Vibrational circular dichroism towards asymmetric catalysis: chiral induction in substrates coordinated with copper(ii) ions†
Cu(II) complexes containing RR- or SS-2,2′-isopropylidene-bis(4-phenyl-2-oxazoline) (denoted as [Cu(RR- or SS-oxa)]2+) are known to catalyse many asymmetric organic reactions. Herein, the source of enantioselectivity was investigated by vibrational circular dichroism (VCD) spectroscopy. An achiral β-diketonato ligand (denoted as LH), such as 1-phenyl-1,3-butanedione and dibenzoylmethane, was added to form [Cu(RR- or SS-oxa)L]+. Clear VCD peaks were obtained from a CDCl3 solution of [Cu(RR- or SS-oxa)]2+ or [Cu(RR- or SS-oxa)L]+ at 1000–1800 cm−1. It is to be noted that when LH was coordinated, a new VCD peak appeared at ∼1380 cm−1, which was assigned to the C–O asymmetric stretching vibration of L−. Theoretical simulation helped rationalise the results in terms of the transformation of coordinated L− into a twisted chiral form. The extent of steric control within the coordination sphere was demonstrated, revealing the first step for enantioselectivity during catalysis.