Gomberg-Bachmann

GOMBERG - BACHMANN - GRAEBE - ULLMANN Arylatwn Atykaryl bond formation via diazonium salts Carbazole synthesis by intramolecular aryl-aryl bond formation. 

Radicaloid substitution has not been extensively studied in the thiophene series. Molecular orbital calculations indicate that substitution should occur in the a-position. This has been found to be the case in the Gomberg-Bachmann coupling of diazohydroxides with thiophenes which has been used for the preparation of 2-(o-nitro-phenyl) thiophene, 2-(p-toluyl) thiophene, " " and 2-(p-chloro-phenyl)thiophene. " Coupling in the /8-position has been used for the preparation of 1,3-dimethyl-4,5-benzisothionaphthene (148) from 2-amino-tt-(2,5-dimethyl-3-thienyl)cinnamic acid (149). A recent investigation describes the homolytic phenylation of 2- and 3-phenyl-... 

Arenediazonium species 1 can be reacted with another aromatic substrate 2, by the Gomberg-Bachmann procedure, to yield biaryl compounds 3. The intramolecular variant is called the Pschorr reaction ... 

In contrast to the Gomberg-Bachmann reaction, the intramolecular variant, the Pschorr reaction, is carried out in strongly acidic solution, and in the presence of copper powder. Diazonium biphenyl ethers are converted to dibenzofurans, e.g. 8 9 ... 

The Gomberg-Bachmann reaction is usually conducted in a two-phase system, an aqueous alkaline solution, that also contains the arenediazonium salt, and an organic layer containing the other aromatic reactant. Yields can be improved by use of a phase transfer catalyst. Otherwise yields often are below 40%, due to various side reactions taking place. The Pschorr reaction generally gives better yields. 

Arylation of Aromatic Compounds The Gomberg-Bachmann and Related Reactions 253... 

Dediazoniation refers to all those reactions of diazo and diazonium compounds in which an N2 molecule is one of the products. The designation of the entering group precedes the term dediazoniation, e. g., azido-de-diazoniation for the substitution of the diazonio group by an azido group, or aryl-de-diazoniation for a Gomberg-Bachmann reaction. The IUPAC system says nothing about the mechanism of a reaction (see Sec. 1.2). For example, the first of the two dediazoniations mentioned is a heterolytic substitution, whereas the second is a homolytic substitution. 

A similar case is the catalysis of Gomberg-Bachmann arylations by A,A-diphenyl-hydroxylamine, which was discovered by Cooper and Perkins (1969). As Scheme 8-46 shows, the covalent adduct cation 8.62 first loses a proton. This facilitates the homolytic dissociation, as a stable radical, A/,A-diphenylnitroxide (8.63), is formed. This... 

The Gomberg-Bachmann reaction is a method for arylation of aromatic compounds and quinones (Gomberg and Bachmann, 1924). Originally this reaction involved adding aqueous sodium hydroxide slowly to an intimate mixture of an aqueous solu-... 

Heteroaromatic diazonium salts can also be used for Gomberg-Bachmann aryla-tions. Fukata et al. (1973) refluxed 3,5-dimethyl-4-diazopyrazole (10.27) in benzene and obtained 3,5-dimethyl-4-phenylpyrazole (10.28, 36%), biphenyl (10.29, 17%), 3,5-dimethylpyrazole (10.30, 12%), and pyrazolo[4,3-c]pyrazole (10.31, 15%). In nitrobenzene the three isomeric 3,5-dimethyl-4-(nitrophenyl)-pyrazoles were formed in the ratio o m p = 10 3 3. In the opinion of Fukata et al. this ratio and the course of the reaction indicate a homolytic process. The present author thinks that the data do not exclude a competitive heterolytic reaction with the pyrazolyl cation, because equal amounts of substitution of nitrobenzene in the 3- and 4-positions are not typical for a homolytic aromatic substitution. 

See the two Gomberg-Bachmann reactions reported in Organic Syntheses (Kaslow and Summers, 1963 Gomberg and Bachmann, 1964). 

The homolytic aromatic substitution part of the Gomberg-Bachmann reaction is, in the opinion of the present author, not sufficiently well understood on the basis of positive experimental data. ... 

The disadvantages of the Gomberg-Bachmann arylations are significantly reduced in their intramolecular analogues, particularly in Pschorr s synthesis of phenan-threne and related compounds. 

There are three areas of activity in the field of arenediazonium salts in interaction with metals and transition elements which have some similarities to metals. First is the use of copper in the reactions of Sandmeyer (1884), Pschorr (1896), Gomberg-Bachmann (1924), and Meerwein (1939). Other transition metal catalysts (Ti and Pd) have been used for such reactions since the 1970s (see Secs. 10.8 and 10.9). Up to now only one intermediate has been directly identified, the aryldiazenido palladium complex (ArN2Pd(PPh3)3]+BF4 (Yamashita et al., 1980 see Sec. 10.9, Scheme 10-64). 

Complexation with crown ethers increases the notoriously low solubilities of diazonium salts in most solvents (with the obvious exception of water). Therefore, it is possible to carry out phase-transfer reactions with complexed diazonium ions (review Gokel et al., 1985). Useful examples can be found in a paper from Gokel s group (Beadle et al., 1984a) on the Gomberg-Bachmann and Pschorr reactions (see Sec. 10.10). 

Gomberg-Bachmann reaction 62, 116, 189, 206, 253ff., 261, 273, 301 Gutmann donor numbers 375... 

Under Gomberg-Bachmann conditions, the species that cleaves is the anhydride ... 

The Pschorr reaction can take place by two different mechanisms, depending on conditions (1) attack by an aryl radical (as in the Gomberg-Bachmann reaction) or (2) attack by an aryl cation (similar to the SnI mechanism discussed on p. 853). Under certain conditions the ordinary Gomberg-Bachmann reaction can also involve attack by aryl cations. 

Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of a Mg—Mgl2 mixture has been called the Gomberg-Bachmann pinacol synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes. 

Free-radical arylation by diazonium salts (Gomberg-Bachmann,... 

By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg-Bachmann, Meerwein reactions). 

In this section we include the intramolecular arylation of the Pschorr type, the inter-molecular arylation (Gomberg-Bachmann reaction), the arylation of alkenes and alkynes (Meerwein reaction) and related processes. 

It is therefore not astonishing that in the Gomberg-Bachmann reaction, the intermolec-ular counterpart of the Pschorr synthesis, yields are generally low. The homolytic part of the Gomberg-Bachmann reaction is, in the opinion of March142 and of the present author, not sufficiently well understood on the basis of the experimental data. Galli117a does not discuss this problem in his review. 

Gomberg-Bachmann-Hey reaction org chem Production of diaryl compounds by adding alkali to a mixture of a diazonium salt and a liquid aromatic hydrocarbon or a derivative. gom,berk bak,man hT re.ak shan )... 

---