Homolytic reactions of ligated boranes. Part 11. Electron spin resonance studies of radicals derived from primary amine–boranes

Abstract

Photochemically generated t-butoxyl radicals react with the primary amine–boranes RNH₂→BH₃(R = Me or Bu^t) to form, initially, the nucleophilic amine–boryl radical RNH₂→BH₂, which subsequently abstracts hydrogen from the parent amine–borane 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 the aminyl–borane radicals show the presence of extensive hyperconjugative delocalisation of the unpaired electron onto the BH₃ group [a(3BH) 64–66 G]. Monoalkylaminyl-borane radicals react readily 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 ionisation potential of the alkene decreases along the series CH₂CH₂ < MeCHCH₂ < Me₂CCH₂≈ Me₂CCHMe < Me₂CCMe₂. The absolute rate coefficient for reduction of propene by MeNH→BH₃ to give the isopropyl radical was determined to be ca. 2 × 10³ dm³ mol^–1 s^–1 at 282 K. Competition experiments have been carried out to determine the relative rates of the various reactions undergone by RNH₂→BH₂ and RNH→BH₃. The results are interpreted with the aid of ab initio molecular-orbital calculations at the 6-31G** level for RNH₂→BH₂, RNH→BH₃, RNH₂→BH₃, and RNH[graphic omitted]BH₂(R = H or Me), together with similar calculations for the isoelectronic organic counterparts in which the NB moiety is replaced by a CC grouping.