Highly Active Cp*Ir Catalyst at Low Temperatures Bearing an N‑Heterocyclic Carbene Ligand and a Chelated Primary Benzylamine in Transfer Hydrogenation

The synthesis of new Cp*Ir complexes bearing an N-heterocyclic carbene ligand and a chelated primary benzylamine is described. The new complexes are chiral at metal and have a stereogenic carbon at the benzylamine ligand. The synthesis is diastereoselective, and the origin is thermodynamically controlled. The chiral complexes have been fully characterized. The catalytic results show that the complexes are very active in transfer hydrogenation: for example, acetophenone is reduced to 1-phenylethanol in 2 h at 50 °C using a catalyst loading of 1 mol %. More interestingly is that no base, apart from the required for catalyst activation, is needed in the process. The enantioselectivities obtained range from low to moderate, with a maximum of 58% ee in the case of 2′-methylacetophenone. Initial mechanistic studies by means of DFT calculations suggest that the mechanism is based on a direct hydrogen transfer via a highly ordered transition state centered at the iridium amido group. The calculations are in good agreement with the experimental data and support a concerted one-step mechanism process.

本文报道了一类新型五甲基环戊二烯基铱（Cp*Ir）配合物的合成方法，该类配合物携带有氮杂环卡宾（N-heterocyclic carbene）配体与螯合型一级苄胺配体。该类新型配合物具有金属手性中心，且其苄胺配体上存在一个手性碳中心。该合成过程具有非对映选择性，其选择性起源于热力学控制。已对该手性配合物进行了完整的结构表征。催化性能测试结果表明，该类配合物在转移氢化反应中展现出优异的催化活性：例如，在50 ℃、催化剂负载量为1 mol%的条件下，苯乙酮可在2小时内被还原为1-苯乙醇。更值得关注的是，除催化剂活化所需的碱之外，该反应体系无需额外添加碱助剂。所得产物的对映选择性处于低至中等水平，其中以2'-甲基苯乙酮为底物时可获得最高58%的对映体过量（enantiomeric excess, ee）值。通过密度泛函理论（Density Functional Theory, DFT）计算开展的初步机理研究表明，该反应的机理为以铱酰胺基为中心的高有序过渡态介导的直接氢转移过程。计算结果与实验数据吻合良好，证实该反应为协同单步机理过程。