Putting the Mechanism of the Soai Reaction to the Test: DFT Study of the Role of Aldehyde and Dialkylzinc Structure

Previous DFT calculations provided support to the proposal that the Soai reaction involves a mechanism in which dimer catalysts serve as templates for the reaction of two molecules of dialkylzinc with two molecules of aldehyde so as to reproduce themselves (ref 11). Here it is shown that, from the point of view of formal kinetics, this mechanism can be reduced to a general model, dubbed the extended dimer model, that has the Blackmond-Brown dimer model as a particular case. Depending on the interplay of kinetic constants, the extended dimer model can give rise to either chiral amplification or depletion. Calculations of the kinetic constants at the M05-2X/6-31G(d) level of theory were carried out in order to theoretically evaluate the effect of the second aza group in the six-membered aromatic ring of the aldehydic substrate and the effect of dialkylzinc structure. Predictions of chiral amplification or depletion are in striking agreement with experimental data thus lending support to the proposed mechanism.