Scope and Mechanistic Analysis for Chemoselective Hydrogenolysis of Carbonyl Compounds Catalyzed by a Cationic Ruthenium Hydride Complex with a Tunable Phenol Ligand

A cationic
ruthenium hydride complex, [(C6H6)­(PCy3)­(CO)­RuH]+BF4– (1), with a phenol ligand was found
to exhibit high catalytic activity for the hydrogenolysis of carbonyl
compounds to yield the corresponding aliphatic products. The catalytic
method showed exceptionally high chemoselectivity toward the carbonyl
reduction over alkene hydrogenation. Kinetic and spectroscopic studies
revealed a strong electronic influence of the phenol ligand on the
catalyst activity. The Hammett plot of the hydrogenolysis of 4-methoxyacetophenone
displayed two opposite linear slopes for the catalytic system 1/p-X-C6H4OH (ρ
= −3.3 for X = OMe, t-Bu, Et, and Me; ρ
= +1.5 for X = F, Cl, and CF3). A normal deuterium isotope
effect was observed for the hydrogenolysis reaction catalyzed by 1/p-X-C6H4OH with an
electron-releasing group (kH/kD = 1.7–2.5; X = OMe, Et), whereas an inverse isotope
effect was measured for 1/p-X-C6H4OH with an electron-withdrawing group (kH/kD = 0.6–0.7;
X = Cl, CF3). The empirical rate law was determined from
the hydrogenolysis of 4-methoxyacetophenone: rate = kobsd[Ru]­[ketone]­[H2]−1 for
the reaction catalyzed by 1/p-OMe-C6H4OH, and rate = kobsd[Ru]­[ketone]­[H2]0 for the reaction catalyzed
by 1/p-CF3-C6H4OH. Catalytically relevant dinuclear ruthenium hydride and
hydroxo complexes were synthesized, and their structures were established
by X-ray crystallography. Two distinct mechanistic pathways are presented
for the hydrogenolysis reaction on the basis of these kinetic and
spectroscopic data.