Asymmetric catalysis of metal complexes with non-planar ONNO ligands: salen, salalen and salan.

Chiral metal (M)-salen complexes are one of the most versatile asymmetric catalysts and the catalysis of trans-M(salen) complexes has been well cultivated. On the other hand, non-planar cis-beta M(salen) complexes were recently found to show unique asymmetric catalysis that cannot be attained by trans-M(salen) complexes. Moreover, related non-planar M(salalen) and M(salan) complexes were also found to exert unprecedented asymmetric catalysis. This Feature Article summarizes the seminal studies on asymmetric catalysis of non-planar M(ONNO) complexes, full utilization of which will provide marked improvement in asymmetric synthesis.