Kinetic Studies On Cu(Ⅱ) Complex-Catalyzed Oxidative Coupling Of 2,6-Dimethylphenol In Aqueous Solution

Enzymatic catalysis for organic synthesis possesses advantages such as high catalytic activity, lack of undesirable sidereactions, facility operations under mild conditions, and high stereo-, region-, and chemo-selectivities of reactions in comparison with those of other chemical catalysis. And the enzymatic polymerization often provides an environmentally benign process, where starting materials and products are within the natural material cycle.The oxidative coupling reaction of 2,6-dimethylphenol(DMP) with the H2O2, catalyzed by five synthetic Copper(â…¡) complexes was investigated kinetically in the aqueous solution. In our work, the optimal formation rate of 3,3',5,5'-tetremethyl-4,4-diphenoquione(DPQ) was at neutral solution. The studies indicated that the catalytic reactions in the buffer system obeyed Michaelis-Menten enzyme-catalysis kinetics. The kinetic parameters k2 and Km of DMP oxidation to DPQ were obtained. The structure and the form of Copper(â…¡) complexes display great influence on the oxidation coupling of DMP. The alkaline chelate grounps, such as pyridine and NH2, have the great catalytic power for DMP oxidation reaction. The"bell-like"curves of R0 vs. pH and that of k2 vs. pH were observed from the data obtained in the solution of complexes with the pyridine, NH2, OH substituted amine ligands. The catalysis of the Copper(â…¡) complexe with acetic acid substituted amine ligands increased with the increasing pH value. Because of its unique structure and characteristic, surfactant and micelle can aggregate the reactant, affect the reaction rate and reaction mechanism, and change the product selectivity and even stererochemistry. In this study, two copper(â…¡) complexes with pyridine substituted amine ligands catalyzed oxidation reaction of 2,6-dimethylphenol was studied kinetically in micellar buffer solutions containing C16-2-C16,2Br-, C12-2-C12,2Br-, CTAB, SDBS,Triton X-100, respectively. The hydrophobic interaction and electrostatic interaction have greatly influence on the reaction.The catalytic mechanism was also investigated and the results showed that the reaction was mainly catalyzed by the first acid ionization species of the Cu(â…¡) complexes. The phenol may experience the proton abstraction by the first deprotonized species (M*S(OH-)), resulting in the formation of phenolate that was possibly still coordinated to activity copper species, as its oxygen atom still bears a partial negative charge. Once formed, phenolate may react with the activity ligand-Cu(â… )-radical species to produce a ligand-Cu(â… )-(phenol radical) complexes. The"control"phenol radical then can be attacked by a phenol or by other phenol radical to give either the DPQ or the PPO.These studies were very important in theory and in application for preparation of functional polymer. The experimental data and studies in this paper provided some useful information for the development of mimetic peroxidase.