Radical cations, formation from

Just as intermolecular interactions result in dimer radical cation formation from a radical cation and a molecule of starting material, the presence of two chalcogens atoms within a molecule in proximity to one another can result in intramolecular dimer radical cation formation. 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

Explanation of the observed de-oxygenation with the formation of the corresponding imines in the photochemical reaction of a-aryl-A-methylaldonitrones, confirms the intermediate formation of radical cations resulting from PET (441, 467). 

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

Similar to the intramolecular addition of neutral carbon-centered radicals to alkenes, the formation of radical cations starting from alkenes with subsequent cyclization offers a convenient method for constructing carbocyclic ring systems. In contrast to the regioselective 1,5-ring closure (5-cxo-trig cyclization) of the... 

Substitution at carbazole nitrogen and the formation of 74, presumably via a radical cation, resulted from reaction with the cation radical salt 75 in acetonitrile. ... 

The unexpected formation of the blue crystalline radical cation (97) from the reaction of triazinium salt (98) with tetracyanoethylene has been reported and the product identified by its EPR spectrum and by X-ray crystallography (Scheme 42).199 Carboxylic acids react with the photochemically produced excited state of N-t-a-phenynitrone (PBN) to furnish acyloxy spin adducts RCOOPBN. The reaction was assumed to proceed via ET oxidation of PBN to give the PBN radical cation followed by reaction with RCO2H.200 The mechanism of the protodiazoniation of 4-nitrobenzenediazonium fluoroborate to nitrobenzene in DMF has been studied.201 Trapping experiments were consistent with kinetic isotope effects calculated for the DMF radical cation. The effect of the coupling of radicals with different sulfur radical cations in diazadithiafulvalenes has been investigated.202... 

As acceptors, they used 9,10-dicyanoanthracene (DCA) or 2,6,9,10-tetracyanoan-thracene (TCA), the donors were methylated benzenes or naphthalenes. The D/A pair was designed to give radical ion pairs on irradiation. In order to determine sep, they monitored the quantum yield of dimethoxystilbene radical cation formation [70], which intercepts the free radical cation of the donor exclusively. Assuming a constant k, from earlier studies [66b], they indirectly obtained k e, according to Eq. (11) ... 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

Triphenylthiopyrylium salts have been employed as efficient electron transfer photosensitizers to promote the [4+2] cycloaddition between thiobenzophenone and substituted styrenes. A radical cation derived from the thiobenzophenone is involved in the formation of separable diastereoisomeric mixtures of 1,3,4-trisubstituted isothiochromans <2007OL3587>. Interest has been maintained in the photosensitizing properties of thiopyrylium salts for blood disinfection <2007BMCL4406>. Several new thioxanthylium salts have been reported <2007JOC2647, 2007JOC2690>. 

We suggest that electron transfer and electrophilic substitutions are, in general, competing processes in arene oxidations. Whether the product is formed from the radical cation (electron transfer) or from the aryl-metal species (electrophilic substitution) is dependent on the nature of both the metal oxidant and the aromatic substrate. With hard metal ions, such as Co(III), Mn(III), and Ce(IV),289 reaction via electron transfer is preferred because of the low stability of the arylmetal bond. With soft metal ions, such as Pb(IV) and Tl(III), and Pd(II) (see later), reaction via an arylmetal intermediate is predominant (more stable arylmetal bond). For the latter group of oxidants, electron transfer becomes important only with electron-rich arenes that form radical cations more readily. In accordance with this postulate, the oxidation of several electron-rich arenes by lead(IV)281 289 and thallium(III)287 in TFA involve radical cation formation via electron transfer. Indeed, electrophilic aromatic substitutions, in general, may involve initial charge transfer, and the role of radical cations as discrete intermediates may depend on how fast any subsequent steps involving bond formation takes place. 

It is also likely that radical cation formation occurs in reactions of very reactive arenes with Pd(II) (see Section II.B.3.b), which would also lead to biaryl formation. That the reactions of arenes with Pd(II) compounds are far from simple is illustrated by the work of Arzoumanidis and Rauch.573,574 In the reaction of Pd(02CCF3)2 with benzene or naphthalene in TFA, a variety of polynuclear complexes, containing both Pd(I) and Pd(II) and arenes, were isolated574 in addition to the usual biaryls. 

An investigation of the spectral and kinetic characteristics of radical cations of para-halogeno-A,A-dimethylanilines and / ara-halogenodiphenylamines in water/terr-butyl alcohol has been performed in view of the fact that such species are probable intermediates in photonucleophilic substitution of halogen638. Bathochromic shifts in the absorption maxima of the radical cations were observed in the order H=F < Cl < Br. The disappearance of the radical cations obeys second-order kinetics and the rate constants are close to the diffusion-controlled values. The radical cations arise from the singlet excited states of the halogenoarylamines and homolysis of the carbon-halogen bond competes with their formation. 

If fcf > feBET- the overall transformation can occur rapidly despite unfavorable driving forces for the electron transfer itself. Only follow-up reactions with high kf can compete with back electron transfer. Different kinds of such unimolecular processes can drive the equilibria toward the final product. A representative example is the mesolytic cleavage of the C-Sn bond in the radical cation resulting from the oxidation of benzylstannane by photoexcited 9,10-dicyanoanthracene (DCA). This is followed by the addition of the benzyl radical and the tributyltin cation to the reduced acceptor DCA [59]. In the arene/nitrosonium system, [ArH, NO+] complexes can exist in solution in equilibrium with a low steady-state concentration of the ion-radical pair. However, the facile deprotonation or fragmentation of the arene cation radical in the case of bifunctional donors such as octamethyl(diphenyl)methane and bicumene can result in an effective (ET) transformation of the arene donor [28, 59]. Another pathway involves collapse of the contact ion pair [D+, A- ] by rapid formation of a bond between the cation radical and anion radical (which effectively competes with the back electron transfer), as illustrated by the examples in Chart 5 [59]. 

In the particular case of the carboxylates of the eosin family, a careful examination of electron transfer paths was made by K. Tokumaru and cowor-kers [158, 159, 160]. They found that eosin Y (EY ) was able to sensitize hydrogen evolution from water under visible light irradiation in a system made of an amine donor, methyl viologen (MV2+) mediator and redox catalyst such as colloidal platinum. In a water-ethanol solution of EY, in the presence of MV2+ and triethanolamine (TEOA), a maximum quantum yield of 0.3 was measured for the MV + radical cation formation. Kinetic measurements of the EY triplet quenching constant by MV2+ (kq = 3.109 M 1 s 1) and TEOA... 

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