Rate Constants for 1,2-Hydrogen Migration in Cyclohexylidene and in Substituted Cyclohexylidenes<sup>1</sup>
作者:John Paul Pezacki、Philippe Couture、James A. Dunn、John Warkentin、Paul D. Wood、Janusz Lusztyk、Francis Ford、Matthew S. Platz
DOI:10.1021/jo990171q
日期:1999.6.1
Laser Bash photolysis (UV-LFP, 308 nm) of suitably substituted oxadiazolines leads to cyclohexylidene (14a), 4-tert-butyl-cyclohexylidene (14b), 2-trifluoromethylcyclohexylidene (14d), 8-aza-8methyl[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 (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 254 and 300 nm) were those of 1,2-migration of hydrogen (1,2-H) in the corresponding carbenes. 2-Trifluromethylcyclohexylidene 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 alpha-position decelerates 1,2-H by roughly: 10-fold, as inferred from the distribution of products.