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

The reactions of silyl radicals R3Si˙(R = Me, CD3, Et, MeO, ButO, 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 But(Me)N[graphic omitted]BH2 is much faster than the corresponding addition to the isoelectronic alkene But(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.