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Issue 12, 1989
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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, 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)C[double bond, length half m-dash]CH2, 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.

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Article type: Paper
DOI: 10.1039/P29890001963
Citation: J. Chem. Soc., Perkin Trans. 2, 1989,0, 1963-1972
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    Homolytic reactions of ligated boranes. Part 13. An electron spin resonance study of radical addition to aminoboranes

    I. G. Green, K. M. Johnson and B. P. Roberts, J. Chem. Soc., Perkin Trans. 2, 1989, 0, 1963
    DOI: 10.1039/P29890001963

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