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 sp ecies. 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 spe ctrum 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 Ra dom (accompanying paper). Irradiation (300 nm) of 5 in solution produces in dane 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) p roduces 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) o f 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 CF2ClCFC l2 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-1 13, 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 rearran gement of the carbene.