Selective Oxidation of 4-Methoxymandelic Acid Catalyzed by Mononucleonic and Binucleonic Metal Complexes

Abstract

Three kinds of mononucleonic metal complexes of L~1Cu,L~1Co,and L~1Mn(L~1=N,N'-bis-(2hydroxyethyl)-malondiamide),and three kinds of binucleonic metal complexes of L~2Cu,L~2Co,and L~2Mn(L~2= N,N'-bis{2-(2-hydroxyethylamino)ethyl}-malondiamide) were prepared.These complexes exhibited good catalytic performance for the selective oxidation of 4-methoxymandelic acid(4-MMA) by H_2O_2 in buffer solution,with96% selectivity to anisaldehyde(AAD) and a small percentage to 4-methoxybenzoic acid(4-MBA).However,the reaction rates were quite different for these catalytic systems,in which the copper complex was the best candidate as the catalyst for the oxidation of 4-MMA.The binucleonic complexes displayed obviously better catalytic performance than the mononucleonic complexes.The effect of the applied buffers(tartaric acid(TA),H_3PO_4,HAc) on the catalytic performance in terms of activities and selectivities for the oxidation of 4-MMA by H_2O_2 were investigated.The reaction kinetics of 4-MMA oxidation catalyzed by six complexes was studied in detail.The apparent first-order rate constants(k_(obs)) were obtained in the pH range of 2.5 to 4.5.The pH effect on the catalytic oxidation was discussed in this paper.