CATALYTIC DISPROPORTIONATION OF ALDEHYDES WITH RUTHENIUM COMPLEXES

It was discovered that the ruthenium complex [(C4Ph4COHOCC4Ph4)(mu-H)][(CO)4Ru2] (2), as well as other isostructural Ru complexes, in the presence of a catalytic amount of formic acid, catalyzes the homogeneous bimolecular disproportionation reaction of aldehydes to give esters: 2RCHO --> RCOOCH2R. The reaction was found to be general and compatible with a variety of aliphatic and aromatic aldehydes and can be carried out in the presence or absence of solvent under mild conditions. It is characterized by an excellent efficiency with an initial turnover frequency reaching 5000 h-1, a measured overall turnover number of ca. 20 000, and high conversion, yield, and selectivity. Increasing the electron density on the metal and the ligand was found to accelerate the reaction. Kinetic studies indicate that the rate = k[catalyst]1/2[aldehyde]. The rate also depends on the initial formic acid concentration. A stoichiometric reaction of complex 2 with formic acid, monitored by infrared spectroscopy, shed light on the identity of the active catalytic species. No kinetic isotope effect could be detected by using PhCDO and DCOOD as reactants. Consequently, a mechanism and a detailed catalytic cycle for the bimolecular transformation of aldehydes to esters were proposed.