Molybdenum(vi) cis-dioxo complexes bearing sugar derived chiral Schiff-base ligands: synthesis, characterization, and catalytic applications

Abstract

Molybdenum(VI)–cis-dioxo complexes bearing sugar derived chiral Schiff-base ligands of general formula MoO₂(L)(Solv) have been synthesized (with L = N-salicylidene-D-glucosamine; N-salicylidene-1,3,4,6-tetraacetyl-α-D-glucosamine; N-5-chlorosalicylaldehyde-1,3,4,6-tetraacetyl-α-D-glucosamine; N-salicylaldehyde-1,3,4,6-tetraacetyl-β-D-glucosamine; N-5-chlorosalicylaldehyde-1,3,4,6-tetraacetyl-β-D-glucosamine; N-salicylidene-4,6-O-ethylidene-β-D-glucopyranosylamine, and Solv = methanol or ethanol). Analytical data including IR, 1D- and 2D-NMR, MS and EA are in accord with their descriptions as monometallic compounds with one ligand L and a coordinated solvent molecule. One of the complexes and two of the chiral ligands have been examined by X-ray crystallography. In the case of the sugar –OH groups being protected as acetyl groups, one of them is selectively deacetylated and coordinates to the metal centre during the reaction process. Furthermore, an inversion takes place at the C1 carbon atom. This uncommon behaviour has been examined in some detail. The high catalytic activity of the title compounds for epoxidation is also described as well as the moderate enantiomeric induction of up to 30% ee for cis-β-methyl styrene.