Molecular Balances Measure Aliphatic CH-π Interactions

A family of conformationally flexible molecules gives chemists a means to measure the weak interactions between aliphatic C–H bonds and the delocalized π-electron systems of arenes in solution (Org. Lett., DOI: 10.1021/ol201657p). The so-called molecular balances, developed by Ken D. Shimizu and coworkers at the University of South Carolina, Columbia, are based on a bicyclic N-arylimide framework. When a balance molecule is in its folded position, an alkoxy group dangles over the molecule’s arene “shelf” so that an aliphatic C–H can interact with the arene. In its unfolded conformation, the arylimide C–N bond rotates so that the alkoxy group is positioned far away from the shelf. Using NMR to determine the ratio of the folded conformation to the unfolded one, Shimizu’s team was able to measure the strength of the aliphatic CH-π interaction. They found that the interaction is quite weak, about 1 kcal/mol relative to control balances lacking the arene shelf or with a simple aromatic shelf that’s not large enough to interact with the alkoxy group, and is easily disrupted by conformational entropy and repulsive steric interactions.