Oxidative coupling reaction of phenol

Finally, mention must be made of possible conjugation reactions in which a covalent bond is formed between a contaminant molecule and a second contami nant molecule or soil organic matter. Oxidative coupling reactions of phenolics and aromatic amines are catalyzed by extracellular enzymes, clays, and oxides (Wang et al., 1986 Liu et al., 1987 Fluang, 1990). The bioavailability of the synthetic organic within the product is reduced or possibly eliminated (Dec et al., 1990 Allard et al., 1994). 

There have been numerous studies in recent years investigating the differences in oxidative coupling reactions of phenols catalyzed by enzymes or mineral colloids. Both mineral colloids and oxidoreductive enzymes contain metals that can act as electron acceptors to catalyze the oxidative transformation of organics hence there are similarities in their reaction products. However, there are differences in the mechanisms by which these catalysts operate. 

Oxidative coupling reactions of phenols are usually performed by treatment of phenols in solution with more than an equimolar amount of metal salts such as FeCF. However, the coupling reaction of some phenols with FeCF was demonstrated to proceed much faster and more efficiently in the solid state than in solution and the reaction in the solid state is accelerated by irradiation with ultrasound. For example, the irradiation with ultrasound of a mixture of finely powdered p-hydroquinone and [Fe(DMF)3Cl2][FeCl4] (2 equiv.) in the solid state (50 °C, 1 h) provided 695 in 64% yield (Scheme 137). 

Several other groups have observed oxidative coupling reactions of phenols (see Equation 5.10) in the presence of ozone. Chrostowski et al. (1983) exposed several polyphenolic compounds to ozone at various pHs and observed changes in color for example, catechol (86) was observed to develop first a red and then a brown color. 

A generalized oxidative coupling reaction of phenolic monomers is shown in Scheme 1. Two kinds of couplings are possible, (a) C-C coupling to form phenylene units, and (b) C-O-C coupling to form oxyphenylene units. 

In phenolic oxidative coupling reactions, these phenol-derived radicals do not propagate a radical chain reaction instead, they are quenched by coupling with other radicals. Thus, coupling of two of these resonance structures in various combinations gives a range of dimeric systems, as shown. The... 

Pal et al. (1994) compared the catalysis of oxidative coupling reactions of various phenolic compounds by the enzymes, laccase and tyrosinase, and mineral catalyst, birnessite. Birnessite acts as a heterogeneous catalyst whereas laccase and tyrosinase function as homogeneous catalysts. Laccase and tyrosinase continue to oxidize catechol after repeated additions of the chemical, while birnessite lost its oxidizing activity after the first addition of catechol (Figure 2.20). In the case of birnessite,... 

Lunarine (26), one of the typical neolignans, is biosynthesized by the ortho-para radical coupling between two molecules of p-hydroxycinnamic acid. In this connection, oxidative coupling reactions of 4-substituted phenols have been extensively stndied using thallium trifluoroacetate (TTFA), potassium ferricyanide (K3[Fe(CN)g]) and other reagents. p-Cresol (27) was also electrolyzed at a controlled potential (+0.25 V vi. SCE) in a basic medium to afford Pummerer s ketone 28 in 74% yield. The snggested mechanism is given in Scheme 4. 

Silica-bound FeCls can act as a one-electron-transfer oxidant, which is very effective for oxidative coupling reactions of aromatic ethers and phenols. 1,2-Diarylethane 691 was oxidized with FeCF supported on silica gel in CH2CI2 to give the corresponding para-para coupled product 692 in almost quantitative yield (98%). Similar oxidation of 2-methoxy-p-hydroquinone (693) provided a dibenzofuran 694 (35%) (Scheme 136). ... 

Coupling reactions of phenol derivatives proceed very efficiently in the solid state. This method is especially useful for the preparation of rac-2,2 -dihydroxy-l, 1 -bi- naphthyl 12 by an oxidative coupling of 2-hydroxynaphthalene 11. For example, when a mixture of finely powdered 11 and 2 mol equiv. of FeCl3-6H20 is kept at 50 °C for 2 h, 12 was obtained in 95% yield [11]. 

The dienone (34), a proposed intermediate in the biosynthesis of Erythrina alkaloids, has been obtained by the phenolic oxidative coupling reaction of the norprotosinomenine derivative (33) with potassium ferricyanide in the presence... 

Discuss and illustrate with chemical equations the formation of poly(phenylene oxide) by the oxidative coupling reaction of 2,6-disubstituted phenols. 

The oxidative coupling reactions of certain electron-rich arenes under suitable reaction conditions proceed, at least partially, via free-radical mechanism. Scheme 3. The phenolate anion is oxidized by suitable one-electron oxidant to the phenoxyl-radical whose tautomeric form is aryl-radical on the adjacent carbon atom. The symmetrical biaryl is formed by coupling of the latter, whereas the unsymmetrical one is produced by free-radical arylation of the second arene molecule, usually in an intramolecular... 

In addition, several other one- and two-electron oxidants have been involved in the oxidative coupling reactions of mainly activated benzenes such as phenols, phenol-ethers, alkylated phenol-ethers, etc. 

The reaction is accompanied with the cleavage of O-isopropyl protecting group, which occured after addition of methanol. Anhydrous iron(lll) chloride efficiently affects the room temperature oxidative coupling reactions of electron-rich aromatics other than phenols and phenol-ethers, e.g. thiophenes [39], as well as the cross-eouplings, e.g. A-alkylcarbazoles with pyridine-A -oxides [40], ete. 

Oxidative coupling reactions of p-phenylenedia-mines with amines and phenols are widely used. As can be seen in Scheme 1 p-phenylene diamine is oxidized to its quinone diimine (QDI) by potassium hexacyanoferrate(III) or a similar oxidant in a weakly basic medium. In the rate-limiting step, QDI reacts with the amine or phenol to give leuco-indamines (indophenols), which are rapidly oxidized to colored indamines (indophenols) with the aid of a QDI molecule. This reaction is chiefly used for resolving a large variety of mixtures of aromatic amines, phenols, and chlorophenols. 

Step. The kinetics and mechanism of QDC oxidation of some benzaldehydes, benzylamines, and para- and mefa-substituted phenols by QDC have been reported. In the case of substituted benzaldehydes and phenols, the reaction is retarded by electron-withdrawing substituents and enhanced by electron-releasing substituents. The oxidative coupling reaction of anilines with QDC in acidic medium gave the... 

Novel heteroquaterphenoquinones were synthesized by a stepwise cross-coupling reaction or by a more convenient one-pot oxidative homocoupling reaction of the heterocycle-substituted phenols (Scheme 20, <96JOC4784 see also 95TL8055>). 

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