From Self-Amplification of Chirality to the Imaging of the Absolute Configuration by Coulomb Explosion

Self-amplifying enantioselective reactions are important in the context of the Origin of Life to explain the formation of homochirality in Nature. The understanding of such complex mechanisms leading to amplification of chirality is the key to a directed design of such reactions and catalysts. The most prominent example of an autocatalytic process is the Soai reaction.¹ In this presentation mechanistic investigations and a novel mechanism of the Soai reaction will be presented with a focus to transfer the knowledge to other reactions and processes.

In particular stereolabile interconverting catalysts open up the possibility of directing enantioselectivity in asymmetric synthesis by formation of diastereomeric complexes with chiral auxiliaries.^2,3 The successful realization of such a system by decoration of the ligand backbone with chiral recognition sites attached to a structurally flexible phoshoramidite type catalyst, that can sense the chirality and induce enantioselectivity, is presented.⁴ Structural flexibility and sensing of the chirality of product molecules result in a rapid increase of enantioselectivity of the dynamic catalysts (Δee of up to 76%) and a shift out of equilibrium.

The second part of the presentation is about the determination of absolute configurations of chiral molecules. In 1951 Bijvoet introduced anomalous single-crystal XRD to determine the absolute configuration of rubidium sodium (+)-tartrate dihydrate crystals to be (R,R), which was correlated to (+)-glyceraldehyde, based on the work of Wohl and Momber in1917.

Recently, we reported the first assignment of the absolute configuration of enantiopure (R,R)-2,3-dideuterooxirane  by direct visualization of the sense of chirality using foil-induced Coulomb explosion imaging (CEI).  In this experiment a small sample of the compound is ionized, accelerated by 2.0 MeV and mass-selected. Coulomb explosion is induced by stripping off the valence electrons within 1 fs during the flight through an ultra thin diamond foil, The expansion of the structure provides an enlarged molecular picture, which allows to directly assign the absolute configuration.