Highly Diastereoselective Formation of Ruthenium Complexes for Efficient Catalytic Asymmetric Transfer Hydrogenation

摘要

The development of more efficient asymmetric catalysts for organic transformations is a topic of great interest for both industrial applications and academic research. The choice of the chiral ligand for transition-metal complexes is a key factor in attaining a high level of asymmetric induction. Complexes containing two appropriate chiral ligands have been successfully employed to increase the level of enantioselectivity in catalytic reactions (the matched-ligands approach). This method is relatively tedious and requires the isolation of a library of enantiomerically pure ligands which need to be correctly assembled. A particularly successful example is the trans-[RuCl2(PP)diamine] (PP = diphosphane) system in which the correct combination of chiral diphosphane and diamine ligands leads to high enantioselectivity in the catalytic hydrogenation of ketones. To overcome the problem of using two precious chiral ligands, different strategies have been developed. Efficient asymmetric catalytic systems have been obtained by reaction of a racemic metal complex, prepared from a racemic ligand, with a suitablechiral auxiliary, leading to deactivation (chiral poisoning) or activation of one metal enantiomer species. Alternatively, efficient chiral catalysts have been prepared from a chirally flexible (tropos) ligand in combination with a rigid one.