Ligand Design for Oxidation

Department of Chemistry, Keio University, Hiyoshi, Yokohama, Japan 

New Frontiers in Asymmetric Catalysis, Edited by Koichi Mikami and Mark Lautens Copyright 2007 John Wiley Sons, Inc. 

In this chapter, the ligand design for the catalytic enantioselective oxidations developed after the Katsuki-Sharpless epoxidation and the Sharpless AD will be discussed. 

The electronic properties of chiral catalysts were examined. Condensation of the optically active 1,2-diphenylethylenediamine with appropriate C5(5 )-substituted terf-butyl salicylaldehyde derivatives followed by complexation with mangane-se(III) center led to the corresponding catalysts 12a-12e. Then three model substrates, 2,2-dimethylchromene, cA-(3-methylstyrene, and cA-2,2-dimethyl-3-hexene, were subjected to enantioselective expoxidation catalyzed by 5-substituted 

In the presence of a catalytic amount of the optically active Mn(III)-saIen complexes 23 and 24, the enantioselective epoxidation of unfunctionalized olefins with the combined use of molecular oxygen and pivaMdehyde was demonstrated. This report revealed that the tert-butyl group on the C3 position of salicylaldehyde in the chiral ligand was essential to realize a high enantioselection, and that pivalaldehyde was the most effective reductant for both the enantioselectivity and chemical yield. It should be noted that the sence of enantioselectivity to form the epoxide from 1,2- 

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The failure to achieve electrochemical oxidation of this latter cobalt(II) complex [to its cobalt(III) state] has been ascribed to the lower overall stability of this species. This result raises the prospect of employing such subtle steric effects in ligand design for the rational tuning of redox behaviour. 

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