Homolytic Reactions of Ligated Boranes. Part 7. an Electron Spin Resonance Study of the T-Butyl Isocyanide–boryl Radical

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 ButNC→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 ButCC–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 ButCC–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 But˙ and other possible origins are discussed. γ-Radiolysis of polycrystalline TBIB affords But˙ 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 But˙.