Organic photochemistry. 91. Photoinduced methyl migrations of methylindenes in the gas phase

Abstract

The mechanisms for photoinitiated methyl migration of methylindenes in solution and the gas phase have been studied with the 13 C-labeled substrates [1- 13 C]-2-methylindene ([1- 13 C]-2-MI) and [3- 13 C]-3-methylindene ([3- 13 C]-3-MI). Photolysis of [1- 13 C]-2-MI in hexane produces 1- and 3-methylindene products labeled at C2, consistent with the previously proposed skeletal rearrangement mechanism. Photolysis of the labeled substrates in the gas phase gives products with labeling patterns consistent with the operation of two mechanisms, skeletal rearrangement and alkyl-shift chemistry (the latter accompanied by hydrogen shifts). The presence of a collisional quenching gas (butane) in the gas-phase photolysis reduces the role of the alkyl-shift mechanism and increases the contribution of skeletal rearrangement. It is proposed that, for these alkylindenes the alkyl and hydrogen shifts derive from an upper electronically or vibrationally excited state while skeletal rearrangement is characteristic of a vibrationally relaxed S 1 state