Chromium- and manganese-salen promoted epoxidation of alkenes

An overview of the development of Cr(salen)- and Mn(salen)-mediated asymmetric epoxidation (AE) from 1999 to March 2004 is given. First, the Cr(salen)- and Mn(salen)-mediated achiral epoxidations that provided the basis for asymmetric variants are described. Second, early work on Mn(salen)-catalyzed AE is outlined focusing on research from the groups of Jacobsen and Katsuki. The development of alternative ligand architectures and Katsuki's second-generation catalysts is also discussed. Third, the variety of stoichiometric oxidants and additives are examined as well as the problem of production of trans-1,2-distributed epoxides and the various solutions to this so far offered. Furthermore, research on the nature of the active species, the trajectory of substrate approach, and the mechanism of the reaction is outlined. Finally, the development of Cr(salen)-mediated AE is also reviewed so as to aid comparison between the two asymmetric systems.