Selective oxidation of Mandelic acids catalyzed by copper (II) complexes

Two copper (II) complexes L1Cu and L2Cu (H2L1 = 4,9-dimethyl-2,11-dioxy-5,8-diazacycldode-2,4,8,10-tetraene; H2L2 = diethyl-3,8-dimethyl-4,7-diazadeca-2,8-diene-1,10-dioat) were prepared and displayed good catalytic efficiency for the oxidations of Mandelic acid (MA), 4-hydroxymandelic acid (4-HMA) and 4-methoxymandelic acid (4-MMA) by H2O2 in aqueous solution at 35 °C. The apparent first-order rate constants kobs of oxidations of MA, 4-HMA and 4-MMA were obtained in the range of pH 2.5–4.5. It was great interesting that the complexes LCu displayed excellent catalytic ability for the selective oxidation of Mandelic acids into aromatic aldehydes (S > 98%). The active species generated by reacting copper (II) complexes LCu with H2O2 was characterized by UV–vis, electrospray ionization mass spectrometry (ESI-MS) and cyclic voltammetry (CV). The LCuI- OOH species was suggested as the possible active species. The geometric configurations of LCu and the LCuI- OOH species were calculated with Density Functional Theory (DFT) method. The LCuI- OOH species is stable and possesses relatively high oxidation potential in aqueous solution at ambient temperature. This study also provides a new insight for the degradation of lignin.