Radical cations aromaticity

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

We postulated a reaction mechanism with participation of an aromatic radical cation which was formed by one electron transfer from an aromatic hydrocarbon to copper(II) chloride. Activated alumina has electron-acceptor properties, and formation of a radical cation of an aromatic hydrocarbon adsorbed on alumina has been observed by ESR (ref. 13). Therefore, it seemed to us that alumina as a support facilitates the generation of the radical cation of the aromatic hydrocarbon. 

Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely (i) direct displacement (S/v2Ar ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR wlAr ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRi w 1 Ar ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR wlAr mechanism, which requires no substituent activation, is insensitive to substituent effects, both the S/v2Ar and the Sr+n lAr mechanisms show strong and opposite substituent effects. 

Scheme 9 Reaction pathways of anodically generated aromatic radical cations.

For example, we can tell that the paramagnetic cation giving rise to the EPR signal in Figure 8.6 is non-planar, which is quite unusual for such an aromatic radical cation, a fact we know from additional computations based on the hyperfine splitting pattern. 

Aromatic radical-cations are generated by pulse-radiolysis of benzene derivatives in aqueous solution. Radiolysis generates solvated electrons, protons and hydroxyl radicals. The electrons are converted by reaction with peroxydisulpbate ion to form sulphate radical-anion, which is an oxidising species, and sulphate. In another proceedure, electrons and protons react with dissolved nitrous oxide to form hydroxyl radicals and water, Hydroxyl radicals are then made to react with either thallium(i) or silver(i) to generate thallium(ii) or silver(ll) which are powerfully... 

A third important reaction of aromatic radical-cations is carbon-carbon bond formation with a further aromatic substrate. This reaction is limited to the oxidation in acetonitrile of substrates with electrondonating substituents. Radical-cations from benzene, naphthalene and anthracene form a-complexes but do not form a a-bonded reaction intermediate. Tlie dimerization reaction has been investigated both by pulse-radiolysis [22] in water and by electrochemical methods [27] in acetoni-... 

The dehydrodimerization reaction involving aromatic radical-cations is fast only when electron donating substituents are present in the benzene ring. These substituents stabilise the a-intermediate. Benzene, naphthalene and anthracene radical-cations form a a-sandwich complex with the substrate but lack the ability to stabilise the a-intermediate so that radical-cation substrate reactions are not observed. The energy level diagram of Scheme 6.4 illustrates the influence of electron donating substituents in stabilising the Wheland type a-intermediate. 

Scheme 6.4. Energy level diagrams for reaclions of aromatic radical-cations with their sub.stratc S (a) Substrates having electron donating nielhoxy substituents, (b) No electron donating substituents present. The wavy lines indicate a single electron tran.sfcrprocess.

Conversion of toluenes to the benzoic acid is also accomplished by anodic oxidation in acetic acid containing some nitric acid. It is not clear if this reaction involves the aromatic radical-cation or if the oxidising agents are nitrogen oxide radicals generated by electron transfer from nitrate ions [66, 67]. Oxidation of 4-fluorotoluene at a lead dioxide anode in dilute sulphuric acid gives 4-fluorobenzoic acid in a reaction which involves loss of a proton from the aromatic radical-cation and them in further oxidation of the benzyl radical formed [68]. 

Table 6.4. Products from reactions between aromatic radical-cations and...

Cleavage of a benzylic carbon-carbon bond is observed from aromatic radical-cations in cases where the resulting fragments are stabilised by substituents. Bond cleavage occurs, for example, with dialkylphenylcarbinols involving loss of the more stable alkyl radical and leaving a phenyl alkyl ketone [85, 86],... 

A recent development in the synthesis of 3//-3-benzazepin-2-ones has been the photocyc-lization of A-(chloroacetyl)phenethylamines (Scheme 25). Ring closure is by homolysis of the alkyl halide followed by intramolecular coupling of the alkyl radical with an aromatic radical cation. Yields are good, especially with a stabilizing electron-donating group (MeO, NMe2) at the position meta to the ethylamino function (i.e. ortho or para to the site of cyclization). Isomeric benzazepinones are normally obtained (Scheme 25) with meta-substituted phenethylamines (80H(14)ll). 

Aromatic radical cation formation on the intracrysta 1s surfaces of transition metal layers lattice silicates, J. Phys. Chera., 78 (1974) 994-999. 

Further examination of "reductive oxidation" ECL using polyaromatic compounds in non-aqueous media has revealed three signific ant features of the luminescence mechanism (10). First, the cyclic voltammograms fojj the reduction of the polyaromatic compounds in the presence of S2O8 were of a highly catalytic type. Second, the efficiency of ECL was qualitatively dependent on the stability of the aromatic radical cation rather than of the aromatic radical anion. Third, the importance of the aromatic radical cation ion in the mechanism for the formation of excited states was illustrated using a tertiary reactant system. The results of these studies are summarized below. 

Aromatic radical cations are readily formed and have a measurable lifetime when they are stabilized by electron-donating substituents [e.g., reaction (55) O Neill etal. 1975],... 

Using a binary sensitizing system (phenanthrene P/DCNB /7-dicyanobenzene) in acetonitrile solution, O-aryl glycosides are transacetalized with alcohols after generation of aromatic radical cations [23], According to kinetic anomeric effects, the a-side attack of nucleophiles to cyclic oxocarbenium ions follows scheme 9. 

Pinnavaia, T. J., Hall, P. L., Cady, S. S., and Mortland, M. M. (1974). Aromatic radical cation formation on the intracrystal surfaces of transition metal layer silicates. /. Phys. Chem. 78, 994-999. 

The same electron transfer mechanism was proposed by Heiba et a/.242 247 and was supported by the observation by ESR of the radical cations of several arenes when they were treated with Co(III) acetate in trifluoracetic acid.248 Cobalt(III) is a stronger oxidant in trifluoracetic acid than in acetic acid217,249 (see later). In some cases (with electron-rich aromatics), radical cations were observed in acetic acid.242 Further evidence for the radical cation mechanism was obtained in the oxidation of p-methoxybenzyl phenyl sulfide.242 The pro-... 

A variety of alkylbenzenes undergo anodic acetoxylation, in which the loss of an a proton and solvation of the radical cation intermediate form the basis of side-chain and nuclear acetoxylation, respectively.30Sa b The nucleophilicity of the solvent can be diminished by replacing acetic acid with TFA. The attendant increase in the lifetimes of aromatic radical cations has been illustrated in anodic oxidations.308 Radical cations also appear to be intermediates in the electrochemical oxidation of alkanes and alkenes.309a-c... 

Analogous experimental conditions (i.e. Cl, 0.1-1 Torr) allowed for the detection by tandem mass spectrometry of the collision complexes formed in the ion-molecule reactions of several aromatic radical cations M+ (M = C6H5X, X= Me, N02, Cl pyridine, furan, thiophene) and neutral iodoalkanes RI (R= n-Pr, 2-Pr, n-Bu, 2-Bu, etc.) en route to areni-um ions34,35. The collision complexes are covalently bonded species, namely nonisomeriz-ing iodonium radical positive ions 4 which dissociate to arenium ions 5 via reductive elimination of I (Scheme 7)34. 

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