Electron-rich aromatic compounds

Coupling reaction of diazoniutn ions with electron-rich aromatic compounds... 

Arenediazonium ions 1 can undergo a coupling reaction with electron-rich aromatic compounds 2 like aryl amines and phenols to yield azo compounds 3. The substitution reaction at the aromatic system 2 usually takes place para to the activating group probably for steric reasons. If the para position is already occupied by a substituent, the new substitution takes place ortho to the activating group. 

In order to achieve high yields, the reaction usually is conducted by application of high pressure. For laboratory use, the need for high-pressure equipment, together with the toxicity of carbon monoxide, makes that reaction less practicable. The scope of that reaction is limited to benzene, alkyl substituted and certain other electron-rich aromatic compounds. With mono-substituted benzenes, thepara-for-mylated product is formed preferentially. Super-acidic catalysts have been developed, for example generated from trifluoromethanesulfonic acid, hydrogen fluoride and boron trifluoride the application of elevated pressure is then not necessary. 

The reaction of electron-rich aromatic compounds with yV,A -dimethylformamide 2 and phosphorus oxychloride to yield an aromatic aldehyde—e.g. 3 from the substituted benzene 1—is called the Vilsmeier reaction or sometimes the Vilsmeier-Haack reaction. It belongs to a class of formylation reactions that are each of limited scope (see also Gattermann reaction). 

Although limited to electron-rich aromatic compounds and alkenes, the Vilsmeier reaction is an important formylation method. When yV,A-dimethylformamide is used in excess, the use of an additional solvent is not necessary. In other cases toluene, dichlorobenzene or a chlorinated aliphatic hydrocarbon is used as solvent. ... 

Electron-rich aromatic compounds such as durene, p-dimethoxybenzene, mesitylene, anisole, thiophene, and fluorene can be benzoylated or acetylated by the corresponding Af-acylimidazole in trifluoroacetic acid to give the corresponding benzophenone or acetophenone derivative in good yield (Method A). As the actual acylating agent, a mixed anhydride of trifluoroacetic acid and benzoic acid has been proposed 1973... 

The formation of 151 from the phosphonate 171 could be proved only by indirect means. Electron-rich aromatic compounds such as N,N-diethylaniline and N,N,N, N -tetraethyl-m-phenylenediamine U0 1I9> and N-methylaniline 120> are phosphorylated in the para- and in the ortho- plus para-positions by 151. Furthermore, 151 also adds to the nitrogen lone pair of aniline to form the corresponding phosphor-amidate. Considerable competition between nucleophiles of various strengths for the monomeric methyl metaphosphate 151 — e.g. aromatic substitution of N,N-diethylaniline and reaction with methanol or aromatic substitution and reaction with the nitrogen lone pair in N-methylaniline — again underline its extraordinary non-selectivity. 

Electron-rich aromatic compounds, such as phenol, anisole and A,./V-dimethylaniline, add to bis(2-trichloroethyl) azodicarboxylate under the influence of lithium perchlorate, boron trifluoride etherate or zinc chloride to yield para-substituted products 74, which are transformed into the anilines 75 by means of zinc and acetic acid86. Triflic acid (trifluoromethanesulphonic acid) catalyses the reactions of phenyl azide with benzene, toluene, chlorobenzene and naphthalene, to give TV-arylanilines (equation 34)87. 

A similar reaction occurs with electron-rich aromatic compounds, such as toluene or anisole, under tin(IV) chloride catalysis, e.g. equation 131 ... 

A. Jordanova, J. Steinbach, B. Johannsen, Radiofluorination of electron rich aromatic compounds with no carrier added [ F]perchlorylfluoride, J. Label. Compds Radiopharm. 44 (2001) S901. 

For the synthesis of the benzosulfonamide subclass of 1,2-thiazines, introduction of the sulfonyl chloride has been effected by treatment of electron-rich aromatic compounds with chlorosulfonic acid. Such is the case for 1,2-benzothiazine 1,1-dioxides 181 which have been accessed from phenylethylamines 182 in 67-92% yields via intermediate 183 (Scheme 23) <1998SC2137>. 

Analogous reactions of electron-rich aromatic compounds can be found in the literature When 4-substituted-N.N-dimethylanilines 231 are treated with dia-zonium salts or some other electrophiles, position 4 is substituted by the electrophile group to 4-substituted dimethylanilines ... 

X - and especially X -phosphorins are electron-rich aromatic compounds, comparable with aniline, whereas pyridine and pyridinium ions are electron-poor and are comparable to nitrobenzene. Many chemical properties can be easily understood once this fact is taken into account. 

The products ArCF2SMe are assumed to be precursors of ArCF3. Indeed, l-(tri-fluoromethyl)naphthalene is obtained in 83% yield by the reaction of l-[difluoro(methylsul-fanyl)mcthyl]naphthalene with tetrabutylammonium dihydrogen trifluoride/ l,3-dibromo-5,5-dimethylhydantoin. It should be noted that under the oxidative desulfurization conditions the electron-rich aromatic compounds can undergo ring bromination. 

Hi) Formation of transition metal carbonyl complexes Ashe and Colburn have reported (77JA8099) the synthesis of molybdenum carbonyl complexes of arsenin and antimonin but were unable to prepare bismin complexes because of its lability (Scheme 23). As expected for electron-rich aromatic compounds, both formed six-electron 7r-complexes (113) by a ligand displacement mechanism. Arsenin also forms a two-electron complex (114) analogous to those formed by pyridine, whereas antimonin did not give a similar complex under the conditions of this reaction. 

The acid-catalyzed dimerization of pyrroles and indoles also involves electrophilic attack by the 2H- or 3//-protonated species upon the non-protonated heterocycles (Schemes 6, 7 and 8, Section 3.05.1.2.2), and 3,3-dimethyl-3//-indole has been reported to react with 7r-electron-rich aromatic compounds to yield the 2-ary.l-3,3-dimethyl-2,3-dihydroindoles (77S343). In the absence of a nucleophile strong acids promote the interchange of substituents at the 2- and 3-positions of 2,3,3-trisubstituted 3//-indoles, e.g. (510) (511) (62JOC1553). 

These polymers, particularly poly(pyrrole), are most conveniently prepared from the parent molecule via electrolysis. So far, furan, pyrrole, thiophene, and various methylated derivatives have been polymerized by this procedure (10). The anodic polymerization apparently also works for relatively electron rich aromatic compounds such as aniline and azulene (11). 

Propargylic alcohols bearing a terminal triple bond react with electron-rich aromatic compounds in the presence of thiolate-bridged diruthenium complexes to give the propargylated aromatic compounds.30 l-Phenylprop-2-yn-l-ol, for example, reacts with 2-methylfuran to form (15). Intramolecular examples of the reaction were also reported. The process is believed to involve electrophilic attack by the ruthenium-stabilized propargyl cation. 

On the contrary, in cytochrome c peroxidase, the intermediate is a (porphy-rin)Fe(IV)=0 complex with a free radical derived from the one-electron oxidation of an amino acid residue in the vicinity of the heme [10], The second step of the HRP catalytic cycle is the one-electron oxidation by compound I of HRP substrates that are generally electron-rich aromatic compounds. This leads to the second intermediate, called compound II, of the catalytic cycle, which is a (porphyrin) Fe(IV)=0 complex. The one-electron reduction of compound II by HRP substrates regenerates HRP in its resting iron(III) state (Figure 5a). 

Lead(IV) acetate is most commonly employed to plumbylate arenes, often using chloroform as the solvent. Being a relatively weak electrophile, Pb(OAc)4 can react only with electron-rich aromatic compounds, for example anisole and polyalkoxy-benzenes. The electrophilicty of the lead reagent is, however, enhanced substan-... 

In addition to the stabilized carbanions, electron-rich aromatic compounds, for example indole derivatives have emerged as new Michael donors [25], In these reactions, aromatic sp2-C-H transformation is involved. These reactions are described in detail in Section 111.1.3.1.3. A highly enantioselective intramolecular Stetter reaction, in which umpolung reactivity of a formyl group was accomplished using a chiral triazolium salt, has also been reported by Rovis [26]. 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

Fig. 5.20. Reactive electrophiles in the nitration of aromatic compounds with nitric acids with different concentrations and with F SOyHNOj. (Reactive electrophiles in the nitration of (more) electron-rich aromatic compounds cf. Figure 5.23 as well as the last equation in this section.)...

There is also some debate over whether nitrations like the one in the last example may alternatively proceed via a nitrosyl cation (NO ) instead of a HN03 molecule as the electrophile. Small amounts of the nitrosyl cation occur in diluted nitric acid and would—via a Wheland complex intermediate—initially lead to a nitrosoaromatic compound (Ar-N=0) as the Ar-SE product. (Remember Figure 5.23 presented a different reaction mode between nitrosyl cations and—less electron-rich—aromatic compounds.) This nitrosoaromatic compound would subsequently undergo rapid oxidation by the diluted nitric acid to finally yield the nitroaromatic compound. 

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