A SIGNIFICANT BARRIER TO 1,2 HYDROGEN MIGRATION IN SINGLET 1-PHENYLETHYLIDENE - A LASER FLASH-PHOTOLYSIS STUDY

Laser flash photolysis of 1-phenyldiazoethane in heptane releases 1-phenylethylidene which can be intercepted with pyridine to form an ylide (lambda(max) = 475 nm). Oxygen trapping experiments indicate that relaxation of singlet 1-phenylethylidene to the ground triplet state is rapid relative to pyridine trapping. Analysis of the data reveals that the (singlet/triplet) carbene equilibrium constant K = 2 X 10(-2) and DELTA-G(ST)(298 K) = 2.3 kcal/mol at ambient temperature in n-heptane. Furthermore, the rate constant for 1,2 hydrogen migration in the singlet state (k1,2) is less than 6 x 10(6) s-1, and the 1,2 hydrogen shift reaction in the singlet state must traverse an activation energy of at least 4.3 kcal/mol in alkane solvent. The 1,2 hydrogen shift reaction is accelerated by more than a factor of 30 in polar acetonitrile solvent and by substitution of an additional methyl group on the migrating CH group (1-phenylpropylidene)