INFLUENCE OF THE GEMINAL BISCYCLOPROPYL GROUP IN THE REACTIONS OF TRIMETHYLENEMETHANE - REVERSIBLE INTERSYSTEM CROSSING

The azo compound precursor to Berson's trimethylenemethane (TMM) birad ical system, 8-methylene-1,3-cyclopentanediyl, was substituted with tw o cyclopropyl groups on the exomethylene. Generation of the TMM specie s by pyrolysis, direct photolysis, and benzophenone-sensitized photoly sis resulted in formation of both cyclopropane ring-opened monomer and cyclopropane ring-unopened dimers whose ratio depended directly on te mperature and extent of dilution. The ground state appears to be tripl et on the basis of ESR spectrometry. The results contrast with previou s work (Waldemar Adam) showing that monocyclopropyl substitution on Pe rson's TMM species gives both ring-opened monomer and ring-unopened di mers upon pyrolysis and gives mostly ring-unopened dimer upon direct p hotolysis at 0-20 degrees C in what appear to be concentration-indepen dent reactions. The observations with the biscyclopropyl-substituted s pecies are consistent with steric effects retarding dimerization of th e triplet allowing reversible intersystem crossing back to singlet and perhaps with reduction in the singlet-triplet gap as well.