Aromatic cations, coupling reactions with

THIS CHAPTER IS CONCERNED WITH A REACTION of aromatic and hetero-cyclic cation radicals about which only little is so far known their ability to react with neutral radicals. The reaction is expressed simply for the coupling of an aromatic cation radical (ArH +) with a radical (R-) in equation 1. This simple equation, presently only poorly documented, is nevertheless part of current thinking in two reactions of wide scope electrophilic aromatic substitution and reactions of cation radicals with nucleophiles. The product of equation 1 is a a complex, (ArHR)+, which is structurally the same as that... 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Basic Red 22 (134), which contains 1 part ia 7 of the yellowish red 1,4-dimethyl isomer, Basic Red 29 (135), and Basic Yellow 25 (136) are all examples of delocalized cationic azo dyes. Dyes of this type can also be synthesized by Hbnig s oxidative coupling reaction of heteroaromatic hydrazones with tertiary aromatic amines. 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

RBSctions of Radical Anions With Radicals. The coupling of arene or alkene radical anions with radicals is an important reaction, and one that has significant synthetic potential. For example, radicals formed by nucleophilic capture of radical cations couple with the acceptor radical anion, resulting in (net) aromatic substitution. Thus, the l-methoxy-3-phenylpropyl radical (113 R = H) couples with dicyanobenzene radical anion loss of cyanide ion then generates the substitution product 132.2 + ... 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Coupling Components With Condensed Cyclic Ammo nium Residues. Heterocyclic compounds in which the condensed benzene ring is substituted by a hydroxyl or an amino group can be coupled with diazonium compounds and may also be quatemized, either prior or subsequent to the coupling reaction, to yield cationic azo dyes. l,2-Dialkyl-6-nitroindazolium salts are reduced to the 6-amino compounds and then coupled with diazonium salts of aromatic amines. These dyes (e g., 21) color polyacrylonitrile in bright yellow to orange shades [64],... 

Diazo Components With Amino alkyl Groups. Yellow dyes are prepared by ami-nomethylation of 2-(4 -aminophenyl)-5-methylbenzothiazole followed by diazoti-zation and coupling with acetoacetarylides. Red cationic dyes are obtained with naphthols or hydroxynaphthoic acid arylides. All are suitable for dyeing paper. The bleachability of these dyes is important for the recycling of waste paper [84], The aminomethylation of aromatic amines is carried out by reaction with formal-... 

In the non-phenolic oxidative coupling reaction the electron-rich arene 19 undergoes electron transfer yielding the radical cation 20, which is preferably treated in chlorinated solvents or strongly acidic media. Attack of 20 on the electron-rich reaction partner 21 will proceed in the same way as an electrophilic aromatic substitution involving adduct 22 which extrudes a proton. The intermediate radical 23 is subsequently oxidized to the cationic species 24 which forms the biaryl 25 by rearomatization. In contrast with the mechanism outlined in Scheme 5, two different oxidation steps are required. 

A mechanism involving the coupling of cation radicals has also been considered for the electropolymerization of benzene compounds [306,313]. This mechanism occurs by a sequence of events similar to those proposed for the electropolymerization of pyrroles. The first step is the oxidation of benzene to a cation radical (471). Two of these cation radicals combine to form a dication dimer (478). The neutral aromatic dimer (479) is formed upon loss of two protons. This dimer is then reoxidized to a cation radical (480). Chain growth is accomplished by the coupling reaction of this cation radical with other cation radicals followed by deprotonation to form aromatic structures. Polymer growth continues by this sequence of steps until precipitation from solution occurs (Fig. 72). 

Diazonium Coupling Reactions. The union of a diazonium ion with a phenol or an amine is known as a diazonium coupling reaction. The process appears to be an example of aromatic substitution in which the reactive species are the diazonium cation and phenoxide ion or the substituted aniline molecule 9... 

Reports of examples of arylphosphonates include those of water-soluble phosphinic-polyphosphonic acids, e.g. 132, and the phosphonate 133 which when coupled to alcohols, to give e.g. 134, act as linkers to proteins in experiments intended to generate antibodies to catalyse cationic cyclisation reactions.Novel water-soluble phosphonate-substituted phthalocyanines have been prepared.The phosphonate esters 135 are insoluble in water but can be hydrolysed by hydrochloric acid to give the water-soluble phosphonic acids 136. Aromatic phosphonate-phosphines 137, and their air-stable complexes, have been obtained from the reaction of 4-halogeno-substituted phenylphosphonates with lithium diphenylphosphide. ... 

Reaction of electron-rich aromatic compounds with TTFA leads to intermolecular oxidative coupling to form the corresponding biaryls without aromatic thallation. The reaction proceeds through one-electron transfer from aromatic compounds to Tl(III) to give an aromatic radical cation which leads to biaryls (Schemes 9.52 and 9.53 [52]). Intramolecular aryl coupling also occurs (Schemes 9.54 [53] and 9.55 [54]) and, further, when the carboxylic acid moiety is present, intramolecular as well as intermolecular lactonization occurs (Schemes 9.56 [55] and 9.57 [56]). 

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