Correlation between the selectivity and the structure of an asymmetric catalyst built on a chirally amplified supramolecular helical scaffold

Abstract: For the first time, supramolecular helical rods composed of an achiral metal complex and a complementary enantiopure monomer provided good level of enantioinduction in asymmetric catalysis. Mixtures containing an achiral ligand monomer (BTAPPh2, 2 mol%) and an enantiopure ligand-free co-monomer (ester BTA, 2.5 mol%), both possessing a complementary benzene-1,3,5-tricarboxamide (BTA) central unit, were investigated in combination with [Rh(cod)2]BArF (1 mol%) in the asymmetric hydrogenation of dimethyl itaconate. Notably, efficient chirality transfer occurs within the hydrogen-bonded co-assemblies formed by BTA Ile and the intrinsically achiral catalytic rhodium catalyst, providing the hydrogenation product with up to 85% ee. The effect of the relative content of BTA Ile compared to the ligand was investigated. The amount of chiral co-monomer can be decreased down to one fourth of that of the ligand without deteriorating the enantioselectivity of the reaction while the enantioselectivity decreases for mixtures containing high amounts of BTA Ile. The non-linear relationship between the amount of chiral co-monomer and the enantioselectivity indicates that chirality amplification effects are at work in this catalytic system. The rhodium complex of BTAPPh2 and BTA Ile co-assemble under the form of right-handed helical rods as confirmed by various spectroscopic and scattering techniques. Remarkably, the major enantiomer and the selectivity of the catalytic reaction are related to the handedness and the net helicity of the co-assemblies, respectively. Further development of this class of catalysts built on chirally-amplified helical scaffolds should contribute to the design of asymmetric catalysts operating with low amounts of chiral entities.