Biaryls from aromatic compounds

Reaction of electron-rich aromatic compounds with TTFA leads to intermolecular oxidative coupling to form the corresponding biaryls without aromatic thallation. The reaction proceeds through one-electron transfer from aromatic compounds to Tl(III) to give an aromatic radical cation which leads to biaryls (Schemes 9.52 and 9.53 [52]). Intramolecular aryl coupling also occurs (Schemes 9.54 [53] and 9.55 [54]) and, further, when the carboxylic acid moiety is present, intramolecular as well as intermolecular lactonization occurs (Schemes 9.56 [55] and 9.57 [56]). 

From the point of view of the synthetic organic chemist, the importance of aromatic thallation, and the remarkable degree of orientation control which can be exercised over this process, lies in the ease with which the resulting ArTlXj compounds can be converted into substituted aromatic derivatives in which the new substituent group has entered the ring at the position to which thallium was originally attached. Syntheses of phenols, nitroso compounds, biaryls, aromatic nitriles, thiophenols, and deuterated aromatic compounds have all been achieved these results are summarized briefly below. 

Diels-Alder Reaction. By means of the Diels-Alder reaction a considerable number of hydrobiphenyls have been prepared,486 from which it should be possible to obtain unsymmetrical biaryls by dehydrogenation. Only a few of the adducts have been converted to the completely aromatic compounds. o-Terphenyl has been prepared from the adduct of maleic anhydride and 3,4-diphenylcyclopentadienone,436 and p-terphenyl has been obtained in practically quantitative yield from the adduct of the methyl ester of acetylenedicarboxylic acid and 1,4-diphenylbutadiene.43d... 

Treatment of diaryl tellurium dichlorides, obtainable from tellurium tetrachloride and aromatic compounds, with carbon monoxide in the presence of palladium(II) chloride/lithium chloride gives arenecarboxylic acids. Biaryls are formed as by-products1,2. Similar products were obtained when the diaryl tellurium dichlorides were reacted with nickel tetracarbonyl3. 

Aryllead tricarboxylates have been shown to react in trifluoroacetic acid with a range of aromatic compounds to give biaryls [31c]. With aromatic compounds that are more electron-rich than toluene the yields are generally good and the method is a useful synthesis of unsymmetrical biaryls (Scheme 13.18). It has been reported that a cationic intermediate generated from aryllead tricarboxylates was probably involved. 

Halogenated aromatic compounds can be condensed with alkyl or aryl halides by using copper to remove the halogen Zincke350 prepared ethyl phenylacetate in this way from bromobenzene and ethyl chloroacetate and Ullmann and his co-workers351 prepared biaryls from 2 moles of aryl halide and copper (for a review see Fanta352). 

Furthermore, different research groups have reported independently the construction of biaryl compounds from unactivated aromatic compounds by direct C—H activation using NaOEu or KOEu and 1,10-phenanthroline [110, 111] or DMEDA [112] as ligands (Eqs. 9.29 and 9.30) ... 

Another, especially historically important copper-mediated radical aryl-aryl bond forming reaction is the Pschorr reaction (Gomberg-Bachmann reaction), which has been first described in 1896 for the preparation of phenanthrene and its derivatives [85]. Later, in 1924, Gomberg and Bachmann described an intermolecular version of this reaction and prepared several biphenyl derivatives in moderate yield [86]. The reaction is initiated by formation of diazonium salts from aromatic amines, which release nitrogen upon reaction with copper salts. The intermediary formed aryl radial then undergoes the desired coupling reaction and allows the isolation of biaryl compounds. An example of such a reaction is outlined in Scheme 12.23 [87],... 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

The two general reactions can be used to prepare a variety of compounds in which two aryl groups are linked. The unsymmetrical biaryls are of particular interest since they are not as accessible through other reactions as the symmetrical compounds. Some conception of the types of compounds available by replacement of the amino group of aromatic amines by aryl groups can be obtained from an examination of the following table, which shows the more important amines from which diazo... 

It is not possible to prepare biaryls containing a free carboxyl group directly by the diazo reaction. No biaryl is formed when (a) diazotized aniline and sodium benzoate, (b) diazotized anthranilic acid and aqueous sodium benzoate, or (c) diazotized anthranilic acid and benzene are used as components in the reaction.13 On the other hand, the reaction proceeds normally if methyl benzoate is used in reaction (a) or when methyl anthranilate replaces the anthranilic acid in (b) and in (c). The success of the diazohydroxide reaction appears to lie in the ability of the non-aqueous liquid to extract the reactive diazo compound from the aqueous layer.4 However, esters and nitriles can be prepared from esters of aromatic amino acids and cyanoanilines and also by coupling with esters of aromatic acids, and from the products the acids can be obtained by hydrolysis. By coupling N-nitrosoacetanilide with ethyl phthalate, ethyl 4-phenylphthalate (VIII) is formed in 37% yield. 

Organocopper reagents, such as methylcopper prepared from MeLi were also used as catalyst recently [148]. Yields could be further improved by ultrasonic irradiation [149],.as well as under phase transfer conditions [150]. It is surprising that, in contrast to the Ullmann biaryl synthesis, activated aromatic fluoro compounds (e.g. fluoro-esters, -nitrils, -aldehydes and -acetophenones) can couple with phenols in good yield. The reaction does not require any catalysts, but only K2CO3 as a base, [146] (Scheme 56). ... 

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