Use of Transition Metals and Enzymes in Tandem

Since the demonstration of the compatibility of enzymes with metal complexes in one pot, this powerful concept has attracted much attention. Indeed, the use of transition metal nzyme combinations, independently highlighted by Reetz and Schimossek, ° Sturmer and BackvalP to effect tandem in situ racemisation and resolution has widely extended the scope of DKRs. In this powerful approach, the enzyme acts as an enantioselective resolving catalyst and the metal serves as a racemising catalyst for the efficient DKR. 

But in order to utilise these reactions, a few conditions must be met. A selective enzyme is crucial and the organometallic catalyst must facilitate a fast racemisation of the substrate. Last, but not least, the catalyst should not influence the enzyme in terms of selectivity and reactivity. In the ideal case, the enzyme transforms one enantiomer of the substrate, giving rise to the corresponding product, which is not susceptible to metal-catalysed racemisation. Three major types of enzyme-metal combinations—lipase-ruthenium, sub-tilisin-ruthenium and lipase combined with a metal other than ruthenium—have been developed primarily as the catalysts for the DKRs of various secondary alcohols but also for diols, amines and esters. Meanwhile, the lipase-ruthenium combination has been the most used method up to the present time. 

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