Esters of α-arylalkanoic acids from ‘masked’α-halogenoalkyl aryl ketones and silver salts: synthetic, kinetic, and mechanistic aspects

Abstract

A method for the synthesis of alkyl esters of α-arylalkanoic acids is given, based on silver-ion-assisted (AgBF₄, AgOSO₂CF₃, AgSbF₆, AgNO₃) solvolysis of alkyl acetals of primary and secondary α-halogenoalkyl aryl ketones (Hal = I, Br, Cl) in an alcoholic medium (methanol, ethanol). The reaction is quite selective and alkyl esters are the only reaction products; ethers, which are possible substitution products, are not found. The importance of masking the carbonyl as the acetal is emphasised. The reaction is found to be first-order in AgBF₄ and in the primary α-halogeno acetal. A three-point Hammett correlation (ρ=–3.29) between σ⁺ and the rate constants suggests a large cationic contribution as well as strong aryl participation in the transition state.

The role played by the oxygen of the acetal group in the specificity of the reaction is discussed in comparison with the reactivity of analogous compounds with saturated skeletons and of α-halogenoalkyl aryl ketones.