Mechanism of dehydroacetoxylation of methyl 3-acetoxy-3-aryl-2-halogenopropanoates

Abstract

The rate of sodium methoxide-induced dehydroacetoxylation of the diesters p-RC₆H₄CH(OAc)CXHCO₂Me (X = Br or Cl) has been studied as a function of the para-substituent. The results correspond to the para-substituent effects expected for proton abstraction from C-2, as estimated from the diastereoisomerisation of model compounds [p-RC₆H₄CH(OMe)CXHCO₂Me] under similar conditions. This suggests that the elimination occurs via a carbanion mechanism, with C(2)–H bond-breaking rate-determining. The apparent insensitivity of the kinetic behaviour to configuration differences, together with the response of the reactions to the effect of the 3-substituent, is interpreted in terms of inductive stabilisation by the 2-halogen atom, which tends to localise the incipient negative charge at C-2, inhibiting distortion of its tetrahedral geometry.