An electron spin resonance study of the generation and reactions of borane radical anions in solution

Abstract

Photochemically or thermally generated t-butoxyl radicals rapidly abstract hydrogen from borohydride or cyanoborohydride anions to form H3B–˙ or H2[graphic omitted]CN–, respectively, which can be detected and their reactions studied in fluid solution using e.s.r. spectroscopy. The spectroscopic parameters confirm that the equilibrium geometry of H3B–˙ is planar, as is that of the isoelectronic H3C·, and the temperature dependences of a(11B) and a(3H) result from Boltzmann population of out-of-plane vibrational states. The extent of conjugative delocalisation of the unpaired electron onto nitrogen is similar in H2[graphic omitted]CN– and in the isoelectronic H2ĊCN. H3B–˙ adds to ethylene, trimethylvinylsilane, benzene, and pyridine, but H2[graphic omitted]CN– is less reactive and addition to only Me3SiCHCH2 was detected. The B–C bond eclipses the orbital of the unpaired electron in the preferred conformation of the ethylene adduct H3H2ĊH2, in contrast to the staggered conformation adopted by the isoelectronic propyl radical. Addition of H3B–˙ to electron deficient arenes is faster than to benzene, and the regioselectivities of these reactions also show that the borane radical anion is highly nucleophilic. Both H3B–˙ and H2[graphic omitted]CN– add to alkyl azides and cyanides to give triazenyl and iminyl radical adducts, respectively, and both borane radical anions displace alkyl radicals from alkyl isocyanides, perhaps through the intermediacy of a transient imidoyl radical adduct. H3B–˙ abstracts a halogen atom from alkyl chlorides, bromides, and iodides, while H2[graphic omitted]CN– reacts rapidly with only bromides and iodides; neither reacts with alkyl fluorides. The reactivities of H3B–˙ and H2[graphic omitted]CN– resemble those of H3Si· and R3Si· much more than those of H3C· and R3C·.