Palladium-catalyzed transfer hydrogenolysis of benzyl acetate with ammonium formate Original Research Article

Transfer hydrogenolysis of benzyl acetate, a model reaction for CO hydrogenolysis, was achieved by ammonium formate and PdC at 20°C. Hydrogen-donating abilities of various formate salts were found to depend on the counter-ion: K+ > NH4+ > Na+ > NHEt3+ > Li+ > H+. Kinetic studies using HCOONH4 revealed that the rate of transfer hydrogenolysis was independent of the substrate (benzyl acetate) concentration. First order dependence was exhibited by both hydrogen donor (HCOONH4) and the catalyst (10% PdC). The initial reaction rate dropped from 46.9 × 10−3 mol L−1 min−1 to 26.8 × 10−3 mol L−1 min−1 when HCOONH4 was replaced with DCOOND4 giving a calculated primary kinetic isotope effect of 1.75. From the kinetic and isotope effect data, a mechanism has been proposed involving abstraction of formyl hydrogen by the catalyst as the rate-limiting step. The rate law derived was R = k′ [HCOONH4] [PdC]. Hydrogen isotope labeling studies using DCOOND4 as hydrogen donor disclosed that the expected monodeuterated toluene (C6H5CH2D) was not formed exclusively. Instead, a mixture of deuterated toluenes (C6H5CHxD3−x) was obtained, demonstrating that benzylic hydrogens are highly labile on the catalyst surface and exchange with the solvent.