EXPERIMENTAL-EVIDENCE FOR NOVEL HYDROGEN METHYL EXCHANGE PROCESSES PRECEDING THE UNIMOLECULAR DISSOCIATION OF SILICON-CONTAINING CATIONS IN THE GAS-PHASE

Isotopic labeling is used to demonstrate that the “nascent” α-silyl-substituted carbenium ion CH3C+(H)Si(CH3)3 prior to dissociation to C2H4 and Si(CH3)3+ rapidly undergoes 1,2-methyl migrations leading to an almost complete exchange of the four methyl groups. The ion also undergoes slow 1,2-hydrogen shifts with a kinetic isotope effect kH/kD of approximately 2.5. These results are in good agreement with previous ab initio molecular orbital (MO) calculations. The tertiary silyl-substituted carbenium ion (CH3)2C+Si(CH3)3 also exchanges, prior to loss of propene, its five methyl groups in a nearly quantitative way. Loss of propene immediately follows upon a rate-determining 1,2-hydrogen shift which has a kinetic isotope effect of 2.5. The unimolecular chemistry of the two homologous ions thus appears remarkably similar. © 1990, American Chemical Society. All rights reserved.