Absolute reactivity of arylallyl carbocations

A series of alpha-vinyl arylmethyl cations were generated and studied using nanosecond laser flash photolysis. Rate constants for the decay of the substituted alpha-vinyl arylmethyl cations were determined under solvolytic conditions in pure solvents and solvent mixtures of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 2,2,2-trifluoroethanol (TFE). In addition the absolute reactivity of the carbocations with added nucleophiles were obtained. The reactivities of the alpha-vinyl arylmethyl cations were then compared to the reactivities of the corresponding alpha-methyl, alpha-phenyl, and alpha-cyclopropyl arylmethyl cations. Hammett sigma(+) plots of each of the series of carbocations were obtained and the substituent effects on carbocation reactivity analyzed. These data show that the influence of substituent on the reactivity of the alpha-vinyl carbocations was different from the substituents effects on the reactivity of the alpha-methyl, alpha-phenyl, and alpha-cyclopropyl series. Copyright (C) 2008 John Wiley & Sons, Ltd.