Pd-Catalyzed Transfer Hydrogenolysis of Primary, Secondary, and Tertiary Benzylic Alcohols by Formic Acid: A Mechanistic Study

A palladium-catalyzed transfer hydrogenolysis of primary, secondary, and tertiary benzylic alcohols by formic acid has been developed and studied. The product hydrocarbons were obtained in excellent yields from both secondary and tertiary benzylic alcohols and in good yields for primary benzylic alcohols. The rate of disappearance of 1-phenylethanol (1) follows zero-order dependence in 1 and first-order dependence in formic acid and palladium. Catalytic amounts of base inhibit a competing disproportionation reaction of alcohol to alkane and ketone, and an optimum was obtained when 5 equiv of base to palladium was used Deuterium kinetic isotope studies for the transfer hydrogenolysis reveal individual isotope effects for the hydridic position (k(CHOH)/k(CDOH) = 2.26 +/- 0.24) and the protic position (k(CHOH)/k(CHOD) = 0.62 +/- 0.06) of the formic acid. Simultaneous deuteration in both positions of formic acid gave a combined isotope effect of (k(CHOH)/k(CDOD) = 1.41 +/- 0.11). We propose a mechanism involving the following steps: a competitive inhibition of the open palladium site by adsorption of the formate anion to generate formato-palladium species, followed by a reversible protonation and a rate-limiting hydride transfer to obtain the active palladium with chemisorbed hydrogen that performs the hydrogenolysis of the alcohol in a fast reaction step.