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

Abstract

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.

Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an alternative hydrogen source

“…The conversion followed a linear relationship with the catalyst amount in range 0-150 mg. The first-order dependence of the catalyst is clearly observed from the plot, which has been widely reported for other transfer hydrogenation processes [41,42]. Phenol conversion increased to 90% with catalyst amount of 150 mg, and the selectivity to cyclohexanone was 95%.…”

“…Interestingly, regardless of the amount (5 mmol or 21 mmol) or groups of reductants (HCOOH, HCOONa, or H 2 ), similar activities were observed under identical conditions in initial period. Previous studies have suggested that the rate-determining step of transfer hydrogenation is the hydride transfer on Pd surface [42]. Therefore this result may also indicate that the H-H and C H (H COO − ) dissociation are both very fast on Pd/AC catalyst.…”

‘‘Hydrogen-Free’’ Hydrogenation of Nitrobenzene Over Cu/SiO2 Via Coupling with 2-Butanol Dehydrogenation

“…Nagaraja et al[10] and Sawadjoon et al[56] reached a similar conclusion in their analysis of the gas phase coupling of cyclohexanol dehydrogenation with furfural hydrogenation (T = 453-523 K, P = 1 atm) over Cu-MgO-Cr 2 O 3 and liquid phase coupling of alcohol hydrogenolysis with formic acid as hydrogen donor (T = 353 K, P = 1 atm) over Pd/carbon, respectively. Moreover, there is evidence in the literature for the formation of transitory metal-H (Cu-H[57], Ru-H[58], Ag-H[59] and Ni-H[60]) species (via interaction with hydrogen abstracted from alcohols) that can serve as a source of reactive hydrogen.…”

Catalytic Transfer Hydrogenolysis as an Effective Tool for the Reductive Upgrading of Cellulose, Hemicellulose, Lignin, and Their Derived Molecules

“…They also tested other heterogeneous catalysts (Ir/C, Ni/C, Pd/C, Re/C, Rh/C) and Pd/C showed the higher reactivity in the cleavage of the β-O-4 C-O bond, allowing an efficient transformation to the corresponding aryl ketones and phenols in high yield, at 80 • C for 1-24 h. They proposed a reaction mechanism in which the first key step is the dehydrogenation of the α-CHO group followed by formation of a Pd-enolate complex, that undergoes a transfer hydrogenolysis process [119,120].…”