Hyperpositive nonlinear effects in asymmetric catalysis

Abstract

Asymmetric amplification is a curious phenomenon that is believed to play a key role in the emergence of biological homochirality, and thus of life itself. In asymmetric catalysis, it is achieved via positive nonlinear effects, which allow high product enantiomeric excesses with a non-enantiopure catalyst. However, it has also been proposed that non-enantiopure catalysts may be even more enantioselective than their enantiopure counterparts, although such a case has never been experimentally observed so far. Here, we present an example of such a hyperpositive nonlinear effect in asymmetric catalysis. We found that addition of dialkylzinc reagents to benzaldehyde gave higher product enantiomeric excesses with only partially resolved chiral N-benzyl-ephedrine ligands. A mechanistic study was carried out and our results point towards a two-component catalysis, where mononuclear as well as aggregated catalysts are in equilibrium and in competition. These results introduce an unprecedented class of asymmetric amplification in enantioselective catalysis. Nonlinear effects in catalysis have been shown to allow for asymmetric amplification. Here the authors report a particularly intriguing case whereby a catalytic asymmetric reaction gives a significantly higher product e.e. when run with a non-enantiopure catalyst compared to the enantiopure version.