BENZYLCHLOROCARBENE - KINETICS PARAMETERS FOR 1,2-H MIGRATION, UV ABSORPTION-SPECTRUM, AND MECHANISM FOR ADDITION TO ALKENES

Laser flash photolysis (LFP) of 3-benzyl-3-chlorodiazirine, 1, in isooctane, produces benzylchlorocarbene which reacts with pyridine to form an ylide or undergoes 1,2-H migration to form the (Z)- and (E)-β-chlorostyrenes. The rate for the 1,2-H migration is determined by plotting the pseudo-first-order rate constants for the growth of the ylide vs [pyridine] and extrapolating to [pyridine] = 0. From such measurements, performed at various temperatures, the kinetic parameters E¡ = 4.5 keal/mol and A¡ = 1011.1 s-1 are obtained. LFP of 1 in the absence of pyridine produces a transient absorption (280-330 nm) assigned to benzylchlorocarbene. Monitoring the carbene decay directly at 310 nm over the same temperature range gives similar values: E¡ = 4.8 keal/mol and A¡= 1011.3 s-1. Three independent methods—LFP, products ratios (Z/E and cyclopropane/chlorostyrenes)—yield a single value, (6.2 ± 0.2) 108 M-1 s-1 for kt, the rate constant for the addition of benzylchlorocarbene to tetramethylethylene. These results are consistent with a mechanism involving the formation of a complex between benzylchlorocarbene and tetramethylethylene. © 1990, American Chemical Society. All rights reserved.