Synthesis and characterisation of Cu(II) complexes bearing N,N′-di(methoxybenzyl)-(R,R)-1,2-diaminocyclohexane as catalysts for the asymmetric nitroaldol reaction

Enantiopure Cu(II) complexes with N,N′-di(p-methoxybenzyl)-(R,R)-1,2- diaminocyclohexane (L¹), N,N′-di(m-methoxybenzyl)-(R,R)-1,2- diaminocyclohexane (L²), and N,N′-di(m-methoxybenzyl)-(R,R)-1, 2-diaminocyclohexane (L³) ligands were synthesised and characterised. The X-ray crystal structures of Cu(II) complexes containing L¹ and L³ were determined. The geometry around the Cu(II) centre for (L ¹)CuCl₂ was distorted square-planar. The crystal structure of (L³)CuCl₂ revealed a square-pyramidal structure with a strong interaction between the Cu and O of the methoxy group. Their double catalytic activation with diisopropylethylamine in the asymmetric nitroaldol reaction of benzaldehyde and nitromethane was examined. The reaction yield was moderate with an enantiomeric excess of up to 87%.