Neighboring Group Participation in Solvolysis. XIII. Kinetic Isotope Effects in Solvolysis of 2-Arylethyl p-Nitrobenzenesulfonates

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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 (ks) pathway. In all other reactions, which proceed mainly via the aryl assisted (kΔ) 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 medium phenyl-1 effect (1.022) were observed for the formolysis of 1b, whose mechanism is thought to be exclusive kΔ 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 kΔ pathway, i.e., advanced Cα–O bond rupture, slight Cα–CPh−1 bond formation, Cα–Cβ double bond formation, slight Cβ–CPh−1 bond rupture, and breakdown of aromaticity at the phenyl-1 position. These characteristics are very similar to those of the phe-nonium ion.