Neighboring group participation in solvolysis, 13

Carbon-14 kinetic isotope effects (k12/k14) were determined for the solvolysis of phenethyl (1a) and 2-(p-methoxyphenyl)ethyl (1b) p-nitrobenzenesulfonates in acetic, formic, and trifluoroacetic acids. Phenyl-1 carbon effects were all in the range of 1.02 - 1.04 except for the acetolysis of 1a (k12/k14 = 1.005), which has been proved to proceed mainly via the aryl unassisted (k sub(s)) pathway. In all other reactions, which proceed mainly via the aryl assisted (k sub(Δ)) pathway, the phenyl-1 carbon apparently changes its bonding in the transition states. A large α-carbon effect (1.131) and a small β-carbon effect (1.013) as well as a mediumphenyl-1 effect (1.022) were observed for the formolysis of 1b, whose mechanism is thought to be exclusive k sub(Δ) without troublesome ion-pair return. These observations, together with α- and β-deuterium effects reported already, enabled us to describe the qualitative nature of the bridged transition-state structure of the k sub(Δ) pathway, i.e., advanced C sub(α)-O bond rupture, slight C sub(α)-C sub(Ph-1) bond formation, C sub(α)-C sub(β) double bond formation, slight C sub(β)-C sub(Ph-1) bond rupture, and breakdown of aromaticity at the phenyl-1 position. These characteristics are very similar to those of the phenonium ion. (author)