Coupling reactions of phenols

The express method of phenolic compounds determination was designed. It is based on heterogeneous azo-coupling reaction of phenols, which different extracts of fresh raw material contain, with aryldiazonium salts grafted on the silica surface. We can carry out phenols analysis on-site, as formation of immobilized azocompounds leads to a drastic change in the sorbent s color. Thus, we pui pose a new method, that allows to indicate herb phenols in aqueous and non-aqueous medias and to compare it with a well-known Folin-Ciocalteau method. 

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). 

Pearl and Dickey (20) isolated 3,3 -dimethoxy-4,4 -dihydroxybenzo-phenone, an analog of the diphenylmethanes apparently present in our product mixture. They suggest that the ketone is a rearrangement product of vanillil. In our system the corresponding diphenylmethanes could also be produced by a coupling reaction of phenolic methylols or quinonemethides, and at present they cannot be considered proved structural elements native to lignin. 

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. 

The coupling reactions of phenols with diazonium salts take place most readily using the phenoxide anions. Many of the products are dyestuffs (see below). 

Ma, D., Cai, Q. N,N-Dimethyl Glycine-Promoted Ullmann Coupling Reaction of Phenols and Aryl Halides. Org. Lett. 2003, 5, 3799-3802. 

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). 

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]. 

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. 

Suggested mechanisms of coupling reaction of phenol derivative of P-lactam analogs were presented in Fig. 15. 

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. 

Scheme 2.16 The Suzuki-Miyaura cross-coupling reaction of phenolic and enol esters.

Scheme 2.23 Direct Suzuki-Miyaura cross-coupling reactions of phenols.

Electrosynthesis Using Diamond Electrode, Scheme 4 Efficient cross-coupling reactions of phenols by adding protic additives to HFIP... 

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