Radical-cations generation from arenes

Chiral amine catalysts have also been used in cascade reactions mediated by SOMO catalysis [143] and Lewis base catalysis [144]. MacMillan s group developed a powerful cascade reaction moderated by SOMO catalysis. The radical cation, generated from an enamine in condensation of imidazolidinone catalyst 208 with aldehyde 207 and subsequent oxidation by Cu oxidant, was expected to engage in a series of 6-endo-trig radical cyclizations terminated by a suitable arene to give a cyclohexadi-enyl radical. After a second oxidation, rearomatization, and liberation of the catalyst, the requisite 209 would be generated (Scheme 1.90). 

In addition to the former example, Pandey et al. achieved efficient a-aryla-tion of ketones by the reaction of silyl enol ethers with arene radical cations generated by photoinduced electron transfer from 1,4-dicyanonaphthalene. Using this strategy various five-, six-, seven-, and eight-membered benzannulated compounds are accessible in yields in the range 60-70% [39],... 

Aryl radical cations generated by electron-transfer processes from methoxy substituted arenes to DCN, tethered by oxygen, nitrogen as well as carbon nucleophile leads to intramolecular cyclizations (Scheme 8.67). The synthetic potentials of... 

However, in certain cases under photolytic conditions, spectra of the corresponding arylmercury radical cations 6 developed, whereas no mercuration occurred in dark [81] signifying collapse of the ArH +,Hg(TFA)2 radical ion pair 4, provides an alternative path way to Wheeland complex 2 and hence to ArHg(TFA) + 6. Arene radical cations can also be generated from arene and thallium(III) tris-(trifluoroacetate) in trifluoroacetic acid [82], but with a different mechanism proposed by Eberson et al. [83]. Oxidation of anthracene showed 9-trifluoroacetoxy and 9,10-bis(trifluoroacetoxy)anthracene [84, 85], benzo[a]pyrene, 7-methylbenzo-[a]pyrene and 12-methylbenzo[a]pyrene yielded radical cations of 7- and/or 12-trifluoroacetates [86], triptycene (9,10-dihydro-9,10-[l,2]benzanthracene) showed... 

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... 

A versatile strategy for efficient intramolecular oc-arylation of ketones was achieved by the reaction of silyle enol ethers with PET-generated arene radical cations. This strategy involved one-electron transfer from the excited methoxy-substituted arenes to ground-state DCN [42]. Pandey et al. reported the construction of five- to eight-membered benzannulated as well as benzospiroannulated compounds using this approach (Sch. 20) [42a]. The course of the reaction can be controlled via the silyl enol ether obtained... 

The benzyl radicals generated by efficient deprotonation or electrofugal group loss from the benzylic position of arene radical cations (Eq.4) have found interesting applications in organic synthesis [25]. Some of the examples pertaining to this class are exemplified in Sect. 2.5. A recent publication of Santamaria et al. [26] illustrates the use of PET generated benzylic radicals (via deprotonation step from arene radical cations) for selective and mild photo-oxidation of... 

The success of the strategy is further applied for the synthesis of carbo-and spiro-aimulated aromatic compounds [146,147] by the intramolecular cyclization of silyl enolethers to PET-generated arene radical cations. Two types of carbocyclic compounds (170 and 173), varying in ring sizes, may be synthesized [146] starting from the same ketone (i.e., 169), as two types of silyl enol ethers can be produced using either thermodynamic or kinetic enolisation procedures. The core spiro structure (177) of the anticancer antibiotic ffed-ericamycin is also prepared [147] by the PET cyclization of 176 (Scheme 36). 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

In keeping with the seminal work of Kita, we proposed that the I(III)-mediated amination involved a radical cation intermediate that was generated by single electron transfer from the arene to the I(III) oxidant. The consequent radical cation should be highly reactive, and the attack of a phthalimide nucleophile would lead to a mixture of regiomeric products, like the 5 6 3 mixture that was observed for our toluene reaction (Scheme 10). This hypothesis contrasts with the mechanisms proposed by Chang and Antonchick, as electrophilic aromatic substitution, even with a reactive R2N species, should favor the para product. 

Under the proposed mechanism (Scheme 2.3), the reaction initiated by pho-toinduceed electron transfer (PET) from an arene to generate an arene cation radical, which was attacked by an amine to give cr-adduct, followed by deprotonation and oxidative aromatization to afford the desired arene. In the presence of TEMPO, dioxygen served as a terminal oxidant and played a role in both the regeneration of the photoredox catalyst and the aromatization. 

Electron-transfer activation. The observation of intense coloration upon mixing the solutions of hydroquinone ether MA and nitrogen dioxide at low temperature derives from the transient formation of MA+ cation radical, as confirmed by the spectral comparison with the authentic sample. The oxidation of MA to the corresponding cation radical is effected by the nitrosonium oxidant, which is spontaneously generated during the arene-induced disproportionation of nitrogen dioxide,239 i.e.,... 

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

Kita and Tohma found that exposure of p-substituted phenol ethers to [bis(tri-fluoroacetoxy)iodo]benzene 12 in the presence of some nucleophiles in polar, less nucleophilic solvents results in direct nucleophilic aromatic substitution [Eq. (84)] [156]. Involvement of a single-electron transfer (SET) from phenol ethers to A3-iodane 12 generating arene cation radicals was suggested by the detailed UV-vis and ESR studies. SET was involved in the oxidative biaryl coupling of phenol ethers by 12 in the presence of BF3-Et20 [157]. 

A stoichiometric example of the combination of two reagents has recently been reported by O Connor in the context of the Bergman cycloaromatization. The generation of the aromatic diradical can be induced by [Cp Ru(CH3CN)3]PF6. Radical reduction occurs through HAT from CpW(CO)3H to provide a cationic Ru-arene complex as shown in Scheme 15 [35]. 

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