Diazonium salts iodination

Chlorobenzenediazonium triiodide, 2137 2,4-Dimethylbenzenediazonium triiodide, 2963 2- or 4-Methoxybenzenediazonium triiodide, 2758 4-Toluenediazonium triiodide, 2756 See other diazonium salts, iodine compounds... 

The diazonium salts usually decompose when warmed with water to give a phenol and nitrogen. When treated with CuCl, CuBr, KI, the diazo group is replaced by chlorine, bromine or iodine respectively (Sandmeyer reaction). A diazonium sulphate and hydroxyl-amine give an azoimide. The diazonium salt of anthranilic acid (2-aminobenzoic acid) decomposes to give benzyne. ... 

Iodine reacted with the indazole silver salt to give 3-iodoindazole. Reaction of potassium iodide with indazole diazonium salts has been reported on a number of occasions [67HC(22)1 90AHC(48)65]. 

Sandmeyer reactions used have included the preparation of the 5-iodo compound from the diazonium salt of 5-amino-8-isoquinolinol further reaction with iodine monochloride gave 5,7-diiodo-8-isoquinolinol (66JMC46). Treatment of 1-chloroisoquinoline with iodide gave the 1-iodo analogue (47%) (67YZ1342). 

It has been suggested that the initial formation of iodine on addition of iodide to a diazonium salt solution is caused by oxidation of the iodide by excess nitrite from the preceding diazotization. Packer and Taylor (1985) demonstrated that, if urea was added as a nitrite scavenger (see Sec. 2.1) to a diazotization solution, that solution produced iodine much more rapidly than a portion of the same diazonium salt solution not containing urea, but eventually the latter reaction too appeared to follow the same course. This confirms the role of excess nitrite, and suggests that the iodo-de-diazoniation steps only occur in the presence of iodine or triiodide (I -). The same authors also found that iodo-de-diazoniation is much slower under nitrogen. All these observations are consistent with radical-chain processes, but not with a heterolytic iodo-de-diazoniation. 

One of the best methods for the introduction of iodine into aromatic rings is the reaction of diazonium salts with iodide ions. Analogous reactions with chloride, bromide, and fluoride ions give poorer results, and 14-25 and 13-20 are preferred for the preparation of aryl chlorides, bromides, and fluorides. However, when other diazonium reactions are carried out in the presence of these ions, halides are usually side products. Aniline has also been converted to fluorobenzene by treatment with t-BuONO and Sip4 followed by heating. A related reaction between PhN=N—N C4Hg and iodine gave iodobenzene. ... 

Let us now consider the formation of aryl iodides from aryl diazonium salts and potassium iodide in methanol (Singh and Kumar 1972a, 1972b). Electron-donor substituents decelerate the process as compared with benzene diazonium (the substituent is hydrogen), whereas electron acceptor substituents accelerate it. Oxygen inhibits the reaction, and photoirradiation speeds it up. As the authors pointed out, in the case of 4-nitrobenzene diazonium, the reaction leads not only to 4-iodonitrobenzene but also to nitrobenzene, elemental iodine, and formaldehyde. All of these facts support the following sequence of events ... 

Replacement of the diazonium group by iodine does not require the presence of cuprous halide and is done simply by shaking the diazonium salt with potassium iodide. 

Displacement of nitrogen from diazonium salts derived from 4- or 5-aminotriazoles can be achieved in the same manner as for other aromatic diazonium salts for example, diazotization of 4-amino-triazole-5-carboxamide and reaction with iodine and potassium iodide gives the 4-iodo derivative. ... 

This preparative scheme leads to only 30% yield due to the side reactions between the meto-astatoaniline diazonium salt and astato-phenol, which cannot be eliminated even by continuous extraction of the product with n-heptane (167). All the astatophenols synthesized to date have been identified by either HPLC (99,104) or TLC (160,166,167). Their dissociation constants (KJ have been established from extraction experiments by measuring the relative distribution of compounds between aqueous borax buffer solutions and n-heptane as a function of acidity. On the basis of these derived values, the Hammett a-constants and hence the field (F) and resonance (R) effects have been estimated for these compounds (167) (see Table VI). The field effect for astatine was found to be considerably weaker than that for other halogens the resonance effect was similar to that for iodine (162). 

Again the Sandmeyer reaction is frequently used to convert amino- to halo-isoquinolines. The diazonium salt of 5-amino-8-isoquinolinol (14) on treatment with potassium iodide and iodine gives the 5-iodophenol (15). Iodination of the hydrochloride of (15) with iodine monochloride in ethanol gives (16 Scheme 9) (66JMC46). 

These reactions are related to the reaction of aryl diazonium salts with iodide yielding iodoaryls, the mechanism of which seems to be a one-electron transfer (radical) reaction and not a nucleophilic displacement. Just as iodide is easily oxi- zed to iodine by the aryl diazonium cation, 2.4.6-triphenyl-X -phosphorin is oxidized to the radical cation 58. 

Diiodobenzo[6 jthiophene is prepared by treating the bis(mercuric chloride) derivative with iodine. Benzo[ > jthiophenes having an iodine atom at the 4-, 5-, 6- and 7-positions are obtained in high yield from the appropriate diazonium salt by the usual replacement reactions. 

A stirred soln of H-L-Phe(4-NH2)-OH (21 54 g, 300 mmol) in 4 M HC1 (600 mL) was cooled in a MeOH/ ice bath and treated slowly with aq NaN02 (20.7 g, 300 mmol). The mixture was kept for 1 h under vacuo (until negative to HN02 in the starch-iodine paper test). The soln of the diazonium salt was mixed slowly into 20% HC1 (400 mL) containing CuCl (10 g) and Cu powder (3g). After the cessation of gas release, the mixture was left overnight at 4 °C, the precipitate was filtered off (53 g of hydrochloride) and the filtrate was concentrated under vacuo and filtered again (9.2g) combined yield 61.2g (86%). 

Aminobenzo[6]thiophene is prepared from 7-hydroxybenzo[6]-thiophene by means of the Bucherer reaction it may be converted into 7-nitrobenzo[6]thiophene via the diazonium salt.84 5,7-Diamino-3-phenylbenzo[6]thiophene and its 2-carboxylic acid are prepared by reduction of the corresponding dinitro compound.334 Partial reduction of o,7-dinitro-3-phenylbenzo[6]thiophene-2-carboxylic acid with ethanolic ammonium sulfide affords 7-amino-5-nitro-3-phenylbenzo-[6]thiophene-2-carboxylic acid, the amino group of which may be replaced by hydrogen or iodine via the diazonium salt.334 7-Amino-4-methoxybenzo[6]thiophene is mentioned in the patent literature.560... 

Keywords solid diazonium salt, potassium iodide, iodination, solid-solid reaction, aryl iodide... 

Aryl iodides.1 DMSO is a particularly useful solvent for decomposition of aryl diazonium salts. Decomposition in the presence of KI (and I2 to minimize formation of iodinated biphenyls) results in aryl iodides in > 85% yield. 

It has been demonstrated that the presence of chlorine or bromine in the nucleus facilitates replacement of the diazo group by hydrogen little or no ether formation occurs.26 Apparently iodine also favors the redudng action of alcohols, but this point has not been investigated carefully.26 27 No attempts to deaminate fluorinated amines are recorded. Representative of the effitiency with which ethanol reduces diazonium salts derived from halogenated amines are the deaminations of m-chloroaniline 26 (87% yield), of 2-bromo-4-methylaniline 28 (67% yield), of 2,4,6-tribfomoaniline 29 (ca. 80% yield), and of 2-carboxy-4-iodoaniline30 (ca. 45% yield) in the biphenyl series the deamination of VIII in 53% yield 31 may be dted. 

In the presence of an alkyl iodide, selective alkyl radical addition to the C-atom of the imine generated in situ occurs, overcoming the competitive phenylation reaction (Equation 14.20) [30]. The Ph- radical, generated by decomposition of the diazonium salt, as described before, generates the alkyl radical by selective iodine atom transfer (Equation 14.21). 

Alkyl iodides have been widely used for selective alkylation of heteroaromatic bases. The method is based on rapid iodine abstraction by aryl radicals (obtained from benzoyl peroxide or diazonium salts) or by a methyl radical (obtained from MeCOOH, t-BuOH, t-BuOOH, (t-BuO)2, (MeCOO)2, MeS0Me/H202, or MeC0Me/H202) [2]. An example is depicted in Eq. (14) of Table 3. 

Because aryl diazonium salts are reasonably stable, other nucleophiles may be introduced to capture the aryl cation when the diazonium salt is heated. Among these, iodide ion is important as it allows the preparation of aryl iodides in good yield. These compounds are not so easy to make by electrophilic substitution (Chapter 22) as aryl chlorides or bromides because iodine is not reactive enough to attack benzene rings. Aryl iodides are useful in the more modern palladium chemistry of the Heck reaction, which you will meet in Chapter 48. 

Another important modern reagent (discovered in 1983) is known as the Dess-Martin periodi-nane, and is an iodine compound that can be made from 2-iodobenzoic acid, itself available from anthranilic acid via the diazonium salt route, as described in the last chapter. 

Copper(I) iodide is unsatisfactory for use in the Sandmeyer reaction because of its insolubility. The iodo group is introduced by warming the diazonium salt solution in aqueous potassium iodide solution (Scheme 8.17). This method is one of the best means of introducing iodine into an aromatic ring. A one-electron reduction by the iodide ion is thought to initiate a radical reaction in a similar way to the Cu(I) ion. 

Aromatic systems substituted with electron-donating groups are more readily halogenated than benzene. Consequently, other synthetic routes or reagents are sometimes used to avoid polyhalogenation and the formation of isomeric mixtures. For example, the iodination of toluene gives a mixture of 2- and 4-iodotoluenes each isomer can be prepared individually from the appropriate toluidine via the diazonium salt (see Chapter 8). 

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