Mechanistic investigations on manganese(II) catalyzed oxidation of substituted 4-oxo-4-arylbutanoic acids by bromate in acid medium

The manganese(II)-catalyzed oxidation of substituted 4-oxo-4-arylbutanoic acids (4-oxo acids) by bromate, in aqueous acetic acid medium containing sulphuric acid, has been studied. The reaction is first order each in [bromate] and [acid], less than unity order in both [substrate] and [catalyst], and also shows solvent isotope effect of 0.60 (k(c)' (H2O)/k(c)' (H2O)). Oxidation rate is not much affected by electron withdrawing substituents, although it is susceptible to electron demand at the scat of the reaction. The linear free-energy relationship is characterized by smooth curves in Hammett plots of log k versus sigma However, the plots are linear with excellent correlation coefficient at all the studied temperatures when Brown's sigma(+) values are used. The reaction constant is negative and increases with decrease in temperature. The mechanism involving decomposition of the complex, formed by participation of the neighboring group of 4-oxo acid and intramolecular catalysis of Mn(II) is proposed, wherein, both the C-C bond-breaking and C-O bond-making in the oxidation are involved. However, these two processes are influenced in different ways by the electron withdrawing and electron releasing substituents. A it-complex formation between Mn(II) and -ene of the enol form of oxo acid is proposed.