Enantioselective Titanium-Catalyzed Sulfides Oxidation: Novel Ligands Provide Significantly Improved Catalyst Life

摘要

Catalytic enantioselective oxidations are among the most interesting transformations in asymmetric synthesis, as indicated by the large amount of research carried out in this field. Although some very satisfactory results have already been obtained, more work is needed before procedures of general applicability and a complete understanding of species involved in the oxidative processes become available. Excellent results have been obtained in allylic alcohols and sulfides oxidation with chiral titanium peroxo species bearing C_2 symmetric diols as ligands (e.g. tartaric.esters). A drawback of such Ti(IV)/(+)-diethyltartrate/alkyl hydroperoxide reagents is that they have a small turnover number. In addition, the structure of the real oxidants and of their precursors is not well defined. In fact, owing to the presence of different species and to the complexity of the equilibrium processes occurring in solution, these systems have not yet been completely characterized.