Radical cation aromatic ring

The mechanism is of the Sn 1 type. That aryl cations are intermediates was shown by the following experiments aryl diazonium chlorides are known to arylate other aromatic rings by a free-radical mechanism (see 14-17). In radical arylation it does not matter whether the other ring contains electron-withdrawing or electron-... 

In the oxidation of aromatic substances at the anode, radical cations or dications are formed as intermediates and subsequently react with the solvent or with anions of the base electrolyte. For example, depending on the conditions, 1,4-dimethoxybenzene is cyanized after the substitution of one methoxy group, methoxylated after addition of two methoxy groups or acetoxylated after substitution of one hydrogen on the aromatic ring, as shown in Fig. 5.55, where the solvent is indicated over the arrow and the base electrolyte and electrode under the arrow for each reaction HAc denotes acetic acid. 

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

Aromatic molecules can be polymerized catalytically on clean metal surfaces, or electrochemically to produce oriented polymer films. Initial adsorption of aromatic molecules occurs by electron donation from the aromatic molecule to the surface. This electron donation creates radical cations that can polymerize. Molecular orientation in the films depends on the stable bonding configuration of the radical cation. Thiophene, pyridines, and pyrrole all polymerize with the ring substantially perpendicular to the surface, whereas aniline polymerizes with the phenyl rings parallel to the surface. The catalytically... 

This novel anodic methoxyiation may proceed via the fluorosulfonium ion B in a Pummerer-type mechanism as shown in Scheme 6.11. In this mechanism, the cation radical A of the sulfide is trapped by a fluoride ion, and this step should suppress side reactions from the cation radical A (such as dimerization and nucleophilic attack on an aromatic ring) even when deprotonation of A is slow due to the weak electron-withdrawing Rf groups or electron-donating substituents on the benzene ring. Since fluoride ions are much weaker nucleophiles compared to methoxide, it is reasonable that the methoxyiation predominates in methanol. 

This proposed reorientation clearly is one explanation for the enhanced stability, compared with radical cations or anions derived from aromatic rings such as found in Bis-A PSF or Hq PSF. 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

The reactions of electrogenerated cation radicals of diarylsulfldes are mainly orbital-controlled and at this level the electronic structure of their frontier orbitals (HOMO-SOMO) has very interesting synthetic consequences. The 3p orbitals of sulfur are conjugated with only one aromatic ring even if there are two aryls bound to sulfur. Therefore, only one ring can be activated electrochemically. The degree of the charge delocalization in the ArS moiety of a cation radical on the one hand, and the availability of p- and o-positions for the substitution on the other, determine quite different reactivity of such species. 

Benzylic CH bonds Benzylic CH bonds can be preferentially substituted at the anode by oxidation of the aromatic ring to a radical cation, which can undergo side-chain substitution at the benzylic carbon atom and/or nuclear substitution. Benzylic substitution preponderates, when there is an alkyl substituent at the aromatic carbon bearing the highest positive charge density in the radical cation, while a hydrogen at this position leads to a nuclear substitution [16]. Anodic benzylic substitution is used in technical processes for the conversion of alkyl aromatics into substituted benzaldehydes [17, 18]. Anodic benzylic substitution has been used for the regioselective methoxylation of estratrienone at C9 (Fig. 4) [19]. 

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings ... 

Pandey and co-workers have generated arene radical cations by PET from electron-rich aromatic rings [119]. The photoreaction is apparently initiated by single-electron transfer from the excited state of the arene to ground state 1,4-dicyanonaphthalene (DCN) in an aerated aqueous solution of acetonitrile. Intramolecular reaction with nucleophiles leads to anellated products regio-specifically. The author explains the regiospecifidty of the cyclization step from... 

The effects of temperature on the shape of intervalence charge-transfer bands for the radical cations of bis(2-f-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)hydrazines that are bridged by 2,5-xylene-1,4-diyl, durene-l,4-diyl, naphthalene-1,4-diyl, biphenyl-4,4-diyl and 9,9-dimethylfluorene-2,7-diyl aromatic rings were studied by ESR. ... 

Scheme 7.56 represents another example of the cleavage of a carbon-carbon bond. The cation-radical of 9-benzyl-iym-nonahydro-lO-selenaanthracene expels a proton and then the benzyl radical (trapped) giving iym-octahydro-lO-selenaanthracene trifluoroacetate in a quantitative yield (Blinokhvatov et al. 1991). The formation of the benzyl radical and aromatization of the central hetero ring of the cation-radical facilitate the development of the reaction. 

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