Salen is a commonly used chelating ligand of coordination chemistry and homogeneous catalysis. The name Salen is a combination of salicylaldehyde and ethylenediamine. Salen has four atoms, and can act as ligands for metals to form metal-Salen complexes in asymmetric catalytic epoxidation, cyclopropanation reaction, silicidation reaction, Strecker reaction, kinetic resolution and other reactions.
- Catalytic asymmetric epoxidation: The Salen-Mn complex gives good enantioselectivity (78% e.e.) in the catalytic asymmetric epoxidation of non-functionalized olefins. Using the Salen-Mn catalyst, a good asymmetric induction effect is obtained in the epoxidation of conjugated monosubstituted, cis disubstituted, trisubstituted and tetrasubstituted olefins.
Figure 1. Salen-Mn complex
- Kinetic resolution: Substances such as epoxy compounds are readily available in racemic form, and resolution by racemic epoxy compounds is another important method for obtaining optically active epoxides. Chiral Salen-Co can catalyze the kinetic resolution of racemic epoxy compounds to give chiral epoxy compounds with good enantioselectivity up to 99% e.e.
- Cyclopropanation reaction: Chiral cyclopropane structures are widely present in natural and synthetic products, many of which have important physiological activities. The development of asymmetric cyclopropane reactions for olefins has extremely important application value. The chiral Salen-Co complex catalyzes the asymmetric cyclopropane reaction of olefins to give products with high cis-trans ratio and high enantioselectivity.
Figure 2. The chiral Salen-Co complex catalyzes the asymmetric cyclopropane reaction
- Silicidation reaction: Optically active cyanohydrins are an important class of organic reaction intermediates that can be used to synthesize a series of important compounds such as A2 amino acids. Cyanohydrin can be obtained by addition of trimethylsilyl cyanide to an aldehyde compound. The use of Salen-Ti and Salen-VO to catalyze such reactions gives very good results.
- Strecker reaction: The asymmetric addition of cyanide to imines is called Strecker reaction, which provides a strategy for the synthesis of A2 amino acid derivatives. Chiral Salen complexes with different metal centers (Ti, Cr, Mn, Co, Ru, Al) have catalytic activity for this reaction. Chiral Salen-Al complexes give the best results.