Carbomethoxychlorocarbene: Spectroscopy, theory, chemistry and kinetics

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 (lambda (max) = 440 nm). It was possible to resolve the growth of the ylide in Freon-113 (CF2ClCFCl2) to measure the lifetime (tau = 114 ns, ambient temperature) of the carbene and the absolute rate constant of its reaction with pyridine (k(pyr) = 2 x 10(9) M-1 s(-1)). A plot of log(1/tau) versus 1/T in CF2ClCFCl2 is linear with Arrhenius parameters E-a = 10.9 +/- 0.8 kJ/mol and A = 10(9.1+/-0.2) s(-1). In perfluorohexane, a less reactive solvent than Freon-113, E-a = 23.4 +/- 1.7 kJ/mol, A = 10(10.6+/-0) s(-1), and tau = 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.