Phenols phenol coupling

One-electron oxidation of an aryl ether, for example at the anode, gives rise to a radical cation whose fate may be radical coupling (shown as dimerization) or substitution into a neutral phenol ether both paths are shown in Scheme 6 and converge to a biaryl product. Other products are possible if coupling at a quaternary center takes place. Mechanisms of this type must operate for the important couplings of phenol ethers with phenol ethers, and phenols with phenol ethers possibly they should not be neglected for phenol-phenol coupling also, in certain cases. 

Few examples, in this class, of phenol-phenol coupling generating six-membered rings, are available the bisarylethane (246) gave (247) only in 11% yield.However biomimetic oxidation of phenolic... 

Alkyl phenols Phenols coupled as p-nitrophenol azo dyes applied to plate of silica gel impregnated with alkali. Separated azo dyes located as yellow/red colours upon exposure of plate to ammonia vapour. ... 

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Generally, phenols (as the phenolate anion) couple more readily than amines, and members of the naphthalene series more readily than the members of the benzene series. 

The early yellow disperse dyes were based on phenolic coupling components, eg, Cl Disperse Yellow 3 (92) (diazotized 4-arninoacetanihde coupled to -cresol) which is still used today for the coloration of cellulose acetate and nylon fibers. 

The general discussion (Section 4.02.1.4.1) on reactivity and orientation in azoles should be consulted as some of the conclusions reported therein are germane to this discussion. Pyrazole is less reactive towards electrophiles than pyrrole. As a neutral molecule it reacts as readily as benzene and, as an anion, as readily as phenol (diazo coupling, nitrosation, etc.). Pyrazole cations, formed in strong acidic media, show a pronounced deactivation (nitration, sulfonation, Friedel-Crafts reactions, etc.). For the same reasons quaternary pyrazolium salts normally do not react with electrophiles. 

In the first of these, the key step in the synthetic sequence involves an oxidative phenol coupling reaction patterned after the biosynthesis of the natural product. Preparation of the moiety that is to become the aromatic ring starts by methyla-tion of phloroglucinol (5) with methanolic hydrogen chloride to give the dimethyl ether (6). Treatment of that intermediate with sulfuryl chloride introduces the chlorine atom needed in the final product (7). 

Organic syntheses based on biosynthetic proposals are often extremely concise and elegant.6 Although the constitution and stereochemical complexity of carpanone may seem formidable, the sequential application of the Diels-Alder and oxidative phenolic coupling transforms7 to the natural product provides an exceedingly efficient solution. Chapman s striking synthesis of carpanone typi-... 

Aromatic amines are in general somewhat less readily attacked than phenols and coupling is often carried out in slightly acid solution, thus ensuring a high [PhN2 ] without markedly converting the amine,... 

Schwartz MA, Holton RA (1970) Intramolecular oxidative phenol coupling. II. Biogenetic-type synthesis of (+-)-maritidine. J Am Chem Soc 92 1090-1092... 

Bisbenzylisoquinoline alkaloids are dimeric benzyltetrahydroisoquinoline alkaloids that are known for their pharmacological activities. A well-described example is the muscle relaxant (+)-tubocurarine, which in crude form serves as an arrow poison for South American Indian tribes. In the biosynthesis of this broad class of dimeric alkaloids, it has been postulated that the mechanism of phenol coupling proceeds by generation of phenolate radicals followed by radical pairing to form either an inter- or intramolecular C - O or C - C bond. Enzyme studies on the formation of bisbenzylisoquinoline alkaloids indicated that a cytochrome P-450-dependent oxidase catalyzes C - O bound formation in the biosynthesis of berbamunine in Berberis cell suspension culture.15 This enzyme, berbamunine synthase (CYP80A1), is one of the few cytochromes P-450 that can be purified to... 

KRAUS, P.F.X., KUTCHAN, T.M., Molecular cloning and heterologous expression of a cDNA encoding berbamunine synthase, a C - O phenol coupling cytochrome P450 from the higher plant Berberis stolonifera, Proc. Natl. Acad. Sci. USA, 1995, 92,2071-2075. 

Scheme 3. Phenolic coupling reactions caused by one-electron oxidations.

Oxidative phenolic coupling. Biosynthesis of the alkaloid narwedine (3) is known to involve oxidative phenolic coupling of norbelladine derivatives (1), but the usual oxidants for such coupling in vitro convert 1(R = H) into the oxomaritidine skeleton (4) rather than 3. A new biomimetic synthesis of 3 involves the palladacycle 2, formed by reaction of 1(R = CH3) with Li2PdCl4, which is known to form complexes with allylic amines or sulfides (8,176-177). Oxidation of 2 with thallium(III) trifluoroacetate effects the desired coupling to give 3. 

Biaryl phenol coupling.1 The natural dibenzylbutanolide lignans, prestegane A (la) and B (lb), are converted to the corresponding bisbenzocyclooctadiene lactones (2) by oxidation with Ru02 in TFA-TFAA in 80-85% yield. The usual reagent for this oxidation, thallium tris(trifluoroacetate), is less efficient (45-50% yields). 

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