Evidence for Concert in the Thermal Unimolecular Vinylcyclopropane to Cyclopentene Sigmatropic 1,3-Shift

Gas phase pyrolysis of cis -2,3-dideuterio- trans -(1‘- tert -butyl-2‘-( Z )-deuteriovinyl)cyclopropane at 290 °C gives trans,trans -3,4,5-trideuterio-1- tert -butylcyclopentene as the major 1,3-shift product with greater than 90% stereospecificity in an orbital symmetry “allowed” suprafacial-inversion sense after correction for the geometric isomerization of starting material and the other materials present. The isotopic substitution at the critical sites of rearrangement eliminates steric or electronic influences of substituents on a biradical pathway as a source of the suprafacial-inversion stereochemistry observed with more highly substituted derivatives. The stereochemical results coupled with a normal deuterium kinetic isotope effect ( k H /2 k D = 1.14 at 311.6 °C) at the exo -methylene carbon are best interpreted in terms of a concerted pathway for rearrangement. A less likely alternative is a stereospecific disrotatory ring opening to a biradical followed by rate-determining closure to a five-membered ring. Accompanying the rearrangement is geometric isomerization of starting material resulting from C-1−C-2 bond fission which favors either the single methylene rotation or a double rotation by a factor of 10 over single rotation at the vinyl-bearing carbon.