1. Ecocatalysts: the first bio-based metal catalysts for green chemistry [1]
A crucial feature of green catalysts is their multi-metallic composition resulting from the combination of transition elements at very high concentrations (e.g. Zn2+, Ni2+, Mn2+, Cu2+, Pd2+, Ce2) with classical elements generally necessary for plant development (e.g. Na+, K+, Ca2+, Mg 2+, Fe3+). The simultaneous presence of a combination of well-defined active sites results from this variety of metallic species. Therefore, sequences of original reaction steps lead to unique selectivities. Indeed, a classic catalyst can be limited to influencing only one or some of the steps of a reaction process, thus limiting the opportunities in organic synthesis. Here, the richness of the different interactions between the present chemical species leads to multiple and unusual metal/ligand interactions in solution.
This peculiarity of composition has been taken advantage of in multi-component reactions and in cascade leading to the direct preparation of complex heterocyclic compounds of pharmaceutical interest with somewhat barbaric names. , such as 2-H-chromenes, 1-H-1,5-benzodiazepines, cannabinoids, tetrasubstituted pyridines, polyhydroxy chiral furans, pyrazoles and 1,2,3- substituted triazoles.
The Suzuki and Heck coupling reactions (Nobel Prize) are carried out without ligands and without additives (important advantage in green chemistry to form carbon-carbon bonds).
2.3. Many mechanisms of organic synthesis revisited
Ecocatalysts allow the synthesis of complex biomolecules with significant industrial and societal impact. In particular, they allow the synthesis of highly sought-after molecules such as antimitotics, DNA and RNA, biocontrol agents, new generation insecticides, cosmetics and key intermediates in the chemical industry.
2.4. Ecocatalyzed reactions can be performed in green solvents, including water.
Thanks to the biosorption process (retention of metals on the biomass), it is possible to recycle and reuse the ecocatalysts, including in the homogeneous phase. This unusual possibility has been tested with the copper-catalyzed alkyne-azide cycloaddition, Suzuki and Sonogashira couplings.
3. Conclusion
The concept of ecocatalysis now makes it possible to free ourselves from all the limitations specific to existing methods thanks to a new generation of functional, ecological, stable and recyclable materials. They present several reactive interfaces whose properties can be controlled from the intensity and the nature of the desired metal/substrate interaction. These are linked to the biodiversity of plant species used in ecological restoration and biosorption. Ecocatalysts are therefore not simple substitutes for catalysts derived from metallurgy, but new tools that integrate a triple vision: chemistry, ecology, environment for sustainable chemistry.
[1] Grison C., Y. Lock Toy Ki. Ecocatalysis, a new vision of Green and Sustainable Chemistry, Current Opinion in Green and Sustainable Chemistry, 2021, 29, 100461. https://doi.org/10.1016/j.cogsc.2021.100461.