Coupling reactions phenolic oxidative

Generally, when two different aryl units are involved in a coupling reaction, the oxidation potentials of the two units will be different. In another mechanistic variant, the more easily oxidized phenol 14 can react with the other unperturbed aromatic unit 15 to furnish a biaryl radical 16, which then oxidizes further, deprotonates, and finally tautomerizes to give the product biaryl (Scheme 4). 

Phenolic Cyclization and Coupling Reactions. The oxidation of phenolic compounds which either do not possess benzylic hydrogen atoms, or which have an alternative reaction pathway, can result in a variety of interesting products. Cyclodehydrogenation reactions leading to oxygen heterocycles represent... 

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). 

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). 

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>). 

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

The features of the electro-oxidative polymerization can he explained as follows. The molecular weight of the obtained polymer stayed constant during the polymerization, because the polymerization proceeds heterogeneously in the diffusion layer of electrode. The C-0 coupling reaction is predominant, probably because the phenol is adsorbed and oriented on the electrode surface. The polymerization started from the dimer is much suppressed, because the dimer diffuses from the bulk phase into the diffusion layer very slowly. 

The polymerization mechanism of phenols is described as follows. The phenol is adsorbed on the electrode surface and accumulated in the diffusion layer. The adsorbed phenol undergoes one-electron oxidation to the phenoxy radical on the electrode surface. The concentrated phenoxy radical is coupled with each other at p-position to form the dimer, and the dimer repeats the electro-oxidation and coupling. The phenoxy radical is assumed to he adsorbed or oriented upon the electrode surface thus resulted in the selective coupling reaction. 

The earlier literature on oxidative coupling of phenols is reviewed in Ref. [168] and that on anodic coupling in Ref. [169, 170] some examples of the coupling reactions are summarized in Table 11, see also Chapter 6. 

Diarylamides with arenes activated by electron-donating substituents can be converted to azacycles by anodic oxidation through phenolic oxidative coupling reactions that can be a key step in the synthesis of alkaloids (Schemes 16 and 17). According to the nature of substituents and the experimental conditions, either spiro compounds [22] or non-spiro compounds [23, 24] were obtained. 

In an effort to explore the factors that govern anodic C-C bond formation, Swenton and coworkers have also been exploring the intramolecular coupling of phenols and olefins (Scheme 28) [44]. In these reactions, initial oxidation of the phenol followed by loss of a proton and a second oxidation led to the formation of a cationic intermediate (26). This intermediate was trapped by the olefin to form a second cation that was in turn trapped by methanol to form the final product 28. When R2 was equal to methyl (25b) or phenyl (25c) the reaction led to a good yield of the cyclized product. Reactions where the R2 was equal to a hydrogen (25a and 25d) were not so successful. The cyclizations were compatible with the incorporation of the olefin into a third ring (25e). 

A number of compounds react rapidly with DDQ at room temperature. They include allylic and benzylic alcohols, which can thus be selectively oxidized, and enols and phenols, which undergo coupling reactions or dehydrogenation, depending on their structure. Rapid reaction with DDQ is also often observed in compounds containing activated tertiary hydrogen atoms. The workup described here can be used in all these cases. 

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