MINDO/3 calculations on enthalpy and entropy effects in the cyclization of the hex-5-enyl, 2-methylhex-5-enyl, and 5-methylhex-5-enyl radicals

Abstract

The intramolecular cyclization of the hex-5-enyl radical has been studied by the MINDO/3 method. The effect of methyl substituents in the alkyl and olefin fragments of the radical has been considered. The different electronic and steric effects competing in these reactions have been analysed by means of an energy-partitioning analysis in local contributions. It is shown that the increase in the reaction rate produced by a methyl group in the alkyl part of the radical is due to both enthalpy and entropy effects. The reversal in the regioselectivity from the hex-5-enyl radical to the 5-methylhex-5-enyl radical is due to an enthalpy effect caused by steric and electronic factors.