Homolytic reactions of ligated boranes. Part 7. An electron spin resonance study of the t-butyl isocyanide–boryl radical

Abstract

Photochemically generated alkoxyl or cyclopropyl, but not n-propyl, radicals abstract hydrogen from t-butyl isocyanide–borane (TBIB) to give the corresponding isocyanide–boryl radical Bu^tNC→BH₂(1), which has been studied in solution using e.s.r. spectroscopy. The structure and electronic configuration of (1) is discussed in relation to the isoelectronic propargylic radical Bu^tCC–ĊH₂. On the basis of qualitative considerations and molecular orbita calculations it is concluded that (1) is probably bent at nitrogen, unlike propargylic radicals which have a linear C–CC–Ċ skeleton, and that the spin population on the β-carbon atom in (1) is much larger than that in Bu^tCC–ĊH₂. In contrast with saturated ligated boryl radicals, (1) does not abstract halogen from alkyl bromides, as judged by e.s.r. spectroscopy, and this low reactivity is probably a consequence of unpaired electron delocalisation in the isocyanide–boryl. The t-butyl radical is formed in lower yield along with (1) when dialkyl peroxides are photolysed in the presence of TBIB. β-Scission of (1) is not the source of Bu^t˙ and other possible origins are discussed. γ-Radiolysis of polycrystalline TBIB affords Bu^t˙ at 77 K, presumably by dissociative electron capture. One-electron reduction of TBIB in a methanol glass at 77 K or by Me₂ĊOH in solution also yields Bu^t˙.