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a b s t r a c tThe mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAA) by oxo(salen)Cr(V) + ion in the presence of ligand oxides has been studied spectrophotometrically in acetonitrile medium. Addition of ligand oxides (LO) causes a red shift in the k max values of oxo(salen) complexes and an increase in absorbance with the concentration of LO along with a clear isobestic point. The reaction shows first-order dependence on oxo(salen)-chromium(V) + ion and fractional-order dependence on PSAA and ligand oxide. Michaelis-Menten kinetics without kinetic saturation was observed for the reaction. The order of reactivity among the ligand oxides is picoline N-oxide > pyridine N-oxide > triphenylphosphine oxide. The low catalytic activity of TPPO was rationalized. Both electron-withdrawing and electrondonating substituents in the phenyl ring of PSAA facilitate the reaction rate. The Hammett plots are non-linear upward type with negative q value for electron-donating substituents, (q À = À0.740 to À4.10) and positive q value for electron-withdrawing substituents (q + = +0.057 to +0.886). Non-linear Hammett plot is explained by two possible mechanistic scenarios, electrophilic and nucleophilic attack of oxo(salen)chromium(V) + -LO adduct on PSAA as the substituent in PSAA is changed from electrondonating to electron-withdrawing. The linearity in the log k vs. E ox plot confirms single-electron transfer (SET) mechanism for PSAAs with electron-donating substituents.

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ABSTRACT. The oxidative decarboxylation of phenylsulfinylacetic acid (PSAA) by Cr(VI) in 20% acetonitrile − 80% water (v/v) medium follows overall second order kinetics, first order each with respect to [PSAA] The product of the reaction is found to be methyl phenyl sulfone. The operation of substituent effect shows that PSAA containing electron-releasing groups in the meta-and para-positions accelerate the reaction rate while electron withdrawing groups retard the rate. An excellent correlation is found to exist between log k2 and Hammett σ constants with a negative value of reaction constant. The ρ value decreases with increase in temperature evidencing the high reactivity and low selectivity in the case of substituted PSAAs.