Oscillatory Symmetry Breaking in the Soai Reaction

Abstract

A kinetic model of spontaneous amplification of enantiomeric excess in the autocatalytic addition of diisopropylzinc to prochiral pyrimidine carbaldehydes is extended by a negative feedback process. Simulations based on the extended model result in large-amplitude oscillations both in a continuous-flow stirred tank reactor (CSTR) and in a semibatch configuration under optimized initial conditions. When sustained oscillations are maintained in a CSTR, no enantiomeric product distribution could be observed in the calculated series; the system keeps its initial enantiomeric ratio endlessly. During damped oscillations, or steady-state conditions, however, chiral amplification from a very small initial enantiomeric excess to more than 99% occurs in a semibatch configuration. Calculations indicated spontaneous enantiomeric product enrichment (i.e., accumulation of one of the enantiomers at the cost of the other one) from strictly achiral starting conditions in a semibatch configuration due to the inherent numerical error of the integrator method, which can be regarded as a model of the statistical fluctuation in the numbers of enantiomeric molecules.