Homolytic reactions of ligated boranes. Part 5. Spin-trapping and other addition reactions of ligated boryl radicals

Abstract

The addition reactions of a variety of amine- and phosphine-boryl radicals (L→ḂH₂; L = R₃N or R₃P) with 2-methyl-2-nitrosopropane (MNP), 2,4,6-tri-t-butylnitrosobenzene (TBN), and phenyl-N-t-butylnitrone (PBN) have been studied in benzene solution using e.s.r. spectroscopy. Spin-trapping of the boron-centred radicals by MNP and PBN affords nitroxides, while addition of Me₃N→ḂH₂ to the ambident TBN yields a mixture of the nitroxide and oxyaminyl adducts, with the latter predominating. Appropriately constituted amine-boryl radicals undergo β-N–C cleavage to give carbon-centred radicals in competition with their spin-trapping by MNP and PBN. In addition to providing mechanistic information, this competition has been used to measure the rate coefficient for spin-trapping of Prⁱ₂EtN→ḂH₂ by MNP. At 314 K, k_trap is ca. 1.6 × 10⁸ dm³ mol^–1 s^–1 and thus amine-boryl radicals are trapped more efficiently than alkyl radicals. Attempts to scavenge H₃N→ḂH₂ were frustrated by the rapid reduction of the spin-traps by ammonia–borane, but this amine-boryl radical did undergo ready addition to alkyl cyanides and isocyanides, to 1,1-di-t-butylethylene, and to 2,6-di-t-butylpyridine as judged by e.s.r. spectroscopy. Addition to cyanides affords transient iminyl radicals H₃N→BH₂(R)CN˙ which undergo subsequent β-scission at a rate which increases along the series R = Et < Prⁱ < Bu^t in parallel with the stabilisation of the departing alkyl radical R˙.