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The reactivity of the cation radical of (4-MeOC6H4)2CH2 photosensitized by 1,4-benzoquinone (BQ), 2,5-dichloro-1,4-benzoquinone (Cl2BQ), and tetrachloro-1,4-benzoquinone (chloranil, CA) was investigated in acetonitrile. The main photoreaction products obtained by steady-state irradiation were identified to be: (4-MeOC6H4)2-CHOC6H4OH, sensitized by BQ; (4-MeOC6H4)2CHCl, sensitized by Cl2BQ; (4-MeOC6H4)2CHOH, sensitized by CA. The mechanism of their formation was investigated by nanosecond laser flash photolysis that allowed transient species (radical ions, neutral radicals, and ions) to be detected and characterized in terms of absorption spectra, formation quantum yields, and decay rate constants. For all systems, the interaction between the triplet quinone (Q) and (4-MeOC6H4)2CH2 produced the corresponding radical ions (quantum yield phi > or = 0.72) which mainly decay by back electron transfer processes. Less efficient reaction routes for the radical ions Q*- and (4-MeOC6H4)2CH2*+ were also: i) the proton-transfer process with the formation of the radical (4-MeOC6H4)2CH* by use of Cl2BQ; ii) the hydrogen-transfer process with the formation of the cation (4-MeOC6H4)2CH+ in the case of CA. Instead. BQ sensitized a much higher yield of BOH* and (4-MeOC6H4)2CH*, mainly by the direct interaction of triplet BQ with (4-MeOC6H4)2CH2. It was also shown that the presence of salts decreases significantly the rate of the back electron transfer process and enhances the quantum yields of formation of the neutral radicals and ions when Cl2BQ and CA are used, respectively. The behavior of BQ*-, Cl2BQ*-, and CA*- appears to be mainly determined by the Mulliken charges on the oxygen atom obtained from quantum mechanical calculations with the model B3LYP/6-311G(d,p). Spin densities seem to be much less important.

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A kinetic and product study of the OH- -induced decay in H2O of the radical cations generated from some di-and tri-methoxy-substituted 1-arylalkanols (ArCH(OH)R*+) and 2- and 3-(3,4-dimethoxyphenyl)alkanols has been carried out by using pulse- and gamma-radiolysis techniques. In the 1-arylalkanol system, the radical cation 3,4-(MeO)2C6H3CH2-OH*+ decay at a rate more than two orders of magnitude higher than that of its methyl ether; this indicates the key role of the side-chain OH group in the decay process (oxygen acidity). However, quite a large deuterium kinetic isotope effect (3.7) is present for this radical cation compared with its a-dideuterated counterpart. A mechanism is suggested in which a fast OH deprotonation leads to a radical zwitterion which then undergoes a rate-determining 1,2-H shift, coupled to a side-chain-to-ring intramolecular electron transfer (ET) step. This concept also attributes an important role to the energy barrier for this ET, which should depend on the stability of the positive charge in the ring and, hence, on the number and position of methoxy groups. On a similar experimental basis, the same mechanism is suggested for 2,5-(MeO)2C6H3CH2OH*+ as for 3,4-(MeO)2C6H3CH2OH*+, in which some contribution from direct C-H deprotonation (carbon acidity) is possible. In fact, the latter process dominates the decay of the trimethoxylated system 2,4,5-(MeO)3C6H2CH2-OH*+, which, accordingly, reacts with OH- at the same rate as that of its methyl ether. Thus, a shift from oxygen to carbon acidity is observed as the positive charge is increasingly stabilized in the ring; this is attributed to a corresponding increase in the energy barrier for the intramolecular ET. When R=tBu, the OH- -promoted decay of the radical cation ArCH(OH)R*+ leads to products of C-C bond cleavage. With both Ar = 3,4- and 2,5-dimethoxyphenyl the reactivity is three orders of magnitude higher than that of the corresponding cumyl alcohol radical cations; this suggests a mechanism in which a key role is played by the oxygen acidity as well as by the strength of the scissile C-C bond: a radical zwitterion is formed which undergoes a rate-determining C-C bond cleavage, coupled with the intramolecular ET. Finally, oxygen acidity also determines the reactivity of the radical cations of 2-(3,4-dimethoxyphenyl)ethanol and 3-(3,4-dimethoxyphenyl)propanol. In the former the decay involves C-C bond cleavage, in the latter it leads to 3-(3,4-dimethoxyphenyl)propanal. In both cases no products of C-H deprotonation were observed. Possible mechanisms, again involving the initial formation of a radical zwitterion, are discussed.

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The H(2)O(2)-promoted oxidations of (R)-[alpha-(2)H(1)]-and (S)-[alpha-(2)H(1)]-arylalkanols catalysed by chloroperoxidase (CPO) from Caldariomyces fumago have been investigated. It has been found that with (R)-[alpha-(2)H(1)]-alcohols, the oxidation involves almost exclusively the cleavage of the C-H bond, whereas in the case of the oxidation of (S)-[alpha-(2)H(1)]-alcohols, the C-D bond is preferentially broken. These results clearly indicate that the reactions of corresponding undeuterated arylalkanols are characterized by a high prochiral selectivity, involving the cleavage of the pro-S C-H bond. This prochiral selectivity is poorly influenced by the electronic effect of ring substituents, whereas it decreases with the length of the carbon lateral chain, in the order: benzyl alcohol > 2-phenylethanol > 3-phenylpropanol. Molecular binding studies showed that the main factor directing the docking of the substrate in such a specific orientation in the enzyme active site is the interaction between the alcoholic OH group and the residue Glu183. This interaction is likely to drive both the stereochemistry and the regiochemistry of these reactions. A bifurcated hydrogen bond involving the OH group, the carboxylate oxygen of Glu183 and the oxoferryl oxygen might also be operating.

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The reaction of H2O2 with 4-substituted aryl alkyl sulfides (4-XC6H4SR), catalysed by lignin peroxidase (LiP) from Phanerochaete chrysosporium, leads to the formation of sulfoxides, accompanied by diaryl disulfides. The yields of sulfoxide are greater than 95% when X = OMe, but decrease significantly as the electron donating power of the substituent decreases. No reaction is observed for X = CN. The bulkiness of the R group has very little influence on the efficiency of the reaction, except for R = tBu. The reaction exhibits enantioselectivity (up to 62% enantiomeric excess with X = Br, with preferential formation of the sulfoxide with S configuration). Enantioselectivity decreases with increasing electron density of the sulfide. Experiments in H218O show partial or no incorporation of the labelled oxygen into the sulfoxide, with the extent of incorporation decreasing as the ring substituents become more electron-withdrawing. On the basis of these results, it is suggested that LiP compound I (formed by reaction between the native enzyme and H2O2), reacts with the sulfide to form a sulfide radical cation and LiP compound II. The radical cation is then converted to sulfoxide either by reaction with the medium or by a reaction with compound II, the competition between these two pathways depending on the stability of the radical cation.

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The mesolytic cleavage of a beta-C-X bond (ArCR(2)-X(*+) --> ArCR(2)(*/+) + X(+/*)) is one of the most important reactions of alkylaromatic radical cations. In this Account, our group's results concerning some fundamental aspects of this process (cleavage mode, structural and stereoelectronic effects, competitive breaking of different beta-bonds, nucleophilic assistance, possible stereochemistry, carbon vs oxygen acidity in arylalkanol radical cations) are presented and critically discussed for reactions where X = H, CR(3), SR, and SiR(3). Several examples illustrating how this information was exploited as a tool to detect electron-transfer mechanisms in chemical and enzymatic oxidations are also reported.

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1. Mechanistic criteria, based on the side-chain fragmentation reactions of aromatic cation radicals, involving the cleavage of a beta bond (i.e. C-H, C-Si and C-S) have been developed for the detection of electron transfer mechanisms in oxidative processes of alkylbenzenes and aromatic sulphides. 2. For benzylic oxidations, the distinction between electron transfer (ET) and hydrogen atom transfer mechanism (HAT) has been based: (a) on studies of intramolecular selectivity, which, with appropriate substrates (5-Z-1,2,3,-trimethylbenzenes and 4-Z-1,2-dimethylbenzenes, where Z = OMe, alkyl), turns out to be much higher in ET than in HAT processes; and (b) on products studies concerning the reactions of bicumyl and benzyltrimethylsilanes since in these systems, the nature of products can be significantly different for ET and HAT mechanisms. 3. These criteria have been applied to the reactions of alkylbenzenes with an NO3 radical (shown to be an ET process) as well as to the microsomal and biomimetic (by iron porphyrins in the presence of PhIO) side-chain oxidation of the same compounds, where the mechanistic probes have suggested a HAT mechanism, with the exception of the biomimetic oxidation of 4-methoxybenzyltrimethylsilane in CH2Cl2-H2O-MeOH, which probably occurs by an ET mechanism. 4. For the enzymatic and biomimetic oxidation of aromatic sulphides an oxygen transfer is suggested, since, with cumyl phenyl sulphide and 4-methoxybenzyl phenyl sulphide, these reactions lead exclusively to the corresponding sulphoxides and sulphones, whereas the same substrates, in genuine ET reactions, form cation radicals which undergo C-H and C-S bond cleavage. 5. An oxygen transfer mechanism is also likely in the biomimetic and enzymatic oxidations of sulphoxides since in these reactions 4-methoxybenzyl phenyl sulphoxide is exclusively converted to sulphone, whereas in ET reactions it forms only C-S bond cleavage products.

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