Chiral dioxovanadium(V) Schiff base complexes of 1,2-diphenyl-1,2-diaminoethane and aromatic o-hydroxyaldehydes: Synthesis, characterization, catalytic properties and structure

A series of dioxovanadium(V) complexes of tridentate ligands obtained by monocondensation of chiral 1,2-diphenyl-1,2-diaminoethane and aromatic o-hydroxyaldehydes was synthesized. The complexes were characterized by spectroscopic methods in the solid state (IR) and in solution (UV–Vis, CD, 1H and 51V NMR). Single crystal X-ray analysis was performed with (VO2L · H2O)2, denoted as (4 · H2O)2, where L is (S,S)-1-amino-2-{(2′-oxido-4′,6′-dimethoxyphenyl)methylene}amino-1,2-diphenylethane. Crystal structure analysis revealed that (4 · H2O)2 contains oxo-bridged dimers of 4 joined with water molecules by hydrogen bonding interactions, and that two five-membered chelate rings in the dimeric molecule adopt different envelope conformations, one on the asymmetric carbon atom linked to the azomethine nitrogen and the other on the asymmetric carbon atom linked to the primary amino nitrogen. The (S,S)- and (R,R)-complexes bearing the methoxy substituent in positions 3 or 5 of the salicylidene moiety catalyze the oxidation of phenyl methyl sulfide by cumene hydroperoxide yielding the corresponding sulfoxide almost quantitatively with 34–39% enantiomeric excess.

Ten new chiral VO2L complexes, where L is the deprotonated product of single condensation of SS(-)-1,2-diphenyl-1,2-diaminoethane and RR(+)-1,2-diphenyl-1,2-diaminoethane with aromatic O-hydroxyaldehydes, were synthesized and characterised by spectroscopic methods. Crystal structure of the (VO2L)2·2H2O complex where L is the SS(–)-1-amino-2-N-[(2′,-oxido-κO-4′,6′,-dimethoxyphenyl)methylene]amino-1,2-diphenylethane was determined. Complexes with ligands bearing methoxy substuents in positions 3 and 5 of the phenyl ring were found to act as catalysts in asymmetric oxidations of prochiral methyl phenyl sulfide to give the enantiometrically enriched sulfoxides.Figure optionsDownload as PowerPoint slide