PHYSICAL ORGANIC-CHEMISTRY OF TRANSITION-METAL CARBENE COMPLEXES .4. KINETICS AND EQUILIBRIA OF METHOXIDE ION ADDITION TO [METHOXY(PHENYL)CARBENE]PENTACARBONYLCHROMIUM(0) AND [METHOXY(PHENYL)CARBENE]PENTACARBONYLTUNGSTEN(0)

A stopped-flow kinetic study of the reversible addition of methoxide ion to [methoxy(phenyl)-carbenelpentacarbonylchromium(0) (2-Cr) and [methoxy(phenyl)carbene]pentacarbonyltungsten-(0) (2-W) in methanol at 25-degrees-C is reported. The reaction was investigated by approaching the equilibrium (eq 1) from either side; due to the rapid decomposition of the adduct, the reaction in the reverse direction had to be measured in a sequential stopped-flow apparatus a few milliseconds after the adduct had been generated. Evidence that the observed reaction refers to the formation of (CO)5--C(Ph) (OCH3)2 (M = Cr, W) includes a comparison of the absorption spectrum of (CO)5W--C(Ph)(OCH3)2 with that of the ylide derived from 2-W and DABCO, low-temperature IR of the adducts, and demonstration of CH3O/CD3O exchange on the same time scale as the kinetic experiments. The following rate and equilibrium constants were obtained: k1 = 77.1 M-1 s-1, k-, = 1.10 s-1, and K1 = 70.1 M-1 for 2-Cr and k1 = 186 M-1, s-1, k-1 = 1.68 s-1, and K1 = 111 M-1 for 2-W. Approximate intrinsic rate constants (k0 = k1 = k-1 when K1 = 1) were calculated from the experimental rate constants. The k0 values are substantially lower than those estimated for the addition of methoxide ion to methyl benzoate. This is consistent with significant charge delocalization in the methoxide ion adducts of the metal carbene complexes. Rate constants, k-1BH, for general acid catalyzed methoxide ion loss from the adducts by phenol and 3,5-dichlorophenol were also determined. They allow an estimate of a Bronsted a value on the order of 0.9, suggesting a transition state in which proton transfer from the catalyst to the incipient methoxide ion is well advanced. The near-independence of K, on the metal contrasts with the strong metal dependence (Cr vs W) of the pK(a) values of (eta5-C5H5)M(CO)3H-type hydrido complexes and supports Norton's view that the acidity of these hydrido complexes is not governed by the stabilization of the negative charge in the respective anions.