Syntheses using diazonium salts

Unsaturated hydrazones, unsaturated diazonium salts or hydrazones of 2,3,5-triketones can be used as suitable precursors for the formation of pyridazines in this type of cyclization reaction. As shown in Scheme 61, pyridazines are obtainable in a single step by thermal cyclization of the tricyanohydrazone (139), prepared from cyanoacetone phenylhydrazone and tetracyanoethylene (76CB1787). Similarly, in an attempted Fischer indole synthesis the hydrazone of the cyano compound (140) was transformed into a pyridazine (Scheme 61)... 

Owing to their particular interest two individual reactions will now be discussed separately. The reaction of methoxycarbonylhydrazine and 3-bromo-2,4-pentanedione affords, in addition to the expected pyrazole (608), a pyrazolium salt (609), the structure of which was established by X-ray crystallography (74TL1987). Aryldiazonium salts have been used instead of arylhydrazines in the synthesis of pyrazolines (610) and pyrazoles (611) (82JOC81). These compounds are formed by free radical decomposition of diazonium salts by titanium(n) chloride in the presence of a,/3-ethylenic ketones. 

Other less widely used alternative diazonium salts for aryl fluoride synthesis include arenediazonium hexafluoroantimonates, ArN2SbFg, arenediazonium hexafluoroarsonates, ArN AsFg, and arenediazonium hexafluorosilicates, (ArNjljSiFg 114]... 

With the synthesis of the benzylidene hydrazones of selenazoles, it was possible to prepare formazans containing a selenazole ring. Of the compounds already described (Section I,C,1), 2-benzylidene-hydrazino-4,5-diphenyl- and 2-benzylidenehydrazino-4-methyl-5-carb-ethoxy-selenazole were used to couple with diazonium salts in order... 

Diazonium salts are important intermediates in organic synthesis, e.g. for the Sandmeyer reaction. The most important use is the coupling reaction with phenols or aromatic amines to yield azo dyes (see Diazo coupling). 

Diazo coupling was used to demonstrate the feasibility of organic multi-step synthesis on a chip [4]. This involved showing the safe quenching of hazardous intermediates such as the diazonium salts needed as precursor for that synthesis. Especially, this involves the in situ generation and consumption of such species. 

Intramolecular radical arylations are found to work quite well, however, e.g. the Pschorr reaction this involves the thermal decomposition of diazonium salts, e.g. (112), in the presence of copper powder as catalyst, and is used in the synthesis of phenathrenes such... 

Furans are even reactive enough to be substituted by some diazonium salts or diazopyrazoles, though other reactions are usual (Section V,A),135 and catalysts are used in response to difficulty in producing an electrophilic species not because the furan ring is unreactive. Scheme 27 shows the synthesis of an interesting a-amino acid derivative136 by electrophilic substitution. 

The synthesis of 4-amino-3,5-diaryl-l,2,4-triazoles is well documented but deamination using sodium nitrite in aqueous nitric acid typically gives low yields and is restricted in its scope. A more efficient method of deamination using aqueous hypophophorous acid has been reported the intermediate diazonium salt formed in this reaction is reduced quickly within the reaction mixture, leading to high yields (Equation 29 and Table 10) <2002JHC93>. 

Triazenes are disguised diazonium ions which can be released under very mild acidic conditions. Inspired by the use of triazenes in natural product synthesis by Nicolaou et al. [127] and the pioneering work of Moore et al. [128, 129] and Tour et al. [130] in the synthesis of triazenes on a solid support and the final detachment to give iodoarenes, a whole set of triazene-based linkers has been developed (Tab. 3.10) [131]. The arene diazonium salts generated from the triazene linkers offer diverse opportunities for multifunctional cleavage. Two linkers based on tria-... 

These agents would be used as adjuncts to beta lactams since they have no antibacterial activity in their own right. A key reaction in the synthesis of each compound involves the replacement of the amine at 6 and the protection of that position as a mono- or di-halide. Thus reaction of 6-APA (2-4) with nitrous acid gives the diazonium salt (9-1) this is converted to the dibromide (9-2) on treatment with bromine. The ring sulfur is then oxidized with permanganate to the sulfone (9-3). Hydrogenolysis of the product replaces the two bromine atoms by hydrogen to afford sulbactam (9-4) [13]. 

Problem 18.35 Using C Hj, C HjCHj, via diazonium salts and any other needed reagents, prepare (a) o-chlorotoluene, (h) m-chlorotoluene, (c) 1,3,5-tribromobenzene, (d) m-bromochlorobenzene, (c) p-iodotoluene, (/) p-dinitrobenzene and (g) p-cyanobenzoic acid. Do not repeat the synthesis of intermediate products. <... 

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