Aryl azides, from arenediazonium salts

The synthesis and mechanism of formation of a triazene from an arenediazonium ion and an amine with one or two aliphatic substituents (see Scheme 13-1, R = alkyl, R = H or alkyl) will be discussed in Section 13.2. Here we will briefly mention Dimroth s method (1903, 1905 a) for synthesis of wholly aliphatic triazenes (Scheme 13-6, R and R = alkyl). Dimroth obtained these by the action of Grignard reagents on alkyl azides followed by isolation via copper(i) salts. The Grignard method can also be applied for the synthesis of triazenes with an aromatic substituent by using an aryl azide. 

A cold, two-phase mixture of the arenediazonium salt in aqueous solution (prepared in situ from an aniline derivative) and hexane was treated with a solution of NaN, in H20. The aryl azide was extracted into the hexane layer as it formed which, after 2 h. was separated and charged to a polyolefin vessel containing excess anhyd HF. After 4 h the mixture was poured into ice water and rendered alkaline, and the fluoroaniline isolated by extraction and purified by either distillation or crystallization. 

The dry azide salt exploded violently with a brilliant flash [1]. Subsequent reports claim it impossible to isolate arenediazonium azides, the attempt giving usually the aryl azide or, under very specific conditions, some arylpentazole. Hantzsch had considered both possibilities and produced qualitative evidence against them. He started from the nominal diazonium hydroxide in dry ether, an unusual azo-coupling method. Whatever he got appeared to revert to the diazonium hydroxide on treatment with alkali [2]. 

Such a mechanism was unlikely as addition of an external trap, 1,1-diphenylethylene, had no effect on the course of the arylation of p-ketoesters.i l A second approach involved the use of an internal trapping system which had been successfully used in the study of the radical reactions of arenediazonium salts.The internal trap containing reagent, ( rf/io-allyloxyphenyl)lead triacetate (94), can be easily prepared from the corresponding boronic acid. 2 Reaction with various types of nucleophiles, such as ethyl 2-oxocyclopentanecarboxylate (86), mesitol (36), the sodium salt of nitropropane, iodide and azide always afforded the C-arylation products in high yield. No trace of the 3-substituted dihydrobenzofurans, expected in a mechanism involving the intermediacy of free radicals, could be detected. 

Fluoroalkanes can be obtained by fluoride transfer from complex fluoride anions to carbocat-ions in the nitrosative decomposition of certain aliphatic azides. Several complex nitrosonium salts, such as NO " BF4, NO PF(, , and NO ShF ". " have been used for this purpose. This reaction is similar to the decomposition of arenediazonium tetrafluoroborate salts to form aryl fluorides (the Balz-Schiemann reaction, discussed in Sections 1.1.8.5. and Vol. F 10a, p686ff). in which fluoride is transferred from a very weak nucleophile, such as the complex tetrafluoroborate anion, to an electron-deficient center. 

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