Concerted and Stepwise Dissociative Electron Transfers. Oxidability of the Leaving Group and Strength of the Breaking Bond as Mechanism and Reactivity Governing Factors Illustrated by the Electrochemical Reduction of α-Substituted Acetophenones

Andrieux, C.P.; Savéant, Jean‐Michel; Tallec, A.; Tardivel, R.; Tardy, Caroline

doi:10.1021/ja963674b

Cited by 115 publications

(96 citation statements)

References 36 publications

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“…For both processes, the values corresponding to the oxidation of the leaving group, E°N u · /Nu -, have previously been reported. [16] However, no E°values have been measured for the σ H complexes; oxidation peak potential values are used as E°values. Fortunately, this will correspond to a systematic and unaffected error as these values would not be meaningfully different in these cases.…”

Section: Resultsmentioning

confidence: 99%

Thermodynamic Study of σ^H Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals

Gallardo

Guirado

2008

Eur J Org Chem

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The mechanism for the electrochemical oxidation of σH complexes, such as 1‐hydro‐1‐alkoxy/sulfoxy or ‐fluoro‐2,4‐dinitro/2,4,6‐trinitrocyclohexadienyl anions, has been widely studied by means of cyclic voltammetry and controlled‐potential electrolysis. Previous studies have shown that the electrochemical oxidation of σH complexes, formed by the addition of carbon or nitrogen nucleophiles followed by a two electron mechanism, corresponding to the formal elimination of the hydride anion (nucleophilic aromatic substitution of hydrogen mechanism, the NASH mechanism). For these σH complexes (Nu– = OH–, –OR, –SR, –F), the electrochemical reaction takes place by a one‐electron mechanism and is followed by the radical elimination of the leaving group with the consequent recovery of the starting material. This mechanism is similar to that proposed for the electrochemical oxidation of σX complexes (nucleophilic aromatic substitution of a heteroatom, the NASX mechanism). The operating mechanism in each case, the NASH or NASX, can be rationalized in terms of thermodynamics. The standard potentials of the σ complex and/or the leaving group as well as the bond dissociation energies (BDEs) are determinant factors. This study has not led to a significant improvement in the electrochemical preparation of aromatic‐substituted compounds, but does help to understand and predict the usefulness or uselessness of using the nucleophilic aromatic substitution route to obtain a desired product. Finally, the current approach extends the electrochemical methodology to different chemical fields, for example, to general nondestructive methods for the detection, identification, and quantification of either organic pollutants or explosives in different solvents. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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Section: Resultsmentioning

confidence: 99%

Thermodynamic Study of σ^H Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals

Gallardo

Guirado

2008

Eur J Org Chem

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“…10 In the case of unsubstituted benzyl halides, the coordination of Ti III to the halogen atom (Cl, Br) is accompanied by the concerted electron transfer to the σ* orbital of the C-Hal bond and cleavage of the latter. 10, 19 In the present work, we compared the processes of direct and electrocatalytic (in the presence of Cp 2 TiCl 2 ) reduction of 1-3 containing the C(sp 3 )-Br and C(sp 2 )-Br bonds activated by the adjacent carbonyl group. These reactions are classified as processes with DET.…”

Section: Resultsmentioning

confidence: 99%

“…This indi cates that halogen is eliminated via the concerted mecha nism, which agrees with published data. 19 The electrochemical reduction of bromocyclohexa dienes has not been studied earlier. In the case of 2,6 dibromo 4,4 dimethylcyclohexa 2,5 dien 1 one (2), the former two peaks (E 1 = -1.48 V, E 2 = -1.72 V) are irreversible and correspond to the consecutive elimina tion of the bromine atoms, and the most negative peak (E 3 = -2.04 V) is reversible to the reduction of 4,4 di methylcyclohexadienone (Fig.…”

Section: Methodsmentioning

confidence: 99%

Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2

2007

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Processes of direct and electrocatalytic (in the presence of electrochemically reduced Cp 2 TiCl 2 ) reduction of three α bromoketones containing the C(sp 3 )-Br or C(sp 2 )-Br bond, viz., 2 bromo and 2,6 dibromo 4,4 dimethylcyclohexa 2,5 dien 1 ones and α bromo acetophenone, were studied by cyclic voltammetry and preparative electrolysis. In all cases, the dissociative electron transfer proceeds via the concerted mechanism. Preparative electrolysis of these α bromoketones in the presence of Cp 2 TiCl 2 affords the reductive debromination prod ucts in 40-80% yield at low cathodic potentials (-0.85 V vs. Ag/AgCl/KCl). In the case of 2,6 dibromo 4,4 dimethylcyclohexa 2,5 dien 1 one in the potentiostatic regime, only one bromine atom can be eliminated selectively.In most cases, the direct electrochemical reduction of alkyl and vinyl halides occurs at rather high negative po tentials 1 and, therefore, the dehalogenation of compli cated polyfunctional compounds is usually nonselective. The use of non direct electrocatalytic reduction (media tor redox catalysis) makes it possible to decrease consid erably the potential of the reaction, which can be substan tial for selectivity of the process.It is known 2 that in the nature many dehalogenation processes proceed via the reduction mechanism involving the catalysts containing the compounds of Co, 3,4 Ni, 5-7 Cu, 8 and other metals. We have previously shown 9,10 that the electrochemically reduced titanocene dichloride (Cp 2 TiCl 2 ) can serve as a catalyst for the reductive dehalogenation of benzylic halides, whereas aryl halides usually undergo no dehalogenation. The present work con tinues these studies and is devoted to the elucidation of the applicability of this approach to cleavage of the C-Hal bond of other types. α Bromoketones containing the C(sp 3 )-Br or C(sp 2 )-Br bond, which is activated by the adjacent carbonyl group, namely, α bromoaceto phenone (1), 2,6 dibromo (2) and 2 bromo 4,4 di methylcyclohexa 2,5 dien 1 one (3), were chosen as ob jects for the study. The latter two compounds are of spe cial interest, because they represent the structural frag ment of discorhabdine molecules C and E (4 and 5, re spectively), which are present in the Lafrunculia sea sponge. It is known 11 that the antimicrobial properties of natural discorhabdines are due to the ability of the C-Hal bond to cleave. This cleavage proceeds presumably via the reduction mechanism. Therefore, it was interesting to reveal whether the electrocatalytic dehalogenation of simi lar compounds is possible in the presence of the electro chemically reduced titanocene dichloride and whether this process is a model of reactions involving discorhabdines in the nature or not. ExperimentalCyclic voltammograms (CV curves) were recorded using an IPC Win potentiostat at the stationary Pt electrode (d = 5 mm) against Bu 4 NBF 4 in anhydrous organic solvents (THF, MeCN) at 20 °С in a special electrochemical cell switched on according

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“…The established mechanism is relevant for the cleavage of carbon-halogen bond and has been successfully applied also for the other categories of compounds (e.g. (Andrieux et al, 1996(Andrieux et al, , 1997Jakobsen et al, 1999)). …”

Section: Alkyl Iodides Electroreduction and  Variabilitymentioning

confidence: 99%

Mathematical Modeling of Electrode Processes – Potential Dependent Transfer Coefficient in Electrochemical Kinetics

Sanecki¹,

Skitał²

2012

Recent Trend in Electrochemical Science and Technology

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Concerted and Stepwise Dissociative Electron Transfers. Oxidability of the Leaving Group and Strength of the Breaking Bond as Mechanism and Reactivity Governing Factors Illustrated by the Electrochemical Reduction of α-Substituted Acetophenones

Cited by 115 publications

References 36 publications

Thermodynamic Study of σ^H Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals

Thermodynamic Study of σ^H Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals

Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2

Mathematical Modeling of Electrode Processes – Potential Dependent Transfer Coefficient in Electrochemical Kinetics

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Concerted and Stepwise Dissociative Electron Transfers. Oxidability of the Leaving Group and Strength of the Breaking Bond as Mechanism and Reactivity Governing Factors Illustrated by the Electrochemical Reduction of α-Substituted Acetophenones

Cited by 115 publications

References 36 publications

Thermodynamic Study of σH Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals

Thermodynamic Study of σH Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals

Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2

Mathematical Modeling of Electrode Processes – Potential Dependent Transfer Coefficient in Electrochemical Kinetics

Contact Info

Product

Resources

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Thermodynamic Study of σ^H Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals

Thermodynamic Study of σ^H Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated Radicals