J. Am. Chem. Soc. 138, 4908–4916 (2016)

Asymmetric catalysis is usually performed using small organometallic complexes with enantiopure ligands. Much rarer, however, are catalysts that use chiral amplification, where a small chiral bias is translated into a large conformational preference. This amplification process is believed to be fundamental to the origin of homochirality, a topic that is still poorly understood. Now, Matthieu Raynal and colleagues at CNRS at the Sorbonne, along with collaborators in Toulouse and Spain, have shown that enantioinduction can be achieved in chirally amplified supramolecular helical catalysts.

The helices are self-assembled from benzene-1,3,5-tricarboxamides (BTA). By mixing achiral BTA phosphine ligands and enantiopure BTA amino acid derivatives, the researchers showed that efficient chirality transfer occurs with just 25% of the chiral monomer. The transfer of stereochemical information is propagated from the amino acid side chains, through the hydrogen-bonded helical assembly, to a peripheral rhodium centre, which in turn catalyses an asymmetric hydrogenation reaction with high enantioselectivity (85% ee). The screw sense (handedness) of the helix directs the stereoselectivity of the catalytic reaction, so both enantiomers of the product can be achieved simply by inverting the amino acid stereochemistry in the BTA comonomer. This illustrates that the 'sergeants and soldiers' effect can be efficiently utilized in asymmetric catalysis. Surprisingly, at higher chiral comonomer concentrations, the enantioinduction actually decreases as the coordination of rhodium by two adjacent phosphine monomers is prevented.