Asymmetric hydrogenation of an a-unsaturated carboxylic acid catalyzed by intact chiral transition metal carbonyl clusters - diastereomeric control of enantioselectivity

Twenty clusters of the general formula [(mu-H)(2)Ru-3(mu(3)-S)(CO)(7)(mu-P-P*)] (P-P* = chiral diphosphine of the ferrocene-based Walphos or Josiphos families) have been synthesised and characterised. The clusters have been tested as catalysts for asymmetric hydrogenation of tiglic acid [trans-2-methyl-2-butenoic acid]. The observed enantioselectivities and conversion rates strongly support catalysis by intact Ru-3 clusters. A catalytic mechanism involving an active Ru-3 catalyst generated by CO loss from [(mu-H)(2)Ru-3(mu(3)-S) (CO)(7)(mu-P-P*)] has been investigated by DFT calculations.