Efficient α-selective chlorination of phenylacetic acid and its para-substituted analogues†

Abstract

α-Chlorophenylacetic acids are synthons with great potential but are limited by the lack of a simple and generalizable method for their preparation from readily available precursors. Therefore, relying on the commercial availability of phenyl acetic acid and a series of para-substituted phenylacetic acids, we explored the practicability of their direct α-selective chlorination without the competing electrophilic aromatic chlorination. Indeed, treatment of these substrates with catalytic PCl₃ and a slight excess of trichloroisocyanuric acid (TCCA) under solvent-free conditions rapidly provided the desired products in high yields with the only condition being that substituents that strongly activate electrophilic aromatic substitution were absent. An efficient preparative method for α-chlorinate phenylacetic acid and its analogues bearing electron-withdrawing or weakly electron-donating para-substituents, such as NO₂, CN, CF₃, COOMe, halogen, and alkyl, was thus developed, making these synthetic intermediates more accessible and exploitable.