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 C_2v and C_4v symmetry denoted as C_2v-OC-4 and C_4v-OC-4, respectively. Heating induces the efficient stepwise conversion of C_2v- to C_4v-OC-4 via a C_s-symmetric intermediate (C_s-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 C_2v- and C_s-OC-4 from C_4v-OC-4. In contrast to C_2v-OC-4, C_4v-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.