CH/#pi# Attraction: The Origin of Enantioselectivity in Transfer Hydrogenation of Aromatic Carbonyl Compounds Catalyzed by Chiral #eta#~6-Arene-Ruthenium(II) Complexes

摘要

In asymmetric catalysis using chiral transition metal complexes, metal ligands generally affect the stability of stereo-determining transition states (TSs) by exerting electronic influences on metallic centers and also by through-space interactions with the assembled substrates. In our enantiose-lective asymmetric transfer hydrogenation of aryl alkyl ketones or [1-~2H]benzaldehydes by using 2-propanol or formic acid, we intuitively selected Ru~II(#eta#~6-arene) catalysts that possess a chiral 2-amino alcohol or related auxiliary. A theoretical study has now revealed that the enantioselectivity originates not only from the chiral geometry of the five-membered chelate ring but also from the CH/#pi# attractive interaction between the #eta#~6-arene ligand and the carbonyl aryl substituent, instead of conventional nonbonded repulsion. This TS stabilization is reminiscent of the origin of endo selectivity in the Diels-Alder reaction that is based on an attractive secondary interaction between nonreacting sites.