Synthesis and characterization of new chiral Cu(ii)–N4 complexes and their application in the asymmetric aza-Henry reaction†
Cu(II) Schiff base complexes Cu(II)-1 and Cu(II)-3 based on 2-acetyl pyridine with both (1R,2R)-1,2-diaminocyclohexane and (1S,2S)-1,2-diaminocyclohexane were synthesized in a single step. Subsequent reduction of ligands 1 and 3 with NaBH4 followed by complexation with Cu(OTf)2 resulted in generation of two more additional chiral centers in complexes Cu(II)-2 and Cu(II)-4. The ligands 1–4 and their corresponding complexes were well characterized by several spectral techniques like 1H-NMR, 13C-NMR, LC-MS, CD, UV-Vis spectroscopy and microanalysis. The respective Cu(II) complexes derived from ligands 2 and 4 were investigated using both the solution and solid state EPR spectra. The particular orientation of the reduced complex with Cu(OTf)2 was confirmed by the X-ray crystal structure of the corresponding complex. All the catalytic protocols were applied in the asymmetric aza-Henry reaction to evaluate the catalytic properties of the Cu(II) complexes in the present study.