Diazonium salts, decomposition

The Sandmeyer reaction utilizes a diazonium salt to produce an aryl halide. The process begins by converting an amine to a diazonium salt. Decomposition of the diazonium salt in the presence of a copper halide places the halide ion into the position originally occupied by the amine. The most useful copper halides are CuCl and CuBr in addition, the copper pseudohalides, such as CuCN, also works by placing a nitrile in the position originally occupied by the amine. Figure 13-27 shows an example of the Sandmeyer reaction. 

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. 

ARYL DIAZONIUM SALTS, DECOMPOSITION Tetrakis(pyridiDe)coppet(I) perchlorate. 

Aryldiazonium salts are more stable than alkyldiazonium salts, which usually react by substitution or elimination under the conditions of their generation. Marked differences are often observed in the pattern of reactivity exhibited by diazonium salt decomposition compared with that seen in solvolysis reactions of the corresponding alkyl halides or arenesulfonates. ... 

The effect of substituents on the rates of diazonium salt decompositions can be accommodated by the SnI mechanism. The essential facts are as follows (10,12,14,15,21) ... 

The diazoamino-compounds are usually yellow in colour, and do not dissolve in acid they can usually be isolated and crystallized without decomposition. When treated with HNO2 two molecules of diazonium salt are formed. Form an azo compound when warmed with an amine and its hydrochloride, e.g. 

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

Gattermann (1890) found that the preparation of the cuprous halide may be avoided by making use of the fact that finely-divided copper (e.g., freshly-precipitated or reduced by hydrogen or copper bronze) acts catal3d.ically in the decomposition of solutions of diazonium salts, for example ... 

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. 

Two mechanisms are among those that have been postulated for decomposition of aryl diazonium salts in aqueous solution containing nucleophilic anions, A ... 

This reaction is commonly achieved by diazotization of the amine in fluoro-boric acid, followed by decomposition of the diazonium salt (Schiemann reaction).Thus, l-amino-4-methylestra-l,3,5(10)-trien-17-one (50) is converted to the fluoro derivative (51) in 29% yield. The synthesis of 3-fluoroestra-l,3,5(10)-triene-17-one (55) from estrone proceeds via (52) and (53)... 

Decomposition of the diazonium salt of 2-amino-iV-methyl-iV-3 -pyridylaniline (205) in aqueous acid solution with copper powder at room temperature gave overall yields of cyclized products consisting of a mixture of i id-iV-methyl-3-carboline (206) (47.5%) and ind-N-methyl-jS-carboUne (207) (25.5%), in agreement with the proposed homolytic character of the reaction under these conditions. This constituted the first unambiguous synthesis of a simple 3-carboline derivative. 

The first preparation of a tribcnz[7>,d,/jazepine was achieved by decomposition of the diazonium salt 29 (R = N2 + X" X = Cl or HSOJ derived from A,7V-diphenyl-2,2 -(l,3-phenylene)diani-line (29. R = NH2).105 9-Phenyl-9//-tribenz[M,/]azepine (30) is obtained along with several other products, and is thought to be formed by ring expansion of the TV.TV-diphenylcarbazolium ion. 

Another possibility for ring closure to annulated 1,3-diazocines employs the thermal decomposition of the diazonium salt derived from arylamine 3.7... 

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]. 

Aryl radicals are produced in the decomposition of alkylazobenzenes and diazonium salts, and by f)-scission of aroyloxy radicals (Scheme 3.73). Aryl radicals have been reported to react by aromatic subsitution (e.g. of Sh) or abstract hydrogen (e.g. from MMA10) in competition with adding to a monomer double bond. However, these processes typically account for <1% of the total. The degree of specificity for tail vs head addition is also very high. Significant head addition has been observed only where tail addition is retarded by sleric factors e.g. methyl crotonate10 and -substituted methyl vinyl ketones 79, 84). 

B. l-Bromo-2-fluorobenzene. Cautionl This step should be carried outm a hood because the PFS evolved on thermal decomposition of the diazonium salt is poisonous. The apparatus consists of a 1-1., three-necked, round-bottomed flask equipped with a thermometer, a condenser, a magnetic stirrer (optional), and a 250-ml. Erlenmeyer flask that is attached by means of a short rubber Gooch connecting tube. The dry powdered hexafluorophosphate salt is placed in the Erlenmeyer flask, and 300 ml. of heavy mineral oil is placed in the round-bottomed flask. The mineral oil is heated to 165-170° by means of an oil bath or electric heating mantle and maintained at this temperature while the salt is added rapidly in portions over a period of 30 minutes. The flask is cooled rapidly to room temperature, the side flask is removed, and 400 ml. of 10% aqueous sodium carbonate is added slowly through the condenser. The mixture is steam-distilled until no more oil is visible in the distillate. 

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