Degenerate Thermal Rearrangement of 1,3-Dimethylenecyclopentane. Evidence for Partially Stereospecific Biradical Formation and Closure in 1,3 Shift

Article abstract of DOI:10.1021/ja00404a028

Parolysis of 1,3-bis(dideuteriomethylene)cyclopentane at 370 deg C resulted in a first-order degenerate rearrangement in which the 1,3- and 3,3-shift products were formed in a 2:1 ratio, respectively, indicating the intermediacy of an effectively orthogonal 2,2'-bis(allylmethane) biradical.Pyrolysis of trans- and cis-4,5-dimethyl-1,3-dimethylenecyclopentane (T and C) gave 1,3- and 3,3-shift products with stereochemistry consistent with predominant conrotatory-bevel ring opening to produce the orthogonal diradical directly.Pyrolysis of optically active T gave the 1,3-shift products with 20.8percent inversion of configuration of the migrating carbon indicative of partial stereospecific closure of the biradical.Comparative pyrolysis of optically active T and its bis(dideuteriomethylene) derivative revealed no secondary kinetic deuterium isotop effect, but an alternation in the extent of racemization of starting material and amounts of 1,3- and 3,3-shift products indicates a product-determining isotope effect.These data provide further evidence for an intermediate.