Decomposition of diazonium salts

Decomposition of Diazonium Salts.—In illustration of another method of following the course of a reaction, we may study the decomposition of diazonium salts, e.g. benzene diazonium chloride. When this salt is warmed with water, it undergoes decomposition according to the equation— 

Since the concentration of benzene diazonium chloride in the solution is proportional to the total volume of nitrogen which the solution is capable of yielding, we can calculate the velodty constant of the decomposition by means of the formula— 

Experiment.—Determine the Velocity of Decomposition oj Bettzene Diazonium Chloride. 

Prepare a solution of benzene diazonium chloride as follows Dissolve 6 64 gms. of aniline in 21 4 c.c. of hydrochloric acid (sp. gr. = i i6) cool in ice-water, and add gradually from a dropping-funnel a cold solution of 4 9 gms. of sodium nitrite 

The solution of benzene diazonium chloride having been placed in the tube A (Fig. 90), the stirrer is inserted, and the apparatus then fitted together, the side tube B being connected with a gas burette, as shown in Fig. ir. The tube with the diazo-solution is immersed up to the level of the cork in a thermostat, the temperature of which is regulated at about 30°, and the stirrer is set in fairly rapid motion by means of an electric motor or engine. During this time the tube with the solution should be in open communication with the air, and the gas evolved allowed to escape. After 5-7 minutes, communication with the gas burette is effected while that with the outside air is stopped, and the time at which this is done is noted.  

---

Decomposition of diazonium salts obtained from 2-aminothiazole (4) (29, 34. 35) could be an interesting reaction to introduce O in A-4-thiazoline-2-one. Acidic hydrolvsis of ethers (36. 37). oxidative hydrolysis... 

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. 

There are several reports on the decomposition of diazonium salts, in particular from pyridines, in anhydrous HF [81JFC( 18)497 88JFC(38)435]. Mildness of conditions and improved yields are benefits. The elusive 4-fluoropyridine can be obtained using this method [81JFC( 18)497]. 

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. 

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... 

Amines are easily identified because they re readily soluble in dilute acid. Sodium fusion converts the cimine to the cyanide ion, which is detectable by a Vciriety of methods. The ready formation and decomposition of diazonium salts (discussed in the earlier section Reactions with nitrous acid ) leads to the identification of primary amines. The Hinsberg test (see the nearby sidebcir) is useful in identifying amines. 

The rarely encountered radical addition to N=N was observed in the decomposition of diazonium salts 297. In some cases unstable derivatives of the initially formed radical 298 were obtained via nitrogen-nitrogen or nitrogen-o-carbon dimerization. ... 

Diazotization has been reported for all the pyrazolopyridines, and in some cases the salts have been isolated. A 7-aminopyrazolo[4,3-c]pyridine (191a) was converted to the corresponding hydroxy compound with sodium nitrite in hot glacial acetic acid.159 Analogous products were obtained from a 3-aminopyrazolo[3,4-b]-15 and 2-aminopyrazolo[l,5-a]pyridine.186 Decomposition of diazonium salts with hydrobromic acid afforded 3-bromo-pyrazolo[3,4-c]-u0 or -[4,3-bjpyridines111 deamination of 3-aminopyra-zolo[3,4-h]pyridines was achieved via treatment of the diazonium salts with hypophosphorous acid,10 titanous chloride,238 or ferrous ammonium sulfate.238 Kocevar et al.236 have made a detailed study of the reactions of the latter diazonium salt. 

Decomposition of diazonium salts in presence of Cu(l) and ammonia leads to biaryls. 

Photospeeds reported were similar to those encountered in photo-reducible dye sensitized polymerizations, and similar photopolymer functionality was demonstrated (e.g., washout development). Reductive decomposition of diazonium salts is well known to produce free radicals (98). 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

The most widely used method for the substitution of nitrogen-containing functional groups by fluorine is the thermal or photochemical decomposition of diazonium salts, the Baltz-Schiemann reaction (see Section 1.1.8 ). 

Cu also finds application in the Sandmeyer reaction involving the decomposition of diazonium salts. In this reaction the free radical Ar is formed as an intermediate. 

The thermal decomposition of diazonium salts in an aqueous solution is a first order reaction as shown in Fig.4. Arrhenius plots of diazonium salts in an aqueous solution are shown in Fig.5. The relationship between the decomposition temperature (Td) obtained from DSC curves and the decomposition rate constant (k) of the diazonium salts at 5 are shown in Fig.6. A good linear relationship is observed between Td and In k for both the aqueous solution and the film from this figure. These linear relationships make it possible to predict the stability of diazonium salts in an aqueous solution and in film from the decomposition temperature Td in the solid state. 

Of course a bridged system must be involved, at least in the transition state, between boron and carbon in the decomposition of diazonium salts (Figure 4.30) and, indeed, transfer from trifluoromethyl has been observed [77] (Figure 4.30). 

Decomposition of Diazonium Salts by Carboxylic Acids (cf. method 93)... 

By the decomposition of diazonium salts. If we examine the structure of an aromatic or aliphatic diazonium salt, we see that the loss of a nitrogen molecule from the cation should give rise to a carbonium ion ... 

As it is well known, nucleophilic substitution of a C-X bond, one of the key synthetic reactions with aliphatic compounds is severely limited with aromatic derivatives, where it occurs thermally only with electron-withdrawing substituted compounds and/or under severe conditions. Alternatives include time honored reactions involving the phenyl radical generated by decomposition of diazonium salts after a reductive step, such as the Meerwein and the Gomberg-Bachmann reactions, as well as the (often photoinitiated) SrnI reaction, where a (usually weak, e.g. carbon-iodine) bond is cleaved after monoelectronic reduction to give an aryl radical as the active inter-mediate that adds to an enolate, cyanide or other nucleophiles (and thus again with an aryl radical as the key intermediate. Scheme S). ... 

Although heat is generally used to promote decomposition of diazonium salts by the Schieman reaction, the reaction has been shown to occur also under the influence of ultraviolet radiation (21, 22) ... 

There are many other reactions that possibly involve aryne intermediates. While some are of mechanistic curiosities, some have been studied in detail, although none are generally synthetically useful. Irradiation of 1,2-diiodobenzene can lead to ortho-benzyne, probably via an aryl radical intermediate resulting from cleavage of the weak C-1 bond (Scheme 7.19). Aryl cations, formed by the decomposition of diazonium salts are also possible intermediates to ortho-benzynes provided that a large ortho-substituent is present, loss of a proton to give an aryne becomes competitive with the normal nudeophihc addition to the cation. 

Still, there are base-free Mizoroki-Heck reactions, which are known to occur with arene diazonium salts. In this particular case, the absence of base is advantageous, as spontaneous decomposition of diazonium salts is known to be accelerated by nucleophiles and bases. High precatalyst loadings are required, as, for example, in the arylation of disubstited acrylates 10 (9 11, Scheme 2.2) [10] a base-free reaction might only be sustained for a few turnovers. 

First attempts to prepare poly(phenylene) date back to 1842 [78]. Riese describes a process in which poly(phenylene) ( =13) is synthesized from 1,4-dibromobenzene and sodium. Further methods are the Ullmann reaction, thermal decomposition of diazonium salts, coupling of phenylene dihalogenide - Grignard compounds. 

The most widely used illustration for the 5 1 mechanism is the uncatalyzed decomposition of diazonium salts. Such substrates carrying amino substituents have scarcely been investigated, except for the magnetochemical study on the photodecomposition of p- N,N-dimethylamino)benzenediazonium chloride . Tlie result of this and other work indicated a free-radical mechanism. 

---