Electro-organic synthesis has attracted many attentions in the last years due to the possibility of using renewable feedstocks like CO₂, CO and N-based compounds to generate valuable products. Among many possible reactions, C-C and C-N bond forming reactions represent an important synthetic class because C2+ products and amine derivatives, respectively, play crucial roles in various applications in synthetic chemistry.

CO₂ emitted in the atmosphere has been used as a feedstock for the synthesis of chemicals and fuels, although C2+ products are more attractive because of they have wider applicability and higher energy density [1], while NH₃ derivatives are used to generate amines and amino acids [2] .

Here we studied both C-C and C-N coupling reactions using reductive electrochemical transformations.

The synthesis of C2+ products from the electrochemical reduction of CO₂ was investigated on Cu-oxide derived nanoparticles. Our results showed the reaction is dependent on the electrode surface structure and proton concentration. Using online GC, ethylene and propanol were detected as products on Cu cubic nanoparticles, whereas on Cu polycrystalline, such C2+ products were not detected.

In order to enhance the spectrum of products, CO₂ has been also considered in the coupling reaction with other organic molecules, i.e. aldehydes, to make carboxylic acids via electrocarboxylation. Our initial results showed CO₂ reacts with butanal via C-C coupling to yield the corresponding carboxylic acid, in absence of CO₂, the electrochemical reduction of butanal leads only to dimers.

We have also investigated the electro-reductive amination of benzyl alcohols to benzylamines in aqueous solvent. Using TEMPO-immobilised graphite electrode, benzyl alcohol was selectively oxidized to benzaldehyde (Figure 1), while in the cathode, t-butylamine was converted into imine, which acts as the main intermediate to the C-N bond formation. Using the electrochemical conditions displayed in scheme 1, MS and NMR data analyses showed faraday efficiency (FE) of 12% towards benzylamine.Despite the low FE, this work opens an opportunity to the synthesis of important fine chemical by using simple renewable feedstocks.

References:

[1] da Silva, A. H. M.; Raaijman, S. J.; Santana, C. S.; Assaf, J. M.; Gomes, J. F.; Koper, M. T. M. Electrocatalytic CO2 Reduction to C2+ Products on Cu and CuxZny Electrodes: Effects of Chemical Composition and Surface Morphology. Journal of Electroanalytical Chemistry 2021, 880, 114750.

[2] Jeong Eun Kim, Seungwoo Choi, Mani Balamurugan, Jun Ho Jang, Ki Tae Nam. Electrochemical C–N Bond Formation for Sustainable Amine Synthesis. Trends in Chemistry, November 2020, 2, 11.