Transition-State Stabilization by n→π* Interactions Measured Using Molecular Rotors

A series of 16 molecular rotors were synthesized to investigate the ability of n→π* interactions to stabilize transition states (TSs) of bond rotation. Steric contributions to the rotational barrier were isolated using control rotors, which could not form n→π* interactions. Rotors with strong acceptor π* orbitals, such as ketones and aldehydes, had greatly increased rates of rotation. The TS stabilization of up to ∼10 kcal/mol was consistent with the formation of a strong n→π* stabilization between the imide carbonyl oxygens and the ortho R group in the planar TS. Computational studies effectively modeled the TS stabilization and geometry, and NBO analysis confirmed the role of n→π* interactions in stabilizing the TS.