Dissociation Of Radical Anions Research Articles

Dissociative Electron Transfer to N‐(Arylthio)phthalimides: Factors Affecting the Formation and Dissociation of Radical Anions

AbstractA series of chemically and biologically relevant N‐(arylthio)phthalimides is synthesized and its electrochemical reduction investigated. The results of the electrochemical study are analysed through application of Savéant's dissociative electron transfer theory and with the assistance of theoretical calculations. Reduction of all investigated N‐(arylthio)phthalimides leads to the corresponding radical anions, followed by the ejection of the phthalimide anion resulting from the dissociation of the N−S chemical bond. This stepwise electron transfer mechanism is evidenced by the electrochemical data which allow determination of the standard reduction potentials of all compounds. In the formation of the N‐(arylthio)phthalimide radical anions, the theoretical calculations show that the incoming electron, is hosted by the phthalimidyl group indicating a homolytic dissociation mechanism. The dissociation rate constants of the radical anions are deduced and show that upon changing the substituent from the MeO, to Me, H, F, Cl and Br the dissociation becomes faster. A good correlation with the Hammett substituent constants is also obtained. Application of Savéant's theory through determination of the associated driving force and the intrinsic barrier energies help rationalize the observed results and show that the change in the dissociation rate constant depends mainly on the standard reduction potential of the parent compounds.

New Insights into the Substituents’ Effect on the Formation and Dissociation of Radical Anions: Dissociative Electron Transfer to Arylsulfonylphthalimides

AbstractThe electrochemical reduction of a series of biologically relevant p‐substituted arylsulfonylphthalimides was investigated and the results were analysed using theoretical calculations and application of Savéant's dissociative electron transfer theory. The dissociation rate constants of the radical anion intermediates were calculated and showed that upon changing the substituent from the MeO, to Me, H, F, Cl and CN the dissociation became faster. A good correlation with the Hammett substituent constants was also obtained. With the NO2 substituent the trend was however inversed. These results and the electrolysis data showed that the dissociation involves the N−S chemical bond. Theoretical calculations showed that for all investigated compounds the extra electron was hosted by the phthalimidyl group except for the p‐NO2 substituted compound for which the extra electron was hosted by the nitrophenyl moiety. Thermodynamic considerations showed that the dissociation of the radical anion of the p‐NO2 compound followed a heterolytic cleavage whereas those of the rest of the investigated compounds followed a homolytic cleavage. The above intriguing result was rationalized through application of the extension of Savéant's dissociative electron transfer theory to the decomposition of radical anions. This study is important not only for understanding substituents’ effect on the biological activity of these compounds but also on the formation and dissociation of radical anions in general.

Electrochemical reduction of N,N'-thiobisphthalimide and N,N'-dithiobisphthalimide: ejection of diatomic sulfur through an autocatalytic mechanism.

The electrochemical reduction of N,N'-dithiobisphthalimide and N,N'-thiobisphthalimide is investigated using electrochemical techniques and theoretical calculations. The results are rationalized using adequate electron transfer theories. The reduction leads to the ejection of diatomic sulfur and involves an interesting autocatalytic mechanism. This mechanism is dependent on the concentration of the initial compound and the cyclic voltammetric scan rate. The starting material is reduced both at the electrode and through homogeneous electron transfer from the produced sulfur. The initial electron transfer follows a stepwise mechanism involving the formation of the corresponding radical anion. This is supported by both the electrochemical data and the theoretical calculation results. The radical anion of the N,N'-dithiobisphthalimide dissociates through cleavage of the N-S chemical bond and not the S-S chemical bond. Application of the extension of the dissociative electron transfer theory to the dissociation of radical anions shows that the N-S chemical bond dissociates despite being stronger than the S-S chemical bond. This is due to the large difference in the oxidation potentials of the two potential anions (the phthalimidyl anion and the phthalimidyl thiyl anion). The electrochemical reduction of N,N'-thiobisphthalimide involves the intermediate formation of N,N'-dithiobisphthalimide and hence the autocatalytic process is less efficient.

The role of the leaving group in the dissociation of radical anions of 9-(aryloxymethyl)anthracenes.

The role of the leaving group in the dissociation of radical anions of 9-(aryloxymethyl)anthracenes.

Radical anions from 5-fluorouracil derivatives. A theoretical study of their cleavage and orbital isomerism

Theoretical calculations were performed on the dissociation of radical anions of 5-fluorouracils substituted at N1. The reaction (1-CR1R2R3-5-fluorouracil)−→(5-fluorouracil)−+R1R2R3C was calculated for several substituents R, including the N1–C5 dimers 1. Two main types of radical anions were observed. Radical anions of π type have the unpaired spin density located at the carbonyl and C–C double bond conjugated system of the uracil ring while the σ species present unpaired spin density mainly at the N1–C(R1R2R3) bond. The electronic nature of the most stable radical anion was found to be determined by the electron affinity of the substituent at N1.The activation barrier for the cleavage of π radical anions arises from its avoided crossing with the σ potential surface. The σ radical anions, which have stabilised SOMOs, dissociate with lower activation barriers than the π intermediates. These observations correlate with the lengthening and increment of unpaired spin density at their N1–C bond. Gas phase calculations are compared with results in aqueous solution. Other fragmentation paths are proposed for substituents like RNO2 or halogen.

Quantum Chemical Investigation of the Electric Field Effect on Activation Barriers for Electrochemical Transformations of Organic Compounds

The MINDO/3 method was used to study the effect of intensity and vector direction of the external electric field on activation barriers of unimolecular reactions which are possible in electrochemical transformations of simple organic compounds : orientation of molecules and radical anions, heterogeneous electron transfer to adsorbed molecules, dissociation of radical anions, cations, and molecules, inversion of anions, free radicals, and neutral molecules, cis-trans isomerization, and chair-boat conformational transition in molecules. The electronic and spatial structure of the molecules, free radicals, and ions, the height of the activation barriers of their reorientation, dissociation, isomerization, and conformational transition, and the configurational stability to change as affected by the field in the range 0.01-0.05 au are determined. Obtained data permit one to theoretically ground different effects under organic compound adsorption on the electrode surface, changes in stereochemistry of the products of alkyl halide electroreduction, and some properties of chemically modified electrodes possibly affected by electrode potential gradient.

Solvent Effects on the Unimolecular Dissociation of Radical Anions Studied by Pulse Radiolysis

Abstract The rate constants of the unimolecular dissociation of radical anions formed by one-electron reduction of chloroacetophenone isomers in dipolar aprotic solvents were measured by a pulse radiolysis technique. The reactivities of these isomers were affected by the polarity of the solvent used.

Some Studies on the Reversibility of the Association Step in SRN 1 Reactions

The association step in SRN1 reactions, in which p- nitrobenzylic radicals are trapped by acinitronate ions or the anions from β- keto esters, is demonstrated to be effectively irreversible under the normal conditions of the SRN1 reaction. The dissociation [i.e. fragmentation of the C(α)-C(β) bond] of C- alkylates [e.g. (2) or (3)] formed from aci - nitronates and p- nitrobenzylic substrates appears to take place in a process involving transfer of two electrons and not through dissociation of radical anions.

Nucleophilic substitution reactions in m-nitrobenzylic substrates

The reaction of m-nitrobenzyl 2,4,6-trimethylbenzoate (11) with the salt (2) of 2-nitropropane, or the thiolate (16), gives moderate yields of substitution products [e.g.(3) or (17)]. However, the mechanism of formation of these products could not be definitively assigned. The m-nitrobenzyl chloride (12), on the other hand, was clearly demonstrated to undergo SRN1 substitution reactions with a variety of anions including the anion from the salt (2), thiolate ion from (16), azide from (22), the malononitrile anion from (20), and the malonic ester anion from (21). Inhibition and entrainment effects clearly rule out an alternative radical anion-radical process involving cage collapse. The occurrence of SRNl reactions in m-nitrobenzylic substrates adds support to a mechanism involving association of radicals with anions, and for dissociation of radical anions, which involves σ* and π* radical anions.