Chiral oxovanadium(V) complexes with a 6-amino-6-deoxyglucopyranoside-based Schiff-base ligand: Catalytic asymmetric sulfoxidation and structural characterization

Two new chiral oxovanadium(V) complexes [VO(OMe)(L)] (1) and [VO(Osal)(L)] (2) derived from the Schiff-base ligand 6-N-[(3,5-di-tert-butyl-salicylidene)amino]-6-deoxy-1,2,3-tri-O-methyl-alpha-D-glucopyranose (H(2)L) were synthesized via two different routes. The reaction of the Schiff-base ligand H(2)L with ammonium metavanadate in hot methanol as well as with tris(isopropoxy)oxovanadium(V) in diethyl ether at room temperature leads to a mixture of complexes I and 2,which can be isolated by means of fractional crystallization. The complexes were characterized with elemental analysis, (51)V, (1)H and (13)C NMR, IR spectroscopy, MS and in case of I by X-ray diffraction. Complex 1 crystalizes in the orthorhombic space group P2(1)2(1)2(1) with a distorted trigonal bipyramidal geometry at the vanadium center (tau = 0.58). Under hydrolytic conditions I forms the cis-dioxovanadium(V) complex [VO(2)(MeOH)(HL)] (3) which can be monitored by NMR spectroscopy. Complexes 1 and 2 were tested as catalysts for sulfoxidation of different sulfide substrates PhSR (R = Me, Bz) utilizing hydrogen peroxide or tert-butyl hydroperoxide (TBHPO) as oxidant in dichloromethane as solvent. The yield as well as the enantiomeric excess were found to strongly depend on the catalyst, substrate and oxidant used. (C) 2008 Elsevier B.V. All rights reserved.