A computational study of stereospecifity in the thermal elimination reaction of menthyl benzoate in the gas phase

This paper reports a theoretical study, at the B3LYP/6-31 + G(dp) and M05-2X/6-31G + (dp) levels, on the thermal decomposition of menthyl benzoate (2-isopropyl-5-methylcyclohexyl benzoate). It undergoes a unimolecular first-order elimination to give 3-menthene (1-isopropyl-4-methylcyclohexene), 2-menthene (3-isopropyl-6-methylcyclohexene), and benzoic acid. We studied two possible mechanisms trying to explain the formation of 2- and 3-menthene, via six-membered or four-membered cyclic transition states. Rate constants were calculated at two temperatures, 587.1 and 598.6 K, and they agree well with the experimentally determined values. We verify that 3-menthene is the product mainly formed at both temperatures. The progress of the reactions has been followed by means of the Wiberg bond indices. Intrinsic reaction coordinate (IRC) calculations have been carried out to verify that the localized transition state structures connect with the reactants and products and also to verify that the parent compound, menthyl benzoate, is taking the cis-configuration needed in the reaction. Copyright (C) 2009 John Wiley & Sons, Ltd.