Side-Chain Fragmentation of Arylalkanol Radical Cations. Carbon−Carbon and Carbon−Hydrogen Bond Cleavage and the Role of α- and β-OH Groups

Baciocchi, Enrico; Bietti, Massimo; Putignani, Lorenza; Steenken, S.

doi:10.1021/ja954236s

Cited by 70 publications

(78 citation statements)

References 52 publications

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“…[9] Then SO 4 .À reacts with the aromatic compounds [10] to form the corresponding radical cations [Eq (5); k 5 Â 10 9 m À1 s À1 for anisole and derivatives [7,11] ]. For reactions carried out at pH 4, Tl 2 , produced by irradiating N 2 O-saturated aqueous solutions of Tl [Eqs.…”

Section: Resultsmentioning

confidence: 99%

“…, centered at 290 and 440 nm, [7,11] are clearly visible. These absorptions reach a maximum 5 ms after the pulse [completion of radical cation formation, Eq.…”

mentioning

confidence: 95%

“…[6] Clear evidence in this respect comes from our previous study on the fragmentation reactions of 1-and 2-arylalkanol radical cations. [7] The important role of the a-OH group was evident in the significantly higher rate of potassium 12-tungstocobalt(iii)ate (Co(iii)W) induced oxidation of 4-MeOC 6 H 4 CH(OH)tBu relative to its methyl ether. On the basis of a pulse radiolysis study of the reactivity of the 4-MeOC 6 H 4 CH(OH)CH 2 Ph radical cation, it was suggested that the favorable effect of the a-OH group is mainly due to the stabilization by hydrogen bonding of the transition state that leads to CÀC bond cleavage.…”

Section: Introductionmentioning

confidence: 99%

“…In all cases they exhibited the characteristic UV/Vis absorption bands, centered around 290 and 440 ± 450 nm, of anisole-type radical cations. [7,11] The rates of decay of the radical cations were determined spectrophotometrically by measuring the decrease in optical density at 440 ± 450 nm or by monitoring the production of H by the ac-conductance technique. In acid media (pH 4), the latter technique was generally preferred, and radical cations were more often generated by method 2 than by method 1.…”

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confidence: 99%

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Kinetic and Product Studies on the Side-Chain Fragmentation of 1-Arylalkanol Radical Cations in Aqueous Solution: Oxygen versus Carbon Acidity

Baciocchi¹,

Bietti²,

Steenken³

1999

Chem. Eur. J.

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A kinetic and product study of the side-chain fragmentation reactions of a series of 1-arylalkanol radical cations (4-MeOC 6 H 4 CH(OH)R . ) and some of their methyl ethers was carried out; the radical cations were generated by pulse radiolysis and g radiolysis in aqueous solution. The radical cations undergo side-chain fragmentation involving the C a À H and/or C a À C b bonds, and their reactivity was studied both in acidic (pH 4) and basic (pH 10 ± 11) solution. At pH 4, the radical cations decay with first-order kinetics, and the exclusive reaction is C a ÀH deprotonation for ) very close to the limit of diffusion control and independent of the nature of the bond that is finally broken in the process (CÀH or CÀC). The methyl ether 8. , which exclusively undergoes CÀH bond cleavage, reacts significantly slower (by a factor of ca. 50) than the corresponding alcohol 1 . . These data indicate that 1-arylalkanol radical cations, which display the expected carbon acidity in water, become oxygen acids in the presence of a strong base such as HO À and undergo deprotonation of the OÀH group; diffusion-controlled formation of the encounter complex between HO À and the radical cation is the rate-determining step of the reaction. It is suggested that, within the complex, the proton is transferred to the base to give a benzyloxyl radical, either via a radical zwitterion (which undergoes intramolecular electron transfer) or directly (electron transfer coupled with deprotonation). The latter possibility seems more in line with the general base catalysis (b % 0.4) observed in the reaction of 5 . , which certainly involves OÀH deprotonation. The benzyloxyl radical can then undergo a b CÀC bond cleavage to form 4-methoxybenzaldehyde and R . or a formal 1,2-H shift to form an a-hydroxybenzyl-type radical. The factors of importance in this carbon/ oxygen acidity dichotomy are discussed.

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Section: Resultsmentioning

confidence: 99%

“…, centered at 290 and 440 nm, [7,11] are clearly visible. These absorptions reach a maximum 5 ms after the pulse [completion of radical cation formation, Eq.…”

mentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Kinetic and Product Studies on the Side-Chain Fragmentation of 1-Arylalkanol Radical Cations in Aqueous Solution: Oxygen versus Carbon Acidity

Baciocchi¹,

Bietti²,

Steenken³

1999

Chem. Eur. J.

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“…43 This proves that the first step of the photoinduced dynamics is an electron transfer (ET) from one of the surrounding MBA molecules to 1 RFTA* yielding the radical anion RFTA À and the radical cation MBA + . The weak signature of the radical cation expected around 440 nm [44][45][46] is not observed spectroscopically as it is masked by the much stronger ground state bleach of flavin. The absence of any residual signals at long delay times indicates a full charge recombination by back electron transfer from RFTA À to MBA + .…”

Section: Femtosecond Dynamics Of Riboflavin Tetraacetate In Pure Methmentioning

confidence: 96%

Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from sub-pico- to microseconds

Megerle

Wenninger

Kutta

et al. 2011

Phys. Chem. Chem. Phys.

102

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Flavin-mediated photooxidations have been described for applications in synthetic organic chemistry for some time and are claimed to be a route to the use of solar energy. We present a detailed investigation of the involved photophysical and photochemical steps in methoxybenzyl alcohol oxidation on a timescale ranging from sub-picoseconds to tens of microseconds. The results establish the flavin triplet state as the key intermediate for the photooxidation. The initial step is an electron transfer from the alcohol to the triplet state of the flavin catalyst with, followed by a proton transfer in B6 ms. In contrast, the electron transfer involving the singlet state of flavin is a loss channel. It is followed by rapid charge recombination (t = 50 ps) without significant product formation as seen when flavin is dissolved in pure benzylic alcohol. In dilute acetonitrile/water solutions of flavin and alcohol the electron transfer is mostly controlled by diffusion, though at high substrate concentrations >100 mM we also find a considerable contribution from preassociated flavin-alcohol-aggregates. The model including a productive triplet channel and a competing singlet loss channel is confirmed by the course of the photooxidation quantum yield as a function of substrate concentration: We find a maximum quantum yield of 3% at 25 mM of benzylic alcohol and significantly smaller values for both higher and lower alcohol concentrations. The observations indicate the importance to perform flavin photooxidations at optimized substrate concentrations to achieve high quantum efficiencies and provide directions for the design of flavin photocatalysts with improved performance.

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One Electron Oxidation of α-Alkylbenzyl Alcohols Induced by Potassium 12-Tungstocobalt(III)ate − Comparison with the Oxidation Promoted by Microsomal Cytochrome P450

Baciocchi¹,

Belvedere

Bietti

et al. 1998

Eur. J. Org. Chem.

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The chemical and microsomal oxidations of a number of 4‐methoxy substituted α‐alkylbenzyl alcohols 4‐MeOPhCH(R)OH (Eox = 1.6−1.7 V vs SCE) were investigated. Using potassium 12‐tungstocobalt(III)ate, a bona fide one electron oxidant, competition between Cα−H and Cα−Cβ bond cleavage in the intermediate radical cation was observed when the side‐chain alkyl group R was Et (2) and iPr (3). With R = Me (1), only C−H bond cleavage took place, whereas with R = tBu (4) C−C bond cleavage was the exclusive fragmentation process. In contrast, the microsomal oxidation of the two substrates 3 and 4 led in both cases to the exclusive formation of the corresponding ketone. Thus, an electron transfer mechanism appears unlikely for the microsomal oxidation of α‐alkylbenzyl alcohols, even though the oxidation potential of these species is lower than or comparable to that of the active oxidant in the enzyme. A hydrogen atom transfer mechanism is more in line with these results.

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Side-Chain Fragmentation of Arylalkanol Radical Cations. Carbon−Carbon and Carbon−Hydrogen Bond Cleavage and the Role of α- and β-OH Groups

Cited by 70 publications

References 52 publications

Kinetic and Product Studies on the Side-Chain Fragmentation of 1-Arylalkanol Radical Cations in Aqueous Solution: Oxygen versus Carbon Acidity

Kinetic and Product Studies on the Side-Chain Fragmentation of 1-Arylalkanol Radical Cations in Aqueous Solution: Oxygen versus Carbon Acidity

Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from sub-pico- to microseconds

One Electron Oxidation of α-Alkylbenzyl Alcohols Induced by Potassium 12-Tungstocobalt(III)ate − Comparison with the Oxidation Promoted by Microsomal Cytochrome P450

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