Optically active metallosalen complexes as catalysts for atom-efficient asymmetric reactions

Salen (N,N′-ethylenebis(salicylideneimine)] ligands are readily prepared by condensation of o-hydroxybenzaldehydes and diamines, and are known to form a variety of metal complexes. In the last decade, optically active metallosalen complexes have emerged as very useful catalysts for various asymmetric transformations, and high enantioselectivities and unique catalytic properties have been acquired by dictating the aldehyde, diamine, and central metal units. Of particular note is their tremendous contribution to recent rapid progress in atom-efficient asymmetric reactions, which emit no waste or only nontoxic byproducts of small molecular weight such as water and nitrogen. Described in this review are (1) asymmetric and chemoselective aerobic oxidations under photoactivation, (2) asymmetric oxidations with hydrogen peroxide regulated by cis-β salen complexes, (3) enantio- and diastereo-selective cyclopropanations of alkenes with diazo compounds, (4) asymmetric aminations with azides, and (5) asymmetric ring opening reactions of epoxides and addition reactions to carbonyl substrates by dual activation methods.