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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 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.
A new water-soluble β-CD-based phosphane consisting of a 3,3'-disulfonatodiphenyl phosphane connected to the primary face of the β-CD by a dimethyleneamino spacer has been synthesized and characterized. Intra- and intermolecular inclusion processes of one of the two sulfophenyl groups into the β-CD cavity were identified in water. However, the association constant (Ka) related to the β-CD/ sulfophenyl group couple was low. Accordingly, the inclusion process was easily displaced upon coordination to rhodium complexes. The efficacy of the resulting Rh-complex coordinated by β-CD-based phosphanes was assessed in Rh-catalyzed hydroformylation of higher olefins. The catalytic system proved to be far more successful and efficient than a system consisting of supramolecularly interacting phosphane and CD. The catalytic activity was up to 30-fold higher while the chemo- and regioselectivities remain rather unchanged.
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.
In 2017, we demonstrated that the Rh-catalyzed hydroformylation of aromatic-substituted alkenes can be efficiently performed in a planetary ball mill under CO/H2 pressure. The dispersion of the substrate molecules and the Rh-catalyst into the grinding jar is ensured by saccharides, namely, methyl-α-D-glucopyranoside, acyclic dextrins (maltodextrins), or cyclodextrins (CDs, cyclic oligosaccharides). The reaction affords the exclusive formation of aldehydes whatever the nature of the saccharide. Acyclic saccharides efficiently disperse the components within the solid mixture leading to high conversions of alkenes. However, they showed typical selectivity for α-aldehyde products. On the other hand, if CDs are used as dispersing additive, the steric hindrance exerted by the CDs with respect to the primary coordination sphere of the metal modifies the selectivity to such an extent that the β-aldehydes were also formed in non-negligible proportions. Such through-space control via hydrophobic effects over reactivity and regioselectivity reveals the potential of such solventless process for catalysis in solid state.
7- Functionalization of naturally occurring vegetable oils
Our research activities are not only focused on supramolecular self-assemblies but also deal with the valorization of naturally occurring vegetable oils. The direct functionalization of the carbon-carbon double bonds of triglycerides is of major interest to access biosourced building blocks with unique molecular and functional properties. We succeeded in synthesizing trihydroxylated triglycerides via a hydrohydroxymethylation reaction which consists in two consecutive Rh-catalyzed reactions, namely, a hydroformylation followed up by a consecutive reduction of the formyl groups.1 Contrary to two-step procedures described in the literature, no phosphane is used to coordinate the Rh-species, thus giving our strategy an industrial potential. Tertiary amines are used as ligands to promote the hydrohydroxymethylation reaction.
The study was extended to the one pot functionalization of the carbon-carbon double bonds of triglycerides via a cascade hydroaminomethylation (HAM) / hydrohydroxymethylation (HHM) reaction. The carbon-carbon double bonds are first hydroformylated using a Rh-catalyst. The produced aldehydes react with secondary amines to yield enamines which are then reduced into tertiary amines. Interestingly, the latter act as ligands of Rh-species capable of hydrogenating the remaining formyl groups into alcohols. As a result, the final triglycerides are substituted by both aminomethyl and hydroxymethyl groups.