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 C 60 or C 70 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.