Stochastic and empirical models of the absolute asymmetric synthesis by the Soai-autocatalysis

Abstract

Absolute asymmetric synthesis (AAS) is the preparation of pure (or excess of one) enantiomer of a chiral compound from achiral precursor(s) by a chemical reaction, without enantiopure chiral additive and/or without applied asymmetric physical field. Only one well-characterized example of AAS is known today: the Soai-autocatalysis. In an attempt at clarification of the mechanism of this particular reaction we have undertaken empirical and stochastic analysis of several parallel AAS experiments. Our results show that the initial steps of the reaction might be controlled by simple normal distribution ("coin tossing") formalism. Advanced stages of the reaction, however, appear to be of a more complicated nature. Symmetric beta distribution formalism could not be brought into correspondence with the experimental observations. A bimodal beta distribution algorithm provided suitable agreement with the experimental data. The parameters of this bimodal beta function were determined by a Pólya-urn experiment (simulated by computer). Interestingly, parameters of the resulting bimodal beta function give a golden section ratio. These results show, that in this highly interesting autocatalysis two or even perhaps three catalytic cycles are cooperating. An attempt at constructing a "designed" Soai-type reaction system has also been made.

Spontaneous Deracemizations

“…The R-dominance may indicate the influence of unknown chiral impurities, which is perhaps a predictable incident by recalling the extremely high sensitivity of the Soai reaction toward a large variety of asymmetric stimuli. Interestingly, in former analyses 618,643 of the earlier Soai group results 137,605 and other experimental studies 644,645 in the homogeneously driven reaction, an S-dominance has been reported. These opposite outcomes, R vs S, may be due to chiral impurities perhaps associated with the specific laboratory environments or different sources of the reactants but practically exclude a universally present asymmetric influence such as the PVED, in effect recently dismissed, 41 which should give rise to a constant R-or Spreference, whenever having a possible impact.…”

“…Since the discovery of the Soai reaction, a number of kinetic, stochastic, statistical, and empirical models have been proposed to rationalize the reaction mechanism (see for recent overviews), , among them also mechanistic principles derived from nonlinear effects (NLE) in asymmetric synthesis. ,, These models extend over early concepts of monomers or dimers to currently tetramers as the catalytic species.…”

Functional Chirality: From Small Molecules to Supramolecular Assemblies

“…An empirical and stochastic analysis of several parallel experiments was performed to investigate the nature of the autocatalytic reactions, and it was found that that the initial steps of the reaction might be controlled by simple normal distribution ("coin tossing") formalism. However, the advanced stages of the reaction appear to be of a more complicated nature, with a high probability of being up to three cooperating catalytic cycles [133].…”

A Quantification of the Soai Reaction

“…To supplement the structural studies mainly established by using DFT calculations and NMR spectroscopy, kinetic models based on the numerical integration of coupled rate equations, as well as various stochastic, statistical, and empirical models, have been proposed . Although such models remain improvable from a structural and kinetic point of view, they provided helpful information on the origin of asymmetric amplification and MISB in the Soai reaction …”