Proximal effect of the nitrogen bases in the oxidative decarboxylation of phenylsulfinylacetic acids by oxo(salen)chromium(V) complexes

• Oxidative decarboxylation is observed in PSAA with oxo(salen)CrV complexes.
• Nitrogen bases viz., imidazole, methyl imidazole and pyridine catalyse the reaction.
• Electron donating and withdrawing substituents in PSAA accelerate the reaction rate.
• Electrophillic attack of oxo(salen)CrV followed by oxygen transfer in the mechanism.
• Proximal effect plays an important role in enhancing the reactivity.

Oxidative decarboxylation of phenylsulfinylacetic acid (PSAA) and several substituted PSAAs with three oxo(salen)chromium(V) complexes in the presence of nitrogen bases are investigated in 100% acetonitrile medium using spectrophotometric technique. The nitrogenous bases such as imidazole, 1-methyl imidazole and pyridine catalyse the reaction and Michaelis–Menten kinetics is observed with respect to these bases. Among the various bases employed, imidazole with strong π donating ability shows the least accelerating effect and the maximum catalytic activity is observed with pyridine. Both the electron donating and electron withdrawing substituents in PSAA accelerate the reaction rate. The Hammett plots for the three oxo(salen)chromium(V) complexes with three nitrogen bases display a nonlinear upward curvature. The Hammett parameter ρ, changes from large negative to small positive values as the substituents are changed from electron donating to electron withdrawing groups. A mechanism involving direct oxygen transfer from oxo(salen)chromium(V)-nitrogen base adduct to PSAA with simultaneous decarboxylation to yield sulfone is proposed. The nonlinear Hammett plot has been ascribed to a shift in the rate determining step caused by the formation and decomposition of the sulfonium ion intermediate which is susceptible to substituent effect.

Figure optionsDownload high-quality image (80 K)Download as PowerPoint slide