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 (t1/2 = 26 min, ΔG‡ = 22.2 kcal/mol). However, upon addition of 3.5 equiv of acid the
rotor rotates rapidly (t1/2 = 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.