Arene radical cations

Kispert, L. D., J. Joseph et al. (1987). EPR study of arene radical cation salt crystals. Synthetic Metals 20 67-72. 

Compounds 24 and 25, containing highly methylated aryl groups, yield the corresponding arene radical cation on treatment with A1C13 in CH2C12 solution, as demonstrated by... 

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... 

McCleland has reported that 3-phenylpropan-l-ol [125] and 3-(p-methyl-phenyl)propan-l-ol 99 [126] cyclize to chromans when oxidized by the radical anion SO4, generated by redox decomposition of S20 with Fe. The intermediate arene radical cation 100 is attacked by the nucleophilic hydroxy group. Whereas 1,6-cyclization yields 7-methylchroman 102, 1,5-cyclization with subsequent C-migration leads to the regioisomer 6-methylchroman 105. A dependence of the isomeric ratio and the combined yields to the pH value is determined. While 7-methylchroman 102 is the main product over a wide pH range, 6-methylchroman 105 is only formed at low pH. When the pH is lowered, the combined yields decrease due to the formation of an a-oxidized non-cyclized product. 

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],... 

When generated in zeolites, alkene or arene radical cations react with the parent molecules to form ti-dimer radical cations. For example, 2,3-dimethyl-l-butene and benzene formed 91 + and 92 +, respectively. The confinement and limited diffusion of the radical cation in the zeolite favor an interaction between a radical cation and a neutral parent in the same channel. 

Bis(trifluoroacetoxy)iodo]benzene (14, BTIB) can be utilized in hexafluoro-2-propanol for the installation of nucleophiles at the ortho-position of para-sub-stituted alkoxyarenes [59-63]. Such reactions have been employed for the construction of carbon-carbon and carbon-heteroatom (N,0,S) bonds, trimethylsi-lyl compounds serving as useful progenitors of the heteroatom nucleophiles (Scheme 20). Oxidative substitutions of this type appear to proceed through arene radical-cations, generated by single electron-transfer within BTIB/sub-strate charge-transfer complexes. 

In a first successful approach, Kochi, Renzepis, and co-workers [41] chose EDA complexes of 9-cyanoanthracene (14) and tetracyanoethene (TCNE, 15) since their charge transfer (CT) absorption bands are well separated from the absorption bands of the monomers. Excitation with a 25 ps laser pulse produced two transient absorption bands near 460 and 750 nm, which decayed simultaneously within ca. 60 ps. As was shown in the chloranil-enolether system 9—10, cf. Fig. 6), the transients can be identified with the arene radical cation (14a+ ) and the olefin radical anion (/5- ), respectively (Scheme 5). 

Pandey, G., Karthikeyan, M., and Mumgan, A. (1998) New intramolecular a-arylation strategy of ketones by the reaction of silyl enol ethers to photosensitized electron transfer generated arene radical cations construction of benzannulated and benzospiroannulated compounds. Journal of Organic Chemistry, 63, 2867-2872. 

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... 

As mentioned in Sec. 15.2.3, benzylic radicals are obtained also from the cleavage of a nucleofugal group from the radical anion. This may lead again to benzylation, and it has been shown that irradiation of 1,4-dimethoxynaphthalene in the presence of substituted benzyl halides leads to benzylated naphthalenes (mainly in position 2) via benzyl radical/arene radical cation combination, which is analogous to the benzyl radical/radical... 

Further functionalizations are obtained via the electron transfer— radical cation fragmentation pathway a typical example is side-chain nitration by irradiation of methyaromatics with tetranitromethane. Aromatics form charge-transfer complexes with C(N02)4 irradiation leads to electron transfer and fragmentation of the C(N02)4 radical anion to yield the triad [Ar + C(NO)J N02], followed by combination between the arene radical cation and the trinitromethanide anion. Thus, cyclohexadienes are formed that generally eliminate and rearomatize at room temperature yielding ring-functionalized products [234] (Sch. 21). 

PET generated arene radical cations and arene radical anions have found significant applications in organic synthesis. We will summarize the important examples of both types in this section. 

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... 

Kochi and co-workers have recently identified and characterized the weak charge transfer complexes between tropylium ion and a series of substituted arenes in acetonitrile solution [74], Photoexcitation of these electron donor acceptor (EDA) complexes leads to an electron transfer from the arene donors to the tropylium ion in accord with Mulliken s theory [75]. Time resolved spectroscopic observation of the arene radical cations (formation within the 30 ps laser pulse) has confirmed their intermediacy. The subsequent decay of the photogenerated radical cation and the concomitant regeneration of the ground state EDA complex occurs with a rate constant, kBET > 4 x 1010 s 1 (Scheme 11). This fast back electron transfer... 

A4.3 Time-resolved Spectra of Arene Radical Cations in Charge-transfer Osmylation... 

The Relevance of Arene Radical Cations in Electrophiiic Aromatic Substitution... 

Indeed the diversion to side products during thallation coincides with the direct obsovation of the arene radical cation as a transient intermediate both by UV-visible and ESR spectroscopy. A similar dichotomy between the products of mercuration and thallation exists with durene, albeit to a lesser degree. Finally no discrepancy is observed with mesitylene, nuclear substitution occuiring exclusively in both mercuration and thallation. Such a divergence between mercuration and thallation can be reconciled by the formulation in Scheme 6 if they difier by the extent to which di sive separation (ki) occurs in equation (31). All factors being the same, diffusive separation of the radical pair from thallium(III) should... 

The study of mercuration and thallation provides a shaip focus on the experimental delineation of stepwise and concerted mechanisms for arene activation. Thus the unequivoc demonstration of arene radical cations as key intermediates in thallation, particularly of durene and pentamethylbenzene, is consistent with a stepwise (electron-transfer) mechanism for arene activation (compare Scheme 6 and equation 39). 

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