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Polarity Reversal Catalysis An.Pdf POLARITY REVERSAL CATALYSIS AND ENANTIOSELECTIVE HYDROGEN-ATOM TRANSFER A thesis submitted for the degree of Doctor of Philosophy of the University of London by PEARL LING HO MOK Department of Chemistry, University College London, 20 Gordon Street, London WCIH OAJ. December 1992 ProQuest Number: 10046085 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10046085 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT This thesis is divided into two sections: Section A Polarity reversal catalysis (PRC) is a method by which sluggish abstraction of electron deficient hydrogen by electrophilic radicals, such as t-butoxyl radical (Bu‘0*), can be accelerated. In this section of the thesis, the effect of PRC by amine-alkylborane complexes on hydrogen-atom transfer from ketones was studied. ESR spectroscopy was used to monitor the radical reaction products. t-Butyl methyl ether was found to be 30 times more reactive than acetone towards hydrogen-atom abstraction by Bu^O* at 190 K, and this difference is attributed entirely to polar effects. In contrast, the highly-nucleophilic amine-boryl radical MegN^BHThx abstracts hydrogen very much more raoidlv from acetone than from the ether. Competitive abstraction from the two different types of a-C-H groups in methyl isopropyl ketone MeC(0)CHMe2 by amine-alkylboryl radicals which have differing steric demands has also been quantified. Specific alkyl radicals can be generated for ESR studies at low temperature by UV irradiation of a solution containing di-t-butyl peroxide, trimethyl amine-butylborane, and the corresponding alkyl chloride. This procedure complements the "triethyl si lane method" of Hudson and Jackson for alkyl radical generation from bromoalkanes. Section B This.section describes the application of PRC to bring about enantioselective a-hydrogen-atom abstraction from esters of the type RIR2CHCO2R3 using optically active amine-alkylboranes to catalyse the reaction of t-butoxyl radicals. This procedurehas been used to bring about kinetic resolution of such esters. The bis(isopinocampheylborane) complex of iV,/V,yV',/V'-tetramethyl- ethylenediami ne and some of its derivatives were investigated as optically active polarity reversal catalysts. After partial consumption of the substrate, the amount remaining and its enantiomeric excess (ee) have been used to derive enantioselectivity constants {k^/s/ks/R) for a-hydrogen-atom abstraction from a variety of esters. Enantioselectivity is sensitive to the substituent bulk at the 2-position of the pinene moiety of the catalyst, and varies considerably with the structure of the substrate. The highest selectivity was observed for hydrogen abstraction from dimethyl 2,2-dimethyl-1,3-dioxolane-trans-dicarboxylate, when after 75% consumption of initially-racemic ester at 183 K, the residual substrate showed an ee of 97%. A transition state model is proposed to account for the observed enantioselectivities. In addition, ESR spectroscopy has been used, for the firs t time, to measure the relative rates of the elementary enantioselective hydrogen-atom abstraction reactions of optically active amine-boryl radicals. ACKNOWLEDGEMENTS I would like to express my sincere thanks to my supervisor. Dr. Brian Roberts, for his continual guidance, encouragement and patience during the course of this project. Thanks are also due to Dang, Neil, Parveen for their fru itfu l discussions, and Claude for his kindness. I would like to thank my family for their constant financial support, and the Department of Chemistry U.C.L. and U.C.L. for providing scholarships. Finally, thanks to Neil for his patience and assistance in the preparation of this thesis. CONTENTS Page ABSTRACT 2 SECTION A : POLARITY REVERSAL CATALYSIS OF HYDROGEN-ATOM ABSTRACTION Chapter 1 Introduction 1.1 Polar Effects in Radical Reactions 10 1.2 Polarity Reversal Catalysis (PRC) 14 1.3 Neutral Ligated Boryl Radicals 19 1.4 Amine-Boryl Radicals 19 1.5 Reactions of Amine-Boryl Radicals with Alkyl Halides 22 1.6 p-Scission of Amine-Boryl Radicals 23 References to Chapter 1 25 Chapter 2 Results and Discussion 2.1 Syntheses of Catalysts 28 2.2 Catalysed Hydrogen-Atom Abstraction by t-Butoxyl Radicals from Ketones 2.2.1 From Acetone 32 2.2.2 From Methyl Isopropyl Ketone 38 2.2.3 From Cyclobutanone 47 2.2.4 From Cyclopentanone 50 2.2.5 From Cyclohexanone 57 2.3 Halogen Abstraction from Alkyl Chlorides 62 References to Chapter 2 65 Chapter 3 Experimental 3.1 ESR Spectroscopy 67 3.2 NMR Spectroscopy 67 3.3 Materials 67 3.3.1 1-Methyl-cis-l-azonia-5-boratabicyclo[3.3.O]octane 68 3.3.2 yV,/V-Dimethylglycinatoborane 69 References to Chapter 3 70 SECTION B ; POLARITY REVERSAL CATALYSIS OF ENANTIOSELECTIVE HYDROGEN-ATOM ABSTRACTION Chapter 4 Introduction 72 References to Chapter 4 79 Chapter 5 Kinetic Resolution of Esters 5.1 Syntheses of Catalysts 80 5.2 Kinetic Resolution of Esters 89 5.3 Model for Enantioselective Hydrogen-Atom Abstraction 109 References to Chapter 5 115 Chapter 6 Experimental 6.1 ESR Spectroscopy 118 6.2 NMR Spectroscopy 118 6.3 Gas Liquid Chromatography Analyses 118 6.4 High-Performance Liquid Chromatography 119 6.5 Column Chromatography and TLC 119 6.6 Optical Rotation Measurements 119 6.7 Differential Scanning Calorimetry 119 6.8 Materials 120 6.9 UV Irradiation 136 6.10 Typical Procedure for Kinetic Resolution 137 References to Chapter 6 140 Chapter 7 ESR Spectroscopic Studies of Enantioselective Hvdroqen-Atom Abstraction Reactions Results and Discussion 142 References to Chapter 7 156 Chapter 8 Experimental 8.1 ESR Spectroscopy 157 8.2 Materials 157 References to Chapter 8 159 Chapter 9 Electron Soin Resonance Spectroscopy 9.1 Principles of ESR Spectroscopy 160 9.2 Methods of Radical Production for ESR Studies 162 9.3 Characteristics of ESR Spectra 164 9.3.1 g-Factors 164 9.3.2 Hyperfine Splitting Constants 165 9.4 Origins of Hyperfine Splitting 166 9.4.1 a-Proton Splittings 167 9.4.2 p-Proton Splittings 168 9.4.3 Long-Range Proton Splittings 169 9.5 Second-Order Effects 170 References to Chapter 9 172 SECTION A : POLARITY REVERSAL CATALYSIS OF HYDROGEN-ATOM ABSTRACTION 10 CHAPTER 1 INTRODUCTION 1.1 Polar Effects in Radical Reactions Although the reactions of neutral free radicals are less subject to polar influences than ionic reactions, such factors can s till strongly affect the courses of homolytic processes. The importance of polar effects in influencing the reactions of uncharged free radicals has been recognised for over forty years.^ A striking illustration of the operation of polar effects can be seen in the phenomenon of alternating radical co-polymerisation. This is the property of certain monomer pairs, for example styrene and maleic anhydride, of forming a co-polymer in which the monomer units tend to alternate along the chain.^ It is also well-established that polar factors play an important role in determining the chemo- and regio-selactivities of hydrogen atom transfer reactions of the type ( 1 . 1 ) . %n valence A + H-B -------- ► A-H + b ‘ (1.1) bond terms, the transition state for such direct atom transfer reactions can be represented as a hybrid of the canonical structures la-d and the stability of the transition state will increase as the contribution from the ionic structures Ic or Id increases. As a [ A H-B ] M ► [ A-H B' ] < --- ► [ A+ H* B “ ] M ► [ A ‘ H B+ ] la lb Ic Id 11 result, the activation energies for a series of similarly exothermic hydrogen atom abstraction reactions would be expected to decrease as the properties of the attacking and departing radicals become more mutually conducive to the participation of charge-transfer structures of the types Ic or Id. Thus, i f Nuc* and El* are nucleophilic and electrophilic radicals, respectively, polar effects w ill favour abstraction of hydrogen from H-Nuc by El* and from H-El by Nuc*, but abstraction of hydrogen from H-Nuc by Nuc* or from H-El by El* will both be disfavoured.* Similar conclusions may be arrived at by consideration of the frontier molecular orbital interactions in the reactants.^ ' The unpaired electron in A* is in a singly occupied molecular orbital (SOMO) which can interact with both the highest occupied molecular orbital (HOMO) and with the lowest unoccupied molecular orbital (LUMO) of the hydrogen donor MB, as shown in Figure 1.1. Both of these interactions are net stabilising and the extent of stabilisation depends upon the reciprocal of the energy difference between the SOMO and the HOMO or LUMO of HB. Radicals with a high-energy SOMO (nucleophilic radicals) w ill react readily with molecules which have a low-energy LUMO, and radicals with a low-energy SOMO (electrophilic radicals) will react *The descriptions "electrophilic" and "nucleophilic" are relative terms. In general, whether a radical A* behaves as a net electrophile [structure Id more important than Ic] or a net nucleophile [structure Ic more important than Id] in reaction (1.1) will depend on the nature of B* (i.e . on the electronegativity difference between A* and B*). 12 preferentially with molecules having a high-energy HOMO. This is illustrated by considering the reactions of the methyl radical and /LU M O r h \ / ' H - ' x / /SO M O \ 4 ^: / ^ 4 f HOMO \ * * * * - H - - H - H — B A* H— B Figure 1.1 : Schematic diagram shoving the interactions between the frontier molecular orbitals of A* and H-B.
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