Diazonium salt rearrangement

Pyridazines are formed from pyrones or their thioxo analogs or from appropriate pyridones. Pyrones or pyridones react with diazonium salts to give the corresponding hydrazones (187) and (188) which are rearranged under the influence of acid or base into pyridazinones as shown in Scheme 107. On the other hand, kojic acid is transformed with hydrazine into a 1,4-dihydropyridazine and a pyrazole derivative. 4H-Pyran-4-thiones... 

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

In all the diazotization reactions discussed in Sections 2.1-2.4 an equimolar amount of water is formed as byproduct. There are two general pathways for obtaining diazonium salts without formation of water. One is based on the rearrangement of 7V-nitroso-7V-arylacetamides, the other on the nitrosation of a monoarylated sp2-hybridized nitrogen compound by nitrosating reagents XNO in which X is a weak nucleophile. 

Research into the mechanism of diazotization was based on Bamberger s supposition (1894 b) that the reaction corresponds to the formation of A-nitroso-A-alkyl-arylamines. The TV-nitrosation of secondary amines finishes at the nitrosoamine stage (because protolysis is not possible), but primary nitrosoamines are quickly transformed into diazo compounds in a moderately to strongly acidic medium. The process probably takes place by a prototropic rearrangement to the diazohydroxide, which is then attacked by a hydroxonium ion to yield the diazonium salt (Scheme 3-1 see also Sec. 3.4). 

This and related syntheses involving diazonium salts were reviewed by Biffin et al. (1971, p. 148). The hypothesis that the triazene occurs as intermediate in Scheme 6-14 has been corroborated by the experiments of Clusius and Hiirzeler (1954) using benzenediazonium salts labeled with 15N at the a- or Impositions. Ammonia adds exclusively at the p-nitrogen no rearrangements were observed. 

The arenediazocyanides have been known since 1879. They played an important role in the Hantzsch-Bamberger debate on the (Z)/( ,)-isomerism of diazo compounds (see Sec. 7.1). When an aqueous solution of a diazonium salt is added to a solution of sodium or potassium cyanide, both in relatively high concentration, at a temperature below 0°C, a yellow to red (Z)-arenediazocyanide starts to crystallize. Hantzsch and Schulze (1895 a) found that these compounds rearrange into the (ii)-isomers, which have a bathochromically shifted visible absorption (see Sec. 7.1). Under strongly alkaline conditions a 1 2 adduct is formed, to which Stephenson and Waters (1939) assigned the structure 6.36. It was never corroborated, however, by modern instrumental analysis. 

In the classical procedure, base is added to a two-phase mixture of the aqueous diazonium salt and an excess of the aromatic that is to be substituted. Improved yields can be obtained by using polyethers or phase transfer catalysts with solid aryl diazonium tetrafluoroborate salts in an excess of the aromatic reactant.177 Another source of aryl radicals is A-nitrosoacetanilides, which rearrange to diazonium acetates and give rise to aryl radicals via diazo oxides.178... 

Electron-rich compounds such as hydrazones react with diazonium salts either at a nitrogen or at a carbon atom to yield formazans, either directly or through a subsequent rearrangement. The use of aldehyde... 

Diazonium salts couple to hydroxy-substituted vicinal triazoles (101) with subsequent rearrangement of the hydroxy arylazo compounds (102) to the carbamoyl tetrazole (104).170 An open-chain intermediate (103) has been proposed.169 This rearrangement is similar to that of the benzoyl... 

Several comparative studies are available in the literature, which clearly shows that, according to the latest results, tetrafluoro-pheny/ azides (AZ), trifluoro-methyl-phenyl diazirines (DZR), and particularly benzophenone (BP) are the best choice (Scheme 3.). Diazocarbonyl compounds, which played a historically role in the evolution of PAL, can also be considered together with aryl-diazonium salts [6] (not shown). For aryl azides the more common unsubstituted photophore is presented. It should be noted that tetrafluoro-phenyl azides have an increased reactivity towards CH-groups and they do not rearrange. Other substituted phenyl azides can readily alter the excitation wavelength allowing the application of milder or differential photoactivation. 

The mechanism of the reaction with ammonia is as follows. The perbromide bromine is converted into hypobromite and at the same time the diazonium salt undergoes rearrangement to syre-diazohydroxide, which immediately couples with NHS to give phenyltriazene ( diazo-benzeneamide ). The latter is then dehydrogenated by the hypobromite yielding phenyl azide (Dimroth) ... 

Benzidine yellow is an example of a large class of organic pigments that contain an azo linkage, -N=N-. Its synthesis relies very heavily on diazonium salt coupling reactions and the benzidine rearrangement. Although benzidine is banned in the U.S. because of suspected... 

Phosphorus H2O2, halogens, diazonium salts lead to 1.1-substituted X -phos-phorins or to rearranged 2-hydrophosphinic acids. 

Perhaps the best-known method of preparing aromatic azo compounds involves the coupling of diazonium salts with sufficiently reactive aromatic compounds such as phenols, aromatic amines, phenyl ethers, the related naphthalene compounds, and even sufficiently reactive aromatic hydrocarbons. Generally, the coupling must be carried out in media which are neutral or slightly basic or which are buffered in the appropriate pH range. The reaction may also be carried out in nonaqueous media. While some primary and secondary aromatic amines initially form an A-azoamine, which may rearrange to the more usual amino-C-azo compound, tertiary amines couple in a normal manner. 

As might be expected, the stability and reactivity of diazonium salts are affected by substituents as well as by the anion. The rate of coupling and the location of the azo bond are influenced by substituents on the coupling reagent. Some primary and secondary aromatic amines derived from benzene initially form A-azo compounds, which rearrange to p-aminoazo compounds in acidic media [8b, c]. Tertiary amines, on the other hand, behave normally. Preparation 2-3, while of a rather complex molecule, illustrates the simple techniques commonly used in coupling in an aqueous system. Note the presumed preferential tendency of coupling to take place predominantly in the para position. 

An old reaction patented by Lange [19] involves the coupling of naphthyl-diazonium salts with sodium sulfite to yield azonaphthalene. A more recent study of this reaction indicates that the first step of the reaction is the formation of an aryl-iyn-diazosulfonate which couples with another molecule of a diazonium salt and by a multicentered rearrangement ultimately affords an azo compound [20, 21]. The reaction is represented in Eq. (7). The validity of... 

Attempts to prepare the isomeric indoloisoquinoline system in the coupling of 3-isoquinolyl triflate and o-anilineboronic acid led to an unexpected result. In the course of the follow-up reactions the nitrogen atom in the isoquinoline ring had to be protected as the A -oxidc to divert the cyclization of the nitrene into the 4-position. The intermediate diazonium salt, however under certain conditions underwent a spontaneous ring closure to the benzofuro[3,2-c]isoquinoline system, which on heating rearranges to the benzisoxazolo[2,3-o]isoquinoline (3.6.),7... 

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