Rate Constants for 1,2-Hydrogen Migration in Cyclohexylidene and in Substituted Cyclohexylidenes¹

Laser flash photolysis (UV-LFP, 308 nm) of suitably substituted oxadiazolines leads to cyclohexylidene (14a), 4-tert-butyl-cyclohexylidene (14b), 2-trifluoromethylcyclohexylidene (14d), 8-aza-8-methyl[3.2.1]oct-3-ylidene (14e), diethylcarbene (14f), and ethyl(methyl)carbene (14h). Carbene intermediates were inferred from the products of steady state photolyses, and their pyridinium ylides were inferred from transient absorption spectra observed when pyridine was present. Yields of the pyridinium ylides 15a−h as a function of pyridine concentration gave the lifetimes (τ) for carbenes 14a−h in cyclohexane, cyclohexane-d₁₂, and benzene solutions, at 22 °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 254 and 300 nm) were those of 1,2-migration of hydrogen (1,2-H) in the corresponding carbenes. 2-Trifluoromethylcyclohexylidene gave 3-trifluoromethylcyclohexene and 1-trifluoromethylcyclohexene in a 9.8:1 ratio. The kinetic data 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 negligible effect on the rate constant for 1,2-H, but the CF₃ group at the α-position decelerates 1,2-H by roughly 10-fold, as inferred from the distribution of products.