Role of Electron-Transfer Processes in Reactions of Diarylcarbenium Ions and Related Quinone Methides with Nucleophiles

Ofial, Armin R.; Ohkubo, Kei; Fukuzumi, Shunichi; Lucius, Roland; Mayr, Herbert

doi:10.1021/ja035191v

Cited by 47 publications

(44 citation statements)

References 35 publications

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“…The electrophilicity of a number of noncharged activated olefins consisting of the three arylidenemalonitriles 11 a – c and of various quinone methides (see Figure 5) have been determined 13f. 26, 27 As revealed by Figure 5, the reactivity of all these compounds is considerably lower than that of NBDF. With an E value of −9.42, the most reactive arylidenemalonitrile studied, namely 11 a , is by more than three orders of magnitude less electrophilic than NBDF.…”

Section: Resultsmentioning

confidence: 99%

Superelectrophilicity of the Nitroolefinic Fragment of 4‐Nitrobenzodifuroxan in Michael‐Type Reactions with Indoles: A Kinetic Study in Acetonitrile

Lakhdar

Goumont

Berionni

et al. 2007

Chemistry A European J

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The kinetics of the coupling of 4-nitrobenzodifuroxan (NBDF) with a series of indoles 8 a-e to give the expected Michael-type adducts 9 a-e have been investigated in acetonitrile solution. No significant influence of the nature of the isotopic substitution at C-3 of the indole ring has been found, indicating that the NBDF addition step is the rate limiting step of the SEAr substitution of the indole moiety. This implies that the measured second-order rate constants (k) for the reactions are identical to the second order rate constants (k1NBDF) associated to the C--C coupling step. By using the known N and s parameters characterizing the nucleophilicity of indoles, the k1NBDF rate constants are found to fit nicely to the three parameters equation logk1=s(N+E) introduced by Mayr to describe the feasibility of nucleophilic-electrophilic combinations. Based on this, the electrophilicity parameter E of NBDF could be determined as E=-6.15. This corresponds to a positioning of the reactivity of the nitroactivated double bond of NBDF in the domain of superelectrophilicity previously defined for nitrobenzofuroxans, in accord with the finding that the rates of coupling of 8 a-e with NBDF are only one order of magnitude lower than those for the coupling of these indoles with 4,6-dinitrobenzofuroxan (DNBF). The theoretical scale of electrophilicity introduced by Domingo et al. on the basis of the global electrophilicity index omega defined by Parr is also a very useful tool to discuss the relative reactivities of NBDF, DNBF, and a number of differently activated C==C double bonds.

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Section: Resultsmentioning

confidence: 99%

Superelectrophilicity of the Nitroolefinic Fragment of 4‐Nitrobenzodifuroxan in Michael‐Type Reactions with Indoles: A Kinetic Study in Acetonitrile

Lakhdar

Goumont

Berionni

et al. 2007

Chemistry A European J

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“…[17b] Recently, we have examined combination reactions of our reference electrophiles 2 with a large variety of nucleophiles, and have found that outer-sphere electron transfer processes do not occur in any of the reactions so far investigated. [18] To examine whether electron transfer takes place in the reactions examined in this work, the actually observed activation free energies, ∆G ‡ obsd. , were compared with the free energies of electron transfer, ∆G°E T .…”

Section: Resultsmentioning

confidence: 99%

“…[18] To examine whether electron transfer takes place in the reactions examined in this work, the actually observed activation free energies, ∆G ‡ obsd. , were compared with the free energies of electron transfer, ∆G°E T .…”

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confidence: 99%

Nucleophilic Reactivities of Ketene Acetals

Tokuyasu

Mayr

2004

Eur J Org Chem

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Keywords: Carbocations / CϪC coupling / Kinetics / Linear free energy relationship / Substituent effectsThe kinetics of the reactions of ketene acetals with benzhydrylium ions have been followed photometrically. The reactions proceed with rate-determining C−C bond formation that is considerably faster than expected for outer-sphere electron transfer processes. Structure−reactivity relationships are discussed. The excellent correlations between the observed second-order rate constants (log k 2 ) and the previ-

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“…Ofial et al [62] have demonstrated that the oxidation potentials of benzhydryl radicals and hence the driving force DG ET for electron transfer depend on the aryl substituents, but experimental data for the m-fluoro compounds are not available.…”

Section: Influence Of Substitution and Solvent Polarity On The Intermmentioning

confidence: 99%

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

et al. 2013

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Bond cleavage and bond formation are central to organic chemistry. Carbocations play a key role in our understanding of nucleophilic substitution reactions that involve both processes. The precise understanding of the mechanism and dynamics of the photogeneration of carbocations and carbon radicals is therefore an important quest. In particular, the role of electron transfer for the generation of carbocations from the radical pair is still unclear. A quantitative femtosecond absorption study is presented, with ultrabroad probing on selected donor and acceptor substituted benzhydryl chlorides irradiated with 270 nm (35 fs) pulses. The ultrafast bond cleavage within 300 fs is almost exclusively homolytic, thus leading to a radical pair. The carbocations observable in the nanosecond regime are generated from these radicals by electron transfer from the benzhydryl to the chlorine radical within the first tens of picoseconds. Their concentration is reduced by geminate recombination within hundreds of picoseconds. In moderately polar solvents this depletion almost extinguishes the cation population; in highly polar solvents free ions are still observable on the nanosecond timescale. The explanation of the experimental findings requires the microscopic realm of the intermediates to be accounted for, including their spatial and environmental distributions. The distance dependent electron transfer described by Marcus theory is combined with Smoluchowski diffusion. The depletion of the radical pair distribution at small distances causes a temporal increase of the mean distance and the observed stretched exponential electron transfer. A close accord with experiment can only be reached for a broad distribution of the nascent radical pairs. The increase in the inter-radical and inter-ion pair distance is measured directly as a shift of the UV/Vis absorption of the products. The results demonstrate that, at least for aprotic solvents, traditional descriptions of reaction mechanisms based on the concept of contact and solvent-separated pairs have to be reassessed.

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Role of Electron-Transfer Processes in Reactions of Diarylcarbenium Ions and Related Quinone Methides with Nucleophiles

Cited by 47 publications

References 35 publications

Superelectrophilicity of the Nitroolefinic Fragment of 4‐Nitrobenzodifuroxan in Michael‐Type Reactions with Indoles: A Kinetic Study in Acetonitrile

Superelectrophilicity of the Nitroolefinic Fragment of 4‐Nitrobenzodifuroxan in Michael‐Type Reactions with Indoles: A Kinetic Study in Acetonitrile

Nucleophilic Reactivities of Ketene Acetals

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

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