Chapter 11. Elucidation of Soai's Asymmetric Autocatalysis

Abstract

Soai's asymmetric autocatalysis is a highly fascinating and striking reaction, showing spontaneous symmetry breaking and amplification of the enantiomeric excess triggered by a small imbalance in the enantiomeric excess of the added product alcohol, many chiral additives or even enantiomorphic surfaces. Understanding the reaction mechanism is of fundamental interest, because it provides not only a conceptional explanation for symmetry breaking, leading to homochirality, but paves the avenue to the design of Soai-type asymmetric autocatalytic reactions to broaden the reaction scope. In this chapter the experimental results of detailed kinetic analyses and in situ reaction analysis by high-resolution Orbitrap mass spectrometry are summarized. These results and structural proof of the formation of a transient hemiacetalate complex allowed us to propose a catalytic reaction cycle that explains the autocatalytic amplification involving these hemiacetal complexes. Detailed kinetic measurements, investigation of the hemiacetal formation equilibria by NMR spectroscopy and comprehensive kinetic analysis provides a mechanistic model of the Soai reaction allowing the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Furthermore, the experimental structural data give insights into the privileged properties of the pyrimidyl moieties and the efficient amplification of even minimal enantiomeric excesses, respectively.