Gomberg coupling reactions

The Gomberg coupling reaction of aryl diazonium salts with arenes is catalysed by quaternary ammonium salts [49], Although yields are variable, they are generally >50% [49, 50]. When aromatic solvents other than benzene are used, the appropriate biaryls are produced, e.g. 4-chlorobenzenediazonium tetrafluoroborate reacts with chlorobenzene to produce the 2,4 -, 3,4 - and 4,4 -dichlorobiphenyls in a ca. 67 18 15 ratio. 

Gomberg-Bachmann reaction When an acidic solution of a diazonium salt is made alkaline, the aryl portion of the diazonium salt may couple with another aromatic ring, i.e. ArH reacting with... 

Over the past century, several methods have been developed for the synthesis of biaryl compounds [7]. Among these are the Ullmann-type coupling [8,9], the Scholl reaction [10], the Gomberg-Bachmann reaction [11], and recently transition-metal-catalyzed cross-coupling reactions [12], In particular, palladium-catalyzed crosscoupling reactions have been successfully applied to the synthesis of biaryls due to their generally high yields and excellent selectivities. 

There are many variations, and a wide range of aromatic diazonium salts can be used. One method for generating the diazonium salt is treatment of an aromatic amine with nitrous acid (HONO via reaction of HNO3/HCI or NaN02/2 HCl).i62 These diazonium salts can be coupled to other aromatic compounds in the presence of base in what is commonly called the Gomberg-Bachmann reaction. 63 Treatment of 4-bromo-aniline with nitrous acid, for example, gave the diazonium salt (231) which reacted with benzene to give the... 

Not just any aryl-aryl-coupling may be suitable for making biphenyls. The construction of the aromatic rings [674] is an interesting alternative in view of the tarry side-products of an unsuccessful Gomberg-Bachmann reaction. Condensation... 

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],... 

In a search for new spasmolytics, Heilbron and Hey and their co-workers synthesized a number of pyridylquinolines by coupling diazotized arainoquinolines with pyridine. Pyridine has also been substituted by the 3-pyridyl radical in the Gomberg reaction, the 2-and 3-substituted products being obtained in 55 and 5% yield, respectively, together with a third product obtained in 20% yield w hich w as tentatively formulated as the 4-isomer. The same radical w as coupled with 4-methyl- and 4-ethyl-pyridine and gave in each case mixtures of the twm possible substitution products in which the radical had coupled predominantly with the carbon atom of pyridine adjacent to the alkyl substituent, as showm in Eq. (19). ... 

Historically, the triphenylmethyl radical (1), studied by Gomberg in 1987, is the first organic free radical. The triphenylmethyl radical can be obtained by the reaction of triphenylmethyl halide with metal Ag as shown in eq. 1.1. This radical (1) and the dimerized compound (2) are in a state of equilibrium. Free radical (1) is observed by electron spin resonance (ESR) and its spectrum shows beautiful hyperfine spin couplings. The spin density in each carbon atom can be obtained by the analysis of these hyperfine spin coupling constants as well as information on the structure of the free radical. 

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