Electrochemical Oxygen Reduction Reaction-Coupled Dechlorination Promoting Acylation Reaction for Aromatic Ester Synthesis

Aromatic ester compounds are one of the most versatile chemicals in many applications. Acylation reactions using acyl chlorides as building blocks are of great significance for their synthesis. However, novel and efficient strategies for acyl chlorides formation are limited. Here, we propose a cascaded route for aromatic ester synthesis via the oxygen reduction reaction (ORR)-coupled electrochemical dechlorination of trichloromethyl compounds, followed by the chemical acylation reaction. A selectivity of 93.2% and a yield of 92.5% for methyl 6-chloronicotinate (MCN) generation are obtained by electrochemical reduction of 2-chloro-5-trichloromethylpyridine over the activated Ag electrode in an O2-saturated methanol solution. Electrochemical in situ characterizations, femtosecond transient absorption spectra, isotope labeling, and theoretical calculations elucidate that the formation of 6-chloronicotinoyl chloride (CNC) is an important step, which is derived from the coupling of in situ generated reactive oxygen species by ORR with a dechlorination intermediate in the electrochemical process. The involvement of O2 alters the electrochemical reaction pathway from conventional hydrodechlorination to an oxygenation-dechlorination process, which is attributed to the preferentially occurring ORR rather than the hydrogen evolution reaction. Subsequently, the in situ generated CNC sparks a chemical acylation reaction with methanol for MCN synthesis. This work provides an efficient electrochemical-chemical cascade strategy for generating a series of aromatic esters, showing promising application potential.