Spontaneous Generation of Aryl Carbocations from Phenols in Aqueous Microdroplets: Aromatic S_N1 Reactions at the Air-Water Interface

Although phenol is stable in bulk water, we report anexceptionalphenomenon in which phenol is spontaneously transformed into a phenylcarbocation (Ph+) in water microdroplets. The high electricfield at the air-water interface is proposed to break the phenolicCsp(2)-OH bond, forming Ph+, which remainsin equilibrium with phenol as deciphered by mass spectrometry. Wedetected up to 70% conversion of phenol to Ph+ in aqueousmicrodroplets, although catalyst-free activation of the phenolic Csp(2)-OH bond is challenging. This transformation is welltolerated by a wide range of electron-donating and -withdrawing substituentsin phenolic compounds. The Ph+ in water microdroplets couldbe reacted with various nucleophiles (amine, pyridine, azide, thiol,carboxylic acid, alcohol, and O-18-water), yielding the ipso-substitution products of phenol through an aromaticS(N)1 mechanism. Despite the fleeting life of Ph+ in the bulk, this study demonstrates its unusual stability at theaqueous microdroplet surface, enabling its detection and transformation.