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 Ea, 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.