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SUPRAMOLECULAR SELF-ASSEMBLY FOR CATALYSIS
(Person in charge: Pr. Frédéric Hapiot)
1- Inversion phenomenon in CD-dimers
For decades the idea that CDs are rigid structures has fueled research into their molecular recognition properties. However, structural alteration of CDs is not inconsequential. For example, we showed that CD dimers undergoes a 360° rotation in water so that the spacer linking the two CDs is deeply included into one of the CD cavities. The magnitude of this inversion phenomenon depends on the nature of the spacer and results in a limited accessibility to the CD cavities. It was shown that the inversion phenomenon occurring in CD-dimers greatly affects their catalytic properties in aqueous biphasic catalysis. This underestimated effect is also the cause of numerous mistakes throughout the scientific literature about CDs.
2- Supramolecular bidentate ligands
Water-soluble self-assembled ligands have been elaborated through hydrophobic effects using appropriate CD/phosphane combinations. The resulting supramolecular PN and PNN ligands are capable of coordinating platinum, palladium and rhodium complexes, with the CD simultaneously acting both as a first- and second-sphere ligand. Such supramolecular ligands find application in hydrogenation and hydroformylation of alkenes, Heck arylation, and a domino reaction where a Pt-catalyzed reduction of nitrobenzene is followed by a Paal–Knorr pyrrole reaction.
3- CD-grafted polymers
CD-grafted polymers functionalized with water-soluble phosphanes were synthesized starting from poly(N-acryloyloxysuccinimide) (polyNAS). They were used as additives in Rh-catalyzed hydroformylation of terminal alkenes. When the catalyst-stabilizing phosphane was not grafted to the polymer chain, a structure−activity investigation provided compelling evidence that positive cooperativity is operative in this catalytic system. Indeed, two close-in-space CDs contribute to improve the recognition process of long alkyl chain alkenes by multivalency. When the phosphane was grafted onto the polymer chain, both the supramolecular properties of the CD and the coordination ability of the phosphane were combined into the same molecular object. During the course of the reaction, the closeness of the three main protagonists (substrate, CD, phosphane) leads to a significant increase in the conversion compared to a catalytic system where the CD and the phosphane are not grafted on the same polymer chain.
Poly(N-isopropylacrylamide) (PolyNIPAM) chain end-decorated by a randomly methylated β-CD (RAME-β-CD) are also effective to favor aqueous biphasic hydroformylation of alkenes using water soluble Rh-catalyst. Above the lower critical solution temperature (LCST), the CD-grafted polyNIPAM self-assemble into aggregates of micrometer size range. These aggregates adsorb at the organic/aqueous interface to form a Pickering emulsion which proved to favor molecular contacts between the substrate and the catalyst.
4- Supramolecular hydrogels
Supramolecular hydrogels are soft-materials generated by the entrapment of large quantities of water within a superstructure made of non-reticulated fibrils of varying dimensions. They exhibit reversible sol–gel transition upon exposure to external stimuli such as temperature. Such physico-chemical property has been exploited in transition metal catalysis, especially to recover and recycle the organometallic catalyst. Pickering-like emulsions are operative in these biphasic systems and promote molecular contacts between hydrophobic substrates and water soluble catalysts, especially in hydroformylation of long chain alkenes.
5- Supramolecular Emulsifiers in Biphasic Catalysis
Hydroformylated triglycerides are of interest because they find application in the synthesis of plasticizers, polymers, and lubricants. We found that the C=C double bonds of triglycerides can be readily converted into aldehydes using an aqueous biphasic system for which an organometallic Rh complex is retained within the aqueous compartment. The triglycerides drive their own conversion because of the transient formation of surface active CD/triglyceride supramolecular complexes during the course of the reaction. Upon stirring, the CD/triglyceride supramolecular complex acts as a self-emulsifier that significantly increases the surface area between the triglyceride organic phase and the catalyst-containing aqueous phase. Because hydroformylated triglycerides do not interact with CDs, the product and the catalyst can be recovered separately by simple decantation once the reaction is complete.
6- Synthesis and catalysis under mechanochemical conditions
Through hydrophobic effects at the solid state, α-, β-, and γ-CDs were modified on their challenging secondary face by mechanosynthesis at room temperature using a laboratory scale ball-mill. Mono-(2,3-manno-epoxide) α-, β-, and γ-CDs were subsequently synthesized by ball-milling a mixture of monotosylated α-, β-, and γ-CDs, respectively, and KOH.
Mechanochemistry also proves effective for Greener Paal-Knorr pyrrole synthesis. Relative to traditional Paal–Knorr methodologies, various N-substituted pyrroles were obtained in very short reaction times. By reaction with unreactive diketones, desymmetrized aliphatic and aromatic compounds were also synthesized.
We also explore the role of CDs and other saccharide additives in the mechanosynthesis of gold nanoparticles (AuNPs) and their use as catalysts in the reduction of substituted nitrobenzene derivatives into their corresponding aniline products. CDs not only allow for the stabilization of the AuNPs but also help diffuse a substrate within a solid mixture via supramolecular means, and orient the chemical reaction to the selective formation of aniline derivatives. the catalytic system could be recycled over three consecutive runs without significant loss in activity, thus highlighting the efficacy of the combination of mechanochemistry, supramolecular chemistry, and catalysis.