Metal hydride reduction diastereoselectivity

Usually, a stoichiometric amount of metal hydride, such as a selectride reagent, is required for diastereoselective reduction of aliphatic ketones to secondary alcohols. Now the same purpose can be achieved by the Ru-catalysed diastereoselective... 

Higher substituted five- and six-membered lactones were also prepared by this procedure, but the enantiomeric excesses of the products were lower. The authors explain the diastereoselectivity of the reduction by a chelate complex in which the metal ion coordinates the hydroxyl group at C-5, one ring oxygen atom and the carbonyl group, which is then attacked by the complex hydride. 

In the presence of a metal cation capable of chelation, transition state (8) was proposed to be favoured the high levels of diastereoselectivity observed in the ZnCyDIBAL reduction can therefore be rationalized on the basis of intermolecular hydride transfer from the least hindered face of the chelated, prochiral carbonyl system. 

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