Carbomethoxychlorocarbene: Spectroscopy, Theory, Chemistry and Kinetics

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Abstract
Photolysis (254 nm) of methyl 8-chloro-3a,7a-methanoindan-8-carboxylate (5) in argon at 14 K produces carbomethoxychlorocarbene (6) as a persistent species. The IR and UV−vis spectra of the carbene were recorded and analyzed with the aid of density functional calculations (B3-LYP/6-31G*). The IR spectrum of 6 is consistent with the carbene having a nonplanar singlet ground state, in agreement with the G3(MP2)//B3-LYP calculations of Scott and Radom (accompanying paper). Irradiation (300 nm) of 5 in solution produces indane in 97% yield. In cyclohexane, carbene 6 is trapped by insertion into a CH bond, whereas in 2,3-dimethylbutene it adds to the double bond to form a cyclopropane. Laser flash photolysis of 5 (308 nm, 17 ns, XeCl excimer) produces carbene 6 which reacts with pyridine to form an ylide (λmax = 440 nm). It was possible to resolve the growth of the ylide in Freon-113 (CF2ClCFCl2) to measure the lifetime (τ = 114 ns, ambient temperature) of the carbene and the absolute rate constant of its reaction with pyridine (kpyr = 2 × 109 M-1 s-1). A plot of log(1/τ) versus 1/T in CF2ClCFCl2 is linear with Arrhenius parameters Ea = 10.9 ± 0.8 kJ/mol and A = 109.1±0.2 s-1. In perfluorohexane, a less reactive solvent than Freon-113, Ea = 23.4 ± 1.7 kJ/mol, A = 1010.6±0. s-1, and τ = 354 ns at 293 K. It is argued that the activation barrier to carbene disappearance in perfluorohexane represents the lower limit to the barrier to Wolff rearrangement of the carbene.