Competitive ring hydride shifts and tolyl-benzyl rearrangements in tolyl and silatolyl cations

Transition states for two plausible isomerization pathways of tolyl and silatolyl cations are predicted at the wB3LYP/6-31(d,p) level of theory. The barrier heights for the interconversion of the ortho- and meta-isomers are high for both tolyl (50 kcal/mol) and silatolyl (57 kcal/mol) cations. Competitive hydride shifts from methyl and silyl groups to the positive charge center at the phenyl ring leading to benzyl and silabenzyl cations have sufficiently different barrier heights: the one for tolyl cation (26 kcal/mol) is close to the upper excess energy limit of the nucleogenic tolyl cation, while this barrier for the silatolyl cation is substantially lower (9 kcal/mol). Stationary points for the path leading from the benzyl cation to the global minimum of the C7H7+ system, i.e. the tropylium cation, were also found. (C) 2000 Published by Elsevier Science B.V.