Aryldiazonium salts substitution reactions

Fig. 5.53. Mechanistic aspects I of nucleophilic aromatic substitution reactions of aryldiazonium salts via radicals introduction of Nu=Cl, Br, CN or N02 according to Figure 5.52. Following step 2 there are two alternatives either the copper(II) salt is bound to the aryl radical (step 3) and the compound Ar-Cu(III)NuX decomposes to Cu(I)X and the substitution product Ar-Nu (step 4), or the aryl radical reacts with the cop-per(II) salt in a one-step radical substitution reaction yielding Cu(I)X and the substitution product Ar-Nu.

Fig. 5.55. Mechanistic aspects III of nucleophilic aromatic substitution reactions of aryldiazonium salts via radicals introduction of Nu = I through reaction of aryldiazonium salts with KI. In this (chain) reaction the radical I2 —apart from its role as chain-carrying radical—plays the important role of initiating radical. The scheme shows how this radical is regenerated the initial reaction by which it presumably forms remains to be provided, namely (1) Ar-N =N + I -> Ar- N=N + h ...

Fig. 5.56. Nucleophilic substitution reactions on a masked aryldiazonium salt. The introduction of fluorine takes place by the SN1 mechanism in Figure 5.50, the introduction of iodine occurs by the radical mechanism of Figure 5.56.

Fig. 5.46. Nucleophilic substitution reactions on a masked aryldiazonium salt.

Although nitriles lack an acyl group, they are considered acid derivatives because they hydrolyze to carboxylic acids. Nitriles are frequently made from carboxylic acids (with the same number of carbons) by conversion to primary amides followed by dehydration. They are also made from primary alkyl halides and tosylates (adding one carbon) by nucleophilic substitution with cyanide ion. Aryl cyanides can be made by the Sandmeyer reaction of an aryldiazonium salt with cuprous cyanide. a-Hydroxynitriles (cyanohydrins) are made by the reaction of ketones and aldehydes with HCN. 

In the presence of metal salts, aryldiazo compounds undergo various coupling reactions in aqueous media. Such reactions have been used extensively in the dye and pigment industry and in anal d-ical chemistry. For example, the AICI3-catalyzed diazo couphng of three different 3-substituted-l//-pyrazol-2-in-5-ones in water with seven different aryldiazonium salts yielded 21 different (5-hydroxy-3-substituted-lFf-pyrazol-4-yl)-azobenzene derivatives as colored products with high yields (Eq. 11.67). ... 

Benzotropolones take part in electrophilic substitution reactions, for example halogenation [293,328,331] and coupling with aryldiazonium salts [332]. They may undergo rearrangement to benzenoid compounds in some instances, for example treatment of 3,4-benzotropolone with excess bromine gives 2,3-dibromonaphtho-quinone and with excess concentrated nitric acid leads to 2,4-di-nitro-l-naphthol [333]. Like other tropolones, 3,4-benzotropolone is photochemically converted into a bicycloheptadienone [334]... 

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. ... 

Another important class of reactions involves the introduction of a cyano group by substitution in an Ar-Z precursor. In fact, novel pathways leading to aromatic nitriles-for example, photosubstitution reactions-are desirable in view of the many applications of aryl cyanides as agrochemicals and pharmaceuticals. Today, the classical copper(l)-mediated Rosenmund-von Braun and Sandmeyer reactions, from aryl halides and aryldiazonium salts respectively, have been supplanted by reactions which employ palladium- or copper-catalysis [57]. The rather common use of excess cyanide anion may lead to a deactivation of the catalyst, and affect to a remarkable extent each of the key steps of the catalytic cycle [58aj. Although the use of complex iron cyanide has been shown to offer an effective solution to this limitation [58b,c], photocyanation provides an equally useful alternative [10],... 

Aromatic amines are usually very reactive towards electrophilic reagents yielding derivatives substituted at carbon atoms. Reactions such as nitration and bromination can be applied to the amine as such, or to the amine previously protected at the nitrogen site. Although the derivatives from these reactions are useful for identification and possibly quantitative purposes, the site and stoichiometry of the substitution are specific for every system, and no generalisation can be made. Aryldiazonium salts undergo coupling reactions with aromatic amines to yield azo dyes, which can be used for detection and determination. In Table 12 some of the aryldiazonium salts used in analysis are listed. 

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