Unexpected Direct Hydride Transfer Mechanism for the Hydrogenation of Ethyl Acetate to Ethanol Catalyzed by SNS Pincer Ruthenium Complexes.

The hydrogenation of ethyl acetate to ethanol catalyzed by SNS pincer ruthenium complexes was computationally investigated by using DFT. Different from a previously proposed mechanism with fac-[(SNS)Ru(PPh3 )(H)2 ] (5') as the catalyst, an unexpected direct hydride transfer mechanism with a mer-SNS ruthenium complex as the catalyst, and two cascade catalytic cycles for hydrogenations of ethyl acetate to aldehyde and aldehyde to ethanol, is proposed base on our calculations. The new mechanism features ethanol-assisted proton transfer for H2 cleavage, direct hydride transfer from ruthenium to the carbonyl carbon, and C-OEt bond cleavage. Calculation results indicate that the rate-determining step in the whole catalytic reaction is the transfer of a hydride from ruthenium to the carbonyl carbon of ethyl acetate, with a total free energy barrier of only 26.9 kcal mol-1 , which is consistent with experimental observations and significantly lower than the relative free energy of an intermediate in a previously postulated mechanism with 5' as the catalyst.