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

Abstract

Although phenol is stable in bulk water, we report an exceptional phenomenon in which phenol is spontaneously transformed into a phenyl carbocation (Ph⁺) in water microdroplets. The high electric field at the air-water interface is proposed to break the phenolic Csp²-OH bond, forming Ph⁺, which remains in equilibrium with phenol as deciphered by mass spectrometry. We detected up to 70% conversion of phenol to Ph⁺ in aqueous microdroplets, although catalyst-free activation of the phenolic Csp²-OH bond is challenging. This transformation is well tolerated by a wide range of electron-donating and -withdrawing substituents in phenolic compounds. The Ph⁺ in water microdroplets could be reacted with various nucleophiles (amine, pyridine, azide, thiol, carboxylic acid, alcohol, and ¹⁸O-water), yielding the <i>ipso</i>-substitution products of phenol through an aromatic S_N1 mechanism. Despite the fleeting life of Ph⁺ in the bulk, this study demonstrates its unusual stability at the aqueous microdroplet surface, enabling its detection and transformation.