Selective Chloroacetylation of Methoxyphenol Isomers in the Presence of Various Catalysts: Product Distribution and Mechanistic Insights

The selective chloroacetylation of methoxyphenol isomers (ortho-, meta-, and para-) was studied under mild conditions using catalytic amounts of various Lewis acids, including FeCl₃, FeCl₃·6H₂O, MoCl₅, WCl₆, ZnCl₂, SnCl₄, VCl₃, TAA, and TSA. The reactivity and selectivity of each isomer toward chloroacetyl chloride were evaluated in nonpolar solvents such as benzene. The influence of catalyst type, molar ratio, temperature, and reaction time on product yield and distribution was systematically investigated. Spectroscopic characterization of the reaction products was performed using IR, UV, and NMR techniques, confirming the formation of O-acylation products (methoxyphenyl chloroacetates) as well as regioisomeric C-acylation products (hydroxy-methoxyphenacyl chlorides). Notably, o-methoxyphenol gave a mixture of three main products, while m- and p-isomers primarily yielded two. Among the catalysts tested, FeCl₃ provided the highest overall yield and favored O-acylation, while stronger Lewis acids like SnCl₄ and MoCl₅ showed greater influence on C-acylation pathways. Mechanistic studies indicate that the reaction proceeds via nucleophilic substitution at the carbonyl carbon of chloroacetyl chloride rather than through a classical acylium ion intermediate, due to the aprotic solvent system and low catalyst concentration. The product distribution correlated strongly with electronic and steric effects of substituents on the aromatic ring. The findings offer valuable insights into structure–reactivity relationships in electrophilic aromatic substitution reactions of substituted phenols and demonstrate the potential of such transformations in the synthesis of bioactive intermediates.