Evidence for silicon-directed acid-catalysed ring opening of a beta,gamma-epoxy silane: Reaction of 1,1-dimethyl-1-silacyclohex-3-ene oxide with p-nitrobenzoic acid

The ring-opening reaction of the beta,gamma-epoxy silane (4) with p-nitrobenzoic acid in chloroform occurs regiospecifically to give the two hydroxy esters (14) and (15). The mechanism involves regiospecific ring opening giving the beta-silicon-stabilized carbenium ion (18) which is captured by the p-nitrobenzoate counter ion. The solution conformation of (14) provides evidence for the presence of sigma(C-Si)-sigma*(C-O) interactions between the ring C-Si bonding orbital and the C-OCOC6H4NO2 antibonding orbital. In acetone, reaction of (4) with p-nitrobenzoic acid takes a different pathway and results in acyclic products from solvent attack at the silicon.