Controlled interconversion of macrocyclic atropisomers via defined intermediates

Macrocyclic conformations play a crucial role in regulating their properties. Our understanding of the determinants to control macrocyclic conformation interconversion is still in its infancy. Here we present a macrocycle, octamethyl cyclo[4](1,3-(4,6)-dimethylbenzene)[4]((4,6-benzene)(1,3-dicarboxylate) (OC-4), that can exist at 298 K as two stable atropisomers with C2v and C4h symmetry denoted as C2v-OC-4 and C4h-OC-4, respectively. Heating induces the efficient stepwise conversion of C4h to C2v-OC-4 via a Cs-symmetric intermediate (Cs-OC-4). It differs from the typical transition state-mediated processes of simple C–C single bond rotations. Hydrolysis and further esterification with a countercation dependence promote the generation of C2v and Cs-OC-4 from C4h-OC-4. In contrast to C2v-OC-4, C4h-OC-4 can bind linear guests to form pseudo-rotaxans, or bind C60 or C70 efficiently. The present study highlights the differences in recognition behavior that can result from conformational interconversion, as well as providing insights into the basic parameters that govern coupled molecular rotations.

Xin Sun, Jin-Ku Bai, Yu-Dong Yang, Ke-Lin Zhu, Jia-Qi Liang, Xin-Yue Wang, Jun-Feng Xiang, Xiang Hao, Tong-Ling Liang, Ai-Jiao Guan, Ning-Ning Wu, Han-Yuan Gong