帶正電有機鋁錯合物在不對稱催化與二氧化碳活化的應用
Aluminum Cation Superacid for Asymmetric Catalysis and CO2 Activation
計畫主持人：臺大化學系-邱靜雯、中研院化學所-陳榮傑
本研究旨在將鋁陽離子與硼陽離子引入手性環境中，探索其進行不對稱催化反應的潛力。在鋁陽離子的研究方面，我們由簡單易得的市售起始物出發，經過兩步合成出具有C2對稱手性的四配位鋁陽離子，藉由比較競爭結合力的實驗，驗證了此類陽離子的高路易士酸度，甚至超過了B(C6F5)的酸度。我們也成功將其應用於催化不對稱硼氫化反應、Diels-Alder反應以及Michael加成反應，獲致相當好的反應轉化率，而鏡像選擇性仍有待進一步提升，相關成果已發表於Organometallics, 2021, 40, 1244-1251以及ChemCatChem, 2022, 14, e202101715.
在三配位硼陽離子的研究方面，我們合成了一系列帶有對稱性[B-Cl-B]+連接的氯硼烷掩護的硼陽離子，並將其用作二烯基苯和2,2,2-三氟乙基丙烯酸酯的不對稱Diels-Alder 環化加成反應的催化劑。發現產生的[B-Cl-B]+陽離子的穩定性取決於配位基團的立體阻礙，而反應的立體選擇性則由手性噁唑烷酮配體控制。而藉由[B-Cl-B]+陽離子的橋接氯原子上配位SnCl4，可以進一步提高立體選擇性，相關成果已發表於Inorg. Chem., 2021, 60,16266–16272。
本項研究突顯了利用陽離子鋁和硼陽離子在有機催化中的潛力，特別是在鏡像選擇性轉化過程。相關研究仍在持續進行中，希望能充分了解這些陽離子複合物在催化中的基礎機制，以期進一步優化其反應效率。
This project aimed to investigate the potential of incorporating an aluminum cation into a chiral environment and utilizing cationic aluminum and borenium ions for catalysis. A C2-symmetric chiral tetra-coordinate aluminum cation was synthesized by a two-step process from commercially available starting materials. The high Lewis acidity of the cation was verified through a competition binding experiment and found to exceed that of B(C6F5). While it demonstrated effectiveness in achieving high conversion in all cases, the presence of a labile N–Al bond resulted in diminished enantioselectivity during the activation process (Organometallics, 2021, 40, 1244-1251; ChemCatChem, 2022, 14, e202101715).
In addition, the project also aimed to examine the application of a tricoordinate borenium ion in Lewis acid catalysis. A series of chloroborane-masked borenium ions featuring the symmetrical [B–Cl–B]+ linkage were synthesized and used as catalysts for the enantioselective Diels–Alder cycloaddition of cyclopentadiene and 2,2,2-trifluoroethyl acrylate. The stability of the resulting [B–Cl–B]+ cation was found to be dependent on the steric bulkiness of the oxazolidinone moiety, while the stereoselectivity of the reaction was controlled by the chiral oxazolidinone ligand. Further improvement of the stereoselectivity could be achieved by coordinating SnCl4 at the bridging chloride of the [B–Cl–B]+ cation (Inorg. Chem., 2021, 60, 16266–16272).
In conclusion, the studies in this project highlight the potential of utilizing cationic aluminum and borenium ions in catalysis, specifically in the enantioselective activation process. Further research is needed to fully understand the underlying mechanisms and optimize the efficiency of these cationic complexes in catalysis.
