Diazonium ions preparation

Aryl diazonium ions prepared by nitrous acid diazotization of primary arylamines are substantially more stable than alkyl diazonium ions and are of enormous synthetic value Their use m the synthesis of substituted aromatic compounds is described m the following two sections... 

The reaction gives better yields with dienes, styrenes, or alkenes substituted with electron-withdrawing groups than with simple alkenes. These groups increase the rate of capture of the aryl radical. The standard conditions for the Meerwein arylation employ aqueous solutions of diazonium ions prepared in the usual way. Conditions for in situ diazotization by /-butyl nitrite in the presence of CuCl2 and acrylonitrile or styrene are also effective.106 107 108... 

Schiemann reaction (Section 22 17) Preparation of an aryl fluonde by heating the diazonium fluoroborate formed by addition of tetrafluorobonc acid (HBF4) to a diazonium ion Schiffs base (Section 17 10) Another name for an imine a compound of the type R2C=NR ... 

Diazonium salt chemistry provides the principal synthetic method for the preparation of aryl fluorides through a process known as the Schiemann reaction. In this procedure the aryl diazonium ion is isolated as its fluoroborate salt, which then yields the desired aryl fluoride on being heated. 

R—N=N=. Aryl diazonium ions are formed by treatment of primary aromatic amines with nitrous acid. They are extremely useful in the preparation of aryl halides, phenols, and aryl cyanides. 

The nitrosation of primary aromatic amines 1 with nitrous acid 2 and a subsequent dehydration step lead to the formation of diazonium ions 3. The unstable nitrous acid can for example be prepared by reaction of sodium nitrite with aqueous hydrochloric acid. 

Diazonium ions generated from ordinary aliphatic primary amines are usually useless for preparative purposes, since they lead to a mixture of products giving not only substitution by any nucleophile present, but also elimination and rearrangements if the substrate permits. For example, diazotization of n-butylamine gave 25% 1-butanol, 5.2% 1-Chlorobutane, 13.2% 2-butanol, 36.5% butenes (consisting of 71% 1-butene, 20% trans-2-butene, and 9% cw-2-butene), and traces of butyl nitrites. ... 

Aromatic diazonium salts can, of course, be isolated (see Chapter 13), but only a few aliphatic diazonium salts have been prepared (see also Ref. 383). For reviews see Laali, K. Olah, G.A. Rev. Chem. Intermed., 1985, 6, 237 Bott, K. in Patai Rappoport The Chemistry of Functional Groups, Supplement C, pt. 1 Wiley NY, 1983, p. 671 Bott, K. Angew. Chem. Int. Ed. Engl., 1979, 18, 259. The simplest aliphatic diazonium ion CH3N2 has been prepared at — 120°C in superacid solution, where it lived long enough for an nmr spectrum to be taken Berner, D. McGarrity, J.F. J. Am. Chem. Soc., 1979, 101, 3135. 

The first widely used intermediates for nucleophilic aromatic substitution were 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.81 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Part A, Section 4.1.5), aryl diazonium ions 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.82 Salts prepared with 0-benzenedisulfonimidate also appear to have potential for synthetic application.83... 

Aryl Halides from Diazonium Ion Intermediates. Replacement of diazonium groups by halides is a valuable alternative to direct halogenation for the preparation of aryl halides. Aryl bromides and chlorides are usually prepared by a reaction using the appropriate Cu(I) salt, which is known as the Sandmeyer reaction. Under the classic conditions, the diazonium salt is added to a hot acidic solution of the cuprous halide.99 The Sandmeyer reaction occurs by an oxidative addition reaction of the diazonium ion with Cu(I) and halide transfer from a Cu(III) intermediate. 

Stable mixtures of anti-diazotates and Naphtols were marketed as Rapid Fast colours for printing onto fabric with development of the azoic dye by steaming. The anti-diazosulphonates (4.113) [113], which were prepared by treatment of a diazonium salt with sodium sulphite and which regenerate the diazonium ion on treatment with an oxidising agent, found similar use. Both ranges are now of only historical interest. 

Diazotized 2- and 4-aminophenols as well as corresponding diazotized aminonaphthols and hydroxy derivatives of higher condensed aminoaromatic systems exist in neutral aqueous solutions as zwitterions (23b) which are mesomeric with the corresponding quinone diazides (23a). They can therefore be classified either as diazonium ions or as diazoketones. Indeed, preparative methods for these compounds include those typical for diazonium ions and those used for diazoketones. 

An alternative synthesis of 4-nitrodibenzothiophene involves heating 2-amino-2 -nitrodiphenyl sulfide in a sealed tube at 190° (20%). The reaction probably proceeds via homolytic cleavage of the derived diazonium ion which could have been formed from nitrous acid liberated during the formation of phenothiazines, which were also detected. Similarly, 2-methyl-4-nitrodibenzothiophene is formed from 2-amino-2 -nitro-4 -methyldiphenyl sulfide (10%), and in this case the intermediacy of the diazonium ion was further indicated in that the same material was obtained by pyrolysis of the separately prepared diazonium salt of the sulfide. Although yields are poor in this reaction, it nevertheless represents the only route to substituted dibenzothiophenes containing a nitro substituent in the 4-position and as such is worthy of further attention. 

After several unsuccessful attempts in our laboratory to prepare authentic a-hydroxydimethylnitrosamine, attention was turned to the preparation of a-aminonitrosamines. The rationale for this work was that these materials might be more stable than the hydroxylated species, but would still lead to the same reactive alkyl diazonium ions. 

These X -phosphorins 720 a-c also fail to react with carbonyl compounds. However, they are attacked by electrophiles (H or alkyl cations) at the C—2 position. In this manner new 1,1-diphenyl-2,3-benzo-X-phosphorins which are sustituted at positions C—2 (and C-4) can be prepared. Diazonium ions attack at C—4 to form azocompounds if an excess is used, C—2 is also substituted Hydrolysis with hot water affords 747. The reaction with ortho-formic acid ester forms a cyanine dye having a bridge at the C—4 positions 142 The experimen-... 

The formation of -butyldiazoate by reaction of [Fe(CN)5(NO)]2 with lithium -butyl amide contrasts with the formation of dibutylamine as the main product in the reaction of the same complex with -butylamine (85). This can be explained if the diazoic/diazoate equilibrium shown in Fig. 18 is shifted to the left far enough to form of a diazenido by loss of hydroxide. Attack of -butylamine on the a-carbon of the diazenido species, produces dibutylamine. DFT computed results suggest that the stabilization by complexation of the intermediate diazonium ion (see Fig. 18) is large for the iron-pentacyano complex, in agreement with the fact that no rearrangement products were observed in the reaction of this species with -butylamine (86). The reaction has been proposed as a good route for the preparation of symmetrical, unsymmetrical, and cyclic secondary amines (85). 

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