The role of a hydroxycyclopentadienyl ruthenium dicarbonyl formate in formic acid reductions of carbonyl compounds catalyzed by Shvo's diruthenium catalyst

Abstract

Addition of excess HCO2H to {2,5-Ph2-3,4-Tol2(η5-C4CO)]Ru(CO)2}2 (6) at -20°C led to the formation of [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2(η1-OCHO) (5), a proposed intermediate in catalytic transfer hydrogenations developed by Shvo. Hydroxycyclopentadienyl formate 5 undergoes rapid reversible dissociation of HCO2H at –20°C, and undergoes decarboxylation at 1°C to form a 1:10 mixture of {[2,5-Ph2-3,4-Tol2(η5-C4CO)]2H}Ru2(CO)4(µ-H) (3):[2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] (4). 5 does not reduce PhCHO below the temperature at which 5 is converted to hydride 4. The catalytic production of benzyl alcohol from 5 and PhCHO in the presence of excess HCO2H is not accelerated by higher concentrations of PhCHO, indicating that 5 does not directly reduce PhCHO. Formate complex 5 is the precursor of hydride 4 which transfers hydrogen to PhCHO. A crucial role for the CpOH proton in the decarboxylation of 5 was indicated by the much slower decarboxylation of the methoxycyclopentadienyl analog [2,5-Ph2-3,4-Tol2(η5-C4COCH3)]Ru(CO)2(η1-OCHO) (7). A mechanism for decarboxylation of 5 is proposed which involves reversible dissociation of formic acid to form the unsaturated dienone dicarbonyl ruthenium intermediate C, followed by simultaneous transfer of hydride to ruthenium from the formic acid carbon and of proton to the carbonyl of C from the formic acid OH group.Key words: Shvo catalyst, ruthenium formate, decarboxylation.