Compound aryl diazonium

In 1991 at the Central Research Laboratories of Hoechst AG we became interested in the palladium-catalyzed olefination (Heck reaction) of aryl halides and aryl diazonium compounds which is arguably one of the most powerfiil methods for the synthesis of substituted olefins. In collaboration with Herrmann and co-workers we have shown that active catalyst mixtures obtained by using in situ mixtures of Pd(II) salts and commercially available tri-o-tolylphosphine consist under the conditions of the Heck reaction primarily of cyclometaUated paUadacycles l... 

There are a number of important reactions of aromatic compounds that involve departure of a group with its bonding pair of electrons. Unlike nucleophilic substitution at saturated carbon, aromatic substitution rarely, if ever, involves a single-step reaction instead, discrete intermediates are usually involved. From a synthetic point of view, the aryl diazonium compounds, in which molecular nitrogen serves as the leaving group, are the most important. 

The Sandmeyer reaction is usually covered in introductory organic chemistry in a chapter on electrophilic aromatic substitution, along with diazonium ion chemistry and the formation of diazo dyes. The Sandmeyer reaction involves the substitution on an aryl diazonium compound with Cu(l) salts, as shown to the right. The mechanism of these transformations is sketchy, but now that we know... 

Sahni, Z., S. Gam-Derouich, S. Mahouche-Chergui, M. Turmine, M. M. Chehimi. On the interfacial chemistry of aryl diazonium compounds in polymer science. Chem. Papers 66, 2012 369-391. 

Substitution reactions involving aryl radicals have been quite important in synthesis. The reason, in part, is that the resistance of aryl halides and related compounds to nucleophilic substitution greatly restricts the utility of Sn2 processes for synthetic purposes. Radical substitution reactions can be carried out with any of the sources of aryl radicals mentioned in Section 12.1.4, but acylnitrosoanilines and aryl diazonium compounds have been most widely used in synthesis. The decomposition of acylnitrosoanilines is a relatively complex process. The principal points of evidence supporting the mechanism shown below have been briefly reviewed ... 

Diazoninm and Azo Compounds. - Aryl diazonium compounds undergo reductive metabolism to aryl radicals, which react with DNA. Studies on the p-hydroxybenzenediazonium ion have shown that such compounds can also undergo conversion to radicals through reactions catalysed by the hydroxyl ion. 

Figure 5.2 Reaction of SWNTs with aryl diazonium compounds. Shown are the electrochemical reactions with pre-formed diazonium salts, and the thermally motivated reaction with in-situ generated diazonium compounds. Also shown are a number of specific moieties that have been attached via these methods.

Aryl diazonium compounds react with hot aqueous acid to give phenols. This is the best way to attach an —OH group to an aromatic ring. 

Treating Aryl diazonium compounds with hypophosphorous acid (H PO ) replaces the diazonium group with a proton. This process can be used to remove the amino substituent from the aromatic ring after its role as a directing group in a synthesis concludes. 

Unsymmetrical diaryls may be prepared by treating an aryl diazonium salt solution with sodium hydroxide or sodium acetate in the presence of a liquid aromatic compound. Thus 2-chlorodiphenyl is readily formed from o-chloro phenyl diazonium chloride and sodium hydroxide solution (or sodium acetate solution) in the presence of benzene ... 

Aryl diazonium ions prepared by nitrous acid diazotization of primary arylamines are substantially more stable than alkyl diazonium ions and are of enormous synthetic value Their use m the synthesis of substituted aromatic compounds is described m the following two sections... 

Azo coupling (Section 22 18) Formation of a compound of the type ArN=NAr by reaction of an aryl diazonium salt with an arene The arene must be strongly activated toward... 

Among the reagents that are classified as weak electrophiles, the best studied are the aromatic diazonium ions, which reagents react only with aromatic substrates having strong electron-donor substituents. The products are azo compounds. The aryl diazonium ions are usually generated by diazotization of aromatic amines. The mechanism of diazonium ion formation is discussed more completely in Section 11.2.1 of Part B. 

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. 

Reaction of aryl organometallic compounds with halogens Aryl halide exchange halo-denitration halo-dehydroxylation Reaction between diazonium salts and iodide ion Heating of diazonium fluoroborates (Schiemann)... 

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