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Electron Spin Resonance Studies of Radicals Derived from Primary Amine-Boranes in Solution

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Electron Spin Resonance Studies of Radicals Derived from Primary Amine-Boranes in Solution ELECTRON SPIN RESONANCE STUDIES OF RADICALS DERIVED FROM PRIMARY AMINE-BORANES IN SOLUTION A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy undertaken at the University of London by JOHN NICHOLAS KIRWAN The Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1 OAJ. November 1990 ProQuest Number: 10609305 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 com plete 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 10609305 Published by ProQuest LLC(2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 ACKNOWLEDGEMENTS I would like to express my sincere thanks to my supervisor Dr. B. P. Roberts for his help and advice throughout the course of my degree. I am also indebted to the technical staff of the department whose services have been invaluble. Abstract The reactions of photochemically generated t-butoxyl radicals with primary amine-boranes (RNH2-»BH3) were studied by electron spin resonance (e.s.r.) spectroscopy. Primary amine-boranes are isoelectronic analogues of the alkanes (RCH2CH3). Much research has been carried out with carbon-centred radicals, the present work set out to investigate the similarities and differences in structures and chemical reactions between these species and the radicals derived from primary amine-boranes. t-Butoxyl radicals abstract hydrogen atoms from RNH2—»BH3 to form, initially, the nucleophilic amine-boryl radical RNH2—>BH2, which subsequently abstracts hydrogen from the parent to give the more stable isomeric aminyl-borane radical RNH—»BH3. The amine-boryl radicals can be intercepted by alkyl bromides or chlorides or by nitriles, with which they react by halogen atom abstraction or by addition to the CN group to give iminyl radicals, respectively. The e.s.r. spectra of RNH—>BH3 show the presence of extensive hyperconjugative delocalization of the unpaired electron onto the BH3 group [a(3Bi/) 64-66 G]. Monoalkylaminyl-borane radicals react rapidly with alkenes with arenes and with conjugated or cumulated dienes to transfer a P-hydrogen atom from boron to give alkyl, cyclohexadienyl, or allyl radicals, respectively. Hydrogen-atom transfer to alkenes from the electrophilic MeNH—»BH3 takes place with high regioselectivity to give the more stable alkyl radical when two adducts are possible; the rate of transfer increases as the ionization potential of the alkene decreases along the series CH2=CH2 < MeCH=CH2 < Me2C=CH2 * Me2=CHMe < Me2C=CMe 2 . The absolute rate for reduction of propene by MeNH—»BH3 was determined to be ca. 2.0 x 103 1 m ol1 s'1 at 282 K. Competition experiments have been carried out to determine the relative rates of various reactions undergone by RNH2—»BH2 and RNH-*BH3. The results are interpreted with the aid of ab initio molecular orbital calculations at the 6-31** level for RNH2-»BH2, RNH—»BH3, RNH2—»BH3, and RNH=BH2 (R = H or Me), together with similar calculations for the isoelectronic counterparts. It was found that the Si-Si bond was cleaved by the same type of reagents that cleave the C—C rc-bond and that aminyl-borane radicals transfered a hydrogen atom to hexamethyldisilane (HMDS) by displacement of Me3Si'. Table of Contents CHAPTER ONE Introduction 1.1 Isoelectronic relationships ....................................................... 1 1.2 Boron-centred r a d ic a ls ............................................................... 3 1.2.1. Reactions of boryl radicals and boryl radicals and borane radical anions .................................................................... 6 Addition to alkenes and arenes ...................................... 6 Reactions with alkyl halides ............................................ 7 Addition to alkyl cyanides ................................................. 8 1.2.2. Ligated boryl radicals ....................................................... 8 1.2.3. Amine-boryl radicals ......................................................... 9 1.2.4. Reactions of amine-boryl radicals ...................................... 10 Addition to alkenes and arenes ......................................... 11 Addition to nitriles ...............................................................11 Reaction with alkyl halides ................................................. 12 P-Scission of amine-boryl ra d ic a ls ...................................... 12 1.2.5. Phosphine-boryl radicals .......................................................14 1.2.6. Sulphide-boryl radicals ..........................................................15 1.3 Carbon-centred radicals ...............................................................15 1.3.1. Reactions of alkyl radicals ................................................ 19 Addition reactions ..................................................................19 1.4 Nitrogen-centred r a d ic a ls ........................................................... 21 1.4.1. Aminyl radicals.......................................................................22 1.4.2. Aminium radical cations .......................................................24 1.4.3. Nitroxide radicals ................................................................. 25 1.4.4. Radical derived from secondary amine-boranes ..............26 1.5 Preparation of Amine-boranes ................................................... 27 References to Chapter 1 .......................................................................30 CHAPTER TWO Radicals Derived From Primary Amine-Boranes 2.1 Methylamine-borane...................................................................... 36 2.2 t-Butylamine-borane...................................................................... 50 2.3 Reaction of primary amine-boryl radicals with nitriles 56 2.4 Addition of primary amine-boryl radicals to alkenes . 62 2.5 Radicals derived from cycloalkylamine-boranes.................63 2.6 Radicals derived from ammonia-borane .................................72 References to Chapter 2 .......................................................................75 CHAPTER THREE Molecular Orbital Calculations .................77 References to Chapter 3 .......................................................................89 CHAPTER FOUR Hydrogen-Atom Transfer Reactions 4.1 Reduction of alkenes, arenes, and dienes by aminyl-borane radicals ....................................................................................................... 90 Relative rate of H-atom transfer to alkenes ............................115 4.2 Reaction of aminyl-borane radicals with other compounds containing carbon-carbon multiple bonds ....................................125 4.3 Reactions of Cycloalkylaminyl-borane radicals................136 4.4 Radicals derived from ammonia-borane ............................. 139 References to Chapter 4 .....................................................................142 CHAPTER FIVE Hydrogen-Atom Transfer From Aminyl-Borane Radicals to Hexamethyldisilane (HMDS).......................................145 References to Chapter 5 .....................................................................159 CHAPTER SIX Experimental 6.1 Electron spin resonance (E.s.r.) spectroscopy ............ 161 6.2 Hyperfine coupling constants ............................................. 162 6.2.2. Second-order effects ............................................................ 163 6.2.3. The g-factor ....................................................................... 166 6.3 Measurement of radical concentrations ............................. 166 Relative reactivities of alkenes towards the methylaminyl- borane radical ............................................................................ 167 6.4 Materials ................................................................................... 167 Purification of D T B P ........................................................ 168 Methylamine-borane......................................................... 168 nB-Enriched methylamine-borane................................. 169 nB-Enriched borane-methyl sulphide com plex 169 B-Deuteriated methylamine-borane (MeNH2—»BD3) . 170 t-Butylamine-borane......................................................... 170 d*,-t-Butylamine-borane.................................................... 170 da-t-Butylacetamide......................................................... 173 ds-t-Butylamine................................................................. 173 Purification of alkylamine-borane complexes .............. 173 (i) Sublimation ................................................................. 173 (ii) P recip atio n ................................................................. 174 (iii) Recrystalisation ......................................................... 174 Dibutanoyl peroxide ......................................................... 174 Tris(trimethylsiloxy)benzene ............................................ 175 References to Chapter 6 ................................................................. 176 1 CHAPTER 1 Introduction 1.1
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