Complementary Double-Stranded Helical Oligomers Bearing Achiral Bifunctional Groups That Catalyze Asymmetric Aldol Reaction
Complementary Double-Stranded Helical Oligomers Bearing Achiral Bifunctional Groups That Catalyze Asymmetric Aldol Reaction
复制标题
带有催化不对称羟醛反应的非手性双官能团的互补双链螺旋低聚物
作者：
D. Taura;K. Shimomura;N. Ousaka;E. Yashima
合成了两种新型的手性二聚体和三聚体链，它们由间三联苯通过对二乙炔基苯单元连接而成，并在链的末端或中心分别引入了手性脒基和非手性哌嗪基，以及与之互补的非手性羧酸二聚体和三聚体.互补的手性/非手性链形成了一个过量的双螺旋结构，这是由手性脒-羧酸盐桥偏置的共轭主链的吸收区中的强烈分裂型科顿效应支持的。发现双螺旋三聚体催化环己酮与4-硝基苯甲醛的直接羟醛缩合反应，并产生具有中等对映选择性的产物，尽管中间引入的催化活性双官能哌嗪/羧酸对是非手性的，表明单手双螺旋框架在超分子双官能有机催化中的关键作用。
Two novel chiral dimer and trimer strands composed ofm‐terphenyl groups linked throughp‐diethynylbenzene units with the chiral amidine group and achiral piperazine group introduced at the terminus or center of the strands, respectively, and its complementary achiral carboxylic acid dimer and trimer were synthesized. The complementary chiral/achiral strands form an excess‐handed double‐helical structure as supported by intense split‐type Cotton effects in the absorption regions of the conjugated backbones biased by the chiral amidinium–carboxylate salt bridges. The double‐helical trimer was found to catalyze the direct aldol reaction of cyclohexanone with 4‐nitrobenzaldehyde and produce the products with a moderate enantioselectivity despite the fact that the catalytically active bifunctional piperazine/carboxylic acid pair introduced in the middle is achiral, indicating the key role of the one‐handed double‐helical framework for supramolecular bifunctional organocatalysis.