Carboxylate ion dependency in the Cu(II) catalysed asymmetric Henry reaction: Structural characterisation of a tridentate Schiff base complex containing a coordinated carboxylic acid

Abstract

Six tridentate Schiff base ligands containing tertiary butyl or benzyl substituents were prepared from chiral amino alcohols and salicylaldehyde derivatives. The ligands were employed as catalysts for the Cu(II) catalysed asymmetric Henry reaction. It was discovered that when different carboxylate salts were used instead of copper acetate as the Cu(II) salt, significant changes in the enantioselectivity of the reactions were observed. Addition of Cu(OAc) 2 to the ligand prepared from salicylaldehyde and α,α-diphenyl-tert-leucinol resulted in the formation of dark green crystals. X-ray structural analysis of these crystals showed that a square planar monomeric complex had been formed rather than the expected dimer. In the structure, the copper(II) centre is bonded to the tridentate ONO ligand and an acetate ion. There is a strong hydrogen bond between the protonated alcoholic oxygen of the Schiff base ligand and the uncoordinated acetate oxygen atom. These results, taken together, indicate that the carboxylate anion may be an important part of the active intermediate when this type of copper complex is used as a catalyst in the asymmetric Henry reaction. KEYWORDSasymmetric Henry reaction, chiral copper (II) catalyst, crystal structure, intramolecular hydrogen bond, Schiff base complex

Synthesis, Characterization, and Biological Activity of Mixed ligand Complexes from 8-Hydroxyquinoline and new Ligand for β-Enaminone

“…The chemical shift at (δ= 32.58 ppm) refers to C20 for aliphatic ring. Finally, The carbon atoms C22 of CH3 groups resonated with the chemical shifts at (δ = 28.41 ppm) [50], [51] . Table (4) gives a brief summary of the results.…”

Enantioselective Henry Reaction Catalyzed by Chiral Piperazine Derivatives

“…In our previous publications we reported the synthesis and catalytic activity of copper(II) complexes of triden-tate Schiff base ligands derived from amino acids. [20][21] Herein, we report novel chiral tridentate Schiff bases which can be readily synthesized from piperazine-amine and can be used as catalysts in enantioselective Henry reactions under mild conditions. We thought that the N group in the ring, which is not coordinated to the metal, will increase the enantioselectivity due to the bifunctional capability of the piperazine derivatives.…”