EVIDENCE FOR CONCERT IN THE THERMAL UNIMOLECULAR VINYLCYCLOPROPANE TOCYCLOPENTENE SIGMATROPIC 1,3-SHIFT

Gas phase pyrolysis of euterio-trans-(1'-tert-butyl-2'-(Z)-deuteriovin yl) cyclopropane at 290 degrees C gives ns,trans-3,4,5-trideuterio-1-t ert-butylcylopentene as the major 1,3-shift product with greater than 90% stereospecificity in an orbital symmetry ''allowed'' suprafacial-i nversion sense after correction for the geometric isomerization of sta rting material and the other materials present. The isotopic substitut ion at the critical sites of rearrangement eliminates steric or electr onic influences of substituents on a biradical pathway as a source of the suprafacial-inversion stereochemistry observed with more highly su bstituted derivatives. The stereochemical results coupled with a norma l deuterium kinetic isotope effect (k(H)/2k(D) = 1.14 at 311.6 degrees C) at the exo-methylene carbon are best interpreted in terms of a con certed pathway for rearrangement. A less likely alternative is a stere ospecific disrotatory ring opening to a biradical followed by rate-det ermining closure to a five-membered ring. Accompanying the rearrangeme nt 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.