Thermal rearrangements of aryldimethylvinylidenecyclopropanes and aryldimethylenecyclopropanes

Abstract

1-Dimethylvinylidene-2-methyl-2-phenylcyclopropane (1a) undergoes thermal rearrangement in solution (130°) or in the vapour phase (350°) to give 2-isopropylidene-1-methylene-3-methyl-3-phenylcyclopropane (6a) exclusively. A kinetic study in solution over the temperature range 120–155° yields values for the activation parameters of 30·4 kcal mol^–1 for E_a, and –2·5 e.u. for ΔS^‡, which are shown to be consistent with the intermediacy of an orthogonal diradical. Similar rearrangements are undergone by a series of six related dimethylvinylidene cyclopropanes. Rearrangement of cis- and trans-1-dimethylvinylidene-2-methyl-3-phenylcyclopropanes (9) and (10) is kinetically controlled at 130° yielding mainly syn-1-ethylidene-2-isopropylidine-3-phenylcyclopropane (11); equilibration of the latter compound with the anti-isomer (12) became significant at 180°. The relative rates of rearrangement of compounds (9) and (10) to products (11) and (12) are explained in terms of the interactions of the substituents bonded to the phenyl-substituted carbon atom and the substituents and the incipient half-filled p-orbital on the methyl-substituted carbon atom. At higher temperatures (530°) the cyclopropanes (1a) and (6a) and 3,3-diphenyl-2-isopropylidene-1-methylenecyclopropane (6e) undergo further rearrangement to 1,2,3-trisubstituted indene derivatives.