MECHANISTIC ASPECTS OF DECARBOXYLATION REACTIONS OF GROUP-10 (PCY3)2M(H)O2CH DERIVATIVES

Kinetic data for the 13CO2 exchange reactions of the trans-(Cy3P)2M(H)O2CH (M = Ni, Pd) derivatives to afford the corresponding trans-(Cy3P)2M(H)O213CH species are presented. These processes were found to exhibit a first-order dependence on the metal complex and to be independent of tricyclohexylphosphine or carbon dioxide concentrations. The activation parameters for the nickel complex were determined to be ΔH* = 22.1 (9) kcal·mol-1 and ΔS* =-5 (3) cal·mol-1·deg-1, whereas the analogous parameters for the palladium complex were found to be ΔH* = 21 (2) kcal·mol-1 and ΔS* = 4 (8) cal·mol-1·deg-1. The reaction pathways are proposed to proceed via rate-determining CO2 extrusion to provide the dihydride complexes, followed by rapid 13CO2 insertion into the M-H bond to yield the formate derivatives. Consistent with this proposal the carbon dioxide exchange rate is greater than 1200 times faster for the palladium derivative, where the (Cy3P)2Pd(H)2 intermediate is a stable species, than for its nickel analogue, where the corresponding dihydride complex is unknown. Kinetic parameters for the intramolecular C-H/Ni-D exchange process involving trans-(Cy3P)2Ni(O2CH)D are indicative of a reaction pathway coincident with that defined for intermolecular CO2 exchange. © 1990, American Chemical Society. All rights reserved.