Comprehensive Analysis Center, ISIR , Osaka University , Mihogaoka , Ibaraki 567-0047 , Japan.
Department of Chemistry , Tokyo Institute of Technology , 2-12-1 Ookayama , Meguro-ku , Tokyo 152-8551 , Japan.
J Am Chem Soc. 2018 Mar 21;140(11):3959-3974. doi: 10.1021/jacs.7b12085. Epub 2018 Feb 19.
Chiral slipped 5,8:9',10'-cyclodimers were preferentially produced over classical 9,10:9',10'-cyclodimers upon supramolecular photocyclodimerization of 2-anthracenecarboxylate (AC) mediated by β-cyclodextrin (β-CD). This photochirogenic route to the slipped cyclodimers, exclusively head-to-tail (HT) and highly enantioselective, has long been overlooked in foregoing studies but is dominant in reality and is absolutely supramolecularly activated by 2:2 complexation of AC with β-CD. The intricate structural and photophysical aspects of this higher-order complexation-triggered process have been comprehensively elucidated, while the absolute configurations of the slipped cyclodimers have been unambiguously assigned by comparing the experimental and theoretical circular dichroism spectra. In the 2:2 complex, two ACs packed in a dual β-CD capsule are not fully overlapped with each other but are only partially stacked in a slipped anti- or syn-HT manner. Hence, they do not spontaneously cyclodimerize upon photoexcitation but instead emit long-lived excimer fluorescence at wavelengths slightly longer than the monomer fluorescence, indicating that the slipped excimer is neither extremely reactive nor completely relaxed in conformation and energy. Because of the slipped conformation of the AC pair in the soft capsule, the subsequent photocyclodimerization becomes manipulable by various internal or external factors, such as temperature, pressure, added salt, and host modification, enabling us to exclusively obtain the slipped cyclodimers with high regio- and enantioselectivities. In this supramolecularly driven photochirogenesis, the dual β-CD capsule functions as a chiral organophotocatalyst to trigger and accelerate the nonclassical photochirogenic route to slipped cyclodimers by preorganizing the conformation of the encapsulated AC pair, formally mimicking a catalytic antibody.
手性滑移 5,8:9',10'-环二聚体优先于经典的 9,10:9',10'-环二聚体通过β-环糊精（β-CD）介导的 2-蒽羧酸（AC）的超分子光环二聚化产生。这种手性光致环二聚体的形成途径，即头对头（HT）和高度对映选择性，在先前的研究中一直被忽视，但在现实中是主要的，并且绝对是由 AC 与 β-CD 的 2:2 络合超分子激活的。这种高阶络合触发过程的复杂结构和光物理方面已经得到了全面的阐明，同时通过比较实验和理论圆二色光谱，明确了滑移环二聚体的绝对构型。在 2:2 络合物中，两个 AC 包装在一个双β-CD 胶囊中，彼此不完全重叠，而是以滑移的反式或顺式 HT 方式部分堆叠。因此，它们在光激发下不会自发环二聚化，而是发射长寿命的激基荧光，波长略长于单体荧光，表明滑移激基既不是极其反应性的，也不是在构象和能量上完全松弛的。由于 AC 对在软胶囊中的滑移构象，随后的光环二聚化可以通过各种内部或外部因素进行操纵，例如温度、压力、添加盐和主体修饰，使我们能够以高区域和对映选择性获得滑移环二聚体。在手性有机光催化剂β-CD 胶囊的作用下，超分子驱动的光致手性诱导中，通过预先组织包裹的 AC 对的构象，来触发和加速非经典的光致手性途径，形成滑移环二聚体，正式模拟了催化抗体。
