PALLADIUM-CATALYZED TRANSFER HYDROGENOLYSIS OF BENZYL ACETATE WITH AMMONIUM FORMATE

Transfer hydrogenolysis of benzyl acetate, a model reaction for C-O hy drogenolysis, was achieved by ammonium formate and Pd/C at 20 degrees C. Hydrogen-donating abilities of various formate salts were found to depend on the counter-ion: K+ > NH4+ > Na+ > NHEt3+ > Li+ > H+. Kineti c studies using HCOONH4 revealed that the rate of transfer hydrogenoly sis was independent of the substrate (benzyl acetate) concentration. F irst order dependence was exhibited by both hydrogen donor (HCOONH4) a nd the catalyst (10% Pd/C). The initial reaction rate dropped from 46. 9 x 10(-3) mol L-1 min(-1) to 26.8 x 10(-3) mol L-1 min(-1) when HCOON H4 was replaced with DCOOND4 giving a calculated primary kinetic isoto pe effect of 1.75. From the kinetic and isotope effect data, a mechani sm has been proposed involving abstraction of formyl hydrogen by the c atalyst as the rate-limiting step. The rate law derived was R = k' [HC OONH4] [Pd/C]. Hydrogen isotope labeling studies using DCOOND4 as hydr ogen donor disclosed that the expected mono-deuterated toluene (C6H5CH 2D) was not formed exclusively. Instead, a mixture of deuterated tolue nes (C6H5CHxD3-x) was obtained, demonstrating that benzylic hydrogens are highly labile on the catalyst surface and exchange with the solven t.