MECHANISM OF PALLADIUM-CATALYZED TRANSFER HYDROGENOLYSIS OF ARYL CHLORIDES BY FORMATE SALTS

Chlorotoluene was hydrodehalogenated using formic acid and its salts in the presence of palladium-on-carbon (10% Pd/C). The hydrogen-donating ability of formate salts was found to depend on the counterion of the formates. The activity decreased in the order: Cs+ approximate to K+ > NH4+ > Na+ > NHEt(3)(+) > Li+ > H+. The addition of a base such as ammonium acetate to HCOOH accelerated the hydrogenolysis reaction rate establishing that HCOO- ion is essential for the reaction to proceed. Similarly, addition of a potassium salt to lithium formate increased the rate, again suggesting the importance of the counterion. Hydrogenolysis experiments using molecular hydrogen revealed that HCl formed during the reaction deactivated the catalyst but did not poison it completely. Kinetic studies showed that the rate of hydrodechlorination by the transfer process was independent of the substrate (2-chlorotoluene) concentration. Both hydrogen donor (HCOONa) and the catalyst (10% Pd/C) exhibited first order dependence. Based on the kinetic data and the observed isotope effect, a mechanism has been proposed involving abstraction of formyl hydrogen by the catalyst as the rate-limiting step. The rate expression derived was R = k' [HCOONa][Pd/C]. At higher concentrations of hydrogen donor, the dissociation of HCOONa was affected and as a consequence the rate expression changed to R = k'alpha[HCOONa][Pd/Cl], where alpha is the degree of dissociation of HCOONa.