Wang Chao, Matile Stefan
Department of Organic Chemistry, University of Geneva, Geneva, Switzerland.
Chemistry. 2017 Sep 4;23(49):11955-11960. doi: 10.1002/chem.201702672. Epub 2017 Aug 10.
The idea of anion-π catalysis is to stabilize anionic transition states by anion-π interactions on aromatic surfaces. For asymmetric anion-π catalysis, π-acidic surfaces have been surrounded with stereogenic centers. This manuscript introduces the first anion-π catalysts that operate with axial chirality. Bifunctional catalysts with tertiary amine bases next to π-acidic naphthalenediimide planes are equipped with a bulky aromatic substituent in the imide position to produce separable atropisomers. The addition of malonic acid half thioesters to enolate acceptors is used for evaluation. In the presence of a chiral axis, the selective acceleration of the disfavored but relevant enolate addition was much better than with point chirality, and enantioselectivity could be observed for the first time for this reaction with small-molecule anion-π catalysts. Enantioselectivity increased with the π acidity of the π surface, whereas the addition of stereogenic centers around the aromatic plane did not cause further improvements. These results identify axial chirality of the active aromatic plane generated by atropisomerism as an attractive strategy for asymmetric anion-π catalysis.
阴离子-π催化的理念是通过芳环表面的阴离子-π相互作用来稳定阴离子过渡态。对于不对称阴离子-π催化，π-酸性表面已被手性中心所包围。本文介绍了首例具有轴手性的阴离子-π催化剂。在π-酸性萘二亚胺平面附近带有叔胺碱的双功能催化剂，在酰亚胺位置配备了一个庞大的芳基取代基，以产生可分离的阻转异构体。将丙二酸半硫酯加成到烯醇负离子受体上用于评估。在手性轴存在的情况下，对不利但相关的烯醇负离子加成的选择性加速比点手性的情况要好得多，并且首次观察到小分子阴离子-π催化剂对该反应具有对映选择性。对映选择性随着π表面的π酸性增加而提高，而在芳环平面周围添加手性中心并没有带来进一步的改善。这些结果表明，由阻转异构产生的活性芳环平面的轴手性是不对称阴离子-π催化的一种有吸引力的策略。
