How Do Benzylic Amide [2]Catenane Rings Rotate?

The structure and dynamics of three benzylic amide catenanes have been investigated using molecular mechanics and calculations employing unimolecular reaction rate theory. The study provides the first complete theoretical description of the lowest energy pathway for the circumrotation of macrocycles in a catenane system. The process is a concerted sequence of several large-amplitude motions which involve rearrangements to minimize steric and electrostatic interactions via the interplay of hydrogen bonding, formation of π-stacks, phenyl−phenyl T-shaped interactions, and amide rotamer interconversions. In spite of this complexity, unifying features are found for the description of the passing of successive moieties through the macrocyclic cavities. Analysis of the structural characteristics of transition states located along the circumrotational pathway of the three catenanes (bearing phenyl-, pyridyl-, and thiophenyl-1,3-dicarbonyl groups) furnishes a simple mechanistic interpretation of the large variation of the dynamical behavior observed in temperature-dependent NMR experiments. Calculations of the rate constants provide the ultimate insight into the complicated dynamics of these molecules, showing that the rate-determining steps do not necessarily correspond to the passage of the bulkiest groups and that, like a cog or rotor system, rotation of one macrocyclic ring induces, or makes easier, rotation of the other.