DOSY NMR for monitoring self aggregation of bifunctional organocatalysts: increasing enantioselectivity with decreasing catalyst concentration
作者:Hyeong Bin Jang、Ho Sik Rho、Joong Suk Oh、Eun Hye Nam、Sang Eun Park、Han Yong Bae、Choong Eui Song
DOI:10.1039/c0ob00047g
日期:——
In this report, we demonstrate that self-aggregation is an intrinsic problem of bifunctional organocatalysts, especially in the case when the substrates do not have functional groups which are able to bind strongly with catalyst. Due to their self-association phenomena, the enantioselectivity of bifunctional catalysts dramatically decreases with increasing catalyst concentration or decreasing temperature. Thus, when the substrate concentration is kept constant, the enantioselectivity of bifunctional catalysts dramatically increases with decreasing catalyst loading. The ee values obtained at different catalyst concentrations are fairly consistent with the diffusion coefficients (D) of the catalysts, strongly indicating that their degree of self-association plays a crucial role in determining their enantioselectivity.
在本报告中,我们证明了自聚集是双功能有机催化剂的一个固有问题,尤其是在底物不具有能够与催化剂强结合的官能团的情况下。由于其自结合现象,双功能催化剂的对映选择性会随着催化剂浓度的增加或温度的降低而急剧下降。因此,当底物浓度保持不变时,双功能催化剂的对映体选择性会随着催化剂负载量的减少而显著增加。在不同催化剂浓度下获得的ee值与催化剂的扩散系数(D)相当一致,这有力地表明催化剂的自结合程度在决定其对映体选择性方面起着至关重要的作用。