DYOTROPIC REARRANGEMENT OF ORGANOSILYLENIUM IONS IN THE GAS-PHASE

The mechanism of ethene extrusion for unimolecular dissociation of (CH3)3Si+ in the gas phase has been studied by using Fourier transform mass spectrometry (FTMS). Collision-activated dissociation (CAD) combined with isotopic exchange reactions suggest that (CH3)3Si+ (4) and (C2H5)(CH3)SiH+ (2) interconvert upon activation. We propose that 2 and 4 interconvert by a concerted 1,2-hydrogen/1,2-methyl migration (dyotropic rearrangement, transition state 3). We envision that the dyotropic rearrangement involves a thermally allowed [sigma(s)2 + sigma(a)2] process. Ethene extrusion from activated 4 proceeds by initial rearrangement to 2 followed by beta-hydrogen migration with ethene elimination. Isotopic labeling studies suggest that (CH3)2-SiH+ decomposes by a similar mechanism with initial rearrangement to (C2H5)SiH2+ by a concerted 1,2-hydrogen/1,2-methyl migration. Subsequent decomposition by beta-hydrogen migration/ethene elimination then yields SiH3+. In contrast to the above rearrangements, activated (C2H5) (CH3)2Si+ eliminates ethene by direct beta-hydrogen migration (no isomerization) and was confirmed by labeling studies. It is argued that the corresponding dyotropic rearrangements for(C2H5)(CH3)2Si+(processes 16 and 17) have prohibitive barriers; consequently, beta-hydrogen migration with ethene elimination is observed directly.