Revised Mechanisms for Aldehyde Disproportionation and the Related Reactions of the Shvo Catalyst

It is widely believed that the Shvo catalyst (1) dissociates to form two active species in solution: the 18-electron hydride RuH­(CO)2[η5-C5(OH)­Ph4] (2) and the naked 16-electron complex Ru­(CO)2[η4-C5(O)­Ph4] (3). This combined experimental/computational study demonstrates that a sustained presence of 3 is not viable in the reactions of alcohols and organic carbonyls; thus, 3 is better treated as nonexistent under the typical catalytic conditions. We propose a modified view where the key catalytic species are the hydride 2 and the 18-electron metal alkoxide intermediate Ru­(OR)­(CO)2[η5-C5(OH)­Ph4] existing in equilibrium with the corresponding alcohol complex. An X-ray crystallographic study of 2 revealed an interesting dihydrogen-bonded dimer structure in the solid state. The mechanistic ideas of this paper explain the highly efficient Tishchenko-like aldehyde disproportionation reaction with the Shvo catalyst. Additionally, our observations explain why 1 is inefficient for hydrogenation of ethyl acetate and for the acceptorless dehydrogenative coupling of ethanol. Our findings provide practical guidance for future catalyst design on the basis of the Shvo ruthenium dimer prototype.