Influence of the Geminal Biscyclopropyl Group in the Reactions of Trimethylenemethane: Reversible Intersystem Crossing

Abstract

The azo compound precursor to Berson's trimethylenemethane (TMM) biradical system, 2-methylene-1,3-cyclopentanediyl, was substituted with two cyclopropyl groups on the exomethylene. Generation of the TMM species by pyrolysis, direct photolysis, and benzophenone-sensitized photolysis resulted in formation of both cyclopropane ring-opened monomer and cyclopropane ring-unopened dimers whose ratio depended directly on temperature and extent of dilution. The ground state appears to be triplet on the basis of ESR spectrometry. The results contrast with previous work (Waldemar Adam) showing that monocyclopropyl substitution on Berson's TMM species gives both ring-opened monomer and ring-unopened dimers upon pyrolysis and gives mostly ring-unopened dimer upon direct photolysis at 0−20 °C in what appear to be concentration-independent reactions. The observations with the biscyclopropyl-substituted species are consistent with steric effects retarding dimerization of the triplet allowing reversible intersystem cross...