Rate constants for 1,2-hydrogen migration in cyclohexylidene and in substituted cyclohexylidenes

Laser Bash photolysis (UV-LFP, 308 nm) of suitably substituted oxadiazoline s leads to cyclohexylidene (14a), 4-tert-butyl-cyclohexylidene (14b), 2-tri fluoromethylcyclohexylidene (14d), 8-aza-8methyl[3.2.1]oct-3-ylidene (14e), diethylcarbene (14f), and ethyl(methyl)carbene (14h). Carbene intermediate s were inferred from the products of steady state photolyses, and their pyr idinium ylides were inferred from. transient absorption spectra observed wh en pyridine was present. Yields of the pyridinium ylides 15a-h as a functio n of pyridine concentration gave the lifetimes (tau) for carbenes 14a-h in cyclohexane, cyclohexane-d(12), and benzene solutions, at 22 degrees C. The intermediacy of cyclohexylidene (14a) was inferred from the observation of cyclohexene formed in both the LFP and steady state (SS) experiments, The major products from dual wavelength irradiation of the oxadiazolines (at 25 4 and 300 nm) were those of 1,2-migration of hydrogen (1,2-H) in the corres ponding carbenes. 2-Trifluromethylcyclohexylidene gave 3-trifluoromethylcyc lohexene and 1-trifluoromethylcyclohexene in a 9.8:1 ratio. The kinetic dat a support the conclusion that 1,2-H in the cyclohexylidenes is accelerated, relative to 1,2-H in dimethylcarbene. A 4-tert-butyl substituent has a neg ligible effect on the rate constant for 1,2-H, but the CF3 group at the alp ha-position decelerates 1,2-H by roughly: 10-fold, as inferred from the dis tribution of products.