Phase transfer-catalyzed reduction of aromatic aldehydes by aqueous sodium formate in the presence of dichlorotris(triphenylphosphine)ruthenium(II): a kinetic study

The kinetics of transfer hydrogenation of aromatic aldehydes by aqueous sodium formate, in the presence of RuCl₂(PPh₃)₃ and a quaternary ammonium salt, have been measured between 20 and 79 °C. Under phase transfer conditions the bicatalytic process was shown to depend on the structure and concentrations of the ammonium salt and of the aldehyde acceptor, on the concentrations of the formate donor and the metal catalyst, and to be sensitive to both the polarity of the organic solvent and the stirring rate. In a solution of concentration up to 5 M, a linear correlation was found to exist between the concentration of the aqueous formate and the reaction rate. The aldehyde substrate proved to exert an autoinhibitory effect on the catalysis. The observed activation energy, E_a = 16.7 kcal mol^-1, suggests that the process is both chemically and diffusion controlled. A general reaction mechanism and rate equation have been proposed.