Proton Grease: An Acid Accelerated Molecular Rotor

A molecular rotor was designed that rotates 7 orders of magnitude faster upon protonation. The quinoline rotor is based on a rigid N-arylimide framework that displays restricted rotation due to steric interaction between the quinoline nitrogen and imide carbonyls. At rt (23 °C), the rotor rotates slowly (t_1/2 = 26 min, ΔG^‡ = 22.2 kcal/mol). However, upon addition of 3.5 equiv of acid the rotor rotates rapidly (t_1/2 = 2.0 × 10^–4 s, ΔG^‡ = 12.9 kcal/mol). Mechanistic studies show that this dramatic acid catalyzed change is due to stabilization of the planar transition state by the formation of an intramolecular hydrogen bond between the protonated quinoline nitrogen (N⁺H) and an imide carbonyl (OC). The acid catalyzed acceleration is reversible and can be stopped by addition of base.