Electric-Field Catalysis of Diels-Alder Reactions.

The dream to catalyze reactions with externally applied electric fields is so obvious and so promising that it has attracted immense scientific attention. The Diels-Alder (DA) reaction has become the primary reaction to elaborate on this dream, but electric-field catalysis (EFC) of DA reactions under practical organic synthesis conditions remained impossible. The objective of this study was to challenge the emerging principles of scalable EFC in microfluidic capacitors with this holy grail in the field. Functional Helmholtz layers are assembled to translate applied electric fields into effective electric fields that are, according to chronoamperometry, strong enough for the EFC. Yields increase nonlinearly from 0% up to 90% with applied voltage. In chiral cationic Helmholtz layers assembled from polyarginines, benzylated quinines, or ammonium binaphthyls, enantioselectivity up to 31% ee is obtained. These results demonstrate that the emerging principles of EFC in microfluidic capacitors pass the ultimate test in the field with distinction: Besides realizing DA-EFC with externally applied electric fields under practical conditions, this study also introduces chiral Helmholtz layers as (i) the primary catalysts, with an impressive efficiency of EC50 = 0.2 mol %, and (ii) the unique source of chirality for asymmetric EFC.