In diazonium coupling

Intermediates in acylation reactions, 47 carbinolamine as, 41 in diazonium coupling, 77 enamine-like, 85... 

Electrophilic substitution (bromination, nitration) of 2-substituted 1,2,3,6,7,116-hexahydro-4//-pyrazino[2,l -a]isoquinolin-4-ones occurred on the aromatic moiety to give either 11- or 8-substituted derivatives the site was not determined (76GEP2441261). The nitro group was reduced to an amino group, which was alkylated, acylated, and converted to different groups via a diazonium group, and involved in diazonium coupling. 

There are numerous multistep processes that generate electrophiles. As examples of these types of reactions, we will consider the diazotization of anilines and the formation of chloroiminium ions in the Vilsmeier-Haack reaction. Aryl diazonium ions are useful in the modification of arenes by the Sandmeyer reaction and as electrophilic intermediates in diazonium coupling reactions for the synthesis of dyes and pigments. Several types of synthetic methods have been developed for this chemistry, and the mechanism varies depending on the methodology [13]. Under some conditions, the nitrosonium ion (18) initiates the process (Scheme 1.5). NjOj and NOCl have also been proposed as intermediates in diazotization—both are considered as nitrosonium ion carriers. The aniline reacts... 

The reactivity of the 5-position of 2-aminothiophene in diazo coupling, which is present also in the acylated derivatives, complicates the formation of a diazonium salt from 2-aminothiophene. Thus Steinkopf and Miiller obtained only an azo dyestuff, although they proved, through the isolation of small amounts of 2-thienyl diazonium chloride, the diazotizability of 2-aminothiophene which had earlier been denied. However, recent Russian work claims the preparation of 2-thienyldiazonium chloride by treating the double salt in 10% hydrochloric acid with sodium nitrite. Amazingly high yields (over 90%) of azo compounds were then achieved by coupling the diazonium salt solution with y9-naphtol, w-toluidine or with the 2-aminothiophene double salt. These authors have also studied the... 

Diazonium coupling reactions are typical electrophilic aromatic substitutions in which the positively charged diazonium ion is the electrophile that reacts with the electron-rich, ring of a phenol or arylamine. Reaction usually occurs at the para position, although ortho reaction can take place if the para position is blocked. 

The high reactivity of heterocyclic diazonium ions in azo coupling reactions is the reason why in some cases the primary diazotization products cannot be isolated. For example, diazotization of 2-methyl-5-aminotetrazole (2.14) directly yields the triazene 2.15, i. e., the N-coupling product, since the intermediate diazonium ion is reactive enough to give the N-coupling product with the parent amine even under strongly acidic conditions (Scheme 2-8 Butler and Scott, 1967). 

In the older literature and in papers by some industrial azo chemists up to the 1960s it was claimed that (Z)-diazoates react in azo coupling processes. This belief can be traced back to the paper by Schraube and Schmidt (1894), who discovered the (Z)/(ii)-isomerism of diazoates (see Sec. 1.1). The most important tool used by Schraube and Schmidt for distinguishing between the two isomers was the (correct) observation that only one of the isomers reacted with coupling components, forming the same azo dye as when diazonium salt solutions were used. The apparent reactivity of the (Z)-diazoate is due to the fact that its equilibrium with the diazonium ion is relatively rapid, whereas the diazonium ion is produced only very slowly from the (ii)-diazoate (see Sec. 7.1). 

Table 12-3. Kinetic hydrogen isotope effects found in azo coupling reactions of 4-chlorobenzene-diazonium ion with naphtholsulfonic acids (Zollinger, 1955 a Ernst et al., 1958).

A large primary isotope effect kH/kD = 3.6 had also been found earlier by Ibne-Rasa122 in the nitrosation of 2,6-dibromophenol in the 4 position which was also shown to be base-catalysed. These values are not unexpected in view of the isotope effect found with diazonium coupling which involves a similarly unreactive electrophile, so that the rate-determining transition state will be displaced well towards products. Furthermore, the intermediate will have a quinonoid structure and will, therefore, be of low energy consequently, the energy barrier for the second step of the reaction will be high. 

Some diazonium couplings are subject to base catalysis and in these cases kinetic isotope effects are observed but since the rate of catalysed reaction is not linearly related to the base concentration (see p. 7), the SE3 mechanism is ruled out and the Se2 mechanism must operate, viz. 

Through the use of arenediazonium salts, the straightforward transformation of amines into cross-coupling products can be realized. Whenever the diazonium salts do not tolerate bases and strong nucleophiles (e.g., phosphines), base- and phosphine-free protocols have to be used. Heterocyclic carbene ligands serve well in cross-coupling of Aryl- and vinylboronic acids, or alkylboronates with arenediazonium salts.369,370 Several convenient phosphine-free protocols have been developed for the same purpose.371-373... 

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