Hydrogen abstraction from silylamines; an investigation of the 1,2-migration of the trimethylsilyl group in aminyl radicals

Abstract

Hydrogen abstraction from a series of silyl-substituted amines, Me₃SiCHRNHR′, by tert-butoxyl radicals has been examined by EPR spectroscopy. For each amine hydrogen is abstracted from both the methylene (or methine) group and from the amino group to give a mixture of aminoalkyl, Me₃SiC˙RNHR′, and aminyl radicals, Me₃SiCHRN˙R′. The proportion of hydrogen abstraction from the amino groups is less than from the methylene (methine) groups. The aminyl radicals are not directly detectable by EPR spectroscopy, but they rearrange above a particular temperature by 1,2-migration of the trimethylsilyl group from carbon to nitrogen to give carbon-centred radicals, ˙CHRNR′SiMe₃, which have been spectroscopically characterised. Mechanistic evidence favours an intramolecular migration of the trimethylsilyl group via a transition state in which the silicon atom expands its valence shell to five, rather than via an elimination–readdition route. Rate parameters for the migrations have been estimated from the temperature development of the rearranged radical concentrations and found to be sensitive to the presence of sterically large groups at the nitrogen terminus.