Homolytic reactions of ligated boranes. Part 14. ESR studies of ring opening of cycloalkylaminyl–borane radicals and reactions of aminyl–borane radicals with silicon-containing compounds

Abstract

ESR spectroscopy has been used to characterise reactions of the aminyl–borane radicals RNH→BH₃(2) in solution. The relative rates of β- hydrogen -atom transfer from [2; R = H, Me, Bu^s, Bu^tC(H)Me, or Bu^t] to 2,3-dimethylbut-2-ene and to furan depend on the nature of the group R, confirming that H-atom transfer is bimolecular and does not involve prior β-scission of (2) to give a free hydrogen atom. Because of favourable polar factors, β-H-atom transfers from (2) to the allylsilanes Me₃SiCH₂CHCH₂ and Me₃SiCH₂CMeCH₂ are particularly rapid. The cycloalkylaminyl–borane radicals (2; R = cyclo-C₃H₅ or cyclo-C₄H₇) undergo rapid ring opening at 282 K, while no spectroscopic evidence was found for opening of the cyclopentyl or cyclohexyl analogues. Alkylaminyl–borane radicals transfer a β-hydrogen atom to hexamethyldisilane to bring about homolytic Si–Si bond cleavage. These radicals also react with trialkylsilanes R₃SiH to give R₃Si˙. Approximate absolute rate coefficients for the reactions of (2) have been determined at 282 or 292 K. Of the primary amine–boranes investigated, s-butylamine– and cyclopentylamine–boranes appear to be the most suitable complexes for ESR spectroscopic work in liquid solution.