Homolytic reactions of ligated boranes. Part 13. An electron spin resonance study of radical addition to aminoboranes

Abstract

The reactions of silyl radicals R₃Si˙(R = Me, CD₃, Et, MeO, Bu^tO, or Me₃SiO) with aminoboranes R¹R²N[graphic omitted]BH₂ in solution have been studied using e.s.r. spectroscopy. The silyl radicals add rapidly at low temperatures to 2,2,6,6-tetramethylpiperidinoborane and to N-t-butyl-N-methylaminoborane to give aminyl-borane radicals of the type R¹R²N→BH₂SiR₃. Di-adducts of the type R¹R²N→BH(SiR₃)₂ are formed as secondary reaction products. Addition of R₃Si˙ to Bu^t(Me)N[graphic omitted]BH₂ is much faster than the corresponding addition to the isoelectronic alkene Bu^t(Me)CCH₂, probably because the NB π-bond is weaker than its CC counterpart. The aminylboranes R¹R²N→BH₂SiR₃ are only weakly bound with respect to aminoborane and the silyl radical, and the extent of binding depends on the nature of R and, because of steric effects, on the bulk of the N-alkyl groups. The radicals R¹R²N→BH₃ are more tightly bound with respect to aminoborane and H˙. Related to this, the extent of hyperconjugative delocalisation of the unpaired electron onto the BH₂M group in R¹R²N→BH₂M is greater when M = R₃Si than when M = H and also varies with the nature of R¹ and R². t-Butoxyl radicals abstract hydrogen from Me₃SiNMe₂→BH₃ to give initially Me₃SiNMe₂→BH₂ which rapidly rearranges, probably by a dissociative mechanism, to the aminylborane Me₂N→BH₂SiMe₃. The results are interpreted with the aid of semi-empirical molecular orbital calculations at the MNDO and INDO levels.