate; 11: free enzyme. Hydrogen bonds are shown in green. They play crucial role in ester recognition, correct positioning in the catalytic pocket and stabilisation of the negatively charged tetrahedral intermediate.

In fact, lipases are potentially ideal because they have been shown to be active in organic solvents.[219] The use of organic solvents is often essential because most organic substrates are insoluble in water. It also excludes the reverse process, namely, the hydrolysis of the newly formed amide. Proteolysis (peptide bond hydrolysis) is catalysed in the cell by a closely related class of proteins, proteases, which share a mechanism that is similar to that of lipases and involves the catalytic triad.

The catalytic triad refers to the three amino acid residues (Asp-His-X, where X is Ser, Asp or Cys) that are commonly found in the active site of the α, -hydrolase superfamily. They work together via formation of a tetrahedral intermediate and activation of small molecules

50

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(Scheme 63).[220] The carboxylic group of aspartic acid forms a low-barrier hydrogen bond with histidine, increasing the pKa of the imidazole nitrogen from 7 to about 12. This makes His act as a strong general base which can deprotonate the acidic hydrogens of serine, cysteine, aspartic acid and acidic substrates.

Scheme 63. Diverse catalytic activities of the α, -hydrolase superfamily. Simplified mechanisms for (a) “classical” serine hydrolase–protease, (b) C-C hydrolase, (c) hydroxynitrile lyase.

Recently, Ema et al. reported the first hydrolase inspired biomimetic trifunctional organocatalysts 112 and 113 (Figure 16), which significantly increased the reaction rate between vinyltrifluoroacetate 114 and methanol or iso-propanol (Scheme 64).[221] Control compounds, lacking either the hydroxyl, pyridine or urea/thiourea moiety were inefficient. This example of trifunctional organocatalysis is perhaps one of the most impressive ones. However, it is very limited. Firstly, vinyl alcohol tautomerises to acetaldehyde and does not act as a competitive nucleophile. Secondly, although vinyltrifluoroacetate 114 showed impressive rate acceleration (reaction was complete in 30 min at 22 °C with 1 mol% 113 and in 1 h with 112), the less reactive vinylacetate remained unreacted even with 1 equiv of 112.

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Figure 16. (a) Lipase active site catalytic triad. (b) Ema’s biomimetic trifunctional organocatalysts for transesterication.

Scheme 64. Rate constants for catalysed transesterification. Urea derivative 113 accelerates reaction i 3–5 times compared to thiourea-based catalyst 112 The kun values for acylations of MeOH and PrOH are 7.6·10–5 and 4.0·10–6 M–1s–1, respectively. un = uncatalysed.

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2.2 Results and Discussion

2.2.1 Introduction

The amide bond is prevalent in nature and its formation is heavily utilised in synthetic organic chemistry.[158,159,178] Despite the multitude of existing methods and strategies, there is a high level of interest, particularly by the industrial community in more cost-efficient ways to perform an amide coupling.[161,166,222]

There are many factors that come into play when determining the cost-efficiency of a process.[222] For chemists, the main issue is of course, atom efficiency, which may be equated in this particular case to sustainability. However, one should not forget about the ease of manipulation, purification and scaling-up, which all contribute to labour and engineering costs; cost of coupling reagent, additives or catalyst; waste disposal costs including solvents; and the energy costs (e.g. cooling or heating the reaction mixture). In this regard, the employment of boronic acids as substoichiometric catalysts and perhaps, other simple boron reagents represents an attractive alternative[131] to already established methods.

In 2007, when this project was initiated, only Yamamoto‟s work,[131a,133,134–138] Tang‟s note of the activity of boric acid[139] and Whiting‟s preliminary results on “Wulff-type” boronic acids[141] were available. Arylboronic/boric acids were shown to act as direct amidation catalysts, however, somewhat inefficiently. In most of the examples, reaction mixtures were heated at temperatures above 100 °C in high boiling point solvents and with water removal using Dean–Stark apparatus. The use of such high temperatures is undesired because of incompatibility with a number of protective and functional groups (e.g. N-Boc). Under these conditions complex starting materials as well as arylboronic acids[140–142,223] may also decompose.

As for the mechanistic rationale behind the direct catalytic amidation of carboxylic acids at that time, Yamamoto was able to synthesise and characterise by crude 1H NMR and IR a monoacyloxyarylboronic ester 115 on heating 4-phenylbutyric acid with 3,5 bis(trifluoro)phenylboronic acid (in ratio 2:1) under reflux in toluene-d8 (Scheme 65). Interestingly, intermediate 115 was very susceptible to hydrolysis and was able to react with benzylamine at ambient temperature. Based on these results, a catalytic cycle, where in situ generation of monoacyloxyarylboronic ester was assumed to be a rate-determining step, was proposed.

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Scheme 65. Proposed mechanism for catalytic boron-based amide bond formation.[134]

The mixed anhydride 115 was supposedly stabilised by an intramolecular hydrogen bond. Later, Whiting and co-workers stated that they could not observe such an intermediate by ESI/MS and 1H NMR.[141] In fact, they detected bis(acyloxy)arylboronate species 116, diboronate 117 and boroxine 118 (Figure 17).

Figure 17. Observed intermediates and species in boronic-acid catalysed carboxamidtions: by (a) Yamamoto and (b) Whiting.

The use of elevated temperatures alongside water removal also raised the question whether a carboxylic acid anhydride could be formed in situ at higher temperatures. Direct condensation of ammonium salts has also been achieved under high temperatures without a catalyst,[180] although this process is highly substrate and temperature-dependent. [185] Notably, the boronic and boric acid-catalysed amidations are also substrate and temperature-dependent.[141,142]

Another interesting issue is solvent effects. Most reactions using arylboronic acids were carried out in arenes (e.g. fluorobenzene, toluene, mesitylene), out of which at ambient temperatures the ammonium salt precipitates rapidly and quantitatively.[244] Improved catalyst, N-methyl-4-boronopyridinium iodide 33, reportedly[224] showed better performance in polar aprotic solvents such as NMP and N-benzylpyrrolidone.[135a] Unfortunately, no

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solvent screening data was provided and boronic acid 33 was used in toluene/[emim][OTf] 5:1 mixture as solvent.

2.2.2 Aims and Objectives

Initially, this project aimed to improve the catalytic performance of boronic acids in direct carboxamidation. This was to be achieved through design and evaluation of a small set of boronic acids, which would also incorporate a general base (Figure 18). Such a strategy is in fact nature-inspired as similar catalytic triads/diads (such as those described in the earlier sections) are found in hydrolases. However, later work by Ema et al. on trifunctional hydrolase-like organocatalysis, showed how highly substrate-dependent these systems are.[221]

Figure 18. Envisaged potential catalysts for direct amide bond formation.

The strategy was based on incorporating additional functionalities into an arylboronic acid skeleton to enhance the overall reaction rate. It was hoped that an intermolecular transfer of the acyl moiety from the acyloxyboron species (regardless whether it is mono or bis) to another intramolecular nucleophile would yield a more readily reactive intermediate 124 (Scheme 66). The transfer step could be favoured as the change in entropy for the intramolecular step would be minimal.

It was envisaged that the additional functionalities (X in Scheme 65) would be based on common acyl transfer reagents, such as 1-hydroxybenzotriazole (catalyst 119) and N,N- dimethylaminopyridine (catalyst 122). In addition, since aminolysis of is a fast process in comparison to that of oxoesters,[225,226] incorporation of a thiol moiety into the boronic acid‟s skeleton was also pursued.

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Scheme 66. Proposed modified catalytic cycle.

Although the structures of the proposed catalysts might look simple, as we found out, they are not easy to prepare. Firstly, because they contain a number of functional groups in close proximity to each other. Secondly, this sets strict limits on how they can be assembled so as not to compromise the neighbouring groups. For instance, we have looked into construction of DMAP-type quinoline-based boronic acid 122 (Scheme 667).[227–231] The original strategy was based on successive intramolecular cyclisation/aromatisation reactions. However, both the corresponding ester and amide failed to cyclise. Another route attempted was via quinoline oxidation, analogous to that reported for a similar substrate.[231] Later in the programme, it was recognised that the proposed catalysts 120–122 were perhaps s too rigid to act as effective catalysts. These conclusions were drawn in part from Hall‟s work that appeared in 2008.[132b] Nevertheless, a full discussion of the strategies employed and/or attempted to synthesise catalysts 119 and 123 are provided below.

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Scheme 67. Strategies to catalyst (4-(dimethylamino)quinolin-8-yl)boronic acid.

2.2.3 Synthesis of Boronic Acids

The high reactivity of boronic acids especially in the presence of transition metals and acids (e.g. protodeboronation) dictated that the boronyl moiety should be introduced towards the end of the synthesis.

2.2.3.1 Synthesis of (1-Hydroxy-1H-benzo[d][1,2,3]triazol-7-yl)boronic acid

It was thought that 1-hydroxybenzotriazole derivative 119 would be accessed via borylation of 7-halosubstituted 125, which would be prepared from 2,6-dihalonitrobenzene 126 (Scheme 68).

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Scheme 68. Principal for (1-hydroxy-1H-benzo[d][1,2,3]triazol-7-yl)boronic acid.

Having taken into consideration that several methods for borylation of aryl chlorides exist,[18,22–24,28] the first strategy that was pursued used commercially available 2,6- dichloroaniline 127a as a starting material. It was oxidised with NaBO3/AcOH to give the corresponding 126a (Scheme 69).[232] While this protocol was previously reported to give 126a in an 87% crude yield, the two unoptimised runs in our hands afforded 126a only in 24% and 31% yields after purification. The starting material was not consumed even after prolonged reaction times and the use of further excess of sodium perborate.

Scheme 69. Synthesis of 7-chloro-1H-benzo[d][1,2,3]triazol-1-ol.

Next, 2,6-dichloronitrobenzene 126a was converted to 7-chloro-1-hydroxybenzotriazole 125a. [223–234] Under optimised conditions, the reaction was carried out under argon in anhydrous ethanol with an excess of hydrazine monohydrate (15–20 equiv) and 5 mol%

NaOAc. The reaction mixture was then quenched with NaHCO3 solution, the aqueous layer was washed with ether, and the product was precipitated out of solution by addition of concentrated HCl solution. The two separate runs afforded 125a in 61% and 58% yields.

The conversion of aryl chloride 125a to arylboronic acid or ester, however, proved unsuccessful (Table 3). Although precedented with aryl chlorides, both insertion of lithium and magnesium into carbon– bond[18] and Pd-catalysed Miyaura borylation,[22–24,28] did not lead to any detectable formation of boronic acid or ester, respectively.

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Table 3. Attempted routes to borylation of 7-chloro-1-benzotriazole 125a.

entry conditions observations ref

1 Pd(dba)2, PCy3, B2pin2, complex mixture, [22] KOAc, 48h, DO, 80 ºC starting material not consumed

2 (i) BuLi, THF, –78 °C; no indication of Cl/Li exchange; [17]

(ii) B(OMe)3 when quenched with D2O, no deuterated product was observed

3 (i) Li granula, THF, –40 °C; no indication of Cl/Li exchange [18]

(ii) B(OMe)3

4 (i) Mg, THF, 70 °C; no indication of Cl/Mg exchange [18]

(ii) B(OMe)3, 16 h, 70 °C

5 Pd(OAc)2, IMes, B2pin2, no product formation [23] KOAc, THF, 10 min, 110 ˚C, MW

Next, a directed metalation strategy was explored. It was hypothesised that the hydroxyl of 1- hydroxybenzotriazole 85 or its MOM-protected variant 128 could act as a directing group for ortho-lithiation (Scheme 70). However, no formation of the desired products 119 or 129 was observed in either of the cases. In addition to standard sequential borylation procedure, an in situ trapping directed lithiation/borylation protocol[235] was tested.

Scheme 70. Attempts towards directed ortho-lithiation/borylation. MOM = methoxymethyl, LTMP = lithium 2,2,6,6-tetramethylpiperidide.

The rising popularity of oxidative C–H activation/functionalisation, tempted us to explore an approach in analogy with Sanford‟s and Yu‟s Pd-catalysed halogenation reports (Scheme 71).[236–239] Ideally, the hydroxyl group could act as a directing group in a manner shown in Scheme 33 promoting C–H activation at the C7 position (Scheme 72). Then, an electrophilic

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halogenating agent (NBS or NIS in our case) could oxidatively add to palladium(II) system 130 to give a Pd(IV) complex 131. As the successive reductive elimination of aryl halide is more favoured from Pd(IV) rather than Pd(II), the reaction would afford the corresponding aryl halide 125. Unfortunately, in systems with glacial acetic acid or acetonitrile as a solvent and NBS, NIS and IOAc (generated in situ from I2 and PhI(OAc)2) as oxidants, only nitrogen–oxygen bond cleavage products were observed by LC/MS. Interestingly, several groups later successfully utilised such N–O bond cleavage in a number of palladium-catalysed oxidative C-H activation/derivatisation approaches (Scheme 73).[240–242]

Scheme 71. (a) Sanford’s and (b) Yu’s approaches to C–H activation/halogenation.[236–239]

Scheme 72. Proposed mechanism for Pd-catalysed C–H activation/halogenation of 30.

Scheme 73. Cleavage of N–O bond in palladium-catalysed (a) amination of aromatic C−H bonds with esters,[240] (b) C−H amidation N-nosyloxycarbamate,[241] (c) synthesis of heteroaryl ethers from pyridotriazol-1-yloxy heterocycles and boronic acids.[242

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Since borylation of the aryl chloride 125a was troublesome, an aryl bromide analogue 125b was synthesised in a similar fashion (Scheme 74). Oxidation of 2,6-dibromoaniline with either sodium perborate[232] or Oxone® in a biphasic system[243] were both unsuccessful. Oxidation [244] with TFA/H2O2 in contrary to a previous report, gave a nitroso compounds 132. Therefore, a two-step oxidation protocol in order to access 126b was applied.[245] Aniline was first converted with MCPBA to the nitroso compound, which was then oxidised to the nitro derivative with catalytic nitric acid and as a terminal oxidant. We subsequently found that 2,6-dibromoaniline could also have been oxidised in one step, if treated with MCPBA for 48 h rather than 4 h (98%).

Scheme 74. Successful synthesis of 7-bromo-1-hydroxybenzotriazole 171.

When 126b was subjected to the same reaction conditions as the dichloro analogue 126a (benzotriazole formation), the product did not precipitate out of the aqueous phase upon treatment with concentrated HCl. The water was therefore removed under reduced pressure and the resulting residue was re-dissolved in a minimum amount of water and only then treated with acid to give the product in 58–65% yield.

To deliver the desired boronic acid via halogen/metal exchange-borylation sequence, the benzotriazole derivative 125b was first protected with a (Scheme 75). To avoid tele-substitution on the heteroaromatics, the Mitsunobu protocol was employed.[246] Such a [247] conversion was thought to be feasible, as the pKa of 1-BtOH was reported to be 4.60; and indeed, the protection worked smoothly. The difficulties with the benzyl-protected compound 133, however, arose on the lithiation/borylation step. Namely, compound 134 was identified as a major product. It formed as a consequence of base promoted formation of , which was later trapped by the aryl lithium species. A number of attempts was made to optimise the reaction. However, conducting the reaction at –100 ºC, slow addition of BuLi as well as addition of BuLi to a mixture of aryl bromide and borate did not lead to detectable amounts of the desired product (by LC/MS and 1H NMR). The use of „turbo-Grignard‟ reagent, iPrMgCl∙LiCl,[248] was also explored. In addition to benzaldehyde-derived

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benzotriazole, formation of 135 was observed by LC/MS and confirmed by HRMS (for + C6H5BrN3 [M+H] found 197.9663, calc. 197.9667).

Scheme 75. Reactions with 1-benzyloxy-1H-benzo[d][1,2,3]triazole.

Hence, there was a need to switch to a protecting group that bears no protons on the carbon adjacent to oxygen and is easily cleavable, i.e. tert-butyl (Scheme 76). Because the Mitsunobu protocol would not lead to the formation of the desired borylated product, to protect the hydroxybenzotriazole 125b Widmer‟s esterification procedure with N,N-dimethylformamide di-tert-butyl was employed.[249] This gave a mixture of 136 and 137. The latter product arises from N-attack (Scheme 77) and was identified by characteristic signal shifts in 1H NMR spectrum in agreement with the previous work by Katritzky.[250] Additionally, it is known that N-oxides arise in alkylation of hydroxybenzotriazoles.[251]

Subsequent lithiation/borylation of 136 gave the corresponding boronic acid 138. Deprotection of the tert-butyl moiety was conducted in the TFA/DCM system. Initially, 138 was treated with 2 equiv of TFA in DCM for ten minutes, however, simultaneous tert-butyl deprotection and protodeboronation was observed. Reducing the amount of acid employed down to 0.2 equiv led to clean formation of the desired material 139 in quantitative yield.

Scheme 76. Synthesis of (1-hydroxy-1H-benzo[d][1,2,3]triazol-7-yl)boronic acid.

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Scheme 77. Proposed mechanism for N-oxide formation.

2.2.3.2 Synthesis of “Sulfur-Armed” Boronic Acid

Thioesters are known to undergo aminolysis much faster than their oxoester counterparts. Hence, it was deemed useful to explore the effect of a sulfhydryl-bearing substituent in the boronic acid. (2-(Mercaptomethyl)phenyl)boronic acid 123 was selected as a most convenient structure to assess the feasibility of the effect (Scheme 78).

Scheme 78. (2-(Mercaptomethyl)phenyl)boronic acid.

Initially, a number of aryl bromides with a free or S-protected thiol were prepared (Scheme 79). Tritylthiol and thioacetate were installed by nucleophilic substitution of 2-bromobenzyl bromide. Thioacetate 141 also underwent deprotection under basic conditions to free thiol 142. However, attempted lithiation/borylation of these aryl bromides failed to give the desired product.

Scheme 79. Synthesis of sulphur-containing bromides.

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However, a complementary route was based on elaboration of benzylic protons in ortho- tolylboronic acid, which exists in the boroxine form (ArBO)3 (Scheme 80). Radical bromination by N-bromosuccinimide in the presence of AIBN as initiator led to benzyl bromide 143,[252–253] which was successfully converted to the corresponding S-protected [254] boronic acid 144. However, attempted deprotection with K2CO3/MeOH led to the formation of a complex mixture of products. As the free thiol 123 could not be isolated in a clean form, it was decided to use the crystalline S-acetylated boronic acid as a pre-catalyst. In the actual amidation system, a sacrificial amount of amine should readily deprotect the thioacetate 144 generating the active catalyst 123 in situ (Scheme 81).

Scheme 80. Synthesis of sulphur-containing bromides.

Scheme 81. Thioacetate-based pre-catalyst activation.

2.2.4 Evaluation of Boronic Acids and Borates for Catalytic Amide Bond Formation

All the experiments, including the screening reactions, discussed below were performed with equimolar amounts of amine and carboxylic acid. In the very first screen, several small molecules (loading 5 mol%) were screened for catalytic activity in amide formation between of 4-phenylbutyric acid and benzylamine (Table 4). These results indicated that at least for this combination of amide coupling partners, the condensation reaction proceeded without a catalyst to a reasonable extent (80% conv). As expected, both PhB(OH)2 and boric acid enhance the reaction progression to give essentially full conversions. Additionally, some catalytic activity was observed with -based reagents (Ph2SiCl2 and PhSi(OH)3). The silicon-based reagents were screened as , PhSiH3, has been reported as a reagent

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for amide bond formation that acted in a fashion similar to that of boranes, i.e. the reaction proceeded via acyloxysilanes.[255]

Table 4. The extent of background reaction in carboxamidation at high temperature.a

entry catalyst conv%b

1 – 80

2 PhB(OH)2 99

3 B(OH)3 97

4 Ph2SiCl2 92

5 PhSi(OH)3 88

6 Ph2Si(OH)2 70 a Reaction conditions: acid (1 equiv), amine (1 equiv), catalyst (5 mol%), PhMe, 130 ºC, carousel tube, 20 h. b 1 Determined by H NMR (DMSO-d6).

Due to the high level of the background reaction, another set of experiments was conducted at lower temperature (80 °C) and shorter reaction times (Table 5). In this case the thermal reaction was suppressed. The activities of phenylboronic acid and boric acid fell drastically in comparison to the reactions at higher temperature, indicating that the reaction temperature has a drastic effect on progression of both the thermal and catalysed reactions. Both trimethyl borate and tetrachlorosilane, although employed in stoichiometric amounts, showed superior results. In addition to activating carboxylic acids directly by forming acyloxyboron and acyloxysilicon species, respectively, they may also promote the reaction by serving as effective dehydrating agents. The use of SiCl4 in pyridine as a reagent for amide formation was reported in the late 1960s.[256]

The conditions used for screening were later modified to allow faster data acquisition and fewer sampling errors, especially to minimise the impact of various work-up manipulations. Hence, amidations were conducted in a microwave (CEM, 150 W) at 100 ºC for 10 min. Then, DMSO was added to dissolve the insoluble material that usually crystallised on cooling to RT. A small sample was then transferred to an NMR tube and diluted with DMSO-d6. In

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the 1H NMR spectrum (600 MHz), a doublet for the benzylic methylene of the product lies downfield of that of the starting material. The ratio of integrals for these two signals was used to determine the extent of the reaction.

Table 5. Progress of the direct carboxamidation under milder conditions.a

entry catalyst/reagent mol% conv%b

1 – 5 2

2 PhB(OH)2 5 6

3 3-NO2C6H4B(OH)2 5 8

4 3-NO2C6H4B(OH)2/PhSH 5/5 8

5 B(OH)3 5 5

6 B(OMe)3 100 38 7 Ph SiCl 5 3 2 2 8 SiCl 100 76 4 9 Ph2Si(OH)2 5 4

a Reaction conditions: acid (1 equiv), amine (1 equiv), reagent (x mol%), 0.5 M PhMe, 80 ºC, 3 h. b 1 Determined by H NMR (DMSO-d6).

Next, a focused subset of boronic acids and boroxoaromatic compounds was screened under these microwave conditions (Table 6). Yamamoto‟s catalyst 28 proved to be superior. Interestingly, Hall‟s ortho-iodophenylboronic acid 47 showed diminished results in comparison to the bromo analogue 48. The sulfur-containing boronic acid 144 was ineffective and this may be due to S→B coordination.

The hydroxybenzotriazole-based boronic acids 138 and 119 did not show significant catalytic effect. This perhaps can be explained by protodeboronation of both 138 and 119, as based on further examination of 1H NMR spectra recorded of the reaction mixtures of these amidation reactions. Namely, the doublet at > 8 ppm, characteristic for the hydrogen bonded to carbon ortho to the carbon–boron bond, had disappeared during the attempted reaction.

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Gratifyingly, alkylboronic acids were also catalytically active. No-one has explored them as potential catalysts because they are regarded to be somewhat unstable. However, they are considerably stable under transition metal-free conditions and are able to catalyse the reaction better than simple arylboronic acids. Hence, it may serve as a good basis for developing new catalysts for carboxamidation. Not only because their aliphatic scaffold allows easier introduction of functional groups, but also the arrangement and placing of these groups can be varied to a larger degree in the three-dimensional space.

Table 6. Focused library screen of potential boron-based catalystsa,b Conversions are shown in blue.

a 1 Conversions were determined by H NMR (DMSO-d6). b Reaction conditions: acid (1 equiv), amine (1 equiv), reagent (5 mol%), 0.5 M MeCN, 100 ºC, MW, 10 min. c with 1 equiv of reagent instead of 5 mol%.

However, we decided to concentrate our efforts on borate esters. They could provide us with a better understanding of factors contributing to successful boron-mediated and/or catalysed direct carboxamidation. Additionally, though required in equimolar amounts, a number of low cost and sustainable borate esters are commercially available except for B(OCH2CF3)3. This, in fact, could facilitate their acceptance as a convenient alternative to the already known classes of coupling agents.

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To date, a single example of direct amidation using trimethyl borate was presented by Pelter et al. in 1970 (Scheme 82).[257] Namely, caproic acid and N-butylamine were heated under reflux in trimethyl borate for 25 h in the presence of catalytic amounts of para- toluenesulfonic acid to gave a mixture of N-butylcaproamide and methyl caproate. In the forty years that followed this precedent, no further use of borate esters as amide coupling agents has been reported.

Scheme 82. Carboxamidation in refluxing trimethyl borate.[257]

Firstly, a number of commonly used solvents were screened (Table 7). At 80 ºC, in the absence of drying agents and/or apparatus, toluene – the most commonly used solvent in boronic acid-catalysed amidations – was one of the least efficient solvents (entry 8). In methanol, no reaction was observed presumably because MeOH coordinated to the borate ester, diminishing its reactivity (entry 10). Amidation reaction conducted in methyl tert-butyl ether (MTBE) turned out to be inefficient as well (entry 9). However, aprotic polar solvents (NMP, MeCN, DMSO, THF and DCE) proved to be significantly better for this transformation (entries 1–2, 4–7). Since both NMP and DMSO are somewhat practically inconvenient due to their high boiling points, acetonitrile was selected as the best solvent for this process. When B(OMe)3 was used as a solvent (at the same concentration, i.e. 18 equiv of borate), a conversion of only 30% was achieved.

We then examined the amount of borate ester added vs. the conversion after three hours under thermal conditions (Graph 1). As expected, an equimolar amount of trimethyl borate was required to reach significant conversion (36%). However, further increasing the amount of borate ester up to two and three equivalents did not lead to any major improvements.

The solvent screen results pointed out a pronounced deactivation of the borate ester by potential by-products of the reaction such as MeOH. This may involve formation of stable tetrahedral alcohol–borate complexes.

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a Table 7. Solvent effects in B(OMe)3-mediated carboxamidations.

entry solvent conv%b entry solvent conv%b

1 NMP 40 7c DCE 19 2 MeCN 36 8 PhMe 12

3 B(OMe)3 30 9 MTBE 4 4 DMSO 27 10 MeOH 0 5 THF 20 11 DMF –d 6 THF (dry) 17 12 MeCN 2e

a b Reaction conditions: acid (1 equiv), amine (1 equiv), B(OMe)3 (1 equiv), 0.5 M solvent, 100 ºC, MW, 10 min. 1 c d Determined by H NMR (DMSO-d6). 1,3,5-(MeO)3C6H3 used as an internal standard. BnNHCHO, a transamidation e product arising from DMF, was the major product. In the absence of B(OMe)3.

Graph 1. Progression of carboxamidation in relation to amount of B(OMe)3 employed with the same initial concentration of phenylacetic acid and benzylamine.a,b

a b Reaction conditions: acid (1 equiv), amine (1 equiv), B(OMe)3 (1 equiv), 0.5 M MeCN, 80 ºC, 3 h. Conversions 1 determined by H NMR (DMSO-d6).

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We then screened for an additive that could potentially enhance borate ester-mediated carboxamidation (Table 8). At first, we observed that protic additives, especially alcohols, strongly diminished conversions (entries 5–13, 15–18). Among compounds screened were diols (entries 11–13), which are known to give rise to boronate esters in situ.[258] It was hoped that they could aid transfer of the carboxylic acid onto boron as shown in Scheme 83a. Use of Brønsted and Lewis acids could potentially stabilise the tetrahedral intermediates as shown in Scheme 83b but this was also ineffective (entries 17–19 and 30–33, respectively). Addition of bases (entries 21–29) or using 0.2 equiv excess of either carboxylic acid or amine (entries 2– 3) did not improve conversion.

Scheme 83. Strategies to affect borate ester-mediated amidation.

We also examined 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 2,3-dihydro-1,4- phthalazinedione as additives (entries 20, 27–28). Although hydrogen bonding catalysis is very rarely observed in amidation reactions,[259] Mioskowski showed that TBD stabilised the tetrahedral intermediate by internal hydrogen bonding in the case of ester amidation (Scheme 84a).[260] Additionally, Iwata and Kuzuhara employed 2,3-dihydro-1,4-phthalazinedione in N- formylation of primary amines (Scheme 84b).[261] However, neither of the reagents proved to be beneficial in our hands.

Finally, the only combination that led to an increase in conversion was the use of 0.2 equiv excess of both the acid and the borate ester.

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a Table 8. Assessment of various additives in B(OMe)3-mediated carboxamidations.

b entry additive x equiv conv%

1 – – 36

2 BnNH2 0.2 24

3 BnCOOH 0.2 33

4 BnCOOH + B(OMe)3 0.2 + 0.2 42

5 H2O 1 19 6 MeOH 1 6

7 B(OH)3 1 14

8 CF3CH2OH 1 15

9 PhOH 1 30

10 4-NO2C6H4OH 0.2 1

11 0.2 36

12 catechol 1 9

13 (CH2OH)2 1 1

14 DMSO-d6 0.2 24 15 PhSH 0.2 28

16 3,4,5-F3C6H2B(OH)2 0.1 8

17 BtOH 0.2 28

18 TsOH∙H2O 0.2 18

19 HCl∙Et O 0.2 26 2 20 phthNHNH 0.2 28 21 DMAP 0.2 29 22 Im 0.2 27 23 N-MeIm 0.2 31 24 N-MeIm 1 29

25 TEA 0.2 37

26 TEA 1 31

27 TBD 0.2 6

28 TBD 1 4 29 DIPEA 1 24 30 LiCl 0.2 6

31 MgCl2 0.2 6

32 MgSO4 0.2 28

33 Mg(ClO4)2 1 11 a Reaction conditions: acid (1 equiv), amine (1 equiv), B(OMe)3 (1 equiv), additive (x equiv), 0.5 M MeCN, 100 ºC, b 1 MW, 10 min. Determined by H NMR (DMSO-d6).

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Scheme 84. Hydrogen bonding catalysis in amidations: (a) TBD-catalysed ester amidation and (b) 2,3- dihydro-1,4-phthalazinedione-catalysed N-formylation of primary amines.

Several boron and silicon reagents were then screened as stoichiometric reagents under microwave conditions (Table 9). Both triphenyl and tris(2,2,2-trifluoroethyl) borates showed significantly improved reactivities (entries 7–8). However, only B(OCH2CF3)3 was chosen to study scope and limitations. Yamamoto‟s catalyst (3,4,5-trifluorophenylboronic acid) did not serve as a good promoter under these reaction conditions (entry 5).

Activation of the carboxylic acid presumably occurs via in situ generation of a four or three- coordinated boron species 145a or 145b (Scheme 85).

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Table 9. Assessment of various simple borate esters and orthosilicate as reagents carboxamidations.a

entry reagent conv%b

1 – 2

2 B(OMe)3 36 i 3 B(OPr)3 9

4 B(OTMS)3 9 5 3,4,5-F3C6H2B(OH)2 19

6 Si(OMe)4 22

7 B(OCH2CF3)3 63

8 B(OPh)3 73

a Reaction conditions: acid (1 equiv), amine (1 equiv), reagent (1 equiv), 0.5 M MeCN, 100 ºC, MW, 10 min. b 1 Determined by H NMR (DMSO-d6).

Scheme 85. Plausible mechanism of borate ester mediated carboxamidation. rds = rate-determining step.

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2.2.5 Borates as a Novel Class of Coupling Reagents for Amide Bond Formation

With the optimised results in hand, it was insightful to look at the substrate scope of both

B(OMe)3 and the borate derived from a more electron-deficient alcohol B(OCH2CF3)3 In order to do so, reactions were carried out with two equivalents of the borate ester. An excess of borate ester ensures that the overall conversion is not compromised by coordination of a by-product (an alcohol or boric acid) to the remaining borate. Additionally, for each carboxylic acid–amine combination, we tested borate-free reaction conditions to assess the thermal contribution towards the carboxamidation reaction.

Tris(2,2,2-trifluoroethyl) borate showed superior activities throughout the series, however, in a number of cases low cost and commercially available B(OMe)3 was sufficient to promote amidation in satisfactory to very good yields (Table 10, entries 1–3, 7–9, 11, 15–16). Reactions with less reactive acids, such as benzoic and pivalic acids were conducted at higher temperatures (entries 5–6); while p-aniside also gave the amide 146n in a good yield (entry 14).

Both volatile substrates (entries 9–10) and those containing polar groups such as free hydroxyl and unprotected indole (entries 7, 11) were tolerated. Tris(2,2,2-trifluoroethyl) borate showed the most improvement over B(OMe)3 in the case of α,β-unsaturated carboxylic acids (Entries 12–14). However, coupling of 3-thiophenylcinnamic acid and pyrrolidine gave only 10% of product 146q (Scheme 86).

Scheme 86. Amide coupling between 3 thiophenylcinnamic acid and pyrrolidine. aIsolated yield.

Importantly, α-branched carboxylic acids and amines were reactive under these conditions. Of note are also results with Boc-L-alanine (enry 16). Trimethyl borate promoted reaction in a reasonable conversion and with no racemisation as determined on Diacel® Chiralpak® OD-H i column (7–10% PrOH/). It was gratifying to see that B(OCH2CF3)3 was also able to promote amidation without N-Boc cleavage. This result is somewhat unexpected as

B(OCH2CF3)3 may act as a strong Lewis acid and this is supported by its Gutmann Acceptor

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Number (AN) value of 66.4.[262] However, under unoptimised conditions, the product was obtained in good yield with a minor loss in enantiopurity (88% ee). The small degree of epimerisation indicates that it takes place in the early steps of amidation, most likely when the carboxylic acid is activated, i.e. forms acyloxyboron species (Scheme 85). Hence, the racemisation-sensitive amidations can be optimised by controlled addition of a borate ester and/or lowering the reaction temperature.

Table 10. Borate-promoted direct amide bond formation.

isolated yield, %a entry product b b c B(OCH2CF3)3 B(OMe)3 thermal

1 146a 91 (74)d 92 (66)d 18

2 146b 70 73 <1

3 146c 70 51 7

4 146d 76 44 5

5 146e 14 (50)e 2 0

6 146f 27 (71)e 12 <1

7 146g 92 45 9

8 146h 82 51 6

9 146i 66 66 6

continued on next page

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continued from previous page

isolated yield, %a entry product b b c B(OCH2CF3)3 B(OMe)3 thermal

10 146j 61 36 3

11 146k 70 quant 9

12 146l 71 17 8

13 146m 95 11 6

14 146n 72 4 0

15f 146o 94 60 5

16 146p 81g 49h 7

a Purified by acid-base extraction, except for entry 14, where flash was used. b Reaction conditions: acid (1 equiv), amine (1 equiv), borate (2 equiv), 0.5 M MeCN, 80 ºC, 15 h. c Reaction conditions: acid (1 equiv), amine (1 equiv), 0.5 M MeCN, 80 ºC, 15 h. d Reaction conditions: acid (1 equiv), amine (1 equiv), borate (1 equiv), 0.5 M MeCN, 80 ºC, 15 h. e Reaction conditions: acid (1 equiv), amine (1 equiv), borate (2 equiv), 0.5 M MeCN, 100 ºC, f i g h caurosel tube, 15 h. From amine-HCl salt, with Pr2NEt (1 equiv). 88% ee. >99% ee.

Hydrochloride salts of amines can also be used as starting materials, although an equivalent of Hünig‟s base is additionally required (entry 15). The base deprotonates the ammonium salt resulting in a free amine that goes into the solution phase. In the absence of a base, the starting material was not soluble and no reaction was observed.

Encouraged by the compatibility of B(OCH2CF3)3 with the Lewis and Brønsted acid-sensitive N-Boc moiety and the low degree of racemisation in the case of N-Boc-L-alanine (entry 16), an experiment to test direct amidation of an unprotected amino acid, namely, L-alanine was conducted with benzylamine (Scheme 87). In the absence of base, the amino acid remained insoluble in acetonitrile and no product was formed. However, when 1 equiv of Hünig‟s base

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was added, pleasingly, the desired amide 146r was observed as a major product in 1H NMR

(DMSO-d6). Dipeptide Ala–Ala was the minor product. Due to time restriction, no further experiments with free amino acids were conducted, though it should be noted that racemisation of the unprotected α-amino acid is not expected to occur under these conditions.

a 1 Scheme 87. Amide synthesis from free α-amino acid. Determined by H NMR (DMSO-d6).

A similar protection/activation[263] strategy was reported by Liskamp and co-workers (Scheme [264] 88). They first prepared a BF2-complex 147 from lithium salt of (S)-phenylalanine, and after purification by flash chromatography treated with 2 equiv of benzylamine, to give the desired amide in 45% over two steps. The same strategy was also pursued with [265] dichlorosilanes (e.g. Ph2SiCl2) in pyridine followed by an excess of amine (3–5 equiv). Unfortunately, couplings with secondary amines were rather poor (with maximum yield up to 17%).

Scheme 88. Stepwise simultaneous protection-activation strategy for amide coupling of amino carboxylates.

It was expected that the Lewis acidity of B(OCH2CF3)3 would be higher than that of other borate esters and boronic acids. Hence, this reagent could potentially activate other carboxyl compounds such as esters and/or amides. When methyl phenylacetate was treated with benzylamine in the presence of B(OCH2CF3)3, no amidation product was observed (Scheme 89). This allowed us to successfully couple L-phenylalanine ethyl ester hydrochloride with phenylacetic acid (Table 10, entry 15) without formation of by-products arising from ester amidation. However, as expected, B(OCH2CF3)3 proved to be successful in activating more nucleophilic primary amides and in the section below we demonstrate several examples of transamidation reaction (Section 2.2.6).

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Scheme 89. Attempted ester amidation.

2.2.6 Tris(2,2,2-trifluoroethyl) Borate as a Reagent for the Activation of Primary Amides

Amides are one of the most widely employed bond types in natural systems. They are often used as linkers in and because they are stable in the cellular environment, i.e. they do not get readily reduced or hydrolysed. In synthetic chemistry, transformations of amides often require highly nucleophilic reagents or pre- activation (Scheme 90). There are two major strategies for this latter approach: (a) use of Lewis acidic catalyst or promoter to generate an activated intermediate 148;[266–268] or (b) formation of an iminium salt 149 upon treatment with triflic anhydride.[269–270] Additionally, amides can be activated with oxalyl chloride[271] but this approach is not compatible with reductions and transamidation reactions.

Scheme 90. Activation of amides. All = allyl, HEH = Hantzsch ester.

In our hands, primary but not secondary or tertiary amides were activated in situ by

B(OCH2CF3) towards transamidation reactions (Table 11). Substrates containing polar groups such as free hydroxyl and indole were tolerated. Moreover, several other boron reagents such

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as Yamamoto and Hall boronic acids, borate esters, B(OR)3 (where R = TMS, Ph, Me) and tetramethyl orthosilicate, Si(OMe)4 did not promote transamidation. Essentially, this is the first example of boron-promoted transamidation as well as a very rare example of the transamidation of primary amides. Notably, Tf2O-mediated activation of primary amides was not reported and metal systems, e.g. Ti(NMe)4 or and Zr(NMe)4 react with primary amides to form imido species ([M]=N) which react further to give amidines.[266] Myers developed a strategy to circumvent this event by pre-forming N‟-acyl-N,N-dialkylformamidines 150 that [268] eventually react with amines in the presence of 0.5 equiv of ZrCl4 (Scheme 91). Bertrand showed that stoichiometric use of AlCl3 can promote such amidations, but the amide/amine ratio was 1:2.5–3.5 (Scheme 92).[266e]

Table 11. Transamidation of primary amides.a,b

entry product yieldc

1 146s 73

2 146t 63

3 146u 82

4 146v 62

a Reaction conditions: amide (1 equiv), amine (1 equiv), borate (2 equiv), 0.5 M MeCN, 100 ºC, carousel tube, 15 h. b Transamidations were performed on 0.5–3.0 mmol scale. cIsolated yield.

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Scheme 91. Myers approach to transamidation of primary amides by in situ formation of N’-acyl-N,N- dialkylformamidines.[268]

Scheme 92. Aluminum chloride-mediated transamidation of acetamide.[266e]

2.2.7 Mechanistic Considerations

As this report was in the final stages, a computational study on the mechanism of boronic acid-catalysed direct amidations was reported (Scheme 93).[143] Marcelli used DFT- experiments (MPW1K, gas-phase) to construct a likely catalytic cycle and interpret the high catalytic ability of ortho-haloboronic acids in this transformation.

Marcelli proposed that the carboxylic acid and not the carboxylate reacts with the boronic acid to give the acyloxyboronate species 151. This is due to a significantly higher activation barrier if the reaction were to proceed from the carboxylate, regardless of several groups' reports that ammonium salt formation on mixing of primary amine and carboxylic acid is instantaneous.[134,141–142] It was then estimated that conversion between acyloxyboronate 151 and acyloxyborate 152 has a low energetic barrier (1.4 kcal/mol), however, nucleophilic attack on the four-coordinated boron species 151 is favoured by 6.8 kcal/mol. The expulsion of water is next, and it is the rate-determining step, at least as calculated for primary amines

(Ea = 20.1 kcal/mol). Hall's ortho-halophenylboronic acids catalyse direct carboxamidation at this stage by acting as mild Lewis bases. Further dissociation of amide from boron is fast. These calculations do not exclude nucleophilic attack on the bisacyloxyboronate species 153.

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Scheme 93. (a) Lowest-energy calculated catalytic cycle.[143] (b) Diagram of the catalytic reaction proceeding via acyloxyboronate species 151 (energy vs. reaction progression)

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These finding are in agreement with previous observations. In his work on boronic acid- catalysed 1,3-dipolar additions to unsaturated carboxylic acids, Hall observed only a three- coordinated species similar to 152 but 13C NMR.[132b] This can be explained by a low energetic barrier between tetra and three-coordinated species 151 and 152 (1.2 kcal/mol) as calculated by Marcelli.

However, in all reported cases employing boronic acid-catalysis removal of water is required to achieved high conversions. This may mean that water and/or other protic substrates may intervene with the catalytic cycle by coordinating to boron or hydrolysing the resulting carboxylic acid/boric acid mixed anhydride. Moreover, our screening data support this for both boronic acid-catalysed and borate ester-mediated conditions. Another factor worth considering is the equilibrium between boronic acids and their trimers, boroxines. Furthermore, dimeric and/or oligomeric species might be involved in catalysis,[141] while for electron-rich boronic acids and some boroxoaromatics protodeboronation may be a reactivity- determining factor .

Additional design of experiments (DoE) experiments were conducted using triphenyl borate under microwave conditions (Table 12). A number of parameters was varied such as initial concentration of the starting material (c 0.15, 1.08 and 2.0 M), reaction temperatures (80, 115 and 150 ºC) and time (5, 18 and 30 min). It was shown that reaction progression did not depend on either concentration or reaction times (Graph 2). However, there was a high correlation with the temperature, the reaction was conducted at. Background reaction, thermal carboxamidation, did not take place at milder temperatures (80 ºC, 115 ºC).

2.2.8 Conclusions and Outlook

From screening a focused library of boronic acids, we have learned that protodeboronation, boroxine formation and complexation of by-products such as water, methanol and boric acid itself may play a substantial role in limiting the overall progression of carboxamidation (Scheme 94). We have shown that alkylboronic acids may act as catalysts in this transformation, opening new possibilities to improve the existing catalytic systems.

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Table 12. Data collected from DoE study by varying concentration, reaction time and temperature demonstrate that there is a dependence on reaction temperature.a,b

entry c, t, time, conv% b entry c, t, time, conv% b x M y ºC z min x M y ºC z min 11 1.08 150 18 85 1 0.15 80 18 61 2 0.15 115 5 73 12 1.08 150 30 85 3 0.15 115 30 76 13 2.0 80 18 62 4 0.15 150 18 83 14 2.0 115 5 75 5 1.08 80 5 61 15 2.0 115 30 79 6 1.08 80 18 62 16 2.0 150 18 84 7 1.08 80 30 67

8 1.08 115 18 78 17c 1.08 80 18 0 9 1.08 115 18 77 18c 1.08 115 18 4 10 1.08 150 5 81 19c 1.08 150 18 24 a Reaction conditions: amine (1 equiv), acid (1 equiv), B(OPh)3 (1 equiv) b 1 Determined by H NMR (DMSO-d6). c Without B(OPh)3.

Graph 2. (a) Temperature effect on reaction progression (see Table 12). All data points have varying concentration (0.5, 1.08 or 2.0 M). (b) Temperature effect in borate ester mediated and non-mediated reactions (c = 1.08 M, t = 18 min).

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Scheme 94. Factors involved in the boronic acid-catalysed carboxamidation.

As a part of work towards rationally designed boronic acid for the catalysis of direct amide bond formation, it was found that simple borates can serve as effective carboxamidation promoters. They act at 80 ºC, a temperature much lower than the one typically used for boronic acid- catalysis and in the absence of any additional drying apparatus or agent.

Borate esters can be easily fine-tuned to deliver a right combination of reactivity. They make up a new family of amide coupling reagents, which are environmentally benign and allow easy purification using aqueous work-up.

Tri(2,2,2-trifluoroethyl) borate, was shown to be superior to trimethyl borate. Not only did it promote carboxamidation with good to excellent yields, it also activated primary but not secondary or tertiary amides towards transamidation. While promoting carbox- and transamidations, it did not cleave the Lewis acid-sensitive N-Boc moiety, pointing out to a window of potential chemoselectivity. This subtle trade-off in the Lewis acidity should be further explored in application to a new platform for oligopeptide synthesis directly from unprotected α-amino acids. The borate reagent in this case will act as a dual C-activation/N- protecting agent (Scheme 95).[263–265]

Apart from surrogate systems for amidations such as (CF3CH2O)2BOBt, a number of transformations, can be envisaged and/or pursued. Most notably, the activation of alcohols and related compounds towards nucleophilic substitution or transmetalation reactions (Scheme 96). The currently employed methods for these reactions are limited to Mitsunobu protocol and formation of pseudohalides (e.g. tosylates, mesylates, nosylates). A process involving borates is most likely to require an additional equivalent of base as depicted in

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Scheme 96 to stabilise the boronate 155. Simple heating of 4-phenyltetrazole, benzyl alcohol and B(OCH2CF3)3 in acetonitrile at 100 ºC in a carousel tube was not effective (Scheme 97). However, due to the severe limitations of alternative methods and their rather poor sustainability it is desirable to look for further elaboration of this system.

Scheme 95. (a) Classical approach to peptide synthesis. (b) Approach incorporating dual C- activation/N-protection strategy.

Scheme 96. Plausible modes of RXH activation by Lewis acid borate towards nucleophilic attack.

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Scheme 97. Attempted direct N-benzylation of 4-phenyltetrazole.

It is of note that a boron-based system, a simple borate buffer, was used in another transformation, where an otherwise hard-to-access phosphorylated substrate, dihydroxyacetone phosphate (DHAP), is required.[272] RhaD, an aldolase, can accept dihydroxyacetone as a substrate when it was in a borate buffer, presumably in the form of a reversible DHA borate (Scheme 98).

Scheme 98. Borate as a phosphate ester mimic in aldolase-catalysed reaction. DHAP = dihydroxyacetone phosphate.

In summary, boron reagents can provide an important tool to mimic the natural reactivity of . Phosphorylation is one of the prevalent ways of activating carboxylic, hydroxyl and keto-substrates in biological systems. Advances in boron catalysis and mediation can provide the synthetic community with better control over desired reactivities in addition to novel reactions.

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Chapter 3

3 Gold-Catalysed Boron Enolate Formation

In the previous Chapter, the Lewis acidity of the boron atom was used to activate carboxylic acids and primary amides and thus promote elimination of a small (water or ammonia), i.e. a condensation reaction. However, boron based systems may be useful in the reverse process; namely, delivering a small molecule in an addition reaction, e.g. water.

Boronic acids bear two hydroxyl groups and they may act as nucleophiles (Scheme 99). So far, few reports of using this reactivity pattern have appeared. Additionally, a water molecule or other oxygen, nitrogen, or sulphur compound (alcohols, amines, , respectively) can coordinate to boron. By donating electron density into the boron system, it may increase the nucleophilicity of the most electronegative substituent, i.e. the hydroxyls. Otherwise, boron may serve as a tether to deliver the O, N, S-nucleophiles from a tetrahedral boronate intermediate to the substrate. An interesting example of hydroxylation would be that of addition of boronic acids across a multiple bond as shown in Scheme 100. This could generate a boron enolate species 156, a highly useful intermediate in synthetic organic chemistry.

Scheme 99. Increasing the nucleophilicity of boronic acids via activation.

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Scheme 100. Trapping an in situ generated boron enolate.

3.1 Background

In the aldol reaction, a β-hydroxy carbonyl compound is produced from two carbonyl- containing compounds (Scheme 101). It is perhaps one of the most widely used methods for new carbon–carbon bond formation.[273] The first notion of such reactivity was described by Borodin[274] and Wurtz[275] independently in 1872. They observed that under acidic conditions aldehydes self-condense giving rise to a new product 157 that acts both as aldehyde and alcohol (hence, the product was named aldol).

A cross-condensation product 158 arising from two different carbonyl compounds is synthetically more valuable than a self-condensation (Scheme 101a). The simplest example of cross-aldol reaction is that between an aromatic and an aliphatic aldehyde (Scheme 101b).[276] Benzaldehyde is not enolisable and thus can only act as an .

Scheme 101. (a) Self vs. cross-aldol reactions. (b) Proline-catalysed cross-aldol reaction.[276]

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To achieve an effective crossed aldol reaction, one of the carbonyl compounds (donor) must be activated as an enolate. A diverse array of enolate equivalents has been explored synthetically (Figure 19). Some of them are commercially available and/or have long shelf- life (e.g. silyl enolates) and can be used directly. Additionally, within the last decade, a number of organocatalytic approaches to crossed aldol reactions have been developed. They often allow perfect control of chemo- and stereoselectivity under milder conditions in a straightforward one-pot manipulation.[273]

However, out of a wide choice of enolate equivalents, boron enolates remain one of most effective ones. Usually, they are generated in situ prior to addition of the aldehyde (acceptor), which itself is added in a controlled fashion and at low temperatures. This prevents transfer of the enolate from one partner to the other and usually gives rise to the kinetic product.

Figure 19. Enolate equivalents and their relative nucleophilicity.

Apart from ascertaining which partner will act as an electrophile and which as a nucleophile, in simple systems the enolate’s geometry controls the stereochemical outcome of the reaction. Up to two new stereocentres can be created and their relative , i.e. syn or anti, is controlled by selective generation of either (E)– or (Z)–enolate, respectively (Scheme 102).[277] To accomplish this, for instance, an appropriate combination of dialkyl boron (pseudo)halide and a base are used. This methodology is largely robust and has been a true workhorse for stereocontrolled aldol reactions and has been widely applied in the synthesis of complex molecules such as natural products.[278]

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Scheme 102. Control of relative stereoselectivity through enolate geometry.[279]

Nevertheless, even for this well established protocol, Abiko et al. reported that in certain cases products of the double aldol reaction 159 are observed. This process was rationalised to proceed via carbon-bound boron enolates 160 (Scheme 103).[280]

Scheme 103. Boron-mediated double aldol reaction of carboxylic esters.

Alternative approaches aim to generate enolates catalytically and by-pass conventional stoichiometric deprotonation at the α-carbon adjacent to the carbonyl.[281] For instance, Motherwell and co-workers[282] pursued a transition metal-catalysed isomerisation of allylic alkoxides (Scheme 104).[283] Lithium alkoxides and potassium triethylboronates were [282a,b] [282a] successfully isomerised with [Rh(dppe)(thf)2][ClO4], Wilkinson’s catalyst and [282b] (Cy3P)2NiCl2 and trapped with aldehydes to give aldol products.

Scheme 104. (a) Rhodium promoted isomerisation of allylic alkoxides[282a] and (b) plausible mechanism. TM = transition metal.

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In 2001, Uma et al. demonstrated that under UV irradiation, Fe(CO)5 can catalyse similar transformation without the need for base (Scheme 105).[284] Next, they introduced a modification for Ru and Rh-catalysed protocols. Hydride, alkyl or aryl-metal species, generated in situ from simple chloride complexes with an equivalent of RLi, catalysed the transformation without stoichiometric amount of base.[285]

Scheme 105. Tandem isomerisation of allylic alcohols/aldol reaction in the absence of base.

Later, Li showed that this transformation can be carried out using commercially available ruthenium-based catalyst in aqueous solvent mixtures or ionic liquids without any other additives (Scheme 106).[286]

Scheme 106. Ruthenium-catalysed aldol-type reactions via olefin migration in polar media.

An approach combining rhodium-catalysed hydrogenation of enones and aldol reaction was thoroughly explored by Krische.[287] In a recent instalment, a diastereo- and enatioselective hydrogenative protocol was developed (Scheme 107).[287h] Namely, TADDOL-like phosphonite ligands allowed smooth hydrogenative aldol coupling between aldehydes and vinyl ketones.

Scheme 107. First enantioselective reductive aldol couplings of vinyl ketones.

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In addition to development of transition metal-catalysed processes, alternative generation of boron enolates were explored by several groups. Hoffmann[288] and Trombini[289] exploited mild oxidation of vinyl boronic esters (Scheme 108). Lipshutz[290] used Stryker’s reagent, [291] [(Ph3P)CuH]6, to reductively alkylate enones with aldehydes (Scheme 109). Reaction proceeded via copper hydride-catalysed 1,4-hydroboration of enones to give boron enolates[292] in situ. A similar process was developed with organosilanes but lacked stereospecifity in the aldol step.[293] Further examples of copper[294] and palladium[295] catalysis have been reported by Koskinen and Oshima, respectively.

Scheme 108. Access to boron enolates via mild oxidation of vinyl boronic acids.

Scheme 109. Stryker's reagent-catalysed reductive hydroalkylation of enones via boron and silyl enolates.

The Reformatsky reaction, zinc-promoted hydroxyalkylation of α-haloesters, can be viewed as another way to construct aldol products.[296] Moreover, other metal systems based on indium,[297] titanium[298] and copper or iron[299] (Scheme 110a) have been used for this transformation. For the synthesis of UCS1025A, a telomerase inhibitor, Danishefsky employed as mediator and α-iodoamide 161 as a substrate (Scheme 110b).[300]

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Scheme 110. (a) Indium and (b) triethylborane-mediated Reformatsky-type reactions.

Mukaiyama explored one-pot deiodination of α-iodoketones by organoboranes (Scheme 111).[301] The enolates thus generated gave preferentially syn-aldol products on reacting with aldehydes. Yanagisawa used activated barium for a similar process employing α- chloroketones (Scheme 112).[302] Importantly, in both these examples, the aldehyde component was present in the reaction mixture from the start.

Scheme 111. Reformatsky-type reaction with α-iodoketone and organoborane.[301]

Scheme 112. One-pot Reformatsky-type reaction with α-chloroketones via barium enolates.[302]

Finally, in 1982, Mukaiyama reported a mercury-mediated stoichiometric hydroboroxylation of an activated alkyne by diphenylborinic acid with subsequent trapping with an aldehyde

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[303] (Scheme 113). Use of equimolar amounts of Hg(OAc)2 led to formation of a partially acetylated aldol product 163.

Scheme 113. Mercury-mediated hydration of an activated acetylene and trapping with aldehyde.[303]

Yet another demonstration of the hydration of activated alkynes was recently reported by Gaunt (Scheme 114).[304] Scandium triflate (or possibly, HOTf) catalysed hydration of ynamide 164 by a silanol to give a silyl enol ether 165, which then underwent scandium- catalysed aldol reaction with an aldehyde present in the reaction mixture.

Scheme 114. Scandium triflate-catalysed silanol-mediated stereoselective anti-aldol reaction of ynamides.[304]

3.2 Aims and Objectives

Cationic gold[305] can efficiently catalyse highly regioselective Markovnikov hydration[306] of unactivated alkynes. At present, it is a highly active area of research because gold offers large functional group tolerability and unlike mercury, is largely non-toxic. Acetylenes are an abundant hydrocarbon source,[307] and their use as starting material for active intermediates is of interest to the chemical community. This can be achieved by design of new dual catalytic

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cycles based on gold and another metal or small molecule. Currently, gold co-catalysed processes are limited to only a few Pd/Au and secondary amine/Au examples.[308–309]

It was envisaged that a boronic acid in combination with gold can lead to an efficient formation of a boron enolate, which, if the aldehyde were present, would undergo an aldol reaction (Scheme 115). Ideally, boronic acid would be regenerated in the presence of a small amount of moisture and thus forming a dual catalytic cycle for both gold and boron catalysts.

Scheme 115. Schematic representation of envisaged gold and boronic acid co-catalysed aldol reaction with alkyne as a starting material.

Earlier in the Sheppard group,[310] it was shown that enolate formation could be achieved intermolecularly (Scheme 116). In the presence of catalytic amounts of PPh3AuCl and AgOTf (used to generate a cationic gold species), ortho-(1-hex-1-enyl)phenylboronic acid cyclised to give a stable boron enolate. In a separate step, on mixing the resulting boron enolate with butyraldehyde for 18 h at RT, a mixture of cis and trans-aldol products was isolated (total 53%). A one-pot transformation starting from boronic acid (–40 ºC to RT, 18 h) afforded the desired products in 70% yield.

Scheme 116. Early results in the Sheppard group.

We therefore wished to optimise the conditions and see whether the reaction can effectively be performed as a one-pot procedure. Additionally, the exploration of ways to improve the diastereoselectivity and define the scope and limitations was desirable.

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3.3 Results and Discussion

The required ortho-alkynylboronic acids 167–171 were prepared in a two-step procedure (Scheme 117). First, 2-iodobromobenzene was alkynylated with terminal acetylenes using a standard Sonogashira coupling protocol.[311,312] Then, the corresponding bromoarenes 166 were subjected to a lithiation/borylation sequence.[15] In the case of TMS-protected boronic acid 171, upon borylation the reaction mixture was quenched with sat. NH4Cl instead of 1 M HCl to avoid protodesilylation.

Scheme 117. Synthesis of ortho-alkynylboronic acids. aIsolated yields over two steps.

3.3.1 Gold-Catalysed Boron Enolate Formation

When 167 was subjected to cationic gold catalysis (1 mol% of both AgOTf and Ph3PAuCl or

0.5 mol% Ph3PAuNHTf2, 10 min), only the 6-endo-dig, i.e. anti-Markovnikov product 172a was formed (Scheme 118). This 6-endo cyclisation was a somewhat unexpected outcome because in several analogous cases, e.g. cyclisation of ortho-alkynylbenzoic acids, usually a mixture of 5-exo and 6-endo-dig products was observed (Scheme 119).[313] For ortho- alkynylbenzyl alcohols, Hashmi observed only the rather unstable 6-endo products, and no reaction was detected when the substituent on the alkyne was hydrogen, TMS or alkynyl. Moreover, intramolecular hydroxylation and hydroamination reactions are believed to proceed via an energetically more viable 5-exo-dig cyclisation followed by gold-catalysed oxygen migration to give the 6-endo product.[314]

Scheme 118. Gold-catalysed cyclisation of ortho-(ethynylphenyl)boronic acid (isolated yields: 84% with

1 mol% Ph3PAuCl, 1 mol% AgOTf and 90% with 0.5 mol% Ph3PAuNTf2).

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Scheme 119. Gold-catalysed cyclisation of ortho-alkynylbenzoic acids. [313]

All the other boronic acids 168–170 smoothly cyclised to give the corresponding 6-endo-dig products 172 which were isolated in excellent yield by column chromatography or, when R = Ph, p-An, by decantation (Scheme 120). In the case of the cyclisation of (2- (cyclopropylethynyl)phenyl)boronic acid 168, both 3-cyclopropyl-1H- benzo[c][1,2]oxaborinin-1-ol 172a and its dimer 173b were obtained and characterised as an inseparable mixture.

Scheme 120. Gold-catalysed intramolecular boron enolate formation. aIsolated yields.

[305d] Similarly, another π-acid, commonly used to activate alkynes, PtCl2, also led to the formation of the 6-endo product (on reaction with 169). However, this transformation was considerably slower and took 17 h to complete at ambient temperature.

Interestingly, although we expected 5-exo-dig cyclisation to occur with TMS-protected 2- ethynylphenylboronic acid, clean formation of 6-endo-dig protodesilylated enolate (R = H) was detected (Scheme 121). The mechanism is further discussed in Chapter 4.

Scheme 121. Gold-catalysed one-pot silyl-deprotection/enolate formation.

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The stability of these compounds on silica gel as well as their tolerance of air- and moisture, can perhaps be attributed to aromatic stabilisation, which has been a focus of several theoretical and experimental studies in the past.[315–316]

Based on ab initio and DFT calculations,[316f] Minyaev et al. predicted that hypothetical 1,2- oxaborabenzene 178 (Figure 20), the simplest ring system not subject to additional stabilisation by fused rings, possesses stable aromatic structure. Relative stabilization energy arising from cyclic π-electron delocalization in 178, ΔEarom, was to calculated to be ca. 50–56 kcal/mol and in the range typical for other cyclic systems exhibiting π-electron delocalisation.[317] Later, Ashe III et al. experimentally studied 179, a chromium tricarbonyl complex of a B-phenylated analogue of 1,2-oxaborabenzene.[316d] Although 179 readily participated in the Diels–Alder reaction with dimethyl acetylenedicarboxylate (DMAD) and was protodeboronated with TFA, its crystal structure revealed that there are features attributable to delocalised π-bonding. Firstly, the endocyclic boron-carbon bond (1.48Ǻ) was significantly shorter than the exocyclic one (1.57 Å), and was closely in line with that calculated by DFT values. Secondly, the oxoborabenzene ring was completely planar.

Figure 20. 1,2-Oxaborabenzenes.

Hence, our boron enolates are potentially boroxoaromatic compounds, similar to borazaaromatics[318] and are isosteric with naphthanols (Figure 21). This feature also influenced the reactivity of the intermediates in the subsequent aldol reactions and will be discussed in the next section.

Figure 21. B–O and B–N as isosteres of a carbon–carbon double bond in aromatic systems.

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3.3.2 One-Pot Boron Enolate Formation/Aldol Reaction

At first, it was decided to work on (2-ethynylphenyl)boronic acid 167, as it bears a small substituent on the alkyne and may result in higher diastereoselectivities upon reaction with aldehyde. However, while this substrate did cyclise to form a boron enolate, no aldol reaction was observed in 24 h (Scheme 122). Numerous attempts to activate the system were made with a number of Brønsted and Lewis bases and acids as well as increasing the reaction temperature. However, the enolate proved to be exceptionally stable and remained unreacted. When strong acids/bases were applied the boron compounds underwent protodeboronation. Similarly, the aryl-substituted boronic acid 175 (R = Ph) did not undergo aldol reaction.

Scheme 122. Unreactive cyclic boron enolates.

It was then necessary to re-examine the original system, that of ortho-(1- hexynyl)phenylboronic acid 169 and butyraldehyde, to learn more about controlling this process. Interestingly, while the results provided earlier were generally reproducible, i.e. the desired product was formed, there was certain variation in isolated yields of the aldol products. Additionally, the cyclic boronic monoesters, e.g. 180 in Figure 22, proved to be difficult to isolate, decomposing both on the silica gel column and under vacuum. Hence, to gain control over this system, we needed to understand it in greater detail.

Gratifyingly, the progression of the reaction could easily be tracked by 1H NMR experiments conducted on a 600 MHz spectrometer equipped with Cryoprobe® that ensured good separation of signals, especially in the aromatic region (Figure 22). When the starting material, i.e. boronic acid 169, was fully consumed, signals for the ortho aromatic protons Ha in the enolate 174a and the trans- and cis-aldol products could be easily resolved for all boronic acid–aldehyde combinations. Signals for hydrogens Hb and Hc bonded to the carbons involved in the new carbon–carbon bond formation were also distinguished. The aldol reaction gave the trans-product 180a as the major product. The stereochemical assignment of the trans- and cis-isomers required follow-on derivatisation and is discussed later in this section.

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nPrCHO CHCl3

tr H b cis H en b Hb

tr c H a CH2Cl2 tr H cis c H cis c H a

en Ha

c

1 Figure 22. Typical H NMR (600 MHZ, CDCl3) spectrum of the reaction mixture (note that starting material 169 is fully consumed). Red, blue and green arrows as well as en, tr and cis labels denote 1H NMR signals corresponding to enolate, trans- and cis-aldol products, respectively. pba = phenylboronic acid.

The choice of solvent was dictated by two factors (Table 13). First was the overall conversion, i.e. from boronic acid to the aldol products, and second was the ratio of diastereomers formed. In regard to conversion, no aldol products 180a and 180b were obtained in trimethyl borate or DMSO, although the cyclisation to form enolate 174a did take place. , though providing the best diastereomeric ratio, was least efficient in the overall conversion. DCM and nitromethane were identified as optimal solvents with acceptable diastereomeric ratio. DCM was selected as it is the most widely used solvent in gold-catalysed processes. All reactions were conducted at ambient temperature.

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Table 13. Solvent screen.a

product (180) entry solvent b trans/cisb conv%

1 DCM 73:27 30

2 MeNO2 70:30 36 3 PhMe 63:37 26 4 MeCN 70:30 10 5 Et2O 79:21 7 6 B(OMe) – 0 3 7 DMSO – 0

a Reaction conditions: boronic acid (1 equiv), butyraldehyde (1 equiv), Ph3PAuNTf2 (1 mol%), 0.5 M solvent, RT, 16h. b 1 Diastereomeric ratios and conversions were determined by H NMR (CDCl3).

Over the course of these reactions, it was noted that there were variations in the conversion to the aldol products and subsequently in the isolated yield. These differed depending on both reaction times and the amount of aldehyde employed. Interestingly, in the case of 180, highest isolated yields were obtained when products were isolated after 5 h and 48 h but not within 10–15 h after the start of reaction. We reasoned that one of the contributing factors might be retro-aldol reaction. However, more information on the progression of the reaction was required.

To find optimal conditions with regard to aldehyde amount, reaction time and gold source, time tracking experiments were conducted. Namely, small aliquots of the reaction mixture (ca 1 0.2 mL) were collected at different time intervals, diluted in CDCl3 and characterised by H NMR . As mentioned above, the proton signals for the enolate and the trans- and cis-aldol products were easily distinguishable. For the boronic acid, the proton peak for Hpba a

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(Figure 22), partially overlaid with the one for the enolate, i.e. Hen . However, the amount of a the remaining boronic acid was estimated by subtracting the integral for Hen from the sum of b

en pba integrals for the two overlaying peaks ( H H ). Further numerical data are provided in a + a the Appendix.

At first, the effect of varying the initial amount/concentration of aldehyde was examined with

Ph3PAuNTf2 serving as a cationic gold source (Graph 3). When 1.2 equivalents of aldehyde were used, the peak for the aldol product was observed after one hour. At the same time, it was noted that the cyclisation to give the boron enolate was complete and no boronic acid remained in the mixture. Over time, however, the product appeared to revert to the enolate and did so rather quickly. Later, the aldol product started to build up again. When two equivalents of aldehyde were used, the peak conversion was observed after three to four hours. After that, it seems that the retro-aldol took place and the enolate became the major species. Only later did the system equilibrate back to give the aldol products 180a and 180b as the predominant species. A dynamic behaviour like this is not common in synthetic chemistry and does not follow monotonic/linear kinetics. However, note that the diastereomeric ratio of the aldol products also changed slowly over time. Further discussion will be provided later in this Section.

It was decided to use 2 equiv of aldehyde in view of the fact that aldol products 180a and 180b were rather unstable and required further derivatisation, for instance, by oxidation (see Section 3.3.3). This would allow a time window of approximately 1 h (s) to trap them in situ, Allowing the reaction mixture to stir for three to four hours proved to be essential in getting high conversions and isolated yields of 180a and 180b.

Finding the optimal gold source was considered next (Graph 4). Of three gold sources compared, AuCl was the only noncationic one and did not allow full conversion of starting boronic acid to boron enolate. Meanwhile, the two cationic gold catalysts – Ph3PAuNTf2 and

Ph3PAuCl/AgOTf – converted boronic acid to enolate in less than one hour. Gagozs catalyst,

Ph3PAuNTf2, was preferred to the Au/Ag system for two reasons: firstly, the NMR spectra of the crude reactions were considerably cleaner, suggesting there were fewer potential by- products; secondly, it was rational not to use Au/Ag because silver triflate, an oxophilic Lewis acid, could influence the progression and/or the outcome of the reaction.

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Graph 3. Progression of one-pot enolate formation/aldol reaction depending on an initial amount of aldehyde ( ∑ [boron-based compounds] = 100).a, b

with 1.2 equiv nPrCHO

time, h 169 174a 180a+180b trans cis

1 2 19 78 62 38 3 0 77 23 67 33 3 0 76 24 68 32 5 0 42 58 56 44 23 0 30 70 70 30

with 2.0 equiv nPrCHO

time, h 169 174a 180a+180b trans cis

1 0 34 66 71 29 3 0 14 86 75 25 4 0 13 87 74 26 7 0 50 50 56 44 24 0 48 52 55 45

a Reaction conditions: boronic acid (1 equiv), butyraldehyde (1.2 or 2.0 equiv), Ph3PAuCl (2 mol%), AgOTf (2 mol%), 1 b 1 M DCM, RT. Conversions and diastereomeric ratios were determined by H NMR (CDCl3).

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Graph 4. Progression of one-pot enolate formation/aldol reaction in relation to the gold source.a, b

with AuCl

time, h 169 174a 180a+180b trans cis

1 77 0 23 79 21 2 73 1 26 78 22 3 70 2 28 78 22 4 69 2 30 77 23 7 69 2 29 75 25 20.5 44 7 49 61 39

with Ph3PAuNTf2

time, h 169 174a 180a+180b trans cis

1 0 28 72 74 26 3 0 15 85 75 25 4 0 19 81 73 27 7 0 50 50 62 38 24 0 30 70 67 33

with Ph3PAuCl/AgOTf

time, h 169 174a 180a+180b trans cis

1 0 34 66 71 29 3 0 14 86 75 25 4 0 13 87 74 26 7 0 50 50 56 44 24 0 48 52 55 45

aReaction conditions: boronic acid (1 equiv), butyraldehyde (2.0 equiv), Au salt (2 mol%), 0.5 M DCM, RT. b 1 Conversions and diastereomeric ratios were determined by H NMR (CDCl3).

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In a small number of cases, while trying to optimise the reaction, it was noted that the diastereomeric ratio had reversed. As one of the potential sources leading to this effect could have been moisture, the effect of water on diastereoselectivity was examined (Graph 5). In the presence of water (5 equiv) and AuCl as the gold source, the cis-aldol product 180b was the major product.

Graph 5. Effect of water on diastereomeric ratio of aldol-product in AuCl-catalysed process.a, b

without H2O

time, h 169 174a 180a+180b trans cis

1 77 0 23 79 21 2 73 1 26 78 22 3 70 2 28 78 22 4 69 2 30 77 23 7 69 2 29 75 25 20.5 44 7 49 61 39

with H2O (5 equiv)

time, h 169 174a 180a+180b trans cis

1 84 0 16 62 38 2 42 36 22 47 53 3 36 36 28 36 64 4 36 36 29 36 64 7 36 28 36 31 69 20.5 36 20 44 30 70

aReaction conditions: boronic acid (1 equiv), butyraldehyde (2.0 equiv), water (none or 5.0 equiv), AuCl (2 mol%), 1 M b 1 DCM, RT. Conversions and diastereomeric ratios were determined by H NMR (CDCl3).

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Altogether, a better understanding of the reaction system allowed us to isolate both trans- and cis-cyclic aldol products as separate compounds in 86% combined yield (Scheme 123).

Scheme 123. Preparation of aldol product 181 under optimised conditions.

What remains is the interpretation of the initial data (Graphs 2 and 3), which implied that the optimal reaction conditions require precise timing. At present, there are not enough data points in Graphs 2 and 3 to unequivocally support the fact that these fluctuations in conversion are systematic. However, reactions done on numerous occasions indicated that leaving the mixture to stir for longer than 5 h always resulted in low conversions and/or isolated yield.

Further examination of the data suggested that in reactions employing 2 equiv of aldehyde in the presence of cationic gold source such as Ph3PAuNTf2 and Ph3PAuCl/AgOTf, the absolute amount of cis-aldol remained constant (ca. 20%) throughout ca. 24 h. The increase and subsequent decrease in the amount of enolate 174a came at the expense of trans-aldol product. In the presence of water with AuCl as a catalyst, on the contrary, it was the trans- isomer 180a whose levels remained constant at ca. 10%, while the amount of cis-isomer increased over time. It is most likely that the changes arise from variation in rates of formation/retro-aldol reaction of the trans and cis-aldol products.

Further experiments on the tandem enolate formation/aldol reaction were conducted in an NMR tube with one and two equivalents of butyraldehyde (i.e. in a sealed tube). The spectra were recorded every ten minutes over the first two hours and are presented in Graph 7. Notably, no epimerisation of the aldol products was observed and the product formation followed linear kinetics.

However, these results cannot be directly compared to the reactions conducted on the bench. In the NMR experiments, stirring was not continuous and the reaction vial was capped.

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Graph 7. Progression of one-pot enolate formation/aldol reaction in an NMR tube.a

with 1 equiv nPrCHO

time, min 169 174a 180a+180b trans cis

10 28 46 26 83 17 20 3 57 40 82 18 30 0 55 45 82 18 40 0 53 47 81 19 50 0 53 47 81 19 60 0 53 47 81 19 70 0 52 48 81 19 80 0 53 47 81 19 90 0 52 48 80 20 100 0 52 48 80 20

210 0 50 50 78 22

with 2 equiv nPrCHO

time, min 169 174a 180a+180b trans cis

15 46 38 16 81 19 25 23 48 29 80 20 35 11 50 39 81 19 45 4 47 48 81 19 55 0 45 55 80 20 65 0 43 57 80 20 75 0 42 58 79 21 85 0 42 58 79 21 95 0 41 59 79 21

105 0 41 59 79 21 115 0 41 59 78 22 215 0 37 63 76 24

a Reaction conditions: boronic acid (1 equiv), aldehyde (1 or 2 equiv), Ph3PAuNTf2 (1 mol%), 0.5 M CD2Cl2, RT. b 1 Conversions and diastereomeric ratios were determined by H NMR (CDCl3).

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Hence, the reaction mixture was not exposed to evaporation and/or adventitious moisture. Notably, the conversion to aldol product was only 50–60% (after ca 4 h), which is less than was afforded under standard reaction conditions (>90 conv%, isolated yield 86% ). This indicates that in bench-top reactions external factors such as the presence of Brønsted or Lewis acids and/or moisture may play a crucial role in driving the system initially out of equilibrium allowing higher than expected conversion to the aldol products.

Overall, this particular system behaves as if it were following either overshoot–undershoot kinetics or damped oscillatory kinetics in respect to 180a (Graph 6).[321] In both of these cases, the product concentration would first overshoot the equilibrium concentration and then undershoot it. Only later would it reach the equilibrium.

Graph 6. Types of kinetics: (a) linear (monotonic), (b) overshoot–undershoot, (c) damped oscillatory, (d) sustained ossicilations. [P] = product concentration, t = time.

This dynamic behaviour was observed exclusively for the combination of 169 and butyraldehyde and keeping track of the reaction time was crucial to obtaining good results in the follow-on one-pot derivatisations described in Section 3.3.3.

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The proposed enolate formation/aldol is shown in Scheme 124. For aliphatic systems, the enolate is formed within the first ten minutes and is followed by the aldol step to give a mixture of trans and cis isomers of the cyclic boronic monoester. However, based on this data alone, it is unclear whether aldol reaction proceeds via open or closed transition state.

Scheme 124. Proposed mechanism.

To determine which diastereomer is formed as a major product, a previously reported[319] mixture of compounds 182a and 182b was prepared via an enolate formation/aldol sequence followed by protodeboronation (Scheme 125). This step had to be performed under mild conditions to avoid retro-aldol and potential dehydration. Kuivila et al. showed that copper is –1 – the most effective cation for protodeboronation (ca. 0.2 mol%, pH 6.7, 90 ºC, kcat ~40 M s 1).[320] Our protodeboronation was conducted with copper(II) sulfate in water as sulfate would lead to slightly acidic pH, and in comparison to other copper salts, minimise formation of undesired Chan–Lam coupling products. Heating at 60 ºC was required as no reaction was detected at 40 ºC. Coupling constants for β-ketoalcohols 182a and 182b were found to be in line with those originally reported by Mukaiyama.[319] Retrospectively, as the anti-isomer 182a was the major product after protodeboronation, trans- boronate 181a was the major aldol-product.

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Scheme 125. Synthesis of 4-hydroxy-3-phenylpentan-2-one via copper(II)-catalysed protodeboronation.

The boronic acid used for this determination was prepared from ((2- bromophenyl)ethynyl)trimethylsilane by TMS deprotection, deprotonation/methylation and subsequent lithiation/borylation (Scheme 126).

Scheme 126. Synthesis of (2-(prop-1-yn-1-yl)phenyl)boronic acid.

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3.3.3 Elaboration of Aldol Products: Oxidation, Suzuki, and Chan-Evans-Lam

As mentioned above, the aldol products were not stable and decomposed on storage and under prolonged exposure to vacuum. It was deemed rational to convert them into more stable derivatives. This was not a straightforward process as the material was susceptible to decomposition and retro-aldol reaction.

Initially, it was proposed to trap the intermediate by acetylating the resulting free alcohol 186 (Scheme 127). Although no free alcohol was observed in crude 1H NMR, it was still assumed that a small proportion might be present. Acetic anhydride and acetyl chloride were used as projected acetylating agents, in the presence of DMAP, tBuOK and CsF both at ambient temperatures and at 37 ºC. The combinations of these reagents were added at the start of the reaction or 3 h later. However, only retro-aldol and/or dehydration products were observed.

Scheme 127. Attempted trapping of the aldol product by in situ O-acylation.

Next, the oxidation strategy was explored. Standard conditions for oxidation of boronic acids

(30% H2O2/1 M NaOH (5 equiv each), RT, 1 h) were first employed (Scheme 128). Although after flash chromatography the oxidised product was obtained, the diastereomeric ratio could not readily be determined in the 1H NMR spectrum prior to isolation.

Scheme 128. Oxidation of cyclic boron aldol under basic conditions.

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Milder oxidants such as sodium perborate and trimethylamine N-oxide (3 equiv, 16 h, RT or 37 ºC), did not cleave the carbon–boron bond. However, in the absence of base, treatment with 30% H2O2, (5 equiv) at ambient temperature for 8 h led to full conversion of aldol product to phenol with unaltered diastereomeric ratio (Scheme 129).

Scheme 129. Telescoping cyclic aldol products to the corresponding phenols using H2O2. *no change in diastereomeric ratio after the derivatisation step.

Having mastered the oxidation, it was thought to be useful to provide an example of a more strategic derivatisation; for example, a Suzuki–Miyaura coupling.[4] Usually this transformation requires a strong base, which in our case would have destroyed the aldol product and/or altered the diastereomeric ratio. With 1 equivalent of p-tolyliodide,

Pd(PPh3)2Cl2 was used as a palladium source, while base, solvent system and reaction temperature were varied (Table 14). Previously reported mild system for Suzuki–Miyaura coupling using sodium borate[322a] as base was tried out first, however, without success. Our attention was then turned to fluorides, namely CsF (entries 2–3), as they were previously reported to activate boronic acids.[322b] Although no coupling was detected at room temperature, heating the same system under reflux in DCM led to clean conversion.

Importantly, in one of these examples, the use of acetaldehyde was demonstrated. This is a good measure of the mildness[323] of these reaction conditions for both the Au-catalysed enolate formation/aldol reaction and subsequent coupling. Acetaldehyde is known to be hard to handle as it easily undergoes acid or base-catalysed self-condensation.

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Table 14. Optimisation of one-pot aldol/Suzuki–Miyaura coupling.a,b

entry base solvent t, ºC comments

c 1 3 equiv Na2B4O7 DO/EtOH 50 nr 5/1 c 2 2 equiv CsF DCM 23 nr d 3 2 equiv CsF DCM 37 78%

a Optimisation of reaction conditions for 189a/189b: (i) boronic acid (1 equiv), aldehyde (2 equiv), Ph3PAuNTf2 (1 mol%), DCM, RT, 3 h. (ii) p-MeC6H4I (1 equiv), Pd(PPh3)2Cl (3 mol%), base (2–3 equiv), solvent, RT–50 ºC, 15 h. bReaction conditions for one-pot aldol/Suzuki–Miyaura coupling: (i) boronic acid (1 equiv), aldehyde (2 equiv),

Ph3PAuNTf2 (1 mol%), DCM, RT, 3 h. (ii) p-MeC6H4I (1 equiv), Pd(PPh3)2Cl (3 mol%), CsF (2 equiv), solvent, 37 ºC, 15 h.c nr = no reaction. d Isolated yield. DO = dioxane.

Finally, it was appealing to explore the possibility of B–O ring contraction (Scheme 130). It was envisaged that such transformation might be mediated or perhaps even catalysed by copper(II) acetate (for a discussion of Chan–Evans–Lam coupling, see Chapter 1.3.2). It would allow an elegant entry into 2,3-disubstituted-2,3-dihydrobenzofurans. Further inspection of literature (mid-2009) implied that this type of core structures has been underexplored in medicinal chemistry due to the limitations of the synthetic methods available to access them. However, an interesting and diverse biological profile has been associated with this scaffold. A selection of natural products isolated to date (Figure 23) have already shown a valuable spectrum of immunosuppressive, antiproliferative, and anti- infective activities such as antimicrobial[325,327,328], antiviral (e.g. anti-HIV activity of (+)- Lithospermic acid[326]) and growth suppression in a human lung adenocarcinoma cell line (A549).[325a]

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Scheme 130. Plausible mechanism for copper-catalysed B-O ring contraction/Chan–Lam–Evans coupling.

Figure 23. Dihydrobenzofuran core-containing natural products and their biological activity.

It was envisioned that Cu(II) will react with the arylboronic acid to give the arylcopper(II) species 191, stabilised by an intramolecular alkoxide (Scheme 128). The initial set up with 5 mol% Cu(OAc)2 was highly likely to fail, because ortho-substituted boronic acids are known

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to give poor conversions under copper catalysis (Scheme 131),[90] and it was also known that aliphatic alcohols are very poor coupling partners.[96] Gratifyingly, the reaction conducted in methanol at 40 ºC was successful from the very first attempt, and the desired product was isolated in 81% yield (192, R1 = cPr, R2 = Me).

Presumably, in the presence of methanol, the cyclic boronic monoester is cleaved, which allowed for copper(II) acetate or methoxide, to exchange ligand with the substrate's free hydroxyl, and that substantially enhances the reactivity. Notably, this process requires only a catalytic amount of copper salt, uses dioxygen as terminal oxidant and does not require any additional base or ligand. However, methanol is essential for this transformation as no reaction took place in DCM, and only partial progression was observed in a 1:1 DCM/MeOH solvent mixture. These products were collected as an inseparable mixture of the cis and trans isomers. However, the cis-product is more stable so that when a mixture of products was treated with TEA and silica gel, only the cis-product was observed. Notably, in 2,3- disubstituted 2,3-dihydrobenzofurans, the dihedral torsion angles are such that the spin–spin coupling constant for the cis-isomer is greater than that for the trans-isomer. This is also in line with the data for several reported natural products containing this core[326,328] and ensure that under our copper-catalysed conditions the substrates do not epimerise.

Scheme 131. Aerobic copper-catalysed B–O ring contraction via Chan–Lam–Evans coupling.

3.3.4 Miscellaneous

Attempts were made to further broaden the scope of reactivity of these enolate systems, and they are discussed below.

Apart from, aldehydes, other potential were briefly explored (Scheme 132). No + – reaction was detected with nitrosobenzene and Eschenmoser’s salt (CH2=N Me2I ).

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Nevertheless, boron enolate 174a reacted with PhCH=NPh, presumably giving rise to 194 (1H NMR). However, on initial attempts the conversion remained below 10% and did not increase on heating to 60 ºC (MeCN) or on addition of 5 mol% Y(OTf)3, which is often used to improve the reactivity of imines in carbon–carbon bond forming reactions.[329]

Scheme 132. Probing other electrophiles.

When BocN=NBoc was used, broad signals were observed in the 1H NMR and a complex inseparable mixture was obtained. It was later proposed that in addition to rotamers (which could explain broadening of the signals in the 1H NMR spectra), the vinyl gold intermediate itself might be trapped by the dialkyl azodicarboxylate reagent giving rise to a complex mixture of products.

It was observed that with methanol, the boron enolates easily form boron enolate esters, so it was interesting to see whether in the presence of allylic alcohol a rearrangement might take place, either by treatment with allyl alcohol prior to cyclisation or by mixing it with the boron enolate itself (Scheme 133). In both cases, only esterification product was observed. Interestingly, not a long time later Blum reported that gold/palladium co-catalysis can address a similar reactivity challenge in ortho-alkynyl allyl benzoates 196 (Scheme 134).[309b]

Scheme 133. Attempted allylation of enolate/hydroxyallylation of alkynylboronic acid.

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Scheme 134. Synthesis of substituted isocoumarin using carbophilic Lewis acidic gold and palladium.

A few experiments to establish conditions for intermolecular enolate formations were conducted but were unsuccessful (Scheme 135). At ambient temperature, alkyne remained mostly unhydrolysed after 24 h and did not incorporate aldehyde when heated in DCM at 37 ºC for the same period of time. Boron compounds screened were 10-hydroxy-10,9- boroxophenanthrene, 2-iodophenylboronic acid, 3,4,5-trifluorophenylboronic and boric acids.

It may be that large presence of boronic acid deactivates the catalyst (Ph3PAuNTf2) and that increased π back donation of boronyl into the vacant orbital on three-coordinated boron.

Scheme 135. Attempted set up for intermolecular enolate formation/aldol reaction. For variations of boronic acids, see the text.

3.4 Summary and Outlook

Cationic gold catalysts can promote highly regioselective hydration of alkynes, which makes this process very valuable for further development into an aldol reaction procedure. Environmentally benign variants of aldol reaction, especially the ones compatible with moisture and air conditions, would be a significant step towards more sustainable formation of carbon–carbon bonds.

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Having spotted this opportunity, an intramolecular system was devised to assess the formation of enolates from a boronic acid and an alkyne. Although initially aiming to simplify the system, additional factors such as boroxoaromaticity came into play and somewhat limited the process to alkynes with alkyl and electron-rich aryl substituents. Nevertheless, the work laid out above, demonstrated the proof-of-concept, namely, that such a multicomponent reaction is feasible. However, the intermolecular version of reaction will face other challenges, such as competing alkyne hydration.

Essentially this strategy builds on gold's ability to activate alkynes towards hydroxylation, while the role of the complementary non-gold co-catalyst is to deliver a water equivalent. Apart from hydration, it also facilitates aldol reaction; and hence, may serve as the most convenient means to control the absolute stereochemistry. This catalyst should be rationally designed to fulfill both of these functions (Scheme 136).

Scheme 136. Hydration/aldol strategy towards one-pot aldol reaction from alkynes. M = metal, L = ligand, X = charged ligand.

It seems reasonable to build such a co-catalyst around a high-valent metal. To control absolute chemistry, it must carry a chiral ligand, bidentate or pincer-like. The complex must have an exposed hydroxyl moiety or coordinated water molecule, to allow participation in Au-catalysed hydration to give a metal enolate directly. To ensure that the process goes via a closed transition state, there needs to be an extra vacant site for aldehyde coordination.

Hence, boron-based catalysts may not be the optimal co-catalysts for this tandem transformation. One of the potential metals is rhodium, as its enolates are rather stable and

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undergo aldol reaction. Additionally, a great number of alternative methods for aldol reactions already exploit rhodium chemistry. Lanthanide-based catalysis, often moisture-tolerant, may serve as yet another option.

Alternatively, hydration can be achieved by one species and then be transmetalated to another for efficient aldol reaction to take place as shown in Scheme 137). However, such step-wise systems are less innovative, as they combine two previously well-described steps.

Scheme 137. Hydration/transmetalation/aldol strategy towards one-pot aldol reaction from alkynes. M = metal, L = ligand, S = solvent., X = charged ligand.

As for the ortho-alkynylboronic acids, it was shown that they can serve as precursors to a variety of scaffolds (Scheme 138). In a simple manipulation, but with accurately chosen conditions, they take place without detectable racemisations. Furthermore, we have gained access to 2,3-disubstitued-2,3-dihydrobenzofurans via an aerobic copper-catalysed ring contraction in a one-pot process. To the best of our knowledge, this constitutes the only example of Chan–Lam coupling of aliphatic alcohols using catalytic Cu. Additionally, preliminary experiments suggest that aza-analogues (2,3-dihydroindoles) can also be accessed.

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Scheme 138. Transformation of ortho-alkynylboronic acids.

Taking into consideration the number of natural products with intriguing biological activities, it might be worth pursuing an asymmetric variant of this enol formation/. Options to control absolute stereochemistry in case of ortho-alkynylboronic acids (presumingly directing aldol reaction to go via open transition state) include employment of chiral organocatalysts (thioureas),[330] Lewis acids (metal–ligand pairs)[331] and Brønsted acids[332] to activate/coordinate to an aldehyde (Figure 24).

Figure 24. Proposed systems to execute stereochemical control in intramolecular enolate formation/aldol reaction via (a) hydrogen bonding and (b) Lewis acid catalysis.

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Chapter 4

4 Observations on the Role of Cationic Gold and Brønsted Acids in Electrophilic Halogenation

4.1 Results and Discussion

In the course of work on gold-catalysed formation of boron enolates, a trimethylsilyl- protected ortho-ethynylphenylboronic acid 171 was subjected to the cationic gold catalysis with the overall goal to drive the 5-exo-dig cyclisation. However, only formation of the desilylated boron enolate 172 was observed (Scheme 139).

Scheme 139. (a) Gold-catalysed protodesilylation/boron enolate formation. aConversion was determined 1 b by H NMR (CDCl3). Isolated yield.

Scheme 140. Plausible mechanism for tandem desilylation/boron enolate formation.

To verify that this process proceeds via hydroxyl coordination to silicon as shown in Scheme 140, silylation of 2-iodophenol with various trialkylsilylacetylenes was examined (Scheme 141). As expected, silylation was less efficient with the alkynyltrialkylsilanes that carried bulkier alkyl substituents.

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a 1 Scheme 141. Gold-catalysed silylation of phenols. Conversions were determined by H NMR (CDCl3).

A more practically interesting example involving silylation of a secondary alcohol, namely, 2- butanol, was attempted with triethylethynylsilane; however, no product formation was observed (Scheme 142).

Scheme 142. Attempted triethylsilylation of 2-butanol.

Next, the focus was drawn to the opposite process, i.e. deprotection of silyl-protected [333] acetylenes. Most oftenly, K2CO3/MeOH and TBAF/THF systems are used to deblock silylacetylenes, which are widely used as ethyne equivalents. Due to the extremely mild conditions, the gold-catalysed process could be of value in the total syntheses of complex targets, for instance, those involving differentially substituted acetylenes, e.g. in neocarzinostatin[334] or a combination of C– and O–silyl groups.

In the presence of 0.5 mol% Ph3PAuNTf2 and 5 equiv of methanol, TMS cleavage proceeded cleanly and was complete within 2 h at ambient temperature in either DCM or acetonitrile (Scheme 143).[335] No reaction took place when either catalyst or methanol were absent or when AuCl (3 mol%) was used.

Scheme 143. Gold-catalysed TMS-deprotection with methanol. aConversion was determined by 1H

NMR (CDCl3).

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It was possible that desilylation proceeded via gold(I) acetylide 198 as shown in Scheme 144. If it were so, we could trap 198 by N-iodosuccinimide (NIS), which is often used to trap vinyl/aryl gold species in situ.[304d,l] Indeed, in the presence of both methanol and NIS (0.5 equiv each), a mixture of proto- and iododesilylated products was afforded. This result alone could not prove direct iodination of proposed Au(I) acetylide 198 because halogenating reagents can also react with TMS-protected acetylene 197 (Scheme 145). Reaction with NIS was complete within 3 h, N-bromosuccinimide (NBS) was less reactive affording the corresponding alkynyl bromide (33 conv% after 3 h; 66 conv% after 15 h). No reaction was observed with N-chlorosuccinimide (NCS).

Scheme 144. Plausible mechanism for protodesilylation.

Scheme 145. Gold-catalysed halodesilylation of TMS-protected alkynes. aConversions were determined 1 by H NMR (CDCl3).

A plausible mechanism for this transformation would involve aurodesilylation followed by iododeauration (Scheme 146). However, there could be another pathway leading to the desired product (Scheme 147). Namely, catalysed by minute amounts of [336] bis(trifluoromethane)-sulfonamide, HNTf2 (pKa 1.7), generated in situ from the gold catalyst.[337] In this case, protonation of N-halosuccinimide's nitrogen or oxygen would increase the electrophilicity of the halo-compound and catalyse halodesilylation.

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Scheme 146. Plausible mechanism for halodesilylation via gold intermediate.

Scheme 147. Plausible mechanism for Brønsted acid catalysed halodesilylation.

Indeed, triflic acid (HOTf) alone was competent to catalyse halodesilylations (Scheme 148). Interestingly, the reactivity of N-halosuccinimides in comparison of that with the gold catalysed examples, was reversed. Chlorination proceeded the fastest, while complete iododesilylation was achieved only after 16 h. This shift in catalytic activity between gold and an acid indicates that different reaction mechanisms are in place. Finally, there were no halogenations observed in the absence of either gold or Brønsted acid catalyst.

a 1 Scheme 148. Triflic acid catalysed halodesilylation. Conversions were determined by H NMR (CDCl3).

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To explore alternative means to confirm that gold(I) acetylenide can be present in the system, direct halogenation of terminal alkynes was examined (Scheme 149). Phenylacetylene was treated with NIS in the presence of either Gagozs catalyst or triflic acid at ambient temperature in DCM. Importantly, formation of iodoalkyne was observed only when the gold(I) catalyst was used. While for phenylacetylene halogenation with NIS was complete in under an hour, a bromination reaction with NBS stopped at 66% conversion (also with 2 mol% of Gagosz catalyst), and no chlorinated product formation was observed with NCS.

Scheme 149. Gold-catalysed direct halogenations of alkynes. Isolated yield in parentheses. a 1 b c Conversions were determined by H NMR (CDCl3). Isolated yield. Conducted at 37 ºC. I

An alkylacetylene, 1-hexyne, did not react with N-bromosuccinimide at either room temperature or when heated under reflux in DCM (Scheme 147). Despite this, it did react with N-iodosuccinimide when the mixture was heated overnight.

Direct iodination of several other arylacetylenes was examined (Figure 25). While the reactions proceeded readily, the signals in the 1H NMR spectra for these compounds were broad and not always clean. Only one further example, that of 4-methoxyacetylene, was fully characterised. Furthermore, the electron-rich substrates underwent partial decomposition on exposure to light, heat and drying under high vacuum. In the later stages, it was found that longer exposure (e.g. for 4-methoxyacetylene, 3 h instead of 1 h) led to decolouration of the reaction mixture, which subsequently led to better spectral data. It is highly likely that for isolation of alkynyl iodides it is important to wait until cationic gold is reduced to the colloidal gold. This ensures that no Au/acetylene complexes are present, and thus the product is more stable.

Figure 25. Products from the substrates explored further for direct iodination (the identity of the corresponding products was proved by HRMS).

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Mechanistically, there are two possibilities for the conversion of gold(I) acetylide 198 to haloalkynes (Scheme 150). In one case, acetylides are undergoing halodeauration via 199. In the other case, gold may be oxidised by NIS to gold(III) species 200, which subsequently undergo fast reductive elimination to form Ar–C≡C–X. Traditionally, gold(I) is known to be resistant to oxidation,[304] so the first explanation could be intuitively favoured. However, a few reports on oxidation of gold(I) complexes with iodine exist,[338–340] and catalytic cycle involving Au(III) cannot be easily disproved.

Scheme 150. Possible mechanisms for halodeauration.

Importantly, in none of the above-mentioned examples for both halodesilylation and direct halogenation, either homocoupled or hydrated products was observed. This is a strong prerequisite to incorporation of gold-catalysed activation of acetylenes into more sophisticated catalytic cycles. Moreover, it may allow controlled oxidation to Au(III) acetylenides and further modifications through Au(III) chemistry.

From a synthetic prospective, haloalkynes are useful intermediates, and the method above potentially provides a very convenient method to access them. During the past decade, alkynyl halides (R–C≡C–X) have found new applications in various transformations,[341] e.g. indium-mediated alkynylation of carbonyl compounds (X = I),[342] rapid and highly regioselective Huisgen /alkyne cycloaddition (X = I),[343] zirconocene-catalysed cyclobutene formation on reaction with EtMgBr (X = Cl, Br, I),[344] synthesis of alkynylepoxides,[345] copper-mediated[346] and catalysed[347] N-alkynylation (X = Cl, Br, I), copper-catalysed cross-coupling with Grignard reagents (X = Cl, Br),[348] rhodium-catalysed intramolecular [4+2] cycloadditions (X = Cl, Br, I),[349] and formal reverse Sonogashira coupling (X = Cl, Br).[350]

As the development of novel C–H activation approaches is in great demand, the reverse Sonogashira cross-coupling reactions caught our attention, In this field, gold catalysis was reported quite recently by two groups (Scheme 151). Waser et al. demonstrated alkynylation

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of heteroaromatics by a hypervalent iodine-based reagent 201,[351] while Nevado showed that alkynes bearing electron withdrawing groups can be used to alkynylate electron-rich arenes and some heterocycles.[352]

Scheme 151. First examples of gold-catalysed alkynylation by (a) Waser[351a] and (b) Nevado.[352]

Encouraged by these two reports, an experiment to investigate the reactivity of an in situ generated haloalkyne with N-methyl and unprotected indoles were set up (Scheme 152). The rationale was that alkynyl iodode would act as an oxidant and add to the indole, after alkynyl- shift delivering the oxidative coupling product 202. However, these initial experiments were not as successful as hoped.

Scheme 152. Attempted one-pot gold-catalysed alkynylation of indoles with in situ generated haloalkynes.

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Another system that was briefly examined was a combination of Au-catalysed in situ alkynyl halide formation/Pd-catalysed direct C–H alkynylation (Scheme 153). In a simplistic set up, only the first transformation took place smoothly. No reaction took place with 2- phenylpyridine; however, with O-methyl benzaldehyde oxime, a complex mixture of products was obtained. Nevertheless, it is worth exploring this directed alkynylation strategy further in the future (e.g. addition of Ag2CO3 as a base), as both the Waser and Nevado systems have great limitations in scope. Waser’s chemistry is based on a single example of an expensive alkynyl derivative. It is far from ideal synthesis as it already incorporates an equivalent of oxidant, and the reaction protocol is limited to heteroaryl systems. Nevado’s catalytic system also has a narrow substrate scope, namely, only electron-rich arenes. Additionally, both of these methods do not allow the introduction of an alkynyl moiety using a more efficient directing metalation group strategy.

Scheme 153. Attempted one-pot directed alkynylation of arenes.

To explore whether haloalkynes may act as oxidants in a fashion similar or perhaps different to that of N-halosuccinimides, we conducted an experiment with a boronic acid as a substrate (Scheme 154). Uncatalysed halogenation of boronic acids was previously reported by Olah to be proceed somewhat slow,[117] so it was decided to set up a competition experiment with a dropwise addition of NIS to a mixture of phenylacetylene and 4-anisylboronic acid over one hour (Scheme 155).

Scheme 154. Uncatalysed coupling of boronic acids and alkynyl halides.

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a 1 Scheme 155. Iodination competition experiments. Conversion was determined by H NMR (CDCl3).

Interestingly, the only product formed was 4-anisyliodide. Further experiments showed that electron-rich boronic acids underwent rapid halodeboronation at ambient temperature and that these reactions were also catalysed by triflic acid (Scheme 156). Due to the time restriction, reactions at elevated temperatures were not examined.

Scheme 156. Halodeboronation: (a) the competition experiment with acetylene, (b) limitations of triflic a 1 b acid catalysed process at RT. Conversions were determined by H NMR (CDCl3). Isolated yield.

At this stage, it is questionable whether this strategy could be of use in converting generally electron-rich alkylboronic acids/esters to the corresponding halides. Such a transformation is not feasible with transition metal catalysts, because many alkylmetal species, for example, alkylcopper and alkylpalladium, would readily undergo β-hydride elimination.[119] Moreover, the synthetic utility of this conversion is potentially enhanced by Hartwig’s highly efficient direct C–H activation/borylation of alkanes.[41–43]

Mechanistically, the Brønsted acid probably activates the electrophile (NXS), and an electron- rich substituent in ortho- and para-positions activates the boronic acid (Scheme 157). For

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activation of an alkylboronic acid, use of electron-donating groups is not a feasible strategy but these boronic acids are more electron-rich than unsubstituted phenylboronic acids in the first place. Secondly, the rate of the reaction may also be enhanced by controlling the borophilic ligand (L).

Scheme 157. Double activation in halodeboronation of arylboronic acids via electron-rich substituent in the substrate and protonation of NXS reagent.

Another conceptually interesting experiment for hydroalkynylation was set up with a styrene derivative (Scheme 158). The idea was to construct a catalytic cycle, where in situ generated HOTf, protonates styrene to give a benzyl cation, which is then trapped by gold acetylide. However, an electron-rich styrene, 4-methoxystyrene, was used as substrate; and only polymerization of this starting material was observed.

Scheme 158. (a) Attempted hydroalkynylation of alkenes and (b) proposed catalytic cycle.

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4.2 Summary and Outlook

In this section, cationic gold and triflic acid were both shown to accelerate halodesilylation and halodeborylation (Scheme 159). This catalysis may well be of synthetic value. Firstly, halodesilylation of silyl-protected alkynes delivers haloalkynes in a simple transformation avoiding common desilylation/deprotonation/halogenation sequence. Secondly, halodeboronation under substantially mild and metal free conditions may allow a simple conversion of boronic acids to the corresponding halides, although the substrate scope remains largely to be determined.

Scheme 159. (a) Triflic acid an (b) cationic gold-catalysed halogenations.

Importantly, free terminal alkynes were activated in the presence of cationic gold and in the presence of an electrophile, NBS or NIS, and gave the corresponding alkynyl halides (Scheme 159b). Typically, gold catalysts are used to activate acetylenes towards nucleophilic attack, but this reaction is most likely proceeding via intermediate gold acetylide, and is mechanistically distinct. Controlling the distribution of intermediate gold–acetylene π- complex vs. gold acetylide will be the key to exploiting this new mechanistic pathway. From a practical prospective, this transformation yields a wide range of alkynyl halides with no homocoupling products observed. The haloalkynes and/or Au-haloalkyne complexes are not very stable (light and temperature-sensitive), and their subsequent use (e.g. alkynylation, heterocycle formation) will fully demonstrate the practical application of this reaction.

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Chapter 5

5 Experimental

5.1 General

Unless otherwise stated all chemicals were used as supplied. THF was used following purification from a zeolite drying apparatus (Anhydrous Engineering, USA). Sonogashira coupling[311] and lithiation/borylation reactions[15] were carried out in dry glassware under a positive pressure of argon.

Chromatographic separations were performed on silica gel (VWR/BDH Prolabo® (40–63 μm) and Merck Silica gel 60 (40–63 μm). Thin-layer chromatography was performed on Merck TLC Silica gel 60 F254 and visualised by UV (254 nm) and/or 10% PMA ethanolic solution (PMA = phosphomolybdic acid).

Melting points were determined using a Gallenkamp apparatus and are uncorrected. Infrared spectra were recorded on Perkin–Elmer Spectrum 100 FTIR ATR spectrometer and are quoted in cm–1. Optical rotations were measured using Perkin–Elmer 343 polarimeter (sodium –1 2 –1 D-line, 529 nm) and [α]D values are reported in 10 deg cm g , c is concentration (g/100 mL). 1H (13C) NMR spectra were recorded at 300 (75), 400 (100), 500 (125) and 600 (150) MHz on Bruker AMX400, Bruker Avance 500 and Bruker Avance 600 spectrometers, respectively. 11B and 19F NMR spectra were recorded at 192 (160) and 282 MHz on Bruker Avance 600 (Bruker Avance 500) and Bruker 300 spectrometers, respectively. Residual solvent peak was used as an internal standard[353]. Chemical shifts are quoted in ppm using the following abbreviations: s singlet, d doublet, t triplet, q quartet, qn quintet, sx sextet, non nonet, m multiplet, br broad; or a combination thereof. The coupling constants J are measured in Hz. Mass spectra were recorded in the Department of Chemistry, University College London.

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5.2 Procedures for Chapter 2

5.2.1 Synthesis of Boron and Silicone Based Reagents

7-Chloro-1H-benzo[d][1,2,3]triazol-1-ol (125a)

A solution of 2,6-dichloroaniline (5.20 g, 32.1 mmol) in glacial acetic acid (50 mL) was added slowly to a stirred suspension of sodium perborate tetrahydrate (24.6 g, 160 mmol, 5.0 equiv) in glacial acetic acid (100 mL) maintained at 55 °C. The reaction mixture was stirred at 50 °C for 21 h and since starting material has not been consumed, additional sodium perborate (5.24 g, 34.1 mmol, 1.06 equiv) was added. The mixture was cooled to RT and the inorganic salts were removed by filtration. Distilled water (50 mL) was added to the filtrate and the mixture was extracted with ether (4×50 mL), washed with brine (30 mL), dried over

MgSO4, filtered and concentrated. The residue was purified by column chromatography (EtOAc/PE 1:25) to give the product as a colourless solid (1.97 g, 32%).

1,3-Dichloro-2-nitrobenzene (126a):[354] Colourless solid. Mp 70–71°C (PE). Lit Mp 71–72 °C (EtOH).[354] 1 H NMR (CDCl3, 400 MHz) δ 7.48–7.33 (m, 3H, ArH). 13 C NMR (CDCl3, 125 MHz) δ 131.1, 129.0, 126.5, carbon adjacent to nitrogen not observed. IR ν 1527, 1372, 1203, 776. + HRMS for C6H3Cl2NO2 [M] found 278.85178, calc. 278.85251.

2,6-Dichloronitrobenzene (484 mg, 2.52 mmol) and hydrazine monohydrate (1.56 mL, 20 equiv) were heated under reflux in anhydrous ethanol (4 mL) in the presence of sodium acetate trihydrate (17 mg, 5 mol%) for 62 h under argon. After removal of the solvent under reduced pressure, the residue was dissolved in 1 M NaHCO3 (20 mL). the solution was washed with ether (2×10 mL) and acidified with conc HCl to precipitate the product,[234] which was washed with water and dried under reduced pressure to give the product as a colourless solid (264 mg, 62%).

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7-Chloro-1H-benzo[d][1,2,3]triazol-1-ol (125a):[233] [233] Colourless solid. Mp 184°C (H2O). Lit Mp 184–185 °C (EtOH/H2O). 1 H NMR (DMSO-d6, 300 MHz) δ 13.88 (s, 1H, OH), 7.98 (dd, 1H, J = 8.4, 0.4, ArH), 7.62 (dd, 1H, J = 7.4, 0.4, ArH), 7.39 (dd, 1H, J = 8.4, 7.4, ArH). 13C NMR (DMSO, 125 MHz) δ 128.0, 125.4, 124.5, 118.6. IR ν 3089, 2322, 1420, 1360, 1167, 979, 783. + HRMS for C6H4ClN3O [M] found 169.00283, calc. 169.00374.

1-(Methoxymethoxy)-1H-benzo[d][1,2,3]triazole (128)

K2CO3 (608 mg, 4.40 mmol), TBAB (35 mg, 0.11 mmol, 5 mol%) and MOMCl (184 μL, 2.42 mmol, 1.10 equiv) were added to a solution of 1-HOBt (297 mg, 2.20 mmol) in MeCN (3 mL) and the mixture was stirred for 1h. The solvent was removed under reduced pressure and the solid residue was dissolved in water and DCM, and extracted with DCM (3 x 10 mL), washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure to give the product as a colourless solid (319 mg, 81%).

Yellow oil. 1 H NMR (CDCl3, 600 MHz) δ 8.02 (d, 1H, J = 8.4, ArH), 7.58 (d, 1H, J = 8.4, ArH), 7.52 (td,

1H, J = 6.6, 0.9, ArH), 7.39 (td, 1H, J = 6.6, 0.9, ArH), 5.47 (s, 2H, CH2), 3.74 (s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 143.7, 128.4, 128.2, 124.7, 120.4, 108.7, 103.8, 58.2. IR ν 2944, 1445, 1368, 1166, 1081, 921, 882. + HRMS for C8H9N3O2 [M] found 179.06680, calc. 179.06893.

7-Bromo-1H-benzo[d][1,2,3]triazol-1-ol (125b)

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30% H2O2 (8.0 mL, 0.26 mol) was added to a solution of 2,6-dibromoaniline (1.03 g, 4.09 mmol) in TFA (7 mL) and the mixture was stirred for 16 h at rt. It was then transferred into ice-cold water and the precipitate was collected by filtration and dried to afford the nitroso- compound 132 as a colourless solid (1.08 g, quant).

2,6-Dibromonitrosobenzene (132):[245] [245] Colourless solid. Mp 134–135°C (H2O). Lit Mp 132–133°C (hexane). 1 H NMR (CDCl3, 400 MHz) δ 7.78–7.70 (m, 2H, ArH), 7.31–7.24 (m, 1H, ArH). 13 C NMR (CDCl3, 125 MHz) δ 134.5, 134.2, 133.4, 132.5, 119.4, 116.7. IR ν 3068, 1562, 1290, 1199, 846, 777.

MCPBA (77%, 7.92 g, 35.4 mmol) was added to a solution of 2,6-dibromoaniline (2.52 g, 10.0 mmol) in DCM (70 mL) and the mixture was left to stir overnight at 40 °C. The precipitate was filtered off. The solution was extracted with 1 M KOH (4×50 mL) until no MCPBA could be detected by TLC. The combined organic layers were concentrated under reduced pressure and the residue was dissolved in glacial AcOH (50 mL) and a solution of

30% H2O2 (25 mL) in glacial AcOH (25 mL) was added at rt. Then HNO3 (1.6 mL) was added. The mixture was left to stir at 90 °C overnight. Water (50 mL) was added, and the precipitate was collected by filtration to give the product as colourless needles (2.29 g, 81%).

2,6-Dibromonitrobenzene (126b):[245] [245] Colourless solid. Mp 77 °C (H2O). Lit Mp 78–79 °C (hexane). 1 H NMR (CDCl3, 400 MHz) δ 7.62 (d, 2H, J = 8.1, ArH), 7.24 (t, 1H, J = 8.1, ArH). 13 C NMR (CDCl3, 75 MHz) δ 151.6 132.7, 131.8, 113.8. IR ν 3078, 1562, 1527, 1436, 1277, 1201, 846, 770. + HRMS for C6H3Br2NO2 [M] found 190.95533, calc. 190.95354.

2,6-Dibromonitrobenzene (2.97 g, 10.6 mmol), hydrazine monohydrate (1.6 mL, 32 mmol) and NaOAc·3H2O (72 mg, 0.53 mmol) were heated under reflux in dry EtOH (2 mL) for 16 h.

The mixture was dissolved in sat NaHCO3 and washed with Et2O. The aqueous phase was concentrated and dissolved in a minimum amount of water. The precipitate formed on addition of conc HCl was filtered off, washed with water and Et2O. It was then washed off the filter with MeOH into a different flask and concentrated to give the product as a colourless solid (1.57 g, 69 %).

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7-Bromo-1H-benzo[d][1,2,3]triazol-1-ol (125b):

Colourless solid. Mp 164 °C (dec; H2O). 1 H NMR (DMSO-d6, 600 MHz) δ 8.02 (d, 1H, J = 8.4, ArH), 7.76 (d, 1H, J = 7.4, ArH), 7.31 (dd, 1H, J = 8.4, 7.4, ArH). 13 C NMR (DMSO-d6, 150 MHz) δ 144.2, 131.3, 126.1, 125.9, 119.0, 101.5. IR ν 3083, 1564, 1419, 1236, 1166, 960, 847. + HRMS for C6H5BrN3O [M+H] found 213.96164, calc. 213.96160.

Attempted Borylation of 1-(Benzyloxy)-7-bromo-1H-benzo[d][1,2,3]triazole (133)

DEAD (1.7 mL, 10.5 mmol) was added to a slurry of 7-bromo-1-hydroxybenzotriazole (1.49 g, 6.97 mmol), PPh3 (2.75 g, 10.5 mmol) and benzyl alcohol (1.08 mL, 10.5 mmol) in THF (10 mL) at 0 °C and the mixture was left to stir for 10 h at RT. The reaction mixture purified by column chromatography (EtOAc/PE 1:30) to give the product as a colourless solid (1.68 g, 79%).

1-(Benzyloxy)-7-bromo-1H-benzo[d][1,2,3]triazole (133): Colourless solid. Mp 120–121 °C (PE/EtOAc). 1 H NMR (DMSO-d6, 600 MHz) δ 8.11 (d, 1H, J = 8.3, ArH), 7.87 (d, 1H, J = 7.4, ArH), 7.60–7.56 (m, 2H, ArH), 7.46–7.43 (m, 3H, ArH), 7.40 (dd, 1H, J = 8.3, 7.4, ArH), 5.61 (s,

2H, CH2). 13 C NMR (DMSO-d6, 150 MHz) δ 144.0, 132.7, 132.3, 130.4, 129.7, 128.7, 126.5, 125.9, 119.4, 101.1, 83.9. IR ν 3036, 1575, 1455, 1345, 1246, 1097, 904, 841. + HRMS for C13H10BrN3O [M] found 303.00143, calc. 303.00018.

A 1.6 M solution of BuLi in (380 μL, 0.62 mmol, 0.95 equiv) was added dropwise at –78 ºC to a solution of 1-(benzyloxy)-7-bromo-1H-benzo[d][1,2,3]triazole (198 mg, 0.65 i mmol) and B(O Pr)3 (180 μL, 0.78 mmol, 1.2 equiv) in THF (7 mL) and the mixture was left to stir overnight. The reaction mixture was quenched with sat NH4Cl solution and extracted

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with ether (3×10 mL), dried over MgSO4, filtered, concentrated and purified by flash chromatography (EtOAc/PE 1:7) to give the product as a colourless solid (64 mg, 59%)

(1-(Benzyloxy)-1H-benzo[d][1,2,3]triazol-7-yl)(phenyl)methanol (134): Colourless solid. Mp 112–113 °C (PE/EtOAc). 1 H NMR (CDCl3, 500 MHz) δ 7.94 (dd, 1H, J = 6.0, 0.8, ArH), 7.46–7.22 (m, 12 H, ArH), 6.35 (d, 1H, J = 4.6, CHOH), 5.40 (d, 1H, J = 10.0, PhCH), 5.37 (d, 1H, J = 10.0, PhCH), 2.66 (d, 1H, J = 4.6, OH). 13 C NMR (CDCl3, 125 MHz) δ 145.5, 143.3, 134.0, 131.23, 131.17, 130.3, 130.1, 129.5, 128.3, 128.2, 128.1, 126.7, 126.2, 121.1, 84.4, 72.6. IR ν 3308, 3027, 1570, 1451, 1343, 1240, 1090/ + HRMS ([M+H] for C20H17N3O2 found 331.13179, calc. 331.13153

(1-Hydroxy-1H-benzo[d][1,2,3]triazol-7-yl)boronic acid (139)

Dimethylformamide di-tert-butyl diacetate (1.6 mL, 6.69 mmol, 4.0 equiv) was added dropwise over 20 min to a solution of 7-bromo-1-hydroxybenzotrizole (352 mg, 1.67 mmol) in toluene (3 mL) and the mixture was left to stir for 4 h at 80 ºC. The mixture was then concentrated and purified by column chromatography (EtOAc/PE 1:20) to give 7-bromo-1- (tert-butoxy)-1H-benzo[d][1,2,3]triazole 136 as a colourless oil (244 mg, 55%) and 4-bromo- 1-(tert-butyl)-1H-benzo[d][1,2,3]triazole 3-oxide 137 as a colourless solid (200 mg, 45%).

7-Bromo-1-(tert-butoxy)-1H-benzo[d][1,2,3]triazole (136): Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 8.00 (dd, 1H, J = 8.3, 0.5, ArH), 7.67 (dd, 1 H, J = 7.4, 0.5,

ArH), 7.25 (dd, 1H, J = 8.3, 7.4, ArH), 1.56 (s, 9H, CH3).

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13 C NMR (CDCl3, 125 MHz) δ 144.1, 132.2, 127.9, 125.4, 119.5, 101.9, 91.0, 27.1. IR ν 2991, 1604, 1555, 1441, 1381, 1372, 1246, 1185, 943. + HRMS for C10H13BrN3O [M+H] found 270.02494, calc. 270.02420.

4-Bromo-1-(tert-butyl)-1H-benzo[d][1,2,3]triazole 3-oxide (137): Colourless solid. Mp 149–151 °C (PE/EtOAc). 1 H NMR (CDCl3, 600 MHz) δ 7.67 (d, 1H, J = 8.6, ArH), 7.46 (d, 1H, J = 7.3, ArH), 7.21

(dd, 1H, J = 8.6, 7.3, ArH), 1.93 (s, 9H, CH3) 13 C NMR (CDCl3, 125 MHz) δ 140.5, 129.7, 128.5, 125.2, 118.5, 106.4, 67.3, 27.0. IR ν 2982, 1575, 1371, 1248, 1162, 1090, 1040, 939. + HRMS for C10H13BrN3O [M+H] found 270.02451, calc. 270.02420.

i A 1.6 M solution of BuLi in hexanes (328 μL, 0.525 mmol, 1.1 equiv) and B(O Pr)3 (121 μL, 0.525 mmol, 1.1 equiv) were added dropwise to a solution of 7-bromo-O-tert- butyloxybenzotriazole (129.0 mg, 0.478 mmol) in THF (3 mL) at –78 °C and the mixture was left to stir for 16 h. The mixture was quenched with 1 M HCl and extracted with Et2O. The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (EtOAc/PE 1:10 then with EtOAc/PE/MeOH 1:10:0.01) to give the product as a colourless solid (71 mg, 63%).

(1-(tert-Butoxy)-1H-benzo[d][1,2,3]triazol-7-yl)boronic acid (138): Colourless solid. Mp 71–72 °C (PE/EtOAc; dec). 1 H NMR (DMSO-d6, 600 MHz) δ 8.50 (s, 2H, B(OH)2), 8.00 (d, 1H, J = 8.3, ArH), 7.59 (d,

1H, J = 6.7, ArH), 7.38 (dd, 1H, J = 8.3, 6.7, ArH), 1.39 (s, 9H, CH3). 13 C NMR (DMSO-d6, 150 MHz) δ 141.3, 132.4, 131.1, 124.1, 119.7, 89.6, 26.7, carbon adjacent to boron not observed. IR ν 3356, 2984, 1599, 1412, 1373, 1329, 1249, 1163, 1105. + HRMS for C10H14BN3O3 [M] found 235.11309, calc. 235.11227.

0.2 M TFA/DCM solution (300 μL, 0.063 mmol, 0.2 equiv) was added to a solution of (1- (tert-butoxy)-1H-benzo[d][1,2,3]-triazol-7-yl)boronic acid (74 mg, 0.31 mmol) in DCM (2 mL) and left to stir for 10 minutes. The mixture was then concentrated to give the product as a colourless solid (56 mg, quant).

(1-Hydroxy-1H-benzo[d][1,2,3]triazol-7-yl)boronic acid (139): Colourless solid. Mp 121–123 °C (DCM, dec).

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(DMSO-d6, 600 MHz) δ 8.46 (s, 2H, B(OH)2), 8.05 (d, 1H, J = 8.2, ArH), 7.57 (d, 1H, J = 6.5, ArH), 7.38 (dd, 1H, J = 8.2, 6.5, ArH). 13 C NMR (DMSO-d6, 150 MHz) δ 141.2, 132.7, 131.0, 124.3, 119.5, carbon adjacent to boron not observed. IR ν 3350, 3293, 1579, 1432, 1370, 1245, 1161, 1090. + HRMS for C6H7N3O3B [M+H] found 180.05763, calc. 180.05805.

2-Iodophenylboronic acid (47)[132b]

iPrMgCl (2.0 M in THF, 1.75 mL, 3.50 mmol) was added dropwise to a solution of 1,2- diiodobenzene (1.156 g, 3.50 mmol) in THF/Et2O (1:1, 30 mL) at –78 °C. The mixture was i stirred at –78 °C for 2 h and then B(O Pr)3 (2.43 mL, 11.50 mmol) was added. The mixture was stirred for further 2h at –78 °C and then was allowed to warm up to RT while being stirred for 16 h. The mixture was acidified with HCl (10%, 40 mL) and extracted with ether (3 x 30 mL). The combined organic layers were dried over Na2SO4, and purified by column chromatography (PE then EtOAc/PE 1:5) to give product as a colourless solid (309 mg, 36%).

Colourless solid. Mp 121–122 °C (PE/EtOAc). 1 H NMR (DMSO-d6, 400 MHz) δ 8.25 (s, 2H, B(OH)2), 7.74 (dd, 1H, J = 7.5, 0.5, ArH), 7.33 (td, 1H, J = 7.5, 1.0, ArH), 7.22 (dd, 1H, J = 7.5, 1.0, ArH), 7.06 (td, 1H, J = 7.5, 0.5, ArH). 13 C NMR (DMSO-d6, 125 MHz) δ 137.6, 133.1, 130.0, 126.8, 99.2, carbon adjacent to boron not observed.

Attempted synthesis of (2-(mercaptomethyl)phenyl)boronic acid

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TrSH (1.34 g, 4.84 mmol) was added to slurry of 2-bromobenzyl bromide (1.21 g, 4.84 mmol) and K2CO3 (1.34 g, 9.68 mmol, 2.0 equiv) in DMF (7 mL) and left to stir at RT for 17 h. The mixture was then washed with 10% LiCl aqueous solution (3×10 mL) and 1 M

NaHCO3 (2×10 mL), dried over MgSO4, concentrated under reduced pressure to give the product as colourless solid (2.12 g, 98%).

(2-Bromobenzyl)(trityl)sulfane (140): 1 H NMR (CDCl3, 400 MHz) δ 7.54–7.48 (m, 7H, ArH), 7.35–7.29 (m, 6H, ArH), 7.27–7.21 (m, 3H, ArH), 7.16 (td, 1H, J = 7.6, 1.3, ArH), 7.05 (td, 1H, J = 7.6, 1.7, ArH), 7.00 (dd, 1H,

J = 7.6, 1.7, ArH), 3.44 (s, 2H, CH2). 1 H NMR (CDCl3, 125 MHz) δ 144.6, 136.9, 131.4, 129.8, 128.7, 128.0, 127.5, 126.8, 124.6, 67.6, 37.3. IR ν 3062, 1487, 1463, 1440, 1023, 766. + HRMS for C20H17BrS [M–PhH] found 368.02260, calc. 368.02289

A mixture of 2-bromobenzyl bromide (1.68 g, 4.67 mmol) and potassium thioacetate (587 mg, 5.14 mmol) was heated under reflux in dry THF (20 mL) for 4 h under argon. The solution was filtered, and the filtrate was concentrated under reduced pressure. The residue was dissolved in ether and washed with sat NH4Cl (10 mL), dried over MgSO4, filtered and concentrated under reduced pressure to give the product as a light brown viscous oil (1.14 g, quant).

S-2-Bromobenzyl ethanethioate (141): 1 H NMR (CDCl3, 600 MHz) δ 7.56 (dd, 1H, J = 7.7, 1.2, ArH), 7.46 (dd, 1H, J = 7.7, 1.7,

ArH), 7.26 (td, 1H, J = 7.7, 1.2, ArH), 7.13 (td, 1H, J = 7.7, 1.7, ArH), 4.25 (s, 2H, ArCH2),

2.36 (s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 195.1, 137.3, 133.0, 131.3, 129.1, 127.8, 124.6, 34.2, 30.5. IR ν 1687, 1469, 1440, 1353, 1130, 1026. + HRMS for C9H9BrOS [M+H] found 244.96357, calc. 244.96333.

S-2-Bromobenzyl ethanethioate (207 mg, 0.844 mmol) and K2CO3 (117 mg, 0.844 mmol) were stirred in MeOH (3 mL) for 2 h at RT. The mixture then purified by flash chromatography (Et2O/PE 1:10) to give the product as a pale yellow oil (162 mg, 94%).

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(2-Bromophenyl)methanethiol (142):[355] Pale yellow oil. 1 H NMR (CDCl3, 400 MHz) δ 7.56 (dd, 1H, J = 7.7, 1.2, ArH), 7.39 (dd, 1H, J = 7.7, 1.6, ArH), 7.30 (td, 1H, J = 7.7, 1.2, ArH), 7.12 (td, 1H, J = 7.7, 1.6, ArH), 3.85 (d, J = 8.1, 2H,

ArCH2SH), 2.02 (t, 1H, J = 8.1, SH). 13 C NMR (CDCl3, 125 MHz) δ 140.6, 133.2, 130.1, 128.8, 128.0, 123.7, 29.7.

(2-((Acetylthio)methyl)phenyl)boronic acid

2-Tolylboronic acid:[356] [356] Colourless solid. Mp 164–166 °C (PE/EtOAc). Lit Mp 168 °C (H2O). 1 H NMR (CDCl3, 600 MHz) δ 8.22 (d, 1H, J = 7.5, ArH), 7.46 (t, 1H, J = 7.5, ArH), 7.31 (t,

1H, J = 7.5, ArH), 7.25 (d, 1H, J = 7.5, ArH), 2.82 (s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 146.4, 137.4, 132.3, 129.4, 125.3, 23.3.

2-Tolylboronic acid (197 mg, 1.45 mmol), NBS (310 mg, 1.74 mmol, 1.2 equiv), and AIBN

(24 mg, 0.15 mmol) were heated in CCl4 (3 mL) at 80 °C for 2 h. The reaction mixture was then cooled to RT, preabsorbed onto silica gel and purified by flash chromatography (EtOAc/PE 1:5) to give the product as a colourless solid (223 mg, 71%).

(2-(Bromomethyl)phenyl)boronic acid (143):[252] [253] Colourless solid. Mp 138–139 °C (EtOAc/PE). Lit Mp 138 °C (DCM/Et2O). 1 H NMR (CDCl3, 400 MHz) δ 8.39 (dd, 1H, J = 7.4, 1.3, ArH), 7.59 (td, 1H, J = 7.4, 1.3,

ArH), 7.53 (dd, 1H, J = 7.4, 1.3, ArH), 7.49 (td, 1H, J = 7.4, 1.3, ArH), 5.16 (s, 2H, CH2). 13 C NMR (CDCl3, 125 MHz) δ 145.5, 138.04, 133.0, 131.0, 128.3, 33.8, carbon adjacent to boron not observed. IR ν 3215, 1597, 1487, 1444, 1335, 1301, 1017, 805

(2-(Bromomethyl)phenyl)boronic acid (724 mg, 3.37 mmol) and potassium thioacetate (423 mg, 3.71 mmol) were heated under reflux in anhydrous THF (6 mL) for 8 h. The reaction

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mixture was then cooled to RT, preabsorbed onto silica gel and purified by flash chromatography (EtOAc/PE 1:1) to give the product as a colourless solid (568 mg, 80%).

(2-((Acetylthio)methyl)phenyl)boronic acid (144): Colourless solid. Mp 46–47 °C (EtOAc/PE). 1 H NMR (CDCl3, 600 MHz) δ 8.01 (dd, 1H, J = 7.6, 1.2, ArH), 7.55 (td, 1H, J = 7.6, 1.2,

ArH), 7.54 (dd, 1H, J = 7.6, 1.2, ArH), 7.47 (td, 1H, J = 7.6, 1.2, ArH), 4.23 (s, 2H, ArCH2),

2.35 (s, 3H, CH3) 13 C NMR (CDCl3, 150 MHz) δ 194.9, 143.4, 137.0, 133.2, 130.8, 128.1, 34.4, 29.9. carbon adjacent to boron not observed. IR 3220, 1594, 1485, 1446, 1334, 1030. + HRMS for C9H11O3BS [M] found 210.05139, calc. 210.05165.

Phenylsilanetriol[357]

PhSi(OMe)3 (1.25 mL, 6.57 mmol) was added dropwise into a 0.5% acetic acid solution (0.75 mL) at 5 °C. The reaction mixture was stirred for 4 h at 5 °C. A colourless crystalline product precipitated. The reaction mixture was cooled to –18 °C for 30 min and then filtered, washed with cold water, toluene and hexane to give the product as colourless needle-like crystals (253 mg, 26%).

Mp 129–130°C (PE). Lit Mp 130 °C (MeOAc, ).[358] 1 H NMR (DMSO-d6, 300 MHz) δ 7.62–7.54 (m, 2H, ArH), 7.36–7.26 (m, 3H, ArH), 6.36 (s, 3H, 3×OH). 13C NMR (DMSO, 125 MHz) δ 137.4, 134.0, 129.0, 127.2.

Phenylsilanediol[359]

Ph2SiCl2 (870 μL, 4.14 mmol) in Et2O (2 mL) was added dropwise over 10 min into a rapidly stirred biphasic system consisting of brine (10 mL), ether (10 mL) and (NH4)2CO3 (240 mg, 3.04 mmol). When addition was complete, the aqueous layer was extracted with ether (3 x 20 mL) and the combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure to give the product a colourless solid (872.3 mg, 97%).

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[360] Mp 164–165°C (Et2O). Lit Mp 161–165 °C (, PE). 1 H NMR (DMSO-d6, 300 MHz) δ 7.71 (d, 4H, J = 7.8, ArH), 7.50–7.30 (t, 6H, J = 7.8, ArH), 2.93 (br s, 2H, 2×OH). 13 C NMR (DMSO-d6, 125 MHz) δ 137.8, 134.1, 129.3, 127.4.

6H-Dibenzo[c,e][1,2]oxaborinin-6-ol[316a]

A solution of 2-hydroxybiphenyl (1.0046 g, 5.90 mmol) in dry hexane (total 80 mL) was added dropwise to a solution of 1 M BCl3 in hexane (9.0 mL, 9.0 mmol, 1.52 equiv) and hexane (40 mL). After the reaction mixture was stirred at 25 ºC for 10 min, AlCl3 (39 mg, 5 mol%) was added. The reaction mixture was heated to 80 ºC for 15 h, and then cooled down. Ice was added to quench the reaction. Diethyl ether (20 mL) was added, and the mixture was stirred for 15 min. The organic layer was separated, and the aqueous phase was extracted with diethyl ether (2×15 mL). Combined organic layers were dried over MgSO4, filtered and concentrated in vacuum to give the product as a colourless solid (1.15 g, quant).

Colourless solid. Mp 213–214 °C (PE). Lit Mp 214–216 °C (PE).[361] 1 H NMR (CDCl3, 600 MHz) δ 8.19 (d, 1H, J = 8.2, ArH), 8.16 (dd, 1H, J = 8.2, 1.4, ArH), 8.09 (d, 1H, J = 7.4, ArH), 7.74 (td, 1H, J = 7.8, 1.4, ArH), 7.50 (td, 1H, J = 7.4, 1.0, ArH), 7.40 (td, 1H, J = 7.4, 1.4, ArH), 7.29 (dd, 1H, J = 7.8, 1.4, ArH), 7.25 (td, 1H, J = 7.4, 1.4, ArH), 4.72 (br s, 1H, OH). 13 C NMR (CDCl3, 150 MHz) δ 151.12, 140.33, 133.34, 132.6, 129.0, 127.3, 123.6, 123.0, 122.7, 121.6, 119.6, carbon adjacent to boron not observed. IR ν 3449, 1604, 1487, 1392, 1018.

Tris(2,2,2-trifluoroethyl) borate[361, 362]

2,2,2-Trifluroethanol (42.7 mL, 58.6 mmol, 3.0 equiv) was added via syringe pump over 30 min to neat BBr3 (48.9 g, 19.5 mmol, 1.0 equiv) at –78 °C and the mixture was allowed to

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warm up to RT overnight under argon flow. The mixture was heated for 1 h at 70 °C under argon before being distilled (120–123 °C; 760 Torr) to give the product as a colourless liquid (54.0 g, 90%).

Colourless liquid. Lit Bp 77 ºC (200 Torr)[362] 1 H NMR (CDCl3, 600 MHz) δ 4.22 (q, 6H, J = 8.4, CH2CF3). 13 C NMR (CDCl3, 150 MHz) δ 123.3 (q, J = 278), 61.9 (q, J = 36). 19 F NMR (CDCl3, 282 MHz) δ –77.2. 11 B NMR (CDCl3, 160 MHz) δ 15.3. IR ν 2974, 1429, 1390, 1264, 1161, 1079, 964, 907, 840, 731. + HRMS for C6H7O3F9B [M] found 309.03461, calc. 309.03445.

5.2.2 Direct carboxamidation

Amidations were performed on 0.5–3.0 mmol scale.

Representative Procedure: Borate (2 equiv) was added to a solution/suspension of carboxylic acid (1 equiv) and amine (1 equiv) in MeCN (0.5 M) and the mixture was heated at 80 °C. After 15 h, the solvent was removed under reduced pressure. The residue was redissolved in

DCM and washed with NaHCO3 (1 M) and HCl (1 M) aqueous solutions, dried over MgSO4, filtered and concentrated under reduced pressure to give the clean amide product.

N-Benzyl-2-phenylacetamide (146a)[204a]

Colourless solid. Mp 118–120 °C (DCM). Lit Mp 118–119 °C (PE/EtOAc).[204a] 1 H NMR (CDCl3, 600 MHz) δ 7.36–7.32 (m, 2H, ArH), 7.32–7.22 (m, 6H, ArH), 7.19–7.15

(d, 2H, J = 7.1, ArH), 5.79 (br s, 1H, NH), 4.41 (d, 2H, J = 5.8, CH2NH), 3.63 (s, 2H,

CH2CO). 13 C NMR (CDCl3, 150 MHz) δ 171.1, 138.2, 134.9, 129.6, 129.2, 128.8, 127.61, 127.56, 127.55, 43.9, 43.7. 1 H NMR (DMSO-d6, 600 MHz) δ 8.56 (br t, 1H, J = 5.5, NH), 7.33–7.26 (m, 6H, ArH), 7.25–

7.20 (m, 4H, ArH), 4.26 (d, 2H, J = 6.0, CH2NH), 3.47 (s, 2H, CH2CO). 13 C NMR (DMSO-d6, 150 MHz) δ 170.1, 139.5, 136.4, 129.0, 128.3, 128.2, 127.2, 126.8, 126.4, 42.4, 42.2.

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IR ν 3286, 1637, 1551. + HRMS for C15H15NO [M] found 225.11483, calc. 225.11482.

N-Butyl-2-phenylacetamide (146b)[363]

Colourless solid. Mp 49–50 °C (DCM). Lit Mp 49°C.[363] 1 H NMR (CDCl3, 600 MHz) δ 7.34–7.30 (m, 2H, ArH), 7.30–7.21 (m, 3H, ArH), 5.71 (br s,

1H, NH), 3.53 (s, 2H, CH2CO), 3.17 (q, 2H, J = 6.5, NHCH2), 1.38 (qn, 2H, J = 7.3, CH2Et),

1.23 (sx, 2H, J = 7.3, CH2CH3), 0.85 (t, 3H, J = 7.3, CH3). 13 C NMR (CDCl3, 150 MHz) δ 171.2, 135.2, 129.5, 129.1, 127.4, 43.9, 39.5, 31.6, 20.1, 13.8. IR ν 3294, 1642, 1549. + HRMS for C12H17NO [M] found 191.12961, calc. 191.13047.

N-Benzyl-3-methylbutanamide (146c)[364]

Colourless solid. Mp 58–59 °C (DCM). Lit Mp 58–60°C.[364] 1 H NMR (CDCl3, 600 MHz) δ 7.34–7.29 (m, 2H, ArH), 7.28–7.23 (m, 3H, ArH), 5.97 (br s ,

1H, NH), 4.41 (d, 2H, J = 5.7, CH2NH), 2.13 (non, 1H, J = 6.7, CHMe2), 2.06 (d, 2H, J = 7.1,

CH2CO), 0.94 (d, 6H, J = 6.6, CH3). 13 C NMR (CDCl3, 150 MHz) δ 172.5, 138.5, 128.8, 127.9, 127.6, 46.2, 43.6, 26.3, 22.6. IR ν 3289, 1635, 1543. + HRMS for C12H17NO [M] found 191.13126, calc. 191.13047.

N-Butyl-3-methylbutanamide (146d)[365]

Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 5.79 (br s, 1H. CONH), 3.20 (q, 2H, J = 6.7, NHCH2), 2.07

(non, 1H, J = 6.7, CH(CH3)3), 1.99 (d, 2H, J = 6.7, CH2CO), 1.44 (qn, 2H, J = 7.2,

NHCH2CH2), 1.30 (sx, 2H, J = 7.2, CH2CH3), 0.90 (d, 6H, J = 6.7, CH(CH3)2), 0.88 (t, 3H, J

= 7.2, CH2CH3). 13 C NMR (CDCl3, 150 MHz) δ 172.7, 46.3, 39.2, 31.9, 26.3, 22.5, 20.2, 13.9. IR ν 3284, 2957, 2871, 1641, 1550, 1465, 1368. + HRMS for C9H20NO [M+H] found 158.15521, calc. 157.15449.

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N-Benzylpivalamide (146e)[366]

Colourless solid. Mp 80–81 °C (DCM). Lit Mp 80–82°C.[366] 1 H NMR (CDCl3, 600 MHz) δ 7.36–7.30 (m, 2H, ArH), 7.30–7.23 (m, 3H, ArH), 5.92 (br s,

1H, NH), 4.43 (d, 2H, J = 5.6, CH2NH), 1.22 (s, 9H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 178.5, 138.7, 128.8, 127.8, 127.6, 43.7, 38.8, 27.7. IR ν 3293, 1634, 1540. + HRMS for C12H17NO [M] found 191.12954, calc. 191.13047.

N-Benzylbenzamide (146f)[204a]

[204a] Colourless solid. Mp 100–101°C (DCM). Lit Mp 98–100°C (H2O/EtOH). 1 H NMR (CDCl3, 600 MHz) δ 7.90–7.76 (m, 2H, ArH), 7.52–7.46 (m, 1H, ArH), 7.45–7.39 (m, 2H, ArH), 7.37–7.32 (m, 4H, ArH), 7.32–7.26 (m, 1H, ArH), 6.56 (br s, 1H, NH), 4.63 (d,

2H, J = 5.1, CH2CO). 13 C NMR (CDCl3, 150 MHz) δ 167.5, 138.3, 134.5, 131.7, 128.9, 128.7, 128.0, 127.7, 127.1, 44.2. IR ν 3318, 1639, 1540. + HRMS for C14H14NO [M+H] found 212.10846, calc. 212.10754.

N-(2-Hydroxyethyl)-2-phenylacetamide (146g)[198]

Colourless solid. Mp 64–66 °C. Lit Mp 65–66 °C.[198] 1 H NMR (CDCl3, 600 MHz) δ 7.38–7.25 (m, 5H, ArH), 5.98 (br s, 1H, CONH), 3.68 (t, 2H, J

= 5.0, CH2OH), 3.61 (s, 2H, PhCH2), 3.38 (q, 2H, J = 5.0, NHCH2), 2.46 (br s, 1H, OH). 13 C NMR (CDCl3, 150 MHz) δ 172.8, 134.6, 129.6, 129.2, 127.6, 62.5, 43.7, 42.9. IR ν 3397, 3278, 3090, 2931, 1635, 1540, 1495, 1454, 1429, 1345, 1061. + HRMS for C10H14NO2 [M+H] found 180.10266, calc. 180.10245.

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(R)-2-Phenyl-N-(1-phenylethyl)acetamide (146h)[204a]

[204a] Colourless solid. Mp 116–117 °C (DCM). Lit Mp 115-116 °C (H2O/EtOH). 25  D [204a] +3.3 (c 1.0, CHCl3). Lit +3.3 (c 1.0, CHCl3).

1 H NMR (CDCl3, 600 MHz) δ 7.37–7.33 (m, 2H, ArH), 7.32–7.27 (m, 3H, ArH), 7.27–7.21 (m, 3H, ArH), 7.20–7.16 (m, 2H, ArH), 5.66 (br s, 1H, CONH), 5.12 (qn, 1H, J = 7.1,

CHCH3), 3.60(d, 1H, J = 16.3, PhCHH)3.58 (d, 1H, J = 16.3, PhCHH), 1.39 (d, 3H, J = 7.1,

CH3). 13 C NMR (CDCl3, 150 MHz) δ 170.2, 143.1, 134.9, 129.5, 129.2, 128.7, 127.5, 127.4, 126.1, 48.9, 43.9, 21.9. IR ν 3284, 3062, 3030, 2974, 1641, 1543, 1495, 1453. + HRMS for C16H18NO [M+H] found 240.13757, calc. 240.13884.

N-Allylhex-5-enamide (146i)

Pale yellow oil. 1 H NMR (CDCl3, 600 MHz) δ 6.09 (br s, 1H, CONH), 5.83–5.66 (m, 2H, 2×CH=CH2), 5.12

(d, 1H, J = 17.2, NHCH2CH=CHH-trans), 5.07 (d, 1H, J = 10.4, NHCH2CH=CHH-cis), 4.97

(d, 1H, J = 17.2, (CH2)2CH=CHH-trans), 4.92 (d, 1H, J = 10.4, (CH2)2CH=CHH-cis), 3.81 (t,

2H, J = 5.6, NHCH2), 2.16 (t, 2H, J = 7.6, COCH2), 2.04 (q, 2H, J = 7.2, CH2CH=CH2), 1.70

(qn, 2H, J = 7.5, CH2CH2CH2). 13 C NMR (CDCl3, 150 MHz) δ 173.0, 138.0, 134.4, 116.3, 115.4, 41.9, 35.9, 33.3, 24.9. IR ν 3289, 3077, 2927, 1640, 1543, 911. + HRMS for C9H14NO [M–H] found 152.10643, calc. 152.10699.

N-Cyclopropylbut-3-enamide (146j)

Yellow oil. 1 H NMR (CDCl3, 600 MHz) δ 6.53 (br s, 1H, CONH), 5.82 (ddt, 1H, J = 17.1, 10.7, 7.1,

CH=CH2), 5.10–5.05 (m, 2H, CH=CH2), 2.88 (d, 2H, J = 7.1, CH2CO), 2.59 (tq, J = 7.3, 3.7, 1H, CHN), 0.66–0.62 (m, 2H, cPr–H), 0.40 (m, 2H, cPr–H). 13 C NMR (CDCl3, 150 MHz) δ 172.5, 131.6, 119.2, 41.4, 22.7, 6.4.

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IR ν 3275, 3081, 1647, 1537, 913. + HRMS for C7H11NO [M] found 125.08291, calc. 125.08352.

N-(2-(1H-Indol-3-yl)ethyl)-4-phenylbutanamide (146k)

[367] Pale yellow solid. Mp 110–111 °C (PE/Et2O). Lit Mp 112–113 °C (MeOH) 1 H NMR (CDCl3, 600 MHz) δ 8.59 (br s, 1H, indole-NH), 7.60 (d, 1H, J = 7.9, indole-CH), 7.37 (d, 1H, J = 7.9, indole-CH), 7.29–7.24 (m, 2H, ArH), 7.24–7.17 (m, 2H, indole-CH), 7.15–7.10 (m, 3H, ArH), 6.97 (s, 1H, indole-CH), 5.69 (br s, 1H, CONH), 3.59 (q, 2H, J =

6.6, NHCH2), 2.98 (t, 2H, J = 6.6, NHCH2CH2), 2.60 (t, 2H, J = 7.6, PhCH2), 2.11 (t, 2H, J =

7.6, CH2CO), 1.94 (qn, 2H, J = 7.6, CH2CH2CH2). 13 C NMR (CDCl3, 150 MHz) δ 173.2. 141.6, 136.5, 128.6, 128.5, 127.5, 126.1, 122.4, 122.2, 119.5, 118.8, 112.8. 111.6, 40.0, 36.1, 35.3, 27.3, 25.4. IR ν 3407, 3284, 2924, 1644, 1526, 1455. + HRMS for C20H22N2ONa [M+Na] found 329.1628, calc. 329.1630.

N-Benzylbut-2-ynamide (146l)

Pale yellow solid. Mp 114–115 °C (DCM). 1 H NMR (CDCl3, 600 MHz) δ 7.37–7.31 (m, 2H, ArH), 7.31–7.26 (m, 3H, ArH), 6.02 (br s,

1H, CONH), 4.47 (d, 2H, J = 5.9, CH2Ph), 1.93 (s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 153.4, 137.4, 128.9, 128.0, 127.9, 83.9, 74.8, 43.9, 3.8. IR ν 3266, 3062, 2253, 1631, 1532, 1287. + HRMS for C11H11NO [M] found 173.08273, calc. 173.08352.

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(E)-N-Benzyl-3-(3-nitrophenyl)acrylamide (146m)[368]

Pale yellow solid. Mp 185–186 °C (DCM). Lit Mp 184–185 °C.[368] 1 H NMR (CDCl3, 600 MHz) δ 8.31 (s, 1H, ArH), 8.15 (d, 1H, J = 8.2, ArH), 7.71 (d, 1H, J = 7.7, ArH), 7.66 (d, 1H, J = 15.6, ArCH=CH), 7.52 (t, 1H, J = 8.2, ArH), 7.34–7.24 (m, 4H,

ArH), 6.60 (d, 1H, J = 15.6, ArCH=CH), 6.48 (br s, 1H, NH), 4.57 (d, 2H, J = 5.8, NHCH2). 13 C NMR (CDCl3, 150 MHz) δ 165.1, 148.7, 138.8, 138.0, 136.7, 134.1, 130.0, 128.9, 128.0, 127.8, 124.1, 123.7, 121.8, 44.8. IR ν 3283, 1656, 1619, 1525, 1349, 1221. + HRMS for C16H15NO [M] found 282.09909, calc. 282.09989.

(E)-N-(4-Methoxyphenyl)-3-(4-(trifluoromethyl)phenyl)acrylamide (146n)

Purified by column chromatography (PE/EtOAc 1:2). Colourless solid. Mp 191–192 °C (EtOAc/PE). 1 H NMR (DMSO-d6, 600 MHz) δ 10.2 (s, 1H, CONH), 7.84 (d, 2H, J = 8.5, ArH), 7.80 (d, 2H, J = 8.5, ArH), 7.66–7.60 (m, 3H, PMPH and ArCH=CH), 6.96–6.90 (m, 3H, PMPH and

ArCH=CH), 3.74 (s, 3H, OCH3). 13 C NMR (DMSO-d6, 150 MHz) δ 162.6, 155.5, 138.9, 137.9, 132.3, 129.4 (J = 32.2), 128.3, 125.9 (J = 3.9), 125.3, 124.2 (J = 271.8), 120.7, 114.0, 55.2. IR ν 3294, 1658, 1622, 1537, 1511, 1326, 1125, 1070. + HRMS for C17H15F3NO2 [M+H] found 322.1057, calc. 322.1055.

(R)-Methyl 3-phenyl-2-(2-phenylacetamido)propanoate (146o)[369]

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Colourless solid. Mp 93–94 °C (DCM). Lit Mp 92–94 °C (EtOAc/hexane).[369]

25 25 [369] αD (c 1.0, CHCl3) = – 49.1. Lit. αD (c 1.0, CHCl3) = – 49.5. 1 H NMR (CDCl3, 600 MHz) δ 7.36.–7.27 (m, 3H, ArH), 7.21–7.16 (m, 5H, ArH), 6.90–6.85 (m, 2H, ArH), 5.80 (d, 1H, J = 7.0, CONH), 4.85 (dt, 1H, J = 7.0, 5.8, NHCH), 3.70 (s, 3H,

CH3), 3.56 (d, 1H, J = 15.9, PhCHHCO), 3.53 (d, 1H, J = 15.9, PhCHHCO), 3.06 (dd, 1H, J = 13.8, 5.8, CHHPh), 2.99 (dd, 1H, J = 13.8, 5.8, CHHPh). 13 C NMR (CDCl3, 150 MHz) δ 171.9, 170.6, 135.6, 134.5, 129.5 129.3, 129.1, 128.7, 127.5, 127.2, 53.1, 52.46, 43.8, 37.7. IR ν 3287, 3063, 3029, 2951, 1744, 1651, 1537, 1496, 1217. + HRMS for C18H20NO3 [M+H] found 298.14468, calc. 298.14431. tert-Butyl (1-(benzylamino)-1-oxopropan-2-yl) (146p)[370]

Colourless solid. Mp 100–102 °C (DCM). Lit Mp 104–106 °C (EtOAc/hexane).[370]

22 22 For B(OCH2CF3)3, αD (c 1.9, CHCl3) = – 22.1. For B(OMe)3, αD (c 1.9, CHCl3) = – 24.1. 22 [363] Lit. αD (c 1.9, CHCl3) = – 24.5. HPLC chromatograms can be found in appendix. 1 H NMR (CDCl3, 600 MHz) δ 7.33–7.28 (m, 2H, ArH), 7.28–7.22 (m, 3H, ArH), 6.63 (br s,

1H, CONHBn), 5.07 (m, 1H, CHCH3), 4.43 (br s, 2H, CH2Ph), 4.20 (br s, 1H, BocNH), 1.40

(s, 9H, C(CH3)3), 1.37 (d, 3H, J = 6.8, CHCH3). 13 C NMR (CDCl3, 150 MHz) δ 172.7, 155.7, 138.1, 128.8, 127.7, 127.6, 80.3, 50.3, 43.5, 28.4, 18.3. IR ν 3304, 1695, 1591, 1497, 1365, 1162. + HRMS for C15H22N2O3Na [M+Na] found 301.15319, calc. 301.15280.

(E)-1-(Pyrrolidin-1-yl)-3-(thiophen-3-yl)prop-2-en-1-one (146q)

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Colourless solid. Mp 94–95 °C (DCM). 1 H NMR (CDCl3, 600 MHz) δ 7.68 (d, 1H, J = 15.5, ArCH=CH), 7.45 (d, 1H, J = 1.8, ArH),

7.33–7.27 (m, 2H, ArH), 6.56 (d, 1H, J = 15.5, ArCH=CH), 3.60 (t, 2H, J = 6.8, NCH2), 3.58

(t, 2H, J = 6.8, N CH2), 1.99 (qn, 2H, J = 6.8, NCH2CH2), 1.89 (qn, 2H, J = 6.8, NCH2CH2). 13 C NMR (CDCl3, 150 MHz) δ 165.1, 138.4, 135.5, 127.2, 126.7, 125.2, 118.5, 46.7, 46.2, 26.3, 24.5. IR ν 2953, 2924, 2872, 1647, 1598, 1436, 1404, 783. + HRMS for C11H14NSO [M] found 208.0805, calc. 208.0796.

5.2.3 Transamidations of Primary Amides

Transamidations were performed on 0.5–3.0 mmol scale.

Representative Procedure: Borate (2 equiv) was added to a solution/suspension of amide (1 equiv) and amine (1 equiv) in MeCN (0.5 M) and the mixture was heated at 100 °C in carousel tube. After 15 h, solvent was removed under reduced pressure. The residue was purified by column chromatography (EtOAc/PE 1:1) to give the product.

N-Benzylpropionamide (146s)[371]

Colourless solid. Mp 51 °C (EtOAc/PE). Lit Mp 49–50 ºC (EtOAc/hexane).[371] 1 H NMR (DMSO-d6, 600 MHz) δ 8.28 (br t, 1H, J = 6.0, CONH), 7.34–7.29 (m, 2H, ArH),

7.25–7.21 (m, 3H, ArH), 4.25 (d, 2H, J = 6.0, NHCH2), 2.14 (q, 2H, J = 7.7, CH2CH3), 1.02

(t, 3H, J = 7.7, CH3). 13 C NMR (DMSO-d6, 150 MHz) δ 172.9, 139.8, 128.3, 127.2, 126.7, 42.0, 28.5, 10.0. IR ν 3282, 3066, 3031, 2977, 2938, 1642, 1541, 1454, 1234, 1029. + HRMS for C10H13NO [M] found 163.09931, calc. 163.09917.

N-Butylpropionamide (146t) [372]

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Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 5.55 (br s, 1H, CONH), 3.23 (q, 2H, J = 6.5, NHCH2), 2.18 (q,

2H, J = 7.5, CH2CO), 1.46 (qn, 2H, J = 7.5, NHCH2CH2), 1.32 (sx, 2H, J = 7.5,

CH2CH2CH3), 1.13 (t, 3H, J = 7.5, CH3CH2CO), 0.90 (t, 3H, J = 7.5, NH(CH2)3CH3). 13 C NMR (CDCl3, 150 MHz) δ 174.2, 39.3, 31.7, 29.7, 20.1, 13.8, 10.1. IR ν 3292, 2960, 2933, 1644, 1550, 1464, 1236. + HRMS for C7H15NO [M] found 129.11468, calc. 129.11468.

N-Benzyl-2-hydroxyacetamide (146u)[373]

Colourless solid. Mp 102–103 °C (EtOAc/hexane). Lit Mp 102–103 °C (DCM).[373] 1 H NMR (CDCl3, 600 MHz) δ 7.35–7.30 (m, 2H, ArH), 7.29–7.24 (m, 3H, ArH), 6.99 (br s,

1H, CONH), 4.45 (d, 2H, J = 5.9, NHCH2), 4.09 (s, 2H, HOCH2), OH not observed. 13 C NMR (CDCl3, 150 MHz) δ 172.0, 137.8, 128.9, 127.9, 127.8 , 62.2, 43.1. IR ν 3317, 3208, 3058, 3031, 2933, 2857, 1633, 1562, 1453, 1424, 1342, 1082. + HRMS for C9H11NO2 [M] found 165.07859, calc. 165.07843.

N-((1H-Indol-3-yl)methyl)-2-hydroxyacetamide (146v)

Purified by column chromatography (PE/EtOAc/MeOH 5:5:1). Colourless solid. Mp 141–142 °C (PE/MeOH). 1 H NMR (DMSO-d6, 600 MHz) δ 10.81 (s, 1H, indole-NH), 7.81 (t, 1H, J = 5.8, CONH), 7.56 (d, 1H, J = 7.9, ArH), 7.33 (d, 1H, J = 7.9, ArH), 7.16 (d, 1H, J = 2.2, ArH), 7.06 (td,

1H, J = 7.4, 0.9, ArH), 6.97 (td, 1H, J = 7.4, 0.9, ArH), 5.49 (t, 1H, J = 5.8, CH2OH), 3.79 (d,

2H, J = 5.8, CH2OH), 3.39 (q, 2H, J = 7.1, NHCH2), 2.83 (t, 2H, J = 7.1, ArCH2) 13 C NMR (DMSO-d6, 150 MHz) δ 171.6, 136.3, 127.2, 122.6, 121.0, 118.4, 118.2, 111.7, 111.4, 61.5, 38.8, 25.4. IR ν 3391, 3301, 3260, 1644, 1619, 1543, 1455, 1353, 1223, 1072.

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4.3 Procedures for Chapter 3

4.3.1 Synthesis of ortho-Alkynylphenylboronic Acids

Representative procedure for Sonogashira coupling: Pd(PPh3)2Cl2 (522 mg, 0.74 mmol, 2 mol%) and CuI (142 mg, 0.74 mmol, 2 mol%) were added to a solution of 2- bromoiodobenzene (10.515 g, 37.17 mmol) in Et2NH (120 mL). After 10 min a solution of 1- hexyne (6.4 mL, 55.8 mmol, 1.5 eq) in Et2NH (3.6 mL) was added dropwise over 3 h via syringe pump and the reaction mixture was left to stir for 18 h. Sat. NH4Cl solution (80 mL) was added and the mixture was extracted with PE (3×60 mL), dried over Na2SO4, filtered, concentrated and purified by flash chromatography (PE) to give the product as a colourless oil (8.81 g, quant).

Representative procedure for lithiation/borylation: A 1.6 M solution of BuLi in hexanes (8.0 mL, 12.8 mmol, 1.2 equiv) was added dropwise to a solution of 1-bromo-2-(hex-1- i ynyl)benzene (2.537 g, 10.70 mmol) in THF (60 mL) at –78 °C. After 30 min, B(O Pr)3 (4.9 mL, 21.4 mmol, 2.0 eq) was added and the reaction mixture was left to stir at –78 °C gradually warming up to RT over 16 h. The reaction was quenched with 1 M HCl aq solution

(40 mL) and then extracted with Et2O (3×30 mL), dried over MgSO4, filtered and concentrated. The residue was purified by flash chromatography (first PE, then PE/Et2O 5:1) to give the product as a colourless solid (1.785 g, 83%).

2-((Trimethylsilyl)ethynyl)phenylboronic acid (171)[374–376]

((2-Bromophenyl)ethynyl)trimethylsilane (183):[374–376] Yield 78%. Yellow oil. 1 H NMR (CDCl3, 600 MHz) δ 7.58 (dd, 1 H, J = 7.7, 1.2, ArH), 7.51 (dd, 1H, J = 7.7, 1.7,

ArH), 7.26 (td, 1H, J = 7.7, 1.2, ArH), 7.17 (td, 1H, J = 7.7, 1.7, ArH), 0.29 (s, 9H, Si(CH3)3). 13 C NMR (CDCl3, 125 MHz) δ 133.7, 132.4, 129.6, 126.9, 125.8, 125.3, 103.1, 99.7, –0.1. IR υ 2959, 2163, 1465, 1248. + HRMS for C11H13BrSi [M] found 251.99521, calc. 251.99644.

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2-((Trimethylsilyl)ethynyl)phenylboronic acid (171):

Yield 63%. Colourless solid. Mp 68–69 ºC (PE/Et2O). 1 H NMR (CDCl3, 600 MHz) δ 7.98 (dd, 1H, J = 7.3, 1.5, ArH), 7.51 (dd, 1H, J = 7.3, 1.5, ArH), 7.42 (td, 1h, J = 7.3, 1.5, ArH), 7.38 (td, 1H, J = 7.3, 1.5, ArH), 5.92 (br s, 2H,

B(OH)2), 0.30 (s, 9H, Si(CH3)3). 13 C NMR (CDCl3, 150 MHz) δ 135.6, 132.6, 130.7, 128.6, 126.6, 106.5, 98.9. –0.3, carbon adjacent to boron not observed. IR υ 3497, 3383, 2143, 1335, 1250. + HRMS for C11H15BO2Si [M] found 218.09185, calc. 218.09289.

2-Ethynylphenylboronic acid (167)[377]

The reaction was quenched with 1 M HCl aq solution (50 mL) and allowed to stir for 30 min. Yield 92%. Colourless solid. Mp 95–96 ºC (PE). Lit Mp 93–95 ºC (THF).[377] 1 H NMR (CDCl3, 600 MHz) δ 8.01 (m, 1H, ArH), 7.56 (m, 1H, ArH), 7.44 (m, 1H, ArH),

7.43 (td, 1H, J = 7.4, 1.9, ArH), 6.19 (br s, 2H, B(OH)2), 3.49 (s, 1H, C≡CH). 13 C NMR (CDCl3, 100 MHz) δ 135.7, 133.2, 130.8, 128.9, 125.3, 85.1, 81.3, carbon adjacent to boron not observed. IR υ 3497, 3358, 3267, 1391, 1339. + HRMS for C24H14O3B3 [3M–3H2O] found 383.12072, calc. 383.12220.

2-(Phenylethynyl)phenylboronic acid (170)[378]

1-Bromo-2-(phenylethynyl)benzene:[375–376] Yield 95%. Yellow oil. 1 H NMR (CDCl3, 600 MHz) δ 7.64 (dd, 1H, J = 7.6, 1.1, ArH), 7.62–7.60 (m, 2H, ArH), 7.58 (dd, 1H, J = 7.6, 1.6, ArH), 7.41–7.37 (m, 3H, ArH), 7.31 (td, 1H, J = 7.6, 1.1, ArH), 7.20 (td, 1H, J = 7.6, 1.7, ArH).

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13 C NMR (CDCl3, 150 MHz) δ 133.2, 132.4, 131.7, 129.4, 128.7, 128.4, 127.1, 125.7, 125.4, 122.9, 93.9, 88.0. IR υ 3058, 2220, 1491 + HRMS for C14H9Br [M] found 255.98796, calc. 255.98821.

2-(Phenylethynyl)phenylboronic acid (170):[378] Yield 88%. Yellow solid. Mp 105–106 ºC [378] (PE/Et2O). Lit Mp 160–161 ºC (EtOH). 1 H NMR (CDCl3, 600 MHz) δ 8.04 (dd, 1 H, J = 7.5, 1.2, ArH), 7.61 (dd, 1H, J = 7.5, 1.0, ArH), 7.59–7.56 (m, 2H, ArH), 7.49 (td, 1H, J = 7.5, 1.2, ArH), 7.43 (td, 1H, J = 7.5, 1.0,

ArH), 7.44–7.39 (m, 3H, ArH), 6.04 (br s, 2H, B(OH)2). 13 C NMR (CDCl3, 150 MHz) δ 138.7, 132.6, 131.6, 130.9, 129.1, 128.7, 128.3, 126.7, 121.9, 93.5, 89.8, carbon adjacent to boron not observed. IR υ 3468, 3347, 3055, 2928, 1590, 1333. + HRMS for C14H11O2B [M] found 222.08524, calc. 222.08466.

2-(Hex-1-ynyl)phenylboronic acid (169)

1-Bromo-2-(hex-1-ynyl)benzene:[374] Yield quant. Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 7.58 (dd, 1H, J = 7.7, 1.2, ArH), 7.46 (dd, 1H, J = 7.7, 1.7, ArH), 7.23 (td, 1H, J = 7.7, 1.2, ArH), 7.12 (td, 1H, J = 7.7, 1.7, ArH), 2.51 (t, 2H, J = 7.2,

C≡CCH2), 1.66 (qn, 2H, J = 7.2, CH2Et), 1.57 (sx, 2H, J = 7.2, CH2Me), 1.00 (t, 3H, J = 7.2,

CH3). 13 C NMR (CDCl3, 150 MHz) δ 133.3, 132.3, 128.7, 126.9, 126.1, 125.5, 95.6, 79.5, 30.7, 22.1, 19.3, 13.7. IR υ 2958, 2931, 2235, 1468, 1026. + HRMS for C12H13Br [M] found 236.02063, calc. 236.01951.

2-(Hex-1-ynyl)phenylboronic acid (169): Yield 93%. Colourless solid. Mp 34–35 ºC

(PE/Et2O). 1 H NMR (CDCl3, 600 MHz) δ 7.96 (dd, 1H, J = 7.6, 1.4, ArH), 7.47 (dd, 1H, J =7.6, 1.2, ArH), 7.41 (td, 1H, J = 7.6, 1.4, ArH), 7.36 (td, 1H, J = 7.6, 1.2, ArH), 5.83 (br s, 2H,

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B(OH)2), 2.51 (t, 2H, J = 7.4, CH2C≡C), 1.65 (qn, 2H, J = 7.4, CH2Et), 1.51 (sx, 2H, J = 7.4,

CH2Me), 0.98 (t, 3H, J = 7.4, CH3). 13 C NMR (CDCl3, 150 MHz) δ 135.4, 132.6, 130.8, 127.63, 127.56, 95.1, 81.7, 30.6, 22.1, 19.1, 13.6, carbon adjacent to boron not observed. IR υ 3504, 3392, 2213, 1560, 1333, 1059. + HRMS for C12H15O2B [M] found 202.11549, calc. 202.11596.

2-(Cyclopropylethynyl)phenylboronic acid (168)

Sonogashira coupling gave an inseparable mixture of 1-bromo-2- + (cyclopropylethynyl)benzene (HRMS for C11H9Br [M] found 219.98782, calc. 219.98821) and 1,4-dicyclopropylbuta-1,3-diyne as a yellow oil that was carried forward to the lithiation/borylation step.

Yield over two steps 74%. Colourless solid. Mp 56–57 ºC (Et2O). 1 H NMR (CDCl3, 600 MHz) δ 7.96 (dd, 1 H, J = 7.5, 1.4, ArH), 7.45 (dd, 1H, J = 7.5, 1.3, ArH), 7.40 (td, 1H, J = 7.5, 1.4, ArH), 7.35 (td, 1H, J = 7.5, 1.3, ArH), 5.95 (br s, 2H,

B(OH)2), 1.54 (tt, 1H, J = 8.3, 5.0, CH2CHCH2), 0.97 (ddd, 2H, J = 8.3, 4.2, 2.0, CHH trans CH), 0.88 (ddd, 2H, J = 5.0, 4.2, 2.0, CHH cis to CH). 13 C NMR (CDCl3, 150 MHz) δ 135.4, 132.7, 130.7, 127.6, 127.4, 98.0, 76.64, 8.9, 0.05, carbon adjacent to boron not observed. IR υ 3417, 3092, 3059, 3015, 2221, 1592, 1444, 1335. + HRMS for C11H11BO2 [M] found 186.08512, calc. 186.08512.

2-(Prop-1-ynyl)phenylboronic acid (181)

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(2-Bromophenylethynyl)trimethylsilane (1.621 g, 6.40 mmol) and K2CO3 (0.885 g, 6.40 mmol, 1 eq) were stirred in MeOH (10 mL) for 2 h. The reaction mixture was then filtered, concentrated and purified by flash chromatography (PE) to give the product as a yellow oil (0.841 g, 67%).

1-Bromo-2-ethynylbenzene (184):[379] Yellow oil. 1 H NMR (CDCl3, 600 MHz) δ 7.61 (dd, 1 H, J = 7.6, 1.2, ArH), 7.55 (dd, 1H, J = 7.6, 1.7, ArH), 7.29 (td, 1H, J = 7.6, 1.2, ArH), 7.22 (td, 1H, J = 7.6, 1.7, ArH), 3.40 (s, 1H, C≡CH). 13 C NMR (CDCl3, 150 MHz) δ 134.1, 132.5, 130.0, 127.0, 125.6, 124.3, 81.9, 81.8. IR υ 3291, 1466, 1027. + HRMS for C8H5Br [M] found 179.95617, calc. 179.95691.

A solution of LHMDS in THF (1.0 M, 6.7 mL, 6.7 mmol, 1.5 eq) was added to a solution of 1-bromo-2-ethynylbenzene (0.807 g, 4.46 mmol) in THF (20 mL) at 0 ºC. After 1 h iodomethane (444 µL, 7.13 mmol, 1.6 eq) was added at 0 ºC and the mixture was left to stir for 16 h. The mixture was quenched with sat. NH4Cl aq solution (7 mL) and extracted with

Et2O (3×10 mL). The combined organic fractions were dried over MgSO4, filtered, concentrated and purified by flash chromatography (PE) to give the product as a colourless oil (598 mg, 69%).

1-Bromo-2-(prop-1-ynyl)benzene (185):[374] Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 7.57 (1H, dd, J = 7.7, 1.2, ArH), 7.44 (1H, dd, J = 7.7, 1.7,

ArH), 7.24 (td, 1H, J = 7.7, 1.2, ArH), 7.13 (td, 1H, J = 7.7, 1.7, ArH), 1.00 (s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 133.4, 132.3, 128.7, 126.9, 126.0, 125.3, 91.0, 78.5, 4.6. IR υ 2916, 2232, 1470, 1433. + HRMS for C9H7Br [M] found 193.97225, calc. 193.97256.

2-(Prop-1-ynyl)phenylboronic acid (181): Yield 86%. Colourless solid. Mp 165–166 ºC. 1 H NMR (CDCl3, 600 MHz) δ 7.97 (dd, 1H, J = 7.5, 1.3, ArH), 7.47 (dd, 1H, J = 7.5, 1.2, ArH), 7.42 (td, 1H, J = 7.5, 1.3, ArH), 7.36 (td, 1H, J = 7.5, 1.2, ArH), 5.92 (br s, 2H,

B(OH)2), 2.16 (s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 135.4, 132.6, 130.8, 127.7, 127.5, 90.5, 80.9, 4.4, carbon adjacent to boron not observed. IR υ 3499, 3363, 1442, 1337. + HRMS for C9H7O2B [M] found 160.06829, calc. 160.06901.

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5.3.2 Boron Enolate Formation

Representative procedure: 2-(Cyclopropylethynyl)phenylboronic acid (105.0 mg, 0.564 mmol) and [Ph3PAuNTf]2∙PhMe (4 mg, 2.8 µmol, 0.5 mol%) were mixed in DCM (0.5 mL) at RT for 1 h. The reaction mixture was preabsorbed onto silica gel and purified by flash chromatography (PE/Et2O 7:1) to give the product as a colourless solid (91.5 mg, 87 %).

1H-Benzo[c][1,2]oxaborinin-1-ol (172a)

Yield 90%. Colourless oil. Characterised as a mixture of monomer/dimer 1:0.2 IR υ 3527, 1585, 1488, 1438, 1401, 1348, 1210, 1132, 961. 1 H NMR (CDCl3, 500 MHz, major) δ 7.99 (d, 1H, J = 7.6, ArH), 7.59 (dd, 1H, J = 7.6, 1.4, ArH), 7.41–7.34 (m, 2H, ArH), 7.03 (d, 1H, J = 5.5, ArCH=CH), 6.30 (d, 1H, J = 5.5, CH=CHO), 4.63 (br s, 1H, OH). 13 C NMR (CDCl3, 150 MHz) 156.1, 144.2, 131.3, 131.9, 125.6, 125.0, 104.8, carbon adjacent to boron not observed. 11 B NMR (CDCl3, 192 MHz) δ 26.0 + HRMS for C16H12O3B2 [2M–H2O] found 274.09596, calc. 274.09670.

3-Cyclopropyl-1H-benzo[c][1,2]oxaborinin-1-ol and its dimer (173a+173b)

Yield 87%. Colourless solid. Mp 71–72 ºC (Et2O). Characterised as a mixture of 173a/173b = major/minor 1:0.36). 11 B NMR δ (CDCl3, 192 MHz, 173a+173a) δ 26.1 IR (173a+173b) υ 3377, 3091, 3056, 3013, 1640, 1478, 1350, 1297.

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3-Cyclopropyl-1H-benzo[c][1,2]oxaborinin-1-ol (173a): 1 H NMR (CDCl3, 600 MHz) δ 7.92 (d, 1H, J = 7.6, ArH), 7.55 (td, 1H, J = 7.6, 1.4, ArH), 7.32–7.28 (m, 2H, ArH), 6.20 (s, 1H, ArCH=C), 4.49 (br s, 1H, OH), 1.79 (tt, 1H, J = 8.3, c c 5.0, CH2CHCH2), 0.98–0.94 (m, 2H, Pr-H), 0.85–0.80 (m, 2H, Pr-H). 13 C NMR (CDCl3, 150 MHz) δ 154.7, 143.3, 132.5, 132.4, 125.0, 124.48, 104.2, 14.7, 6.0, carbon adjacent to boron not observed. + HRMS for C11H11O2B [M] found 186.08523, calc. 186.08466.

1,1'-Oxybis(3-cyclopropyl-1H-benzo[c][1,2]oxaborinine) (173b) 1 H NMR (CDCl3, 600 MHz) δ 7.97 (d, 1H, J = 7.6, ArH), 7.59 (td, 1H, J = 7.6, 1.3, ArH), 7.36 (d, 1H, J = 7.6, ArH), 7.32–7.28 (m, 1H, ArH), 6.32 (s, 1H, ArH), 1.83 (tt, 1H, J = 8.2, c c 5.1, CH2CHCH2), 0.98–0.94 (m, 2H, Pr-H), 0.85–0.80 (m, 2H, Pr-H). 13 C NMR (CDCl3, 150 MHz) δ 155.4, 144.0, 133.5, 132.6, 124.9, 124.46, 104.6, 14.9, 6.3, carbon adjacent to boron not observed. + HRMS for C22H20O3B2 [M] found 354.15931, calc. 354.15849.

3-Butyl-1H-benzo[c][1,2]oxaborinin-1-ol (174a)

Yield 85%. Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 7.93 (d, 1H, J = 7.5, ArH), 7.31 (td, J = 7.5, 1.4, ArH), 7.33–

7.28 (m, 2H, 2×ArH), 6.08 (s, 1H, ArCH=C), 4.47 (s, 1H, BOH), 2.44 (t, 2H, J = 7.6, CH2Pr),

1.64 (qn, 2H, J = 7.6, CH2CH2Et), 1.38 (sx, 2H, J = 7.6, CH2CH3), 0.94 (t, 3H, J = 7.6,

CH2CH3). 13 C NMR (CDCl3, 150 MHz) δ 155.1, 143.2, 132.4, 132.3, 125.4, 124.9, 105.4, 34.7, 29.3, 22.2, 13.9, carbon adjacent to boron not observed. 11 B NMR (CDCl3, 192 MHz) δ 25.9 IR υ 3416, 1645, 1480, 1396, 1302. + HRMS for C12H15O2B [M] found 202.11534, calc. 202.11596.

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3-Phenyl-1H-benzo[c][1,2]oxaborinin-1-ol (175a)

2-(Phenylethynyl)phenylboronic acid (142 mg, 0.639 mmol) and [Ph3PAuNTf2]2-PhMe (5.0 mg, 3.2 µmol, 0.5 mol%) were mixed in DCM (0.6 mL) at RT for 1 h. The reaction mixture was concentrated and the crude residue washed with PE to give the product as colourless needles (113.9 mg, 80 %).

3-Phenyl-1H-benzo[c][1,2]oxaborinin-1-ol: Colourless needles. Mp 144–145 ºC (PE). 1 H NMR (DMSO-d6, 600 MHz) δ 9.35 (s, 1H, BOH), 8.04 (d, 1H, J = 7.6, ArH), 7.93 (dd, 1H, J = 7.6, 1.3, ArH) 7.61 (td, 1H, J = 7.6, 1.3, ArH), 7.53 (d, 1H, J = 7.6, ArH), 7.48 (t, 2H, J = 7.6, ArH), 7.39 (td, 1H, J = 7.6, 1.0, ArH), 7.37 (td, 1H, J = 7.6, 1.0, ArH), 7.14 (s, 1H, ArCH=C). 13 C NMR (DMSO-d6, 150 MHz) δ 149.7, 142.6, 134.9, 132.9, 132.2, 128.8, 128.7, 126.1, 126.0, 124.7, 104.3, carbon adjacent to boron not observed. IR υ 3206, 1626, 1475, 1369, 1263, 1003, 767. + HRMS for C14H11O2B [M] found 222.08562, calc. 222.08466.

3-(4-Methoxyphenyl)-1H-benzo[c][1,2]oxaborinin-1-ol (176a)

Yield 82%. Colourless solid. Mp 167–168 °C (DCM). 1 H NMR (DMSO-d6, 600 MHz) δ 9.29 (s, 1H, BOH), 8.01 (d, 1H, J = 7.6, ArH), 7.86 (d, 2H, J = 9.0, ArH), 7.59 (td, 1H, J = 7.6, 1.0, ArH), 7.48 (d, 1H, J =7.6, ArH), 7.32 (td, 1H, J =

7.6, 1.0, ArH), 7.04 (d, 2H, J = 7.6, ArH), 6.99 (s, 1H, ArCH=C), 3.81 (s, 3H, OCH3). 13 C NMR (DMSO-d6, 150 MHz) δ 159.8, 149.9, 142.9, 132.9, 132.1, 127.5, 126.2, 125.8, 125.6, 114.1, 102.6, 55.3, carbon adjacent to boron not observed. IR υ 3391, 2931, 2840, 1625, 1601, 1510, 1450, 1357, 1246, 1174, 1035, 990, 898, 758. + HRMS for C15H13O3B [M ] found 252.09632, calc. 252.09632.

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3-Methyl-1H-benzo[c][1,2]oxaborinin-1-ol (177a)

Yield 92%. 1 H NMR (CDCl3, 600 MHz) δ 7.94 (d, 1H, J = 7.8, ArH), 7.54 (td, 1H, J = 7.8, 1.2, ArH), 7.31 (t, 1H, J = 7.8, ArH), 7.28 (d, 1H, J = 7.8, ArH), 6.09 (s, 1H, ArCH=C), 4.61 (br s, 1H,

BOH), 2.19 (s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 151.5, 143.3, 132.6, 132.4, 125.5, 124.8, 106.2, 21.2, carbon adjacent to boron not observed. 11 B NMR (DMSO-d6, 192 MHz) δ 26.2 IR υ 3141, 3055, 2917, 1651, 1604, 1476, 1346, 1302, 1245, 918. + HRMS for C9H10O2B [M+H] found 161.07590, calc. 161.07684.

5.3.3 One-Pot Boron Enolate Formation/Aldol Reaction

Representative procedure: Butyraldehyde (107 µL, 1.19 mmol, 2 equiv) and

[Ph3PAuNTf2]2∙PhMe (5 mg, 3 µmol, 0.5 mol%, 1 mol% [Au]) were added to a solution of 2- (hex-1-ynyl)phenylboronic acid (120 mg, 0.59 mmol) in DCM (0.5 mL) and the mixture was allowed to stir at RT for 3 h. The mixture was concentrated and purified by flash chromatography (PE/Et2O 9:1 to 4:1) to give trans (112 mg) and cis (28 mg) aldol products (combined 140 mg, 86%).

trans-1-(1-Hydroxy-3-propyl-3,4-dihydro-1H-benzo[c][1,2]oxaborinin-4-yl)pentan-1-one (171a): Colourless viscous oil. 1 H NMR (CDCl3, 600 MHz) δ 7.81 (dd, 1H, J = 7.5, 1.0, ArH), 7.45 (td, 1H, J = 7.5, 1.4, ArH), 7.35 (td, 1H, J = 7.5, 1.0, ArH), 7.16 (d, 1H, J = 7.5, ArH), 4.97 (br s, 1H, BOH), 4.63 (ddd, 1H, J = 8.5, 5.2, 2.2, 1H, CHOB), 3.68 (d, 1H, J = 2.2, ArCH), 2.43 (dt, 1H, J = 17.5, 7.4, COCHH), 2.37 (dt, 1H, J = 17.5, 7.4, COCHH), 1.56–1.52 (m, 1H, CHCHH), 1.50–1.45

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(m, 3H, COCH2CH2 and CHCH2CHH), 1.39-1.32 (m, 2H, CHCHHCHHCH3), 1.25–1.18 (m,

2H, COCH2CH2CH2), 0.89 (t, 3H, J = 7.2 Hz, CH(CH2)2CH3), 0.81 (t, 3H, J = 7.2 Hz,

CO(CH2)3CH3). 13 C NMR (CDCl3, 125 MHz) δ 209.8, 141.2, 133.2, 132.1, 128.6, 127.6, 74.7, 59.3, 41.4, 38.0, 25.6, 22.2, 19.1, 13.9, 13.8, carbon adjacent to boron not observed. IR υ 3387, 3062, 2958, 2932, 2871, 1706, 1605, 1450. + HRMS for C16H23O3B [M] found 274.17436, calc. 274.17348. cis-1-(1-Hydroxy-3-propyl-3,4-dihydro-1H-benzo[c][1,2]oxaborinin-4-yl)pentan-1-one (171b): Colourless viscous oil. 1 H NMR (CDCl3, 600 MHz) δ 7.84 (dd, 1H, J = 7.4, 1.1, ArH), 7.41 (td, 1H, J = 7.4, 1.4, ArH), 7.35 (td, 1H, J = 7.4, 1.1, ArH), 7.15 (d, 1H, J = 7.4, ArH), 4.53 (br s, 1H, BOH), 4.33 (ddd, 1H, J = 8.7, 4.4, 3.4, CHOB), 3.83 (d, 1H, J = 3.4, ArCH), 2.44 (ddd, 1H, J = 17.9, 8.2,

6.4, COCHH), 2.35 (ddd, 1H, J = 17.9, 8.2, 6.4, COCHH), 1.67–1.57 (m, 3H, CHCH2CHH),

1.48–1.35 (m, 3H, COCH2CH2 and CHCH2CHH), 1.22–1.14 (m, 2H, COCH2CH2CH2), 0.95

(t, 3H, J = 7.2, CH(CH2)2CH3), 0.81 (t, 3H, J = 7.2, CO(CH2)3CH3). 13 C NMR (CDCl3, 125 MHz) δ 209.1, 143.6, 133.7, 131.7, 127.6, 127.2, 75.8, 59.5, 42.7, 36.5, 25.3, 22.2, 19.4, 13.9, 13.8, carbon adjacent to boron not observed. IR υ 3422, 3061, 2959, 2933, 2871, 1701, 1603, 1451 + HRMS for C16H23O3B [M] found 274.17372, calc. 274.17348.

5.3.4 Aldol/Oxidation

Representative procedure: Butyraldehyde (134 µL, 1.5 mmol, 2 equiv) and

[Ph3PAuNTf2]2∙PhMe (6 mg, 4 µmol, 0.5 mol%) were added to a solution of 2-(hex-1- ynyl)phenylboronic acid (151 mg, 0.75 mmol) in DCM (0.5 mL) and the mixture was left to stir for 3 h at RT (crude aldol product dr trans/cis 79:21). Then, 30% H2O2 (200 μL, 1.9 mmol) and 100 μL MeOH were added and the mixture was left to stir for 8 h. The reaction mixture was purified by flash chromatography (Et2O/PE 1:7) to give anti (149 mg) and syn aldol (37 mg) products (combined 186 mg, 94%).

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Chapter 5

7-Hydroxy-6-(2-hydroxyphenyl)decan-5-one (187)

Aldol reaction time 3 h. anti-7-Hydroxy-6-(2-hydroxyphenyl)decan-5-one (187a): Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 8.92 (br s, 1H, ArOH), 7.24 (td, 1H, J = 7.8, 1.5, ArH), 7.10 (dd, 1H, J = 7.8, 1.5, ArH), 6.95 (dd, 1H, J = 7.8, 1.2, ArH), 6.92 (td, 1H, J = 7.8, 1.2, ArH), 4.57–4.52 (m, 1H, CHOH), 4.07 (br s, 1H, CHOH), 3.51 (d, 1H, J = 3.6, COCH), 2.32–2.25

(m, 2H, COCH2), 1.50–1.44 (m, 3H, COCH2CH2 and CH(OH)CH2CHH), 1.39–1.29 (m, 2H,

CH(OH)CHHCHHCH3), 1.24–1.16 (m, 3H, COCH2CH2CH2 and CH(OH)CHH), 0.88 (t, 3H,

J = 7.2, CH(OH)(CH2)2CH3), 0.80 (t, 3H, J = 7.4, CO(CH2)3CH3). 13 C NMR (CDCl3, 125 MHz) δ 213.2, 155.4, 133.7, 129.8, 120.6, 118.7, 122.0, 71.3, 62.7, 41.2, 36.0, 25.9, 22.1, 18.9, 13.9, 13.7. IR υ 3302, 2958, 2932, 2873, 1698, 755. + HRMS for C16H24O3 [M] found 264.17194, calc. 264.17200. syn-7-Hydroxy-6-(2-hydroxyphenyl)decan-5-one (187b):

Colourless needles. Mp 50 ºC (PE/Et2O). 1 H NMR (CDCl3, 600 MHz) δ 7.81 (br s, 1H, ArOH), 7.19 (td, 1H, J = 7.8, 1.7, ArH), 7.02 (dd, 1H, J = 7.8, 1.6, ArH), 6.91–6.87 (m, 2H, ArH), 4.36 (td, 1H, J = 7.8, 2.9, CHOH), 3.87

(d, 1H, J = 7.8, COCH), 2.93 (br s, 1H, CHOH), 2.56–2.44 (m, 2H, COCH2), 1.58–1.48 (m,

3H, COCH2CH2 and CH(OH)CH2CHH), 1.30–1.18 (m, 5H, CH(OH)CH2CHH and

CO(CH2)2CH2), 0.82 (t, 6H, J = 7.2, CO(CH2)3CH3 and CH(OH)(CH2)2CH3). 13 C NMR (CDCl3, 125 MHz) δ 215.4, 155.1, 131.3, 129.4, 120.8, 118.0, 122.3, 72.3, 61.9, 43.4, 37.1, 25.7, 22.1, 18.7, 14.0, 13.8. IR υ 3276, 3068, 2958, 2933, 2873, 1698. + HRMS for C16H24O3 [M] found 264.17148, calc. 264.17200.

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1-Cyclopropyl-3-hydroxy-2-(2-hydroxyphenyl)hexan-1-one (188)

Aldol reaction time 1 h. anti-1-Cyclopropyl-3-hydroxy-2-(2-hydroxyphenyl)hexan-1-one (188a): Colourless needles. Mp 72 ºC. 1 H NMR (CDCl3, 600 MHz) δ 8.94 (br s, 1H, ArOH), 7.28 (td, 1H, J = 7.8, 1.6, ArH), 7.16 (dd, 1H, J = 7.8, 1.6, ArH), 6.98 (dd, 1H, J = 7.8, 1.2, ArH), 6.94 (td, 1H, J = 7.8, 1.2, ArH), 4.53–4.50 (m, 1H, CHOH), 4.20 (br s, 1H, CHOH), 3.74 (d, 1H, J = 3.7, ArCH), 1.82 (tt, 1H,

J = 7.3, 4.6, COCH(CH2)2), 1.54–1.46 (m, 1H, CH(OH)CH2CHH), 1.40–1.33 (m, 2H, CH(OH)CHHCHHMe), 1.28–1.23 (m, 1H, CH(OH)CHH), 1.13–1.10 (m, 1H,

COCH(CHHCH2)), 1.05–1.00 (m, 1H, COCH(CH2CHH)), 0.85–0.82 (m, 1H,

COCH(CHHCH2)), 0.79–0.74 (m, 1H, COCH(CH2CHH)), 0.89 (t, 3H, J = 7.2 Hz, CH3). 13 C NMR (CDCl3, 125 MHz) δ 212.5, 155.7, 134.0, 129.9, 120.6, 121.9, 118.7, 71.2, 63.6, 36.0, 20.1, 18.9, 13.9, 12.6, 12.0. IR υ 3304, 3012, 2958, 2926, 2873, 1677, 1456. + HRMS for C15H20O3 [M] found 248.14097, calc. 248.14069. syn-1-Cyclopropyl-3-hydroxy-2-(2-hydroxyphenyl)hexan-1-one (188b): Colourless viscous oil. 1 H NMR (CDCl3, 600 MHz) δ 7.90 (br s, 1H, ArOH), 7.19 (td, 1H, J = 7.8, 1.0, ArH), 7.06 (dd, 1H, J = 7.8, 1.0, ArH), 6.91–6.87 (m, 2H, ArH), 4.35–4.31 (m, 1H, CHOH), 4.11 (d, 1H,

J = 7.4, ArCH), 3.50 (br s, 1H, CHOH), 1.94 (tt, 1H, J = 7.8, 4.5, COCH(CH2)2), 1.56–1.49

(m, 1H, CH2CH3), 1.45–1.37 (m, 1H, CH(OH)CHH), 1.35–1.29 (m, 2H, CHHCHHCH3),

1.16–1.07 (m, 2H, COCH(CHHCHH)Me), 0.94–0.91 (m, 1H, COCH(CHHCH2), 0.85–0.80

(m, 1H, COCH(CH2CHH), 0.85 (t, 3H, J = 7.3 Hz, CH3). 13 C NMR (CDCl3, 125 MHz) δ 214.6, 155.2, 131.2, 129.3, 120.8, 117.5, 122.8, 72.5, 61.4, 36.7, 21.7, 19.6, 13.9, 12.36, 12.33. IR υ 3241, 2976, 2872, 1683, 1455. + HRMS for C15H21O3 [M+H] found 249.15021, calc 249.14907.

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5.3.5 Aldol/Suzuki–Miyaura Coupling

Representative procedure: Acetaldehyde (71 µL, 1.269 mmol, 2 eq) and

[Ph3PAuNTf2]2∙PhMe (5 mg, 3 µmol, 0.5 mol%) were added to a solution of 2- (cyclopropylethynyl)phenylboronic acid (118 mg, 0.634 mmol) in DCM (0.5 mL) and the mixture was allowed to stir at RT for 1 h (crude aldol product dr trans/cis 62:38). p-

Iodotoluene (138 mg, 0.634 mmol, 1 eq), CsF (193 mg, 1.27 mmol, 2 eq) and Pd(PPh3)2Cl2 (13 mg, 19 mmol, 3 mol%) were then added and the mixture was heated at 40 ºC for 10 h (Suzuki coupling product with dr anti/syn 62:38). The reaction mixture was then cooled to

RT, absorbed onto silica gel and purified by flash chromatography (PE/Et2O 10:1) to give the product as a yellow oil (139 mg, 74%).

1-Cyclopropyl-3-hydroxy-2-(4'-methylbiphenyl-2-yl)butan-1-one (190)

1-Cyclopropyl-3-hydroxy-2-(4'-methylbiphenyl-2-yl)butan-1-one: Yellow oil. Characterised as a mixture of diastereomers (190a/190b= anti/syn 59:41). 1 H NMR (CDCl3, 600 MHz, 190a) δ 7.50–7.11 (m, 8H, Ar), 4.39 (app qn, 1H, J = 6.3,

CHOH), 4.18 (d, 1H, J = 6.0, ArCHCO), 2.72 (br s, 1H, OH), 2.45 (s, 3H, ArCH3), 1.72 (tt, c 1H, J = 7.8, 4.5, CH2CHCH2), 1.08–0.94 (m, 2H, Pr–H), 1.00 (d, 3H, J = 6.4, CHCH3), 0.88– 0.72 (m, 2H, cPr–H). 1 H NMR (CDCl3, 600 MHz, 190b) δ 7.50–7.11 (m, 8H, Ar), 4.32 (app qn, 1H, J = 8.8,

CHOH), 4.07 (d, 1H, J = 8.7, ArCHCO), 3.49 (br s, 1H, OH), 2.45 (s, 3H, ArCH3), 1.80 (tt, c 1H, J = 7.8, 4.5, CH2CHCH2), 1.08–0.94 (m, 2H, Pr–H), 0.91 (d, 3H, J = 6.4, CHCH3), 0.88– 0.72 (m, 2H, cPr–H). 13 C NMR (CDCl3, 150 MHz, 190a+190b, overlapping peaks) δ 213.2, 211.7, 144.0, 143.3, 138.5, 138.2, 137.1, 137.0, 134.1, 132.7, 131.0, 129.6, 129.4, 129.14, 129.05, 128.8, 127.9, 127.8, 127.7, 127.3, 127.2, 70.0, 68.0, 62.1, 60.55, 21.22, 21.20, 21.2, 20.5, 19.8, 12.3, 12.0, 11.9, 11.8. IR (190a+190b) υ 3459, 3020, 2972, 2926, 1682, 1481, 1379, 1050. + HRMS (190a+190b) for C20H22O2Na [M+Na] found 317.1510, calc. 317.1517.

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7-Hydroxy-6-(4'-methylbiphenyl-2-yl)decan-5-one (189)

Aldol reaction time 3 h. Yield 78%. 7-Hydroxy-6-(4'-methylbiphenyl-2-yl)decan-5-one: Yellow oil. Characterised as a mixture of diastereomers (major/minor = anti/syn 77:23). 1 H NMR (CDCl3, 600 MHz, 189a) δ 7.50–7.1 (m, 8H, Ar), 4.26 (td, 1H, J = 6.8, 2.7, CHOH),

4.04 (d, 1H, J = 6.6, ArCHCO), 2.45 (s, 3H, ArCH3), 2.30–2.14 (m, 2H, CH2CO), 1.69 (br s,

1H, OH), 1.52–1.30 (m, 8H, 4×CH2), 0.88–0.76 (m, 6H, 2×CH3). 1 H NMR (CDCl3, 600 MHz, 189b) δ 7.50–7.1 (m, 8H, Ar), 4.18 (td, 1H, J = 8.9, 2.8, CHOH),

3.94 (d, 1H, J = 8.8, ArCHCO), 3.03 (br s, 1H, OH), 2.44 (s, 3H, ArCH3), 2.30–2.14 (m, 2H,

CH2CO), 1.52–1.30 (m, 8H, 4×CH2), 0.88–0.76 (m, 6H, 2×CH3). 13 C NMR (CDCl3, 150 MHz, 189a+189b, overlapping peaks) δ 213.1, 212.0, 143.9, 143.2, 138.3, 138.1, 137.1, 137.0, 133.9, 132.7, 131.0, 130.9, 129.6, 129.4, 129.1, 129.0, 128.3, 128.2, 127.9, 127.8, 127.4, 127.21, 127.17, 73.2, 71.8, 60.0, 58.7, 42.4, 42.3, 37.0, 35.7, 25.7, 25.6, 22.1, 21.3, 19.1, 19.0, 14.0, 13.82, 13.79. IR (189a+189b) υ 3503, 2957, 2931, 2871, 1705. + HRMS (189a+189b) for C23H30O2Na [M+Na] found 361.2142, calc. 361.2144.

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5.3.6 Aldol/Intramolecular Chan–Evans–Lam Coupling

Representative procedure: Butyraldehyde (86 µL, 0.97 mmol, 2 eq) and

[Ph3PAuNTf2]2∙PhMe (4 mg, 2 µmol, 0.5 mol%) were added to a solution of 2-(hex-1- ynyl)phenylboronic acid (97 mg, 0.48 mmol) in DCM (0.5 mL) and the mixture was allowed to stir at RT for 3 h (crude aldol product dr trans/cis 79:21). Cu(OAc)2∙H2O (5 mg, 24 µmol, 5 mol%) and MeOH (1 mL) were then added and the mixture was heated at 40 ºC for 10 h (Chan–Lam coupling product dr trans/cis 79:21). The reaction mixture was then cooled to

RT, absorbed onto silica gel and purified by flash chromatography (PE/Et2O 25:1) to give the product as a yellow oil (89 mg, 75%).

1-(2-Propyl-2,3-dihydrobenzofuran-3-yl)pentan-1-one (192)

Aldol reaction time 1h. 1-(2-Propyl-2,3-dihydrobenzofuran-3-yl)pentan-1-one: Yellow oil. Characterised as a mixture of diastereomers (192a/192b= trans/cis 79:21). 1 H NMR (CDCl3, 600 MHz, 192a) δ 7.20–7.15 (m, 2H, ArH), 6.86 (td, 1H, J = 7.5, 1.0, ArH), 6.81 (dd, 1H, J = 7.7, 1.0, ArH), 5.02 (dt, 1H, J = 7.7, 5.6, CHOAr), 3.95 (d, 1H, J = 5.6, ArCHCO), 2.59 (dt, 1H, J = 17.5, 7.4, COCHH), 2.49 (dt, 1H, J = 17.5, 7.4, COCHH),

1.82–1.38 (6H, 3×CH2), 1.27 (sxd, 2H, J = 7.4, 1.8, CH2Me), 0.96 (t, 3H, J = 7.4, CH3), 0.87

(t, 3H, J = 7.4, CH3). 1 H NMR (CDCl3, 600 MHz, 192b) δ 7.20 (t, 1H, J = 7.5, ArH), 7.12 (d, 1H, J = 7.5, ArH), 6.89 (td, 1H, J = 7.5, 1.0, ArH), 6.88–6.84 (m, 1H, ArH), 4.83 (td, 1H, J = 8.7, 4.3, CHOAr), 4.10 (d, 1H, J = 8.6, ArCHCO), 2.29 (ddd, 1H, J = 17.8, 8.8, 6.8, COCHH), 2.23 (ddd, 1H, J

= 17.8, 8.8, 6.8, COCHH), 1.82–1.38 (6H, 3×CH2), 1.21 (sxd, 2H, J = 7.3, 1.5, CH2Me),

0.98–0.95 (m, 3H, CH3), 0.82 (t, 3H, J = 7.3, CH3). 13 C NMR (CDCl3, 150 MHz, 192a) δ 207.8, 159.7, 129.57, 125.5, 124.88, 120.5, 110.3, 84.4, 61.2, 40.3, 38.0, 25.7, 22.4, 18.5, 14.00, 13.98. 13 C NMR (CDCl3, 150 MHz, 192b) δ 209.7, 160.6, 129.60, 126.8, 124.85, 121.1, 110.4, 86.2, 58.9, 42.0, 33.0, 25.5, 22.3, 20.0, 14.00, 13.98. IR (192a +192b) υ 2958, 2932, 2872, 1715, 1675, 1479, 1462, 1236. + HRMS (192a +192b) for C16H22O2 [M] found 246.16143, calc. 246.16143.

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Cyclopropyl(2-methyl-2,3-dihydrobenzofuran-3-yl)methanone (193)

Aldol reaction time 1 h. Yield 81%. Cyclopropyl(2-methyl-2,3-dihydrobenzofuran-3-yl)methanone: Colourless oil. Characterised as a mixture of diastereomers (193a/193b= trans/cis 67:33). 1 H NMR (CD2Cl2, 600 MHz, 193a) δ 7.28 (d, 1H, J = 7.6, ArH), 7.24–7.17 (m, 1H, ArH), 6.94–6.85 (m, 1H, ArH), 6.84 (d, 1H, J = 7.9, ArH), 5.25 (qn, 1H, J = 6.3, CHCH(OAr)Me),

4.09 (d, 1H, J = 6.3, COCHCH), 2.07 (tt, 1H, J = 7.7, 4.5, CH2CHCH2), 1.49 (d, 3H, J = 6.3, c c c CH3), 1.16–1.09 (m, 1H, Pr-H), 1.09–1.02 (m, 1H, Pr-H), 1.02–0.95 (m, 1H, Pr-H), 0.95– 0.90 (m, 1H, cPr-H). 1 H NMR (CD2Cl2, 600 MHz, 193b) δ 7.24–7.17 (m, 2H, 2× ArH), 6.94–6.85 (m, 2H, 2×ArH), 5.11 (dq, 1H, J = 9.1, 6.7, CHCH(OAr)Me), 4.25 (d, 1H, J = 9.1, CHCHMe), 1.78 c (tt, 1H, J = 7.7, 4.4, CH2CHCH2), 1.53 (d, 3H, J = 6.7, CH3), 1.16–1.09 (m, 1H, Pr-H), 1.09– 1.02 (m, 1H, cPr-H), 0.91–0.86 (m, 1H, cPr-H), 0.86–0.79 (m, 1H, cPr-H). 13 C NMR (CDCl3, 150 MHz, 193a) δ 207.5, 160.4, 129.48, 125.8, 125.1, 120.6, 110.2, 80.8, 63.2, 21.3, 19.1, 12.2, 11.7. 13 C NMR (CDCl3, 150 MHz, 193b) δ 209.0, 159.4, 129.46, 126.5, 125.0, 121.1, 110.1, 81.7, 60.0, 20.2, 16.8, 12.8, 12.1. IR (193a+193b) υ 3008, 2979, 2929, 1694, 1595, 1477, 1380, 1232. + HRMS (193a+193b) for C13H14O2 [M] found 202.09792, calc. 202.09883.

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5.3.7 Aldol/Protodeboronation

Representative procedure: Acetaldehyde (69 µL, 1.2 mmol, 2.0 eq) was added to a suspension of 2-(prop-1-ynyl)phenylboronic acid (99 mg, 0.62 mmol) and

[Ph3PAuNTf2]2∙PhMe (5 mg, 3 µmol, 0.5 mol%) in DCM (0.5 mL) and the mixture was allowed to stir at RT for 10 h (crude aldol product dr trans/cis 75:25). H2O (1 mL) and

CuSO4∙5H2O (15 mg, 0.062 mmol, 10 mol%) were added and the mixture was heated at 60 ºC for 16 h (crude protodeborylation product dr anti/syn 75:25). The reaction mixture was extracted with DCM (3×5 mL), dried over MgSO4, concentrated and purified by flash chromatography (PE/Et2O/Et3N 50:50:1) to give the product as a colourless oil (61 mg, 55 %).

4-Hydroxy-3-phenylpentan-2-one (182)[319]

4-Hydroxy-3-phenylpentan-2-one: Colourless oil. Characterised as a mixture of diastereomers (182a/182b= anti/syn 75:25). IR (182a+182b) υ 3412, 2971, 2927, 1705, 1355. + HRMS (182a+182b) for C11H14O2 [M] found 178.09810, calc. 178.09883.

anti–4-hydroxy-3-phenylpentan-2-one (182a): 1 H NMR (CDCl3, 600 MHz) δ 7.38–7.24 (m, 5H, ArH), 4.43 (app qn, 1H, J = 6.0, CHOH),

3.61 (d, 1H, J = 5.4, ArCHCO), 2.66 (br s, 1H, OH), 2.08 (s, 3H, COCH3), 1.09 (d, 3H, J =

6.3, CHCH3). 13 C NMR (CDCl3, 150 MHz) δ 210.0, 134.5, 129.8, 129.1, 128.01, 67.5, 65.6, 30.2, 20.5. syn–4-hydroxy-3-phenylpentan-2-one (182b): 1 H NMR (CDCl3, 600 MHz) δ 7.38–7.24 (m, 3H, ArH), 7.18–7.14 (m, 2H, ArH), 4.36 (dq, 1H, J = 9.2, 6.2, CHOH), 3.56 (d, 1H, J = 9.2, ArCHCO), 3.11 (br s, 1H, OH), 2.05 (s, 3H,

COCH3), 0.98 (d, 3H, J = 6.2, CHCH3). 13 C NMR (CDCl3, 150 MHz) δ 210.3, 136.0, 129.3, 128.8, 127.95, 69.0, 67.9, 29.9, 20.0.

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5.4 Procedures for Chapter 4

Protodesilylation procedure: [Ph3PAuNTf]2∙PhMe (0.5 mol%) was added to a solution of ((2- bromophenyl)ethynyl)trimethylsilane (1 equiv) and MeOH (5 equiv) in DCM (1 M) and was left to stir at RT. After designated time, the mixture was filtered through a silica pad and concentrated to afford the product.

Representative procedure for halogenations: [Ph3PAuNTf]2∙PhMe (0.5 mol%) or TfOH (1 mol%) was added to a solution of trimethylsilylalkyne, a terminal alkyne or a boronic acid (1 equiv) in DCM (1 M) and was left to stir at RT. After designated time and if reaction was complete, the mixture was filtered through a silica pad and concentrated to afford the product.

(2-Iodophenoxy)trimethylsilane: [380] Colourless oil. 1 H NMR (CDCl3, 600 MHz) δ 7.67 (dd, 1H, J = 7.8, 1.5, ArH), 7.25 (td, 1H, J = 7.8, 1.2,

ArH), 7.00 (d, 1H, J = 7.8, ArH), 6.69 (td, 1H, J = 7.8, 1.5, ArH), 0.21 (s, 9H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 155.2, 139.3, 129.3, 123.2, 119.1, 91.3, 0.5.

1-Bromo-2-(iodoethynyl)benzene:[381] Colourless oil 1 H NMR (CDCl3, 600 MHz) δ 7.60 (d, 1H, J = 7.7, ArH), 7.47 (d, 1H, J = 7.7, ArH), 7.25 (t, 1H, J = 7.7, ArH), 7.15 (t, 1H, J = 7.7, ArH). 13 C NMR (CDCl3, 150 MHz) δ 132.4, 129.1, 128.3, 123.7, 94.3, 6.4. + HRMS for C8H4BrI [M] found 305.85391, calc. 305.85356.

1-Bromo-2-(bromoethynyl)benzene:[382] Yellow oil. 1 H NMR (CDCl3, 600 MHz) δ 7.57 (d, 1H, J = 7.7, ArH), 7.47 (d, 1H, J = 7.7, ArH), 7.25 (t, 1H, J = 7.7, ArH), 7.19 (t, 1H, J = 7.7, ArH). 13 C NMR (CDCl3, 150 MHz) δ 134.1, 132.6, 130.0, 127.2, 125.8, 124.9, 78.8, 55.0. + HRMS for C8H4Br2 [M+H] found 257.86812, calc. 257.86743.

(Iodoethynyl)benzene:[343] Yellow oil. Mp 118–120 °C (DCM). Lit Mp 118–119 °C (PE).[343] 1 H NMR (CDCl3, 600 MHz) δ 7.45–7.41 (m, 2H, ArH), 7.33–7.28 (m, 3H, ArH).

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13 C NMR (CDCl3, 150 MHz) δ 132.4, 128.9, 128.4, 123.5, 94.2, 6.3. IR ν 3055, 1597, 1488, 1442, 753, 689. + HRMS for C8H5I [M] found 227.94390, calc. 227.94304.

1-(Iodoethynyl)-4-methoxybenzene:[383] Yellow solid. Mp 62–63 °C (PE/DCM). Lit Mp 61–62°C (DCM).[383] 1 H NMR (CDCl3, 600 MHz) δ = 7.38 (d, 2H, J = 7.8, ArH), 6.84 (d, 2H, J = 7.8, ArH), 3.82

(s, 3H, CH3). 13 C NMR (CDCl3, 150 MHz) δ 160.0, 133.7, 115.5, 94.0, 55.3, 3.7. + HRMS for C9H7IO [M] found 257.95290, calc. 257.95361.

174

References

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Appendix

Appendix

B(OCH2CF3)3

Racemate Spiking experiment

197

Appendix

B(OMe)3

Racemate Spiking experiment

198

Appendix

time, h 180 174a 180a+180b trans cis 1 2 19 78 62 38 3 0 77 23 67 33 3 0 76 24 68 32 5 0 42 58 56 44 23 0 30 70 70 30

Reaction conditions: boronic acid (1 equiv), aldehyde (1.2 equiv), Ph3PAuCl (2 mol%), AgOTf (2 mol%), 1 M DCM, RT.

90

80

70

60 aldol enolate 50 pba 40

30

20

10

0 0 5 10 15 20 25

time, h

199

Appendix

time, h 180 174a 180a+180b trans cis 1 0 34 66 71 29 3 0 14 86 75 25 4 0 13 87 74 26 7 0 50 50 56 44 22 0 48 52 55 45

Reaction conditions: boronic acid (1 equiv), aldehyde (2 equiv), Ph3PAuCl (2 mol%), AgOTf (2 mol%), 1 M DCM, RT.

100 90 80 70 60 50 40 aldol 30 enolate 20 pba 10 0 0 5 10 15 20 25

time, h

200

Appendix

time, h 180 174a 180a+180b trans cis 1 0 28 72 74 26 3 0 15 85 75 25 4 0 19 81 73 27 7 0 50 50 62 38 24 0 30 70 67 33

Reaction conditions: boronic acid (1 equiv), aldehyde (2 equiv), Ph3PAuNTf2 (2 mol%), 1 M DCM, RT.

100 90 80 70 60 50 40 aldol 30 enolate 20 pba 10 0 0 5 10 15 20 25

time, h

201

Appendix

time, h 180 174a 180a+180b trans cis 1 77 0 23 79 21 2 73 1 26 78 22 3 70 2 28 78 22 4 69 2 30 77 23 7 69 2 29 75 25 20.5 44 7 49 61 39

Reaction conditions: boronic acid (1 equiv), aldehyde (2 equiv), AuCl (2 mol%), 1 M DCM, RT.

90

80 aldol

70 enolate pba 60

50

40

30

20

10

0 0 5 10 15 20 25

time, h

202

Appendix

time, h 180 174a 180a+180b trans cis 1 84 0 16 62 38 2 42 36 22 47 53 3 36 36 28 36 64 4 36 36 29 36 64 7 36 28 36 31 69 20.5 36 20 44 30 70

Reaction conditions: boronic acid (1 equiv), aldehyde (2 equiv), AuCl (2 mol%), H2O (5 equiv), 1 M DCM, RT.

90

80 aldol

70 enolate pba 60

50

40

30

20

10

0 0 5 10 15 20 25

time, h

203

Appendix

time, min 180 174a 180a+180b trans cis 10 28 46 26 83 17 20 3 57 40 82 18 30 0 55 45 82 18 40 0 53 47 81 19 50 0 53 47 81 19 60 0 53 47 81 19 70 0 52 48 81 19 80 0 53 47 81 19 90 0 52 48 80 20 100 0 52 48 80 20 210 0 50 50 78 22

Reaction conditions: boronic acid (1 equiv), aldehyde (1 equiv), Ph3PAuNTf2 (1 mol%), 0.5 M, CD2Cl2, RT.

60

50

40

30

aldol 20 enolate 10 pba

0 0 50 100 150 200 250

time, min

204

Appendix

time, min 180 174a 180a+180b trans cis 15 46 38 16 81 19 25 23 48 29 80 20 35 11 50 39 81 19 45 4 47 48 81 19 55 0 45 55 80 20 65 0 43 57 80 20 75 0 42 58 79 21 85 0 42 58 79 21 95 0 41 59 79 21 105 0 41 59 79 21 115 0 41 59 78 22 215 0 37 63 76 24

Reaction conditions: boronic acid (1 equiv), aldehyde (2 equiv), Ph3PAuNTf2 (1 mol%), 0.5 M, CD2Cl2, RT.

70

60

50

40

30 aldol 20 enolate pba 10

0 0 50 100 150 200 250

time, min

205

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