Electron Transfer in Radicals

Bietti, Massimo; Steenken, S.

doi:10.1002/9783527618248.ch23

Cited by 4 publications

(3 citation statements)

References 247 publications

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“…However, both hypotheses are unlikely. An increase in the rate of β ‐cleavage of the intermediate benzyloxyl radical as we move from 4 .+ to 11 .+ and 16 .+ can be excluded since the rates of β ‐cleavage of some ring‐substituted cumyloxyl radicals have been found to be practically unaffected by the nature and number of substituents 30. Likewise, a significantly lower rate of OH deprotonation with 11 .+ and 16 .+ than with 4 .+ is made unlikely by the results obtained with the α ‐ tert ‐butyl derivatives 8 .+ and 13 .+ which were found to react with OH − at rates three orders of magnitude higher than those measured for 11 .+ and 16 .+ , respectively (Table 2).…”

Section: Discussionmentioning

confidence: 99%

Structural Effects on the OH−-Promoted Fragmentation of Methoxy-Substituted 1-Arylalkanol Radical Cations in Aqueous Solution: The Role of Oxygen Acidity

Baciocchi¹,

Bietti²,

Gerini³

et al. 2001

Chem. Eur. J.

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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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Section: Discussionmentioning

confidence: 99%

Structural Effects on the OH−-Promoted Fragmentation of Methoxy-Substituted 1-Arylalkanol Radical Cations in Aqueous Solution: The Role of Oxygen Acidity

Baciocchi¹,

Bietti²,

Gerini³

et al. 2001

Chem. Eur. J.

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“…The key step of the photocatalytic oxygenation reactions is the deprotonation of substrate radical cations in relatively nonpolar solvents such as chloroform (CHCl 3 ) to produce deprotonated radicals that can readily react with molecular oxygen . Although the deprotonation dynamics of substituted toluenes has been examined in detail in an aqueous solution, , the deprotonation dynamics in aprotic or nonpolar solvents has yet to be reported.…”

Section: Introductionmentioning

confidence: 99%

Selective Oxygenation of 4,4‘-Dimethylbiphenyl with Molecular Oxygen, Catalyzed by 9-Phenyl-10-methylacridinium Ion via Photoinduced Electron Transfer

2005

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Photooxygenation of 4,4'-dimethybiphenyl with oxygen occurs efficiently in the presence of 9-phenyl-10-methylacridinium perchlorate (AcrPh(+)ClO(4)(-)) under visible light irradiation in O(2)-saturated chloroform (CHCl(3)) to yield 4-(4'-methylphenyl)benzaldehyde as a main oxygenated product. Prolonged photoirradiation afforded the further oxygenated product, 4,4'-diformylbiphenyl. The reactive radical intermediates involved in the photocatalytic cycle have successfully been detected by laser flash photolysis and electron spin resonance (ESR) measurements. The photocatalytic mechanism for the oxygenation of 4,4'-dimethybiphenyl via photoinduced electron transfer from 4,4'-dimethybiphenyl to the singlet excited state of AcrPh(+) is clarified based on the dependence of quantum yields on concentrations of substrates and the detected radical intermediates.

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“…1 Pulse radiolysis and laser flash photolysis techniques were usually employed to produce and characterize such intermediates 2 that display typical unimolecular decay reactions in polar solvents, due to side chain deprotonation or C–X bond cleavage. 3 Furthermore, the addition of nucleophilic media may yield a substituted cyclohexadienyl radical causing a decrease in the radical cation lifetime. 4…”

Section: Introductionmentioning

confidence: 99%

Photogenerated aryl mesylate and aryl diethyl phosphate radical cations: a time-resolved spectroscopy investigation

et al. 2022

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Electron Transfer in Radicals

Cited by 4 publications

References 247 publications

Structural Effects on the OH−-Promoted Fragmentation of Methoxy-Substituted 1-Arylalkanol Radical Cations in Aqueous Solution: The Role of Oxygen Acidity

Structural Effects on the OH−-Promoted Fragmentation of Methoxy-Substituted 1-Arylalkanol Radical Cations in Aqueous Solution: The Role of Oxygen Acidity

Selective Oxygenation of 4,4‘-Dimethylbiphenyl with Molecular Oxygen, Catalyzed by 9-Phenyl-10-methylacridinium Ion via Photoinduced Electron Transfer

Photogenerated aryl mesylate and aryl diethyl phosphate radical cations: a time-resolved spectroscopy investigation

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