Characterization of a ‘hypersensitive’ probe for single electron transfer to carbonyl compounds

Tanko, James M.; Brammer, Larry E.; Hervás, Manuel; Campos, Kevin R.

doi:10.1039/p29940001407

Cited by 17 publications

(20 citation statements)

References 23 publications

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“…The unimolecular cleavage of radical anions has been suggested as suitable clock-reactions to distinguish hydride transfer from electron transfer/radical pathways in the reduction of carbonyls with different hydride donors. − The idea follows from the well established free radical clocks that led to the development of a number of useful chemical probes to report the existence of short lived free radical intermediates. Free radical clocks are now available that cover a range in radical lifetime from milliseconds down to subpicoseconds …”

Section: Introductionmentioning

confidence: 99%

Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions¹^a

1997

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The cleavage of radical anions of substituted α-phenoxyacetophenones, X-C6H4COCH2OPh, IIa−k, has been studied in DMF by voltammetric and coulometric techniques. The standard potentials (E°) for formation of and rate constants, k, for the cleavage of the radical anions were determined using linear sweep voltammetry, LSV, together with digital simulation and previously reported laser flash photolysis data. The rate constants cover a range of almost eight orders of magnitude (0.4 s-1 for X = p-MeCO- to 1.3·107 s-1 for X = p-MeO−). The relative driving forces, ΔΔG°het(RX•-), for the heterolytic cleavage of the radical anions (to give R• + X-) were estimated from thermochemical cycles. A combined plot of log(k) versus ΔΔG°het(RX•-) for the radical anions of IIa − k and of α-aryloxyacetophenones gave a curve with α = 0.5 at high driving forces and α = 1 at low driving forces, where α = ∂Δ /∂ΔG°. The plot was analyzed using a model in which reversible cleavage of the radical anions takes place inside the solvent cage followed by (counter)diffusion of the fragments out of the solvent cage. The change in the value of α is interpreted as a change in the rate limiting process from chemical activation (i.e.,, fragmentation) to counterdiffusion. The model allowed the determination of the absolute values of ΔG°het(RX•-) and the intrinsic barrier, Δ , for the fragmentation of the radical anions (8 ± 1 kcal mol-1, 0.35 eV). This leads to an estimate of the homolytic bond dissociation free energy of the C−OPh bond in the unsubstituted α-phenoxyacetophenone.

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Section: Introductionmentioning

confidence: 99%

Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions¹^a

1997

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“…In 1994, we suggested that spiro[2.5]octa-3,6-dien-5-one 1 may be an excellent probe for the detection of SET in reactions of nucleophiles with carbonyl compounds …”

Section: Introductionmentioning

confidence: 99%

Radical Ion Probes. 7. Behavior of a “Hypersensitive” Probe for Single Electron Transfer in Reactions Not Involving Electron Transfer

Tanko

Brammer

1997

J. Org. Chem.

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1-Methyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (8) and 1,1-dimethyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1) react with thiophenoxide ion to produce cyclopropane ring-opened products. Thermodynamic considerations effectively rule out any possibility that single electron transfer is involved in theses reactions; the process PhS- + substrate → PhS• + substrate•- is endothermic by over 50 kcal/mol! Nucleophilic attack occurs both at the least- and most-hindered carbons of the cyclopropyl group, and the product ratio (R(1°/2°) from 8 and R(1°/3°) from 1, where 1°, 2°, and 3° refer to the regioisomeric phenyl sulfides formed from these substrates) is found to vary with solvent. In dipolar, aprotic solvents, nucleophilic attack occurs preferentially at the least-hindered carbon of the cyclopropyl group (R(1°/2°) and R(1°/3°) ≈ 4−5), consistent with an SN2 mechanism. In protic solvents, products arising from nucleophilic attack at the more-substituted carbon of the cyclopropyl group become increasingly important, consistent with the onset of a carbocationic (SN2(C+)) pathway. The strengths and weaknesses of 1 and 8 as probes for single electron transfer are discussed in the context of these results.

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“…Since the electron transfer steps in all of these cases were known to be endergonic processes (i.e. the reverse reaction will be much faster than the forward), there have been a number of attempts to “calibrate” the mechanistic probes, by determining the rate constant for fragmentation of the radical anions. − The ideal mechanistic probe is one in which the rate of oxidation of the radical anion (i.e. back electron transfer) cannot compete with fragmentation.…”

Section: Introductionmentioning

confidence: 99%

Electrochemistry of Electron-Transfer Probes. The Role of the Leaving Group in the Cleavage of Radical Anions of α-Aryloxyacetophenones¹

1996

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The formal reduction potential (E°) of α-phenoxyacetophenone has been determined from the voltammetric peak potential obtained by linear sweep voltammetry in combination with the rate constant for fragmentation of the radical anion which had been determined by laser flash photolysis. The E° values of a number of α-aryloxyacetophenones were then estimated from a correlation of the 13C chemical shifts of the carbonyl carbon and a similar correlation (E° versus 13C chemical shift) within a series of substituted α-anilinoacetophenones. Using these potentials (which vary by only 34 mV over a wide range of substituents) the rate constants for fragmentation of the α-aryloxyacetophenone radical anions were determined from digital simulation of the corresponding voltammetric waves. The fragmentation rate constants were shown to correlate with the pK a of the corresponding phenols. However, the kinetic range was too small and the experimental errors too large to allow a distinction between a linear and quadratic free energy dependence. A thermochemical analogy between the leaving group ability in reactions of radical anions and that in simple heterolysis of closed shell compounds is developed. The utility of these compounds as potential electron transfer probes is discussed.

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Characterization of a ‘hypersensitive’ probe for single electron transfer to carbonyl compounds

Cited by 17 publications

References 23 publications

Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions¹^a

Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions¹^a

Radical Ion Probes. 7. Behavior of a “Hypersensitive” Probe for Single Electron Transfer in Reactions Not Involving Electron Transfer

Electrochemistry of Electron-Transfer Probes. The Role of the Leaving Group in the Cleavage of Radical Anions of α-Aryloxyacetophenones¹

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Characterization of a ‘hypersensitive’ probe for single electron transfer to carbonyl compounds

Cited by 17 publications

References 23 publications

Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions1a

Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions1a

Radical Ion Probes. 7. Behavior of a “Hypersensitive” Probe for Single Electron Transfer in Reactions Not Involving Electron Transfer

Electrochemistry of Electron-Transfer Probes. The Role of the Leaving Group in the Cleavage of Radical Anions of α-Aryloxyacetophenones1

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Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions¹^a

Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions¹^a

Electrochemistry of Electron-Transfer Probes. The Role of the Leaving Group in the Cleavage of Radical Anions of α-Aryloxyacetophenones¹