Asymmetric Henry reaction catalyzed by chiral Cu(II) salalen and salan complexes derived from ( S )-proline

Abstract

Single chiral center C1 symmetric salalen and salan ligands were synthesized from (S)-proline and their Cu(II) complexes were used as catalysts for the asymmetric Henry reaction between aromatic aldehydes and nitromethane/nitroethane. The reaction of 4-nitrobenzaldehyde and nitromethane using salalen ligand (10 mol%) and Cu(OAc)2·H2O (10 mol%) in isopropanol with 4-methoxyphenol (10 mol%) as an additive at room temperature, afforded the (S)-2-nitro-1-(4-nitrophenyl)ethanol in 82% yield and 81% ee. We have also generalized the catalysis of nitro-aldol reaction for a variety of substrates using nitromethane, gave 67–94% yields with 49–98% ee after 96–120 h. The absolute configuration of nitro-aldol product was governed by the use of the metal, Mn(III) complex of the ligand 2 gives (R)-enantiomer while Cu(II) complex of same ligand gives the (S)-enantiomer.