Kirwan, John Nicholas; (1991) Electron spin resonance studies of radicals derived from primary amine-boranes in solution. Doctoral thesis (Ph.D), UCL (University College London).

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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?BH2 to form, initially, the nucleophilic amine-boryl radical 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 [α(3BH) 64-66 G]. Monoalkylaminyl-borane radicals react rapidly with alkenes with arenes and with conjugated or cumulated dienes to transfer a β-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=CMe2. The absolute rate for reduction of propene by MeNH→BH3 was determined to be ca. 2.0 x 103 1 mol-1 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 ah 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 π-bond and that aminyl-borane radicals transfered a hydrogen atom to hexamethyldisilane (HMDS) by displacement of Me3Si.

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