Effect of para‐Substituents on meso‐Functionalized Oxidovanadium(IV) Porphyrins as Catalysts for Oxygen Atom Transfer Mediated Oxidation of Benzoin to Benzil under Mild Conditions

Abstract

AbstractSystematic analysis of the effect of para‐substituents (H, Cl, Br and OMe) on the meso‐phenyl group in vanadyl meso‐tetraphenylporphyrins ([VIVO(TPP)] (R=H, 1), [VIVO(TCPP)] (R=Cl, 2), [VIVO(TBPP)] (R=Br, 3) and [VIVO(TMPP)] (R=OMe, 4)) on their properties and catalytic oxygen atom transfer (OAT) for oxidation of benzoin to benzil using DMSO as well as 30 % aqueous H2O2 as the sacrificial oxygen source have been studied. Electrochemical and theoretical (density functional theory) studies are in good agreement with the influence of these substituents on the catalytic property of these complexes. Complex [VIVO(TCPP)] (2) displayed the best catalytic activity for the conversion (92 %) of benzoin to benzil in 30 h with >99 % product selectivity when DMSO was used as an oxygen source, whereas excellent conversion (~100 %) of benzoin to benzil was noticed in 18 h with 95 % product selectivity when 30 % aqueous H2O2 was used as a source of oxygen. Furthermore, among these complexes, the electron‐withdrawing nature of the chloro substituent at the p‐position of meso‐phenyl group significantly influences the oxygen atom transfer. Experimental and simulated EPR studies confirmed the +4 oxidation of vanadium in these complexes. The structure of 2, 3 and 4, confirmed by single crystal X‐ray diffraction method, are domed in shape, and the displacement of V(IV) ion from the mean porphyrin plane follows the order: 2 (0.458 Å) <3 (0.459 Å) <4 (0.479 Å). We observed that the electron‐withdrawing nature of chloro substituent at the p‐position of meso‐phenyl group influence the oxygen atom transfer from vanadyl porphyrin to dimethyl sulfide much.