The Mechanistic Study of Palladium-Catalyzed Conversion of Glycerol to Dihydroxyacetone

Glycerol can be efficiently converted to dihydroxyacetone
using
organometallic Pd catalysts. However, the entire catalytic cycle and
reaction mechanism have not been comprehensively studied. In this
work, we performed detailed theoretical calculations of the mechanism
of glycerol conversion to dihydroxyacetone catalyzed by the Pd/2,9-dimethyl-1,10-phenanthroline
complex. The results demonstrate that when benzoquinone serves as
the oxidant, C–H bond activation constitutes the rate-determining
step of the overall reaction. The activation barrier for the midposition
C–H in glycerol is lower than that of the terminal C–H
bond, which explains the observed high selectivity toward dihydroxyacetone
formation. Mechanistic analysis reveals that protons generated during
glycerol oxidation are efficiently transferred to benzoquinone via
water-mediated proton transport. In contrast, when molecular oxygen
is employed as the oxidant, the rate-limiting step shifts to the hydrogen
atom abstraction step that involves an oxygen molecule directly abstracting
a hydrogen atom coordinated on a Pd catalyst through an intermolecular
hydrogen transfer mechanism. The acetic acid molecules can promote
the catalytic cycle of the reaction.