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.
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
Shuji
S
Fujita
Hisako
H
Sato
Norishige
N
Kakegawa
Akihiko
A
Yamagishi
D001546:Bentonite / Q000737:chemistry*; D002384:Catalysis; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010777:Photochemistry; D012428:Ruthenium / Q000737:chemistry*; D013237:Stereoisomerism; D013440:Sulfides / Q000737:chemistry*; D013499:Surface Properties
D009942:Organometallic Compounds; D013440:Sulfides; C521593:alpha-ethylbenzyl phenyl sulfide; D001546:Bentonite; D012428:Ruthenium