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

Addition of excess HCO2H to {2,5-Ph-2-3,4-Tol(2)(eta (5)-C4CO)]Ru(CO)(2)}(2 ) (6) at -20degreesC led to the formation of [2,5-Ph-2-3,4-Tol(2)(eta (5)-C 4COH)]Ru(CO)(2)(eta (1)-OCHO) (5), aproposed intermediate in catalytic tran sfer hydrogenations developed by Shvo.Hydroxycyclopentadienyl formate 5 und ergoes rapid reversible dissociation of HCO2H at -20degreesC, and undergoes decarboxylation at 1degreesC toform a 1:10 mixture of {[2,5-Ph-2-3,4-Tol(2 )(eta (5)-C4CO)](2)H}Ru-2(CO)(4)(mu -H) (3):[2,5-Ph-2-3,4-Tol(2)(eta (5)-C4 COH)Ru(CO)(2)H] (4). 5 does not reduce PhCHO below the temperature at which 5 is converted to hydride 4. The catalytic production of benzyl alcohol fr om 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 t o PhCHO. A crucial role for the CpOH proton in the decarboxylation of 5 was indicated by the much slower decarboxylation of the methoxycyclopentadieny l analog [2,5-Ph-2-3,4-Tol(2)(eta (5)-C4COCH3)]Ru(CO)(2)(eta (1)-OCHO) (7). A mechanism for decarboxylation of 5 is proposed which involves reversible dissociation of formic acid to form the unsaturated dienone dicarbonyl rut henium intermediate C, followed by simultaneous transfer of hydride to ruth enium from the formic acid carbon and of proton to the carbonyl of C from t he formic acid OH group.