Dediazoniation reaction

Aromatic diazonium salts are almost as important for reactions in which the diazonio group is lost as molecular nitrogen and in which aryl cations and radicals are the reagent proper (dediazoniation reactions, see Chs. 8 and 10). 

The most comprehensive modern works on the subject are the relevant volumes of Patai s series The Chemistry of Functional Groups, namely the two volumes on diazonium and diazo groups (Patai, 1978), the two volumes on hydrazo, azo, and azoxy groups (Patai, 1975) and the two Supplement C volumes on triple-bonded groups (Patai and Rappoport, 1983). Supplement C contains chapters on arene- and alkene-diazonium ions and on dediazoniation reactions. 

More recently Gorelik s group determined the structure of 1-phenylsulfonyl-2-pyrazoline-3-diazonium tetrafluoroborate by X-ray crystallography (Gorelik et al., 1989) and calculated (Glukhovtsev et al. 1990) the heats of the dediazoniation reaction of seven cyclic five-membered diazonium ions (including 2.13, R=H) relative to that of the benzenediazonium ion by the MNDO method (see Sec. 8.4). 

The experimental work of the groups of Swain and Zollinger on the dediazoniation mechanism of arenediazonium ions, which started in 1975, provided good evidence for the existence of aryl cations as steady state intermediates (see Sec. 8.3). These results also initiated theoretical work on aryl cations, in part combined with further calculations on the structure and reactivity of arenediazonium ions. Publications that contain data on arenediazonium ions and aryl cations will therefore be discussed in the chapter on dediazoniation reactions (Sec. 8.4). In the rest of this section we will concentrate on investigations that are concerned with the geometries and electron densities of diazonium ions but not, or only marginally, with energetics of the dediazoniation reaction. 

The second test for the mechanism shown in Scheme 8-8 is to apply equation (c) to (8-10) with kinetic data for dediazoniations with varying concentrations [N2] of molecular nitrogen. As the solubility of N2 is quite low in most solvents, kinetic measurements must be made under N2 pressure. The dediazoniation reaction has a... 

All these results are consistent with the hypothesis that aryl cations react in aqueous media at diffusion-controlled rates with all nucleophiles that are available in the immediate neighbourhood of the diazonium ion. On this basis Romsted and coworkers (Chaudhuri et al., 1993a, 1993b) used dediazoniation reactions as probes of the interfacial composition of association colloids. These authors determined product yields from dediazoniation of two arenediazonium tetrafluoroborates containing ft-hexadecyl residues (8.15 and 8.16) and the corresponding diazonium salts with methyl groups instead of Ci6H33 chains. ... 

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

Table 10-9. Yields in homolytic dediazoniation reactions of 2-(2 -propenyl)benzenediazonium salts (10.55, Z = 0, 7i = l, R=H or CH3) and the related diazonium salt 10.59.

An important use of the dediazoniation reaction is to remove an amino group after it has been used to direct one or more other groups to ortho and para positions. For example, the compound 1,3,5-tribromobenzene cannot be prepared by direct bromination of benzene because the bromo group is ortho-para directing however, this compound is easily prepared by the following sequence ... 

Many other compounds that would otherwise be difficult to prepare are easily synthesized with the aid of the dediazoniation reaction. 

The dediazoniation reaction is mainly dependent on the temperature, especially in the case of 4-aminophenol at low temperature 4-hydroxybenzenediazonium fluoride loses hydrogen fluoride to deliver a stable quinone-diazide which gives fluorophenol at 140°C only.93 The mechanisms involved in the synthesis of 4-fluorophenol are summarized in Scheme 5.93... 

In contrast to aromatic diazo compounds, diazoalkanes are less important as large-scale industrial intermediates. Nevertheless, their dediazoniation reactions offer a series of important applications in organic synthesis, since Wolff (1902) discovered the rearrangement of diazo ketones into carboxylic acids (1-3). After... 

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