Chapter 4 - Kinetic and Structural Aspects of Mirror-Image Symmetry Breaking in the Soai Reaction

Abstract

The mechanism of the Soai reaction remains a challenge for experimentalists and theorists. It appears to be much more complex than simple models describing asymmetric autocatalyis in generalized terms. Numerous oligomer species obeying the principles of chiral combinatorics are likely involved. A number of these intermediates have been either isolated or recognized by nuclear magnetic resonance (NMR) studies. A combination of kinetic analyses and density functional theory (DFT) calculations could shed some light on the deeper understanding of this asymmetric autocatalysis. A recent breakthrough was provided by the identification of X-ray structures of various oligomers. To rationalize its mechanism, kinetic modeling of the Soai reaction has been developed in two complementary directions: a structural approach validated by DFT calculations and a functional approach by numerical studies and the reproduction of experimental features. Recently, we introduced a new mechanistic feature related to the concept of kinetic proofreading. We showed that whatever the values of the kinetics parameters, an increase in the catalytic oligomer size from one to three lead to a higher tolerance to poorer chiral recognition between the diastereoisomers, which is a key element for asymmetric amplification and spontaneous symmetry breaking. In the present contribution, we also show that numerical simulation is useful to understand the ee amplification during the stepwise addition of aldehyde and organozinc in a typical Soai reaction experimental procedure. Also, we present a first modeling approach in respect to the observed high sensitivity of the Soai reaction towards chiral additives. Our model predicts that the aldehyde concentration plays the role of a bifurcation parameter inducing a transition from a racemic state to an optically active one. Our results also confirm that in such complex systems, kinetic orders must be handled with care.