Racemization of chiral secondary alcohols

The remarkable activity of copper catalysts in carbonyl hydrogenation and alcohol dehydrogenation prompts their use also for the racemization of chiral secondary alcohols. Actually, since the first report on chemoenzymatic dynamic kinetic resolution [68], racemization of alcohols via the corresponding ketone has attracted considerably attention, owing to its role as backbone in this resolution [69, 70]. 

Some of these catalyze the smooth racemization of chiral secondary alcohols at room temperature. However, a major problem which needed to be solved in order to design an effective combination of ruthenium catalyst and lipase in a DKR of secondary alcohols was the incompatibility of many of the ruthenium catalysts and additives, such as inorganic bases, with the enzyme and the acyl donor. For example, the ruthenium catalyst may be susceptible to deactivation by the acetic acid generated from the acyl donor when it is vinyl acetate. Alternatively, any added base in the racemization system can catalyze a competing selective transesterification of the alcohol, resulting in a decrease in enantioselectivity. Consequently, considerable optimization of reaction protocols and conditions was necessary in order to achieve an effective DKR of secondary alcohols. 

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