Supramolecular Axial Chiral Ligands for Asymmetric Catalysis

在本项目中，我们将制备一类新型的基于互补氢键的超分子配体，并将其用于不对称催化。在这些配体中，自组装主链的轴向手性扭曲是由配体单体上的手性取代基控制的。初步的催化实验表明，这种新型配体在α-脱氢氨基酸的不对称氢化反应中具有良好的立体选择性，ee高达87%。通过计算研究、核磁共振光谱和圆二色谱相结合的方法研究了过渡金属配合物的结构。在这里介绍的项目中，我们计划根据我们在这一领域的经验制备一个超分子、轴向-手性配体文库：将制备基于AT碱基对中观察到的互补氢键的同二聚体和异二聚体配体，并在前手性烯烃的不对称氢化、氢甲酰化和氢氰化反应中进行测试。此外，我们计划对这些新配体的过渡金属配合物进行深入的结构研究：将通过核磁共振光谱、X射线分析和CD光谱来探索具有催化活性的Rh配合物和模型铂配合物的结构。主要的重点在于分析活性络合物的结构与观察到的催化选择性和活性之间的关系。在这方面，我们还将使用密度泛函方法计算阿托品的相对稳定性，并研究手性取代基在不对称氢化反应中引起的立体诱导。

In this project a new class of supramolecular ligands based on complementary hydrogen-bonding will be prepared and evaluated in asymmetric catalysis. In these ligands an axial-chiral twist of the self-assembled backbone is controlled by chiral substituents on the ligand monomers. Preliminary catalysis experiments with first examples of this new type of ligand showed good stereoselectivity in the asymmetric hydrogenation of alpha-dehydroamino acids with up to 87 % ee. The structure of the transition metal complexes was investigated by a combination of computational studies, NMR spectrometry and circular dichroism spectroscopy. In the project presented here, we plan to prepare a library of supramolecular, axial-chiral ligands based on our previous experience in this field: homodimeric ligands as well as heterodimeric ligands based on the complementary hydrogen bonding observed in AT base pairs will be prepared and tested in asymmetric hydrogenation, hydroformylation and hydrocyanation reactions of prochiral alkenes. Furthermore, we plan to perform an in-depth structural study of the transition metal complexes of these new ligands: The structure of the catalytically active Rhodium complexes as well as model Platinum complexes shall be explored by NMR-spectroscopy, X-Ray analysis and CD-spectroscopy. A main focus lies on analysing the relationship between the structure of the active complex and the observed catalytic selectivity and activity. In this regard we will also use DFT methods to calculate the relative stability of atropisomers, and to investigate the stereo induction caused by the chiral substituents in asymmetric hydrogenation.