2-aryl phenols

Dimroth, K. Arylated Phenols, Aroxyl Radicals and Aryloxenium Ions Syntheses and Properties. /29, 99-172 (1985). 

Acceptor-substituted haloarenes have been successfully used to O-arylate phenols by aromatic nucleophilic substitution (Table 7.14). The most common arylating agents are 2-fluoro-l-nitroarenes, 2-halopyridines, 2-halopyrimidines, and 2-halotriazines. When sufficiently reactive haloarenes are used, the reaction proceeds smoothly with either the arylating agent or the phenol linked to the support. The thallium(III) nitrate catalyzed arylation of phenols with aryl iodides has been used for macrocycli-zations on solid phase [184], Burgess and co-workers have developed a solid-phase synthesis of 3-turri mimetics based on ring-closure by aromatic nucleophilic substitution (Entry 4, Table 7.14 see also Table 10.5). Phenols, alkylamines, and thiols have been successfully used as nucleophiles for this type of macrocyclization [185],... 

Asymmetric amplification, in diisopropylzinc additions, 2, 386 Asymmetric arylation, phenols with lead triacetates, 9, 399 Asymmetric carboalumination, Zr-catalyzed, alkenes, 10, 272 Asymmetric carbonyl-ene reaction, characteristics, 10, 559 Asymmetric catalysis... 

Arylated Phenols, Aroxyl Radicals and Aryloxenium Ions... 

In spite of the immense quantity of literature available on alkyl-substituted phenols including their syntheses, chemical and physical properties, their great importance as intermediates in the synthesis of natural products, as well as their many applications in science or industry, much less is known about aryl-substituted phenols and their derivatives. Even in recent comprehensive reviews on phenols1,2) no chapter specifically concerning arylated phenols has been included. What may be the reasons for this strange gap ... 

Contrary to the easy synthesis of alkylated phenols, e.g. via the electrophilic alkylations of phenols using alkyl halides, alkenes or other alkyl derivatives, there is no universal method for the preparation of arylated phenols. Radical or other arylation procedures usually lead to a mixture of difficult to separate isomers. Aryl-substituted phenols have, therefore, to be prepared by special routes. 

It is hoped that further study of the neglected class of arylated phenols and their oxidation products will lead to a revival in their research and practical applications. 

Arylated Phenols, AroxyJ Radicals and Aryloxenium Ions Syntheses and Properties... 

Finally, in sect. 2.9, the conversion of 2,4,6- or higher substituted pyrylium salts into the corresponding nitrobenzene derivatives through reaction with nitromethane 27) will be discussed in more detail. Since pyrylium salts of any desired complexity can be easily prepared 28), the conversion of the nitroarenes leads to a large number of phenols of the desired constitution. This method is by far the most versatile in the preparation of a desired arylated phenol. These syntheses, however, involve many steps. 

In spite of the essential difficulties mentioned above concerning the direct substitution of a particular hydrogen atom of multi phenylated benzenes giving rise to a well defined phenol or phenol ester, much work has been done29). Its emphasis, however, has been on mechanism rather than on synthesis. In this account we will only discuss some of the more promising synthetic routes of arylated phenols. An interesting example may be that of 2,4,6-triphenylphenol via direct hydroxylation or acyloxylation of 1,3,5-triphenylbenzene. 

When N-tosylated phenylhydroxylamines are treated with trifluoroacetic acid in the presence of benzene, 2- and 4-hydroxybiphenyl are found in yields of 25% and 20%, respectively. The aryloxenium ion is believed to be the intermediate56). No higher arylated phenol derivatives have been examined. 

Dehydrogenation to give the corresponding arylated phenols has so far not been reported, but this should be possible using established routes. 

Similar syntheses of arylated 1,3-dioxo-cyclohexanone derivatives also lead to arylated phenols. The enolic methoxyl group of the enolketone is removed using lithiumaluminiumhydride. Acetone cyanhydrin transforms the enolic methoxyl group into a cyano group. Arylated cyano phenols are also available109) by this route ... 

Addition of vinylene carbonate to 1,2,3,4-tetraphenylcyclopentadieneone gives 80 % of the Diels Alder addition product which upon thermolysis or photolysis yields 95 % of 2,3,4,5-tetraphenylphenol105). Other substituted alkyl-aryl-phenols are synthesized by a similar route, using 2-alkyl-3,4,5-triphenylcyclopentadienones110). 

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