Trimethoxy N-phenylphenothiazine 1 (25 mol%) is found to be the most effective catalyst for this transformation and is proposed to undergo electrochemical oxidation before being trapped by a carboxylic acid-derived nucleophile to form 2. A nucleophilic substitution reaction between an alcohol and 2 would then produce the ester product and a sulfoxide 3. The sulfoxide 3 can then undergo electrochemical reduction to regenerate 1. Interestingly, phosphorus-based catalysts prove to be ineffective for this transformation, likely owing to the challenging reduction of P=O bonds compared to S=O bonds.
A reticulated vitreous carbon anode and a stainless steel cathode are used for the electrolysis, along with Bu4NPF6 as an electrolyte and acetonitrile as a solvent. The reactions are conducted under a N2 atmosphere; however, the reactions also proceed when performed in air, albeit with lower yields. These conditions are amenable to a range of carboxylic acids, providing ester products in good yields. Substrates that are susceptible to decarboxylation, such as benzylic, tertiary or α-heteroatom-containing acids, yield the ester products in reduced yields compared to other acids. A variety of alcohols are also tolerated, though it is noted that hindered alcohols provide lower yields and phenol, which is redox active, is incompatible with these conditions.
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