posted on 1999-05-27, 00:00authored byJohn Paul Pezacki, Philippe Couture, James A. Dunn, John Warkentin, Paul D. Wood, Janusz Lusztyk, Francis Ford, Matthew S. Platz
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.