Recent Advances in Asymmetric Transfer Hydrogenation

Professor Takao Ikariya discussed his work on including a dynamic kinetic resolution of a–substituted ketones.  If a racemic a–substituted ketones is subjected to transfer hydrogenation conditions using a chiral ruthenium catalyst [RuCl[(S,S)-Tsdpen](p-cymene)] and triethylamine / formic acid, four possible products ([R,R], [R,S], [S,R], [S,S]) can be formed as shown below.  The relative amounts of each product depends on the relative rates of hydrogenation of the two enantiomeric ketones, the rate of inversion of the ketones and the effectiveness of the catalyst in asymmetric induction.

In many cases (X = OH, OCH3, OiPr) over 93% of the product is the [R,R] diastereomer, for X = OAc 85% of the product is the [R,R] product, whilst for X = OBz the reaction is far less effective.  In this case the product contains all 4 possible diastereomers ([R,R] 32%, [R,S] 30%, [S,R] 29%, and [S,S] 9%).