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

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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-α- d-glucopyranose (H 2L) were synthesized via two different routes. The reaction of the Schiff-base ligand H 2L 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 1 and 2, which can be isolated by means of fractional crystallization. The complexes were characterized with elemental analysis, 51V, 1H and 13C NMR, IR spectroscopy, MS and in case of 1 by X-ray diffraction. Complex 1 crystalizes in the orthorhombic space group P2 12 12 1 with a distorted trigonal bipyramidal geometry at the vanadium center ( τ = 0.58). Under hydrolytic conditions 1 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.