Catalysis of copper(II) chelate‐amine complexes in the oxidative coupling of 2,6‐dialkylphenols

Abstract

The oxidative coupling reaction of 2,6‐dimethylphenol and 2,6‐di‐tert‐butylphenol with molecular oxygen was performed by using a series of copper(II) chelate complexes as a catalyst, derived from copper(II), β‐diketone, and some Shiff bases. Under the applied reaction conditions, the reaction products of 2,6‐dimethylphenol were poly(2,6‐dimethyl‐1,4‐phenylene oxide) (CO coupling product) and 3,3′,5‐5′‐tetramethyl‐4,4′‐diphenoquinone (CC coupling product), and that of 2,6‐di‐tert‐butylphenol oxidation was only 3,3′,5‐5′‐tetra‐tert‐butyl‐4,4′‐diphenoquinone (CC coupling product). The catalytic activity has been shown to be dependent on the properties of the copper(II) chelates used as catalysts and the mole ratios of amine ligand to copper(II) chelate (ligand ratio). The basicity and the steric bulkiness of the amine used as a ligand for copper(II) β‐diketonato catalysts were found to be two of the main factors that govern the oxidative coupling mode (CO and/or CC coupling) of 2,6‐dimethylphenol. The oxidative coupling activity of 2,6‐dialkylphenol is discussed in terms of both the stabilities of the copper(II) chelates and of the copper(II) chelate‐amine adducts. The rate of oxygen absorption for 2,6‐dimethylphenol catalyzed by the copper(II) acetylacetonato‐piperidine system is first order in oxygen partial pressure and zero order in 2,6‐dimethylphenol concentration, respectively. A Cu(II)‐oxygen, as an intermediate is suggested on the basis of the results obtained.

Aerobic Copper-Catalyzed Organic Reactions

“…The reaction can be performed using CuCl, CuBr, CuPh, or copper(II) sources. ,, The rate is increased when a high trans -effect counterion is used . The phenoxide monomer itself can also be used as a counterion, though larger counterions are not as effective because they cannot form the necessary μ-X – bridges …”

Valmet Chiral Schiff‐Base Ligands And Their Copper(II) Complexes as Organo, Homogeneous and Heterogeneous Catalysts for Henry, Cyanosilylation and Aldol Coupling Reactions

“…Copper coordinated by various ligands such as chelate and chelate‐amine, [1] or N‐heterocyclic carbenes [2] have been largely investigated as homogeneous catalysts for various reactions with the advantage of replacing expensive and scarce noble metals. More recently, organocatalysis elicited an additional interest by offering a green alternative to metal‐complex catalysis through replacing the metal by organo‐functionalities such as amine or tio‐ groups [3] .…”