Diazonium coupling activity

Iodination of 2-benzyl-2-hydroxypyridine (9.101) occurs at the 6- and subsequently at the 4-position. Under similar conditions, the corresponding N-oxide gave only the 4,6-diiodo product. This does not necessarily prove that the N-oxide function is more effective in activating the 4-position steric hindrance to 6-substitution may be involved (cf. 75CHE478). Whereas both aminomethylation and diazonium coupling were easier for the N-oxide than for 9.101, the 4- 6-isomer ratio for both reactions was similar for both pyridine and its N-oxide (75CHE478). 

With 20% oleum at 200°C, 3-hydroxyquinoline and 4-hydroxyisoquino-line undergo sulfonation in the 5- and 8-positions in 61 and 57% yield, respectively (72BAU406). These results contrast with the behavior of these molecules in nitration (Section 4.A), halogenation (Section 5.A), and diazonium coupling (Section 6.C), and demonstrate the usual high steric hindrance to sulfonation. However, under basic conditions, the very powerful activation by O is sufficient to produce the normal 4- and 3-substitution, respectively, in yields of 80 and 60% (72BAU404). 

As has been mentioned previously, the reactivity of halogeno- and methylsulfonyl-l,3,4-thiadiazoles toward nucleophilic reagents is in agreement with the low electron density on the carbon atoms in the ring. The high coupling activity of the diazonium salts can be ascribed to the same cause. 

Wet silica gel has been successfully utilised as a diazonium salt activator for a solid-liquid interfacial azo-coupling reaction, and has considerable advantages over previously reported phase-transfer catalysts (Vol. 5, p. 198 Vol. 6, p. 211) for the same process. The commercially available silica gel has the apparent advantage of simplicity of handling and recycling, and yields of azo-coupled products are excellent (Scheme 43). ... 

A reaction of aryl diazonium salts that does not involve loss of nitrogen takes place when they react with phenols and arylamines Aryl diazonium ions are relatively weak elec trophiles but have sufficient reactivity to attack strongly activated aromatic rings The reaction is known as azo coupling two aryl groups are joined together by an azo (—N=N—) function... 

Azo coupling (Section 22 18) Formation of a compound of the type ArN=NAr by reaction of an aryl diazonium salt with an arene The arene must be strongly activated toward... 

Other typical electrophilic aromatic substitution reactions—nitration (second entr-y), sul-fonation (fourth entry), and Friedel-Crafts alkylation and acylation (fifth and sixth entries)—take place readily and are synthetically useful. Phenols also undergo electrophilic substitution reactions that are limited to only the most active aromatic compounds these include nitrosation (third entry) and coupling with diazonium salts (seventh entry). 

Diazonium salt 185 (R = H) when coupled with different CH active compounds yielded 3-hydrazono derivatives (e.g. 186 and 187). Hydrazono derivatives 186 and 187 were cyclized into 3-(l,4-dihydropyridazin-l-yl) 188 and 3-(pyrrolin-l-yl) derivatives 189, respectively (00MI33). 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

Compounds acting as coupling components must have a high electron density on the reacting carbon atom. Therefore diazonium ions react only at aliphatic carbon atoms which are activated by electron-withdrawing groups (usually acyl or nitro). There is... 

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