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 Bu^tO˙, although the steric congestion at the radical centre in the former results in a preference for attack at less hindered sites. For example, whilst Bu^tO˙ and H₂N˙ both react with isobutane to give mainly Bu^t˙, (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.