Nanoparticules de métaux nobles stabilisées par des cyclodextrines modifiées chirales. Application en catalyse supramoléculaire asymétrique – SUPRANANO

Owing to their original surface properties, metallic nanoparticles are considered as reference catalysts in arene hydrogenation with great activities. However, in the literature, no example of enantiodiscrimination for such reactions has been reported in solution. Our approach relies on the use of chirally functionalized cyclodextrins as protective agents, which could induce the desired enantiodiscrimination, avoid the NPs aggregation and also be used as mass transfer shuttle towards the synthesis of optically enriched compounds. Two routes were explored for the synthesis of nanocatalysts in water according to the nature of the metal precursors (metallic salts or organometallic compounds). The organometallic method is more tedious for the synthesis and the transfer of nanospecies in water and the development of these systems has first been performed through the use of water-soluble diphosphines as protective agents of Ru NPs. The addition of cyclodextrins to these developed systems leads to the formation of supramolecular structures which proved to be highly active in the hydrogenation of model substrates.
A good knowledge of the number of ligands and of the occupied sites has enabled a better understanding these systems and to tune the properties of the supramolecular edifices (relation structure / properties).
In parallel, Rh nanocatalysts have been developed for investigations in hydroformylation reaction.

A family of amino-acids functionalized cyclodextrins (CDs) has been synthesized with high yields. In parallel, tetrasulfonated diphosphines (dppe, dppp, dppb) have been prepared to assess their ability to stabilize metallic nanoparticles (NPs) obtained by the organometallic route in water. Stable water-soluble Ru and Rh NPs, with small sizes (?1.3 nm) have been synthesized from organometallic precursors. The characterizations highlighted the strong interaction of the ligand with the particle surface. Promising results have been achieved, using CDs alone or in combination with diphosphines as protective agents, but further experiments are still going on.
The synthesis of Ru NPs in aqueous media through the reduction of metallic salts is easier and the NPs stabilisation, optimized in achiral way, has been extended to chiral CDs. The two developed approaches (in two steps – cascade route – or in one step - one pot -) enable a control of the amount of CDs to assure a good activity/stability compromise. The obtained catalysts have been investigated in the hydrogenation of prochiral model substrates. The extension to Rh needed the combination of surfactants to CDs in order to increase the systems stability. In all cases, great activities have been achieved, with no asymmetric induction.
Organometallic Rh NPs stabilized by diphosphines and/or CDs afforded interesting but weakly reproducible results in hydroformylation, owing to the weak stability of these systems in catalysis. Rh NPs have thus been synthesized by reduction of salts and stabilized by water-soluble polyvinylpyrrolidone. These more stable systems lead to more reproducible results in catalysis. Further works need to go on in order to better understand the mechanisms

An exhaustive bibliography review resulting form the collaboration of the different partners has showed that only a few catalytic systems based on rhodium nanoparticles are known for hydroformylation reactions. The development of stable rhodium nanocatalysts in the catalysis conditions are particularly promising. The weak stability observed with rhodium colloids prepared through the organometallic route in the hydroformylation conditions has showed the need to develop protective agents that may be more adapted to metallic surfaces to increase their stability without hampering their catalytic activity.
Moreover, the development of nanoparticles in a chiral environment remains a challenge, particularly for the synthesis of optically enriched cyclohexanic compounds by hydrogenation of arene derivatives in aqueous media.

1. The synthesis of amino-acids modified cyclodextrins has been published by the different partners in Carbohydrate Research review. «Using click chemistry to access mono- and ditopic ß-cyclodextrin hosts substituted by chiral amino acids «Carbohydrate Research 2011, 346, 210-218.
2. The results on the synthesis of sulfonated diphosphines-stabilized Ru nanoparticles NPs and their investigation in catalysis has been reported in a multi-partner publication in Catalysis Today. “Alkyl sulfonated diphosphines-stabilized ruthenium nanoparticles as efficient nanocatalysts in hydrogenation reactions in biphasic media” Catal. Today, 2012, 183, 34-41
3. The different partners have written a review on their competences on rhodium nanoparticles for applications in hydrogenation and hydroformylation “About the Use of Rhodium Nanoparticles in Hydrogenation and Hydroformylation reactions” est à paraître à Current Organic Chemistry - Special issue «Nanoscale Catalysts as Tools for Synthesis«, Martin H. G. Prechtl (Ed.)
4. Two partners of the project have contributed to two chapters in Nanomaterials in Catalysis, P. Serp and K. Philippot (Eds.), à paraître chez Wiley-VCH en 2012 : « Concepts in Nanocatalysis » par K. Philippot et « Metal nanoparticles in neat water for catalytic applications » par A. Denicourt-Nowicki et A. Roucoux

In a Green Chemistry context, the use of metallic nanoparticles in aqueous catalytic systems constitutes an ecological alternative to classical polluting industrial processes. One of the more attractive advantages of nanoparticles lies in the growing number of metallic centres and consequently in potential active sites on their surface when decreasing their size, giving them interesting catalytic properties in terms of activity and selectivity. Nevertheless, their stability and properties in water strongly depend on the nature of the stabilising agents. Thus, we recently demonstrated that the use of modified cyclodextrins (cyclic oligosaccharides) not only favoured the stabilisation of the nanoparticles in water but also induce discriminant reactions. In this project, we propose to extend this result to asymmetric catalysis. More precisely, the aim of the study is to elaborate supramolecular multifunctional nanocatalysts constituted of metallic colloids stabilised by chiral cyclodextrins and to apply them to the asymmetric catalysis in biphasic media (organic/aqueous phases) of reactions such as the hydrogenation of disubstituted benzenes or the hydroformylation of styrene derivatives. The multifunctionality of these nanocatalysts is closely related to the presence of chiral cyclodextrins adsorbed on the metal surface and whose role is to ensure: i) a high stability of the metallic colloids in water without affecting their catalytic activity ii) the mass transfer between the two phases of the biphasic system iii) the preferential formation of one of the two possible enantiomers starting from a prochiral substrate. The weak interaction between the chiral cyclodextrins and the metallic nanoparticles is the key parameter giving rise to this multifunctionality. The project is divided in three steps. First, chiral cyclodextrins will be synthesised by mono- or polysubstitution of their primary or secondary faces by chiral amino-acids. This part of the project will be achieved by the group of Pr Monflier from Lens whose expertise in cyclodextrin chemistry has been widely recognised during these last ten years. Once synthesised, the chiral cyclodextrins will be used as protecting agents towards metallic nanoparticles to avoid any aggregation and/or sedimentation process. Two routes will be investigated according to the nature of the precursors such as metallic salts (approach developed at Rennes ' Team of Pr Roucoux) or organometallic compounds (approach developed at Toulouse ' Team of Dr Philippot). The precise knowledge of the number of ligands and their position on the metal would allow understanding and tuning the properties of the supramolecular edifices (structure/properties relationships). The last step of the project is devoted to the catalytic application of the colloidal system elaborated in the second step at Rennes and Toulouse. Two model reactions will be simultaneously dealt with on both sites Rennes and Lens. The expertises and know-how of the two involved teams appeared to be complementary. Thus, the group of Rennes will develop the asymmetric hydrogenation of disubstituted arenes by aqueous suspensions of metallic nanoparticles, reaction for which the group has gained a world-wide recognised experience for its results in the non-prochiral series. Similarly, the asymmetric hydroformylation will be performed by the group of Lens which is used to perform hydroformylation as a model reaction. Based on the use of liquid-liquid biphasic systems, the reusability of the nanocatalysts will be of great importance in the catalytic study.