Mechanistic Aspects on Cyclopentadienylruthenium Complexes in Catalytic Racemization of Alcohols

Cyclopentadienylruthenium complexes commonly serve as efficient transition metal catalysts in the racemization of alcohols. The combination of the racemization reaction with enzymatic resolution leads to dynamic kinetic resolution (DKR). In DKR, a theoretical yield of 100% is possible, making it a powerful tool for enantioselective synthesis. In this Account, we summarize the most important mechanistic aspects of racemization of alcohols reported over the past decade based on both experimental and computational results. Precatalyst activation is often necessary, either by heating the reaction or by adding an alkoxide-type base. The subsequent alcohol-alkoxide exchange is rapid and introduces the substrate into the catalytic cycle. This exchange requires a free coordination site, which may be created via several different mechanisms. Following alkoxide formation, racemization occurs via beta-hydride elimination and subsequent readdition. In cyclopentadienyldicarbonylruthenium alkoxide complexes, which are 18-electron complexes, researchers originally considered two mechanisms for the creation of the free coordination site required for beta-hydride elimination: a change in hapticity of the cyclopentadienyl ligand from eta 5 to eta 3 and dissociation of a CO ligand. Based on computational and experimental results, we have found strong support for the pathway involving CO dissociation. Researchers had also wondered if the substrate remains coordinated to the metal center (the inner-sphere mechanism) during the hydrogen transfer step(s). Using competition and crossover experiments, we found strong evidence for an inner-sphere mechanism. In summary, we have obtained a detailed picture of the racemization of alcohols by cyclopentadienylruthenium catalysts, leading to the development of more efficient catalytic systems for racemization.