Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan.
Department of Chemistry, Kitasato University, Sagamihara 252-0329, Japan.
Phys Chem Chem Phys. 2020 Nov 4;22(42):24393-24398. doi: 10.1039/d0cp04827e.
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.
含有 RR- 或 SS-2,2'- 亚异丙基双（4-苯基-2-恶唑啉）（表示为 [Cu（RR- 或 SS-oxa）] 2+）的 Cu（ii）配合物已知能够催化许多不对称有机反应。在此，通过振动圆二色性（VCD）光谱研究了对映选择性的来源。添加非手性β-二酮配体（表示为 LH），例如 1-苯基-1,3-丁二酮和二苯甲酰甲烷，以形成 [Cu（RR- 或 SS-oxa）L]+。从 CDCl3 溶液中的 [Cu（RR- 或 SS-oxa)] 2+或 [Cu（RR- 或 SS-oxa）L]+在 1000-1800 cm-1 处获得了清晰的 VCD 峰。值得注意的是，当 LH 配位时，在约 1380 cm-1 处出现了一个新的 VCD 峰，这归因于 L-的 C-O 不对称伸缩振动。理论模拟有助于根据配位 L-转变为扭曲的手性形式来解释结果。展示了配位球内空间控制的程度，揭示了催化过程中对映选择性的第一步。
