Trialkylsilylaminyl radicals. Part 1. Electron spin resonance studies of the photolysis of silylated hydrazines, hydroxylamines, triazenes, and tetrazenes

Abstract

Radicals produced by liquid-phase u.v. photolysis of a number of trialkylsilylated hydrazines, hydroxylamines, triazenes, and tetrazenes have been studied using e.s.r. spectroscopy. The bis(trimethylsilyl)-aminyl radical (Me3Si)2N˙, generated by photolysis of (Me3Si)2NN(SiMe3)2, (Me3Si)2NOSiMe3, or (Me3Si)2NNNN(SiMe3)2, is much more reactive than a dialkylaminyl radical. It abstracts hydrogen from aliphatic C–H groups, adds to ethylene, t-butyl isocyanide, and trialkyl phosphites; the bis(triethylsilyl)-aminyl radical behaves similarly. Differences between (R3Si)2N˙ and (R3C)2N˙ are attributed to the σ-donor–π-acceptor substituent effect of the trialkylsilyl ligand. The reactivity of (R3Si)2N˙ is generally similar to that of ButO˙, although the steric congestion at the radical centre in the former results in a preference for attack at less hindered sites. For example, whilst ButO˙ and H2N˙ both react with isobutane to give mainly But˙, (Me3Si)2N˙ gives mainly Bui˙ below room temperature. The trimethylsilyl(methyl)aminyl radical was generated by photolysis of Me3SiN(Me)OSiMe3 or Me3SiN(Me)NNMe and, although its e.s.r. spectrum was not detected, it could be trapped by trialkyl phosphites to give the phosphoranyl radicals Me3SiN(Me)P(OR)3, implying a reactivity greater than that of Me2N˙. Hydrogen abstraction from R3SiN(H)OSiR3 affords R3SiNOSiR3 which rearranges to (R3Si)2NO˙, but RNOSiR3 does not rearrange to R3Si(R)NO˙. The e.s.r. spectra of RNOR, ROSiR3, and R3SiOSiR3 are compared and differences are accounted for in terms of the π-acceptor nature of the R3Si group.