Synthesis,characterization and catalytic oxidations of oxovanadium(IV), oxotitanium(IV) and dioxomolybdenum(VI) complexes with chiral imines of L-amino acids

Representative oxovanadium(IV), oxotitanium(IV), and dioxomolybdenum(VI) complexes of N-salicylidene-L-amino acids (the amino acids are: valine, leucine, except for Mo(VI), and histidine) have been synthesized and characterized by various spectroscopic techniques. It has been found that while in the histidine complexes of titanium(IV) and vanadium(IV) the amino acid residue is bound in the expected glycine-like mode, in the molybdenum(VI) complex it is bound in the unusual histamine-like mode. Also, the structure of this molybdenum(VI) complex contains imidazolate-bridged polymeric units in the solid state, while the carboxyl group of the amino acid residue is protonated. In solution the polymeric structure is cleaved and the monomers contain carboxylate and protonated imidazole groups at the histidine residue. The histamine-like structure of the histidine complex was probcd by comparison with that of the chiral dioxomolybdenum(VI) complex of N-salicylidene-L-histidinol. While the structure of the metal centers is six-coordinate for the dioxomolybdenum(VI) complexes and the histidine complexes of oxotitanium(IV) and oxovanadium(IV), it is likely that the complexes of the latter metals derived from nonpolar amino acids do not achieve coordination numbers higher than five. The present oxometal complexes are catalytically active in the sulfoxidation of sulfides and in the epoxidations of activated olefins by tert-butyl hydroperoxide, but in general exhibit a low degree of asymmetric induction in these reactions.