Supramolecular catalysis based on dynamic hydrogen-bonded assemblies – SupraCatal

The catalytic system is composed of a metallic ion (Rh for olefin hydrogenation, Cu in the case of ketone hydrosilylation), an achiral BTA bearing a phosphine function (the ligand) and a chiral BTA that does not directly interact with the metal (the sergeant). The helicity of the assembly is imposed by the chiral BTA.

Three concepts have been demonstrated in this project: - chirality transfer: the chirality of the sergeant imposes the handedness of the helical assembly, which then determines the chirality of the product of the catalytic reaction. - chirality amplification: 20% of chiral sergeant (and 80% of achiral ligand) is enough to yield the maximum enantioselectivity of the catalytic reaction. Thus, fewer chiral precursors than catalytic sites are needed. - chirality inversion: addition of a third BTA (of opposite chirality than the sergeant) switches the handedness of the helix and thus the selectivity of the catalytic reaction. This inversion is fast and can be performed several times without erosion of the selectivity.

-Optimization of the catalytic systems identified. -Correlation of the enantioselectivity of catalytic reactions and the amplification of chirality at the supramolecular level.

5 articles have been published (+1 in preparation) and 1 dissemination article is in preparation.

Currently, intense research efforts are made towards the discovery of innovative catalysts. Among the strategies used, catalysts that can be tuned by non-covalent interactions are more and more investigated. Such supramolecular catalysts are designed in the aim of modifying the second coordination sphere of a metal center; an unsolvable challenge with classical covalent metal-catalysts. Up to now, most of the supramolecular catalysts are built by means of electrostatic interactions or metal-ligand bonds which are poorly dynamic. Moreover, the architectures developed so far are difficult to modulate because their functionalization requires tedious synthetic steps. Finally, these systems are not reversible due to the fact that their chemical or catalytic properties are not modified by changes of their environment. Here, we describe how nanotubes and nanohelices, based on the hydrogen-bonded assemblies of bis-ureas and benzene 1,3,5-tricarboxamide (BTA) monomers, can constitute a new class of supramolecular catalysts. These assemblies are dynamic (since based on hydrogen bonds), functionalizable (by simple chemical modification of the monomer) and reversible (their architecture varies according to the concentration and the temperature among other parameters). By means of these unique properties, we will demonstrate that the self-association can be used to modulate the electronic, steric and chiral properties of a metal catalyst. Our catalysts will be statistical copolymers including phosphine-functionalized and classical bis-urea or BTA monomers. We plan to apply this system in three catalytic reactions: the asymmetric hydrogenation of olefins, the hydroformylation of olefins and the carbonylation of epoxides. In all cases, the same ligands will be used and a common d8-catalytic resting state will be involved in all the catalytic experiments. An additional feature of our system is the possibility to design an allosteric catalyst i.e. with catalytic properties that can be modified along the time by adding a cofactor or modifying the environment. The properties of these nanotubes and nanohelices (reversibility, dynamism and functionalization) and their use for the construction of several types of catalysts will constitute a breakthrough in the domain and thus deserve an urgent and intense investigation.