A New Chiral Rh(II) Catalyst for Enantioselective [2 + 1]-Cycloaddition. Mechanistic Implications and Applications

A novel chiral Rh(II) catalyst (1) is introduced for the [2 + 1]-cycloaddition of ethyl diazoacetate to terminal acetylenes and olefins with high enantioselectivity. The catalyst 1 consists of one acetate bridging group and three mono-N-triflyldiphenylimidazoline-2-one bidentate ligands (DPTI) spanning the Rh(II)−Rh(II) metallic center in a structure that was determined by single-crystal X-ray diffraction analysis. A rational mechanism is advanced that provides a straightforward explanation for the enantioselectivity and absolute stereochemical course of the [2 + 1]-cycloaddition reactions. A key element in this explanation is the cleavage of one of the Rh−O bonds of the bridging acetate group in the intermediate Rh−carbene complex to form a new pentacoordinate Rh carbene complex (formally 1.5 valent Rh) that can undergo [2 + 2]-cycloaddition with the C−C π-bond of the acetylenic or olefinic substrate. Reductive elimination of the resulting adduct affords the cyclopropene or cyclopropane product. The C2-symmetry of the two DPTI ligands orthogonal to the bridging acetate also contributes to the high observed enantioselectivity and mechanistic clarity. The catalyst 1, which functions effectively at 0.5 mol %, can be recovered efficiently for reuse. Its ready availability, robustness, and effectiveness suggest it as a useful addition to the list of practical chiral Rh(II) catalysts for synthesis.

一种新型的手性 Rh(II) 催化剂（1）被引入，用于与末端炔烃和烯烃进行 [2 + 1]-环加成反应，展现出极高的对映选择性。催化剂 1 由一个乙酸桥连基团和三个单-N-三氟甲基二苯基咪唑-2-酮双齿配体（DPTI）组成，这些配体跨越 Rh(II)−Rh(II) 金属中心，其结构通过单晶X射线衍射分析确定。提出了一种合理的机理，为 [2 + 1]-环加成反应的对映选择性和绝对立体化学路径提供了直观的解释。该解释的关键要素是中间体 Rh-碳烯复合物中桥连乙酸基团的一个 Rh−O 键的断裂，形成一个新的五配位 Rh 碳烯复合物（形式上为 1.5 价 Rh），该复合物可以与炔烃或烯烃底物的 C−C π 键进行 [2 + 2]-环加成。桥连乙酸基团中垂直于桥连基团的两个 DPTI 配体的 C2 对称性也促进了所观察到的对映选择性和机理的清晰性。催化剂 1 在 0.5 mol % 的浓度下高效地发挥作用，并且可以高效地回收以供重复使用。其易得性、稳定性和有效性表明，它有望成为合成中实际应用的手性 Rh(II) 催化剂列表的有用补充。