Synthesis and evaluation of hemisalen type ligands based on chiral diamine and their use with ruthenium (II) as water-soluble catalysts for the ATH of aromatic ketones

Abstract

We have developed a robust ruthenium (II) complex catalyst coordinated by chiral Schiff bases for the reduction of acetophenone by ATH in water. The results show a significant effect of the nature of the chiral entity on the reactivity and the selectivity of the catalytic system. The Schiff base synthesized from the chiral diamine is more reactive and selective than the Schiff base synthesized from chiral amino alcohol. The reduction of acetophenone by catalysts coordinated with hemisalen ligands synthesized from chiral cyclohexyl diamine (1–4) gives high asymmetric inductions with ee up to 84% and total conversion. The effectiveness of the best ligand has been evaluating on a set of aromatic/heterocyclic ketones and was resulted to the corresponding alcohols with high enantioselectivities and good yields. A multigram-scale of the reduction process was applied on chroman-4-one and giving access to (R)-(+)-chromanol-4-ol with 87% ee and yield of 79%.