Mechanistic Insights into the Soai Reaction from Formal Kinetics and Density Functional Theory Calculations

The Blackmond-Brown dimer model (DM) is one of the most credited mechanism proposed to explain the asymmetric autocatalysis in the Soai reaction. It is based on the hypothesis that the homochiral dimers of the zinc alkoxide product are the catalitically active species in statistical equilibrium with the inactive heterochiral dimer. In this chapter, we show that the OM model may be viewed as a particular case of an "extended dimer model", the outcome of which depends on the result of the competition between a homochiral homocatalytic cycle and two other catalytic cycles, the homochiral enantiocatalytic and the heterocatalytic ones. We also present a thorough DFT computational study on the mechanism of the Soai reaction, aimed at locating the relevant transition states of the catalytic cycles. On the basis of computational results, a mechanism detailed at the molecular level is proposed, that sheds light on the origin of enantioselectivity and lends support to the main hypotheses of the DM. This mechanism reproduces the evolution of enantiomeric excess (ee) over the reaction progress at ambient temperature, i.e. as long as no significant formation of oligomers higher than the dimers takes place in solution.