Concepts and tools for mechanism and selectivity analysis in synthetic organic electrochemistry

Abstract

As an accompaniment to the current renaissance of synthetic organic electrochemistry, the heterogeneous and space-dependent nature of electrochemical reactions is analyzed in detail. The reactions that follow the initial electron transfer step and yield the products are intimately coupled with reactant transport. Depiction of the ensuing reactions profiles is the key to the mechanism and selectivity parameters. Analysis is eased by the steady state resulting from coupling of diffusion with convection forced by solution stirring or circulation. Homogeneous molecular catalysis of organic electrochemical reactions of the redox or chemical type may be treated in the same manner. The same benchmarking procedures recently developed for the activation of small molecules in the context of modern energy challenges lead to the establishment and comparison of the catalytic Tafel plots. At the very opposite, redox-neutral chemical reactions may be catalyzed by injection (or removal) of an electron from the electrode. This class of reactions has currently few, but very thoroughly analyzed, examples. It is likely that new cases will emerge in the near future.

Keywords: mechanism; molecular catalysis; organic electrochemistry; synthetic electrochemistry.

Fig. 1.

Electrolysis under force convection for a simple competitive reaction scheme. Steady-state concentration profiles and expression of the yields.

Fig. 2.

Oxalate (as opposed to CO and carbonate) yield in the preparative electrolysis of CO₂ in DMF on a mercury electrode at a current density of 1.6 mA/cm² at 0 °C as a function of CO₂ concentration. The fitting with the theoretical curve implies that k_rr/k_rs^3/2 = 8.5 × 10⁵ M^1/2s^−1/2 and D = 10⁻⁵ cm²/s.

Fig. 3.

Reductive electrolysis of aromatic halides. The representative points are derived from deuteration experiments carried out at a potential located past the cyclic voltammetric peak potential. The compass rose on the top right summarizes the effect of the various rate and operational parameters.

Fig. 4.

Molecular catalysis of electrochemical reactions. (A) Reactions schemes and (B) potential energy profiles.

Fig. 5.

Homogeneous molecular catalysis of an electrochemical reaction. (A) Concentrations profiles. (B) catalytic Tafel plots.

Fig. 6.

cis-trans isomerization of olefins.

Fig. 7.

S_RN1 aromatic nucleophilic substitution.