Stabilizing Highly Reactive Aryl Carbanions in Water Microdroplets: Electrophilic <i>Ipso</i>-Substitution at the Air–Water Interface

The fleeting existence of aryl carbanion intermediates in the bulk phase prevents their direct observation and spectroscopic measurement. In sharp contrast, we report the direct interception of such unstable species at the air–water interface of microdroplets. We observed the transformation of three types of aryl acids (benzoic, phenylsulfinic, and phenylboronic acids) into phenyl carbanion (Ph–) in water microdroplets, as examined by mass spectrometry. Experimental and theoretical evidence suggests that the high intrinsic electric field at the microdroplet surface is likely responsible for cleaving the respective acid functional groups of these substrates, generating Ph–, which can subsequently be trapped by an electrophile, including a proton, to yield the corresponding ipso-substitution product. While catalyst-free decarboxylation at ambient temperature is challenging in the bulk phase, we report over 30% instantaneous conversion of benzoic acid to Ph– in sprayed aqueous microdroplets in less than a millisecond. Thus, this study lays the foundation of a green chemical pathway for the aromatic electrophilic ipso-substitution reaction by spraying an aqueous solution of aryl acids, eliminating the need for any catalyst or reagent.