Water reaction with aryl diazonium salts

The most broadly useful intermediates for nucleophilic aromatic substitution are the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.75 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Section 10.1), aryl diazonium are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.76 The steps in forming a dizonium ion are addition of the nitrosonium ion, +NO, to the amino group, followed by elimination of water. 

An amino group can also be acylated <84JHC697, 86JHC935) it reacts with sulfonyl chlorides and aryl isocyanates <86JHC935>, and also with nitrous acid to give, via the diazonium salt (281), either the triazinone by reaction with water or the chloro-triazine by reaction with chloride ions (Scheme 50). Nitrosation of 3-amino-1,2,4-triazine 2-oxides (282) and subsequent thermolysis of the diazonium tetrafluoroborate salts (283) afforded 3-fluoro-1,2,4-triazine 2-oxides (284) (Scheme 51). In one instance the diazonium tetrafluoroborate was isolated <85H(23)1969>. 

Owing to the easy electroreduction of such species, the electrografting of aryl diazonium salts has been quite easily achieved in aqueous media [15]. For this purpose, the pH of the solution has to be kept below 2 (e.g., by the addition of H2SO4) in order to avoid degradation of the diazonium salt by its reaction with water. This advantage is of importance for both ecologic and economic impacts of the method. 

Some observations are important for improvement of the yield and for the elucidation of the mechanism of the Meerwein reaction. Catalysts are necessary for the process. Cupric chloride is used in almost all cases. The best arylation yields are obtained with low CuCl2 concentrations (Dickerman et al., 1969). One effect of CuCl2 was detected by Meerwein et al. (1939) in their work in water-acetone systems. They found that in solutions of arenediazonium chloride and sodium acetate in aqueous acetone, but in the absence of an alkene, the amount of chloroacetone formed was only one-third of that obtained in the presence of CuCl2. They concluded that chloroacetone is formed according to Scheme 10-50. The formation of chloroacetone with CuCl2 in the absence of a diazonium salt (Scheme 10-51) was investigated by Kochi (1955 a, 1955 b). Some Cu11 ion is reduced by acetone to Cu1 ion, which provides the electron for the transfer to the diazonium ion (see below). 

The most satisfactory method of preparation of a copper(i) cyanide solution is to dissolve the copper(i) cyanide (90 g, 1 mol) in a solution of sodium cyanide (125 g, 2.5 mol) (CAUTION) in 600 ml of water. If it is desired to avoid the preparation of solid copper(i) cyanide, the following procedure may be adopted. Copper(i) chloride, prepared from 35 g of copper(n) sulphate pentahydrate as described under 22 above, is suspended in 60 ml of water contained in a 500-ml round-bottomed flask, which is fitted with a mechanical stirrer. A solution of 18.5 g of sodium cyanide (96-98%) in 30 ml of water is added and the mixture is stirred. The copper(i) chloride passes into solution with considerable evolution of heat. As the copper(i) cyanide is usually employed in reactions with solutions of aryl diazonium salts it is usual to cool the resulting copper(i) cyanide solution in ice. 

The diazonium salt 42 is stable at 0-5 °C but decomposes to N2 and an unstable aryl cation 43 on warming to room temperature. The empty orbital of 43 is in an sp2 orbital in the plane of the aromatic ring, quite unlike the normal p orbital for cations like 20. Reaction occurs with any available nucleophile, even water, and this is a route to phenols 45. 

The diazonium group can be replaced by a number of groups. " Some of these are nucleophilic substitutions, with S l mechanisms (p. 432), but others are free-radical reactions and are treated in Chapter 14. The solvent in all these reactions is usually water. With other solvents it has been shown that the Sj-jl mechanism is favored by solvents of low nucleophilicity, while those of high nucleophilicity favor free-radical mechanisms. The N2 group can be replaced by CP, Br, and CN, by a nucleophilic mechanism (see OS IV, 182), but the Sandmeyer reaction is much more useful (14-20). Transition metal catalyzed reactions are known involving aryl-diazonium salts, and diazonium variants of the Heck reaction (13-10) and Suzuki coupling (13-12) were discussed previously. As mentioned on p. 866 it must be... 

Heck reactions are compatible with water (see Chapter 3.2.4) [35], which increases the speed of reaction in the presence of quaternary ammonium salts [36 a]. It is not surprising, then, that aqueous solvents (e. g. CH3CN/H2O) and water-soluble catalysts such as Pd(TPPTS)3 where TPPTS = P(C6H4-m-S03Na)3 [35, 67] can be employed successfully (eq. (10)). However, only aryl and vinyl iodides and aromatic diazonium salts (generated in situ from arylamines in aqueous media) are, up to the present, accessible to this method [36 b-h]. 

Diazonium salts also readily react with nickel carbonyl, yielding mainly carboxylic acids and ketones in the presence of water and hydrochloric add (26, 27). Iron pentacarbonyl and dicobalt octacarbonyl with diazonium salts behave similarly, but the hexacarbonyls of chromium and molybdenum are virtually ineffective. This reaction may be considered as a transition metal-catalyzed carbonylation of aryl radicals, and is closely related to the Meer-wein reaction (26). 

W. A. Waters (Oxford University) Investigations of the extent to which complexes such as (CuCl)+ and undissociated CuCL affect the chain length in the polymerization associated with the Sandmeyer reaction are in progress at Oxford. It is well known that ions that complex well with cupric, e.g., (CN) , can be introduced into aryl nuclei by the Sandmeyer procedure in preference to chloride even when diazonium chlorides have initially been taken. The system, however, is complicated by the fact that the complexing of cuprous and cupric salts alters the redox potential, and this affects the facility of both stages 1 and 3 of the reaction sequence. The effects of introducing polar substituents into the aryl nuclei (Table I) indicates the importance of such effects. 

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