Insights into the Free-Energy Dependence of Intramolecular Dissociative Electron Transfers

Abstract

To study the relationship between rate and driving force of intramolecular dissociative electron transfers, a series of donor-spacer-acceptor (D-Sp-A) systems has been devised and synthesized. cis-1,4-Cyclohexanedyil and a perester functional group were kept constant as the spacer and acceptor, respectively. By changing the aryl substituents of the phthalimide moiety, which served as the donor, the driving force could be varied by 0.74 eV. X-ray diffraction crystallography and ab initio conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido)-axial(perester) conformation and the same D/A orientation. The intramolecular dissociative electron-transfer process was studied by electrochemical means in N,N-dimethylformamide, in comparison with thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular process consists of the electron transfer from the electrochemically generated phthalimide-moiety radical anion to the peroxide functional group. The electrochemical analysis provided clear evidence of a concerted dissociative electron-transfer mechanism, leading to the cleavage of the O-O bond. Support for this mechanism was obtained by ab initio MO calculations, which provided information about the LUMO of the acceptor and the SOMO of the donor. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter data being obtained by using the model molecules. As long as the effective location of the centroid of the donor SOMO does not vary significantly by changing the aryl substituent(s), the intramolecular dissociative electron transfer obeys the same main rules already highlighted for the corresponding intermolecular process. On the other hand, introduction of a nitro group drags the SOMO away from the acceptor, and consequently, the intramolecular rate drops by as much as 1.6 orders of magnitude from the expected value. Therefore, a larger solvent reorganization than for intermolecular electron transfers and the effective D/A distance and thus electronic coupling must be taken into account for quantitative predictions of intramolecular rates.

Structural Effects in Radical Clocks and Mechanisms of Grignard Reagent Formation: Special Effect of a Phenyl Substituent in a Radical Clock when the Crossroads of Selectivity is at a Metal/Solution Interface

“…Such examples of intramolecular redox catalysis in the vicinity of a cathode have been explored. [71,72] This intramolecular redox catalysis is to be distinguished from the intermolecular redox catalysis involved in the arene catalysis of formation of organolithium compounds. [73] The electrochemical treatment of the cathodic reactivity of 9-chloroanthracene was a simplified case of intramolecular redox catalysis: here the cascade consisted of only two electron transfers.…”

Study of the electrochemical reduction of amoebicide teclozan and its amperometric determination in pharmaceutical formulations

“…On the other hand, the authors have proposed that the electrochemical reduction of aromatic halides occurs according to a stepwise pathway as in the classical mechanism. The literature provides several discussions about concerted and stepwise pathways for dissociative electron transfer, [33][34][35] including the use of the electrochemical transfer coefficient (α) as a tool to detect the transition between both pathways. 36 The electrochemical reduction of several polychloroacetamides was studied by Constentin et al 37 on glassy carbon electrode in dimethylformamide by cyclic voltammetry.…”

Electroreduction of halogenated fumaric esters: a combined electrochemical and computational investigation

“…The cathodic reduction of phenacyl halides [5][6][7][8][9] and phenacyl haloacetates [10,11] using a mercury electrode and aprotic medium provided an interesting route for the electrosynthesis of heterocycles such as tetrahydrofurans and coumarins, respectively. Additionally, in respect mainly to halides, a number of studies have been concerned with dissociative electron transfer (DET) [12,13], where the dissociation of the fragile bond may follow or even be concerted to the electron transfer itself. This is an important issue which could rationalise and help to predict the outcome of chemical transformation triggered by the injection of an electron into a molecule [12,13].…”

“…Additionally, in respect mainly to halides, a number of studies have been concerned with dissociative electron transfer (DET) [12,13], where the dissociation of the fragile bond may follow or even be concerted to the electron transfer itself. This is an important issue which could rationalise and help to predict the outcome of chemical transformation triggered by the injection of an electron into a molecule [12,13]. When considering such mechanisms, several questions arise, in particular, the number of electrons transferred, the nature and site of the electron transfer [12,13], whether the transfer occurs directly from the electrode or indirectly through another chemical species, and the involvement of the electrode material itself, especially if the electrode is a reactive metal [3].…”