Aldehydes from diazonium salts

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

Addition of Vinyl and Aryl Groups. The reaction of aromatic radicals, generated by decomposition of diazonium salts, with iminium salts in the presence of TiCE in aqueous media produces secondary amines (Eq. 11.53).91 The iminium salts are formed in situ from aromatic amines and aldehydes. 

Titanium plays a key role in this process owing to its reducing power in the Ti(III) state, it acts as radical initiator generating Ar- by decomposition of arene-diazonium salts (Equation 14.16) owing to its Lewis acid character in the Ti(IV) state, it promotes the formation of the inline [29] from a generic aldehyde and a primary aromatic amine in the aqueous medium (Equation 14.17) and increases... 

Diazonium intermediates have also been employed in the synthesis of pyrazoles. A convenient one-pot procedure for the preparation of 3-phenyl- or 3-pyridylpyrazoles 27 from the 1,3-dipolar cycloadditions of phenylacetylene or 3-(pyridyl)acetylene with diazo compounds 26 generated in situ from aldehydes 25 has been reported <03JOC5381>. Cyclization of ortho-(arylethynyl)benzene diazonium salts 28 having substituents at the para-position of the aryl ring furnished indazoles 29 <03TL5453>. 

Incidentally, 34 contributes more to the hybrid than 35, as shown by bond-distance measurements. In benzenediazonium chloride, the C—N distance is 1.42 A, and the N—N distance 1.08 A, which values fit more closely to a single and a triple bond than to two double bonds (see Table 1.5). Even aromatic diazonium salts are stable only at low temperatures, usually only < 5°C, although more stable ones, such as the diazonium salt obtained from sulfanilic acid, are stable up to 10 or 15°C. Diazonium salts are usually prepared in aqueous solution and used without iso-lation, although it is possible to prepare solid diazonium salts if desired (see 13-23). The stability of aryl diazonium salts can be increased by crown ether complexion. For aromatic amines, the reaction is very general. Halogen, nitro, alkyl, aldehyde, sulfonic acid, and so on, groups do not interfere. Since aliphatic amines do not react with... 

Many patents, especially from Agfa and Kodak, deal with the preparation of dyes for color photography by coupling with diazonium salts (Table III). Dyes have also been obtained by condensations with aromatic aldehydes these may be salts (e.g., 431)"" or neutral molecules (432,"" 433 ). Cyanine dyes containing a polymethine chain between the azapentalene ring and the aryl moiety are also known. "" "" ... 

Synthesis of aromatic aldehydes and of alkyl aryl ketones from aldoximes or semicarbazones and aromatic diazonium salts (see 1st edition). 

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