Enantioselective Photooxidation of a Sulfide by a Chiral Ruthenium(II) Complex Immobilized on a Montmorillonite Clay Surface: The Role of Weak Interactions in Asymmetric Induction

The present work pursued a possibility that enantioselectivity was achieved through weak intermolecular interactions between a catalyst and a substrate. For that purpose, we studied the photooxidation of alpha-ethylbenzyl phenyl sulfide catalyzed by a polypyridyl ruthenium(II) complex as a chiral photosensitizer. No covalent bonding was formed between a catalyst and a substrate, because the complexes used ([Ru(phen)(3)](2+) or [Ru(bpy(3))(2+)]) were coordinatively saturated. Enantiomer excess (ee) was attained to be 30% when a chiral photosensitizer was immobilized on montmorillonite clay. It was even improved to 43% in the presence of an additional chiral auxiliary, dibenzoyl-D(+)-tartaric acid. Notably, no enantioselectivity was achieved when the reaction took place in homogeneous solutions. The ab initio calculations were performed on the stability of an associate composed of a catalyst (metal complex) and a product (sulfoxide) to obtain a clue to reaction mechanisms. The calculations suggest that chiral discrimination is achieved even through noncovalent interactions between a substrate and a chiral sensitizer when the attacking direction by a substrate toward a catalyst is limited sterically on a solid surface.