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MOLECULAR HOSTS FOR ORGANOMETALLIC CATALYSIS
(Person in charge: Pr. Sébastien Tilloy)
1- Phosphane-Based Cyclodextrins
In biphasic aqueous organometallic catalysis, phosphanes based on a cyclodextrin skeleton are an interesting alternative since these compounds can simultaneously act as mass transfer agents and as coordinating species towards transition metals. For twenty years, various cyclodextrin-functionalized phosphanes have been described in the literature. Nevertheless, while their coordinating properties towards transition metals and their catalytic properties were fully detailed, their mass transfer agent properties were much less discussed. As these mass transfer agent properties are directly linked to the availability of the cyclodextrin cavity, we have shown that the nature of the reaction solvent and the nature of the linker between cyclodextrin and phosphorous moieties can deeply influence the recognition properties. In addition, a crucial impact on the catalytic activity was also discussed..
A new diphenylphosphane based on β-cyclodextrin skeleton possesses the interesting property of a solvent-dependent conformation change. Indeed, the self-inclusion of a phenyl group of the phosphane moiety into CD cavity observed in water disappeared in organic solvent resulting in change of conformation. Hydrogenation or hydroformylation reactions were performed with this original ligand in water or in organic solvent.
A new triphenylphosphane based on a β-cyclodextrin skeleton (PM-β-CD-OTPP) was synthesized. This ligand can be dispersed in water by using the nanoprecipitation method. Transmission electron microscopy and NMR spectroscopy showed that PM-β-CD-OTPP is aggregated in water and forms a stable dispersion. Its aqueous solubility can be dramatically increased in the presence of selected watersoluble guests by formation of inclusion complexes. Associated to a rhodium precursor, PM-β-CD-OTPP is able to generate soluble rhodium species in water. In addition, NMR experiments showed that the cyclodextrin cavity remains accessible for a guest even when PM-β-CD-OTPP is coordinated to rhodium. Finally, this ligand was efficient for rhodium-catalyzed hydrogenation and hydroformylation performed in aqueous medium.
The inclusion of a guest inside the cavity of a new water-soluble cyclodextrin-phosphane obtained by click-chemistry allows controlling the natural conformation of this ligand leading to an inversion of the regioselectivity during aqueous hydroformylation reaction.
2- Cyclodextrin dimer as a supramolecular reaction platform
A supramolecular reaction platform based on a CD dimer was elaborated and was able to simultaneously host the substrate and the catalyst. Schematically, the catalyst was included inside one cavity while the substrate was included in the other. The proximity between the catalyst and the substrate allowed reaching higher catalytic activities than those reported for classical systems based on CD.
3- Low Melting Mixtures (LMMs) based on Cyclodextrin derivatives
Native and modified cyclodextrins (CDs) associated to N,N’-dimethylurea formed low melting mixtures able to immobilize organometallic species based on sulfonated phosphanes. Interestingly, hydroformylation and Tsuji–Trost reactions were efficiently performed in these new solvents, which led to new recyclable catalytic systems. Indeed, at 90°C, the catalytic layer was a liquid and the reactions were conducted in a biphasic liquid medium, the reaction substrate and products forming a non-miscible layer. After the reaction, the catalytic layer solidifies by cooling at room temperature, permitting the reaction products to be easily isolated.