Asymmetric oxidation of aromatic sulfides with hydrogen peroxide catalyzed by transition metal complexes of polyphenol-appended β-cyclodextrin derivatives

The six modified beta-cyclodextrin (beta-CD) derivatives, which have neighboring phenolic oxygens (GA, PA, NH-PA and 2,3-HBA), one phenolic oxygen (SA) and no phenolic oxygen (BA), were synthesized, and the catalytic activity of their transition metal ion complexes in the asymmetric oxidation of aromatic sulfides having phenyl, naphthyl, and phenanthryl rings was examined in the pH range of 1-7. The hydroxy groups attached at the 3,4 and 5 positions of the phenyl ring in the polyphenol-appended beta-CD derivatives were found effective for the asymmetric oxidation of methyl 1-naphthyl sulfide in the presence of Mo(V) ion at pH of 6 to form the corresponding (R)-sulfoxide preferentially in the optical yields of 20-55%. Conversely use of the beta-CD derivatives with the phenolic hydroxy group at position of 2 resulted in the formation of (S)-sulfoxide in 19-35% ee. The optical yields also depend on the bridging group with which the 3,4-dihydroxybenzoyl group (PA) is bound to the primary C-6 position of beta-CD, that is, the Mo (V) complex of beta-CD-PA (ester bonding, optical yield 55%: the highest ee value obtained by the oxidation of methyl 1-naphthyl sulfide in a series of aromatic sulfides) shows higher stereoselectively than that of beta-CD-NH-PA (amide bonding, 41% ee). No chiral induction is observed in the presence of beta-CD-BA which has no phenolic hydroxy group. The complexes of beta-CD-PAA with Mo and W (4d- and 5d-blocks) metal ions accelerate the formation of (R)-sulfoxide with 36-55% ee, while those with 3d-block metal ions favor the (S)-sulfoxide production with 10-45% ee.