Copper oxidative phenol coupling

SCHEME 41. Copper-promoted oxidative phenol-couplings of 2,6-dimethylphenol... 

The last example to be mentioned deals with the application of coordination compounds attached to polymers and their use as immobilized catalysts. This technique has been used for a long time in organometallic catalysis. Similar reactions with biomimetic catalysts, as with Cu(II) oxidases, are less well known, and a review for polymeric copper imidazole complexes used in oxidative phenol coupling is available. 

Kozlowski and co-workers reported asymmetric oxidative phenolic coupling reactions using the chiral 1,5-diaza-ds-decalin copper catalyst (Scheme 3.19). ... 

Enantioselective synthesis of axially chiral biaryls include all approaches where the starting material is achiral and the chirality is induced by an influence of chiral catalyst or reagent. Among enantioselective reactions that have been successfully used in the synthesis of axially chiral biaryls are the Kharasch reaction [34-36], Suzuki-Miyaura reaction [37,38], oxidative phenolic coupling mediated by copper complexes... 

Oxidative phenoiic coupling. Copper complexes have been used under aerobic conditions to oxidize phenols to quinones and to products of cleavage. Feringa and Wynberg have now found that Cu(lll amine complexes under anaerobic conditions can effect oxidative coupling of phenols. Some typical... 

This chiral copper(ll) complex was also effective in the oxidative cross-couplings between two electronically different polycyclic phenols, generally giving good yields and high enantioselectivity [42]. 

Two copper(ll)-mediated coupling reactions and one Fe(III)-catalyzed reaction are outlined in Scheme 12.22. An example of the stoichiometric use of CuCl in the oxidative coupling of phenol 98 and amine 99 is shown in Equation 12.22-1, Scheme 12.22 [81]. This reaction also demonstrates the potential of copper-mediated processes in the formation of unsymmetrical adducts (with the formation of only minor amounts of homodimers as side products). The authors supported their experimental findings with calculations of the frontier orbital interactions and showed that the HOMO and LUMO interaction with the SOMO of the initially formed radical is favored in differently substituted coupling partners. The success of the reaction also strongly depends on the additive and the yield dropped significantly when the reaction was carried out without the chelating primary amine. 

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

We were interested in the behaviour of polymeric catalysts in order to confirm that typical polymer effects may occur. Oxidative coupling of 2,6-disubstituted phenols, as developped by Hay (7), was chosen as a model reaction and the catalytic activities of coordination complexes of copper with several polymeric tertiary amines were compared with the activities of their low molecular weight analogs. The overall reaction scheme is presented in scheme 1. 

Figure 15.15. Copper cataly sed oxidative coupling of phenols...

The following questions on the electro-oxidative polymerization arose. First, why various phenol derivatives were smoothly polymerized which could not occur by the oxidation with the copper catalyst or lead dioxide. Secondly, why the activated phenol was reacted preferentially through C-0 coupling to form the poly(phenyleneoxide). The mechanism of the electro-oxidative polymerization is discussed below by using the example of 2,6-dimethylphenol. 

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