Chiral Titanium Coordination Assemblies: Robust Cooperative Self-Supported Catalysts for Asymmetric Ring Opening of meso-Epoxides with Aliphatic Amines
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- 作者：Zheming Sun ; Jiangbo Chen ; Yaoqi Liu and Tao Tu
- 刊名：Advanced Synthesis & Catalysis
- 出版年：2017
- 出版时间：February 2, 2017
- 年：2017
- 卷：359
- 期：3
- 页码：494-505
- 全文大小：1842K
- ISSN：1615-4169
文摘
By utilizing the oxygen bridge in dimeric μ-oxo-titanium-salen complexes as an efficient cross-linkage, a series of robust chiral titanium coordination assemblies has been successfully fabricated with the ditopic bridging ligands derived from BINOL and salen derivatives via coordination polymerization, which are fully characterized by IR, elemental analysis, XRD and microscopic studies. Because of their insolubility in most organic solvents and water, these metal-organic assemblies can successfully function as robust self-supported chiral catalysts, allowing an asymmetric ring opening (ARO) of meso-epoxides with aliphatic amines. Remarkably, owing to the linkage effects and the cooperation of two kinds of chiral titanium moieties in the metal-organic assemblies, the self-supported chiral catalysts demonstrate extremely high stability. They not only show high tolerance towards various meso-epoxides and nucleophilic aliphatic amines, but also can be reused in more than 20 runs without obvious metal leaching and loss in yields and enantioselectivities. Furthermore, the self-supported catalyst accomplished a one-pot tandem olefin epoxidation and ARO of an epoxide sequence starting from the olefin, 30% hydrogen peroxide and benzylamine. In marked contrast, the reaction failed to work when using the two corresponding homogeneous catalysts under identical reaction conditions. Good yields and enantioselectivities were obtained by the robust self-supported catalyst, which clearly indicates the cooperative effects between two chiral moieties within the metal-organic assemblies. The two catalytic centers can perform their own duties without interference and this further supports our strategy for the self-supported catalyst design.