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Numerical kinetic simulations have been employed to understand the origin of enantioselectivity reversal of chiral beta-amino alcohol catalysts occurring in the presence of their achiral analogs in the autocatalytic addition of diisopropylzinc to pyrimidine carbaldehydes (Soai reaction). In a preceding work (Rivera Islas, J.; Lavabre, D.; Grevy, J. M.; HernAndez Lamoneda, R.; Rojas Cabrera, H.; Micheau, J. C.; Buhse, T. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 13743-13748), we provided a possible explanation for the phenomenon of spontaneous mirror-symmetry breaking observed in this reaction. The effect of enantioselectivity reversal, giving rise to steep and abrupt transitions between opposed optically active states, has been identified by our current approach as another manifestation of such mirror-symmetry breaking capability. In the present case, we considered the involvement of the chiral catalyst and its achiral analog in the reaction network. Our modeling indicates that the enantioselectivity reversal can be either caused (i) by an interaction between the chiral and achiral additives or (ii) by an interaction of the customary products of the Soai reaction with the additives. The second option, which stands for the reversible inhibition of the chirally selective autocatalysts by the additives, could also explain a number of further additive effects in the Soai reaction reflecting the extraordinary sensitivity of this system toward the initial addition of almost any, even catalytically inactive, chiral substance.

The autocatalytic biphasic alkaline hydrolysis of C-4 to C-8 ethyl alkanoates was studied in a CSTR. In our experimental conditions, the two shorter esters (C-4 and C-5) showed monostable behavior ...

Autocatalytic kinetics are observed during the biphasic alkaline hydrolysis of C-4, C-6 and C-8 (butanoate, hexanoate and octanaoate) ethyl esters. Ethanol and alkanoate anions are the products of the reaction. A kinetic model including spontaneous dissolution of the organic phase into the aqueous phase, aqueous hydrolysis of the ester, formation of ester containing aggregates and micellisation has been proposed. For C-4, taking into account salting-in and solvent effects is sufficient to reproduce the autocatalytic behaviour. For C-8, the major autocatalytic process is an ester containing aggregates (ECA) mediated phase transfer. For C-6, it appears that the role of ECA is double. Small sized ECA which are formed at the beginning of the reaction do not perform rapid phase transfer and trap surfactant molecules. Larger ECA which appear at the end of the reaction are the main phase transfer agent. Using an empirical formula to describe the size of the interface, the model is able to reproduce the effect of a variation of the volumic ratio of the two phases as well as the effect of stirring rate and non-emulsified experiments.

The alkaline hydrolysis of C-4 to C-8 (butanoate to octanoate) ethyl esters shows autocatalytic kinetics when performed under two-phase conditions without any mixing solvent. Alkanoate anions and ethanol are the products of the reaction. A dynamic model is proposed that describes quantitatively this kinetic behavior. The model includes the main processes occurring in the biphasic medium and the corresponding thermodynamic calculations of the average size and stoichiometry of the molecular aggregates. Modeling indicates that salting-in and solvent effects caused by the alkanoate anions and ethanol determine the autocatalytic kinetics in the hydrolysis of C-4 ethyl ester where no aggregation occurs. In the C-5 to C-8 experiments, ester-containing micelles (ECM) are mainly formed by cooperative aggregation of alkanoate anions with ester molecules. ECM is formed only after a threshold concentration of the alkanoate anion has been reached. In a phase-transfer-like process, ECM carries ester molecules into the ...

The biphasic alkaline hydrolysis of ethyl caprylate (EC) exhibits highly nonlinear kinetics that has been attributed in the literature to “micellar autocatalysis” (Bachmann, P. A. et al. Nature 1992, 357, 57). New experimental results enable us to establish a macroscopic kinetic model quantitatively accounting for the dynamics of this reaction. According to the model, EC is carried from the organic to the aqueous phase by a transient micelle−EC complex (MEC). Curve fitting of the experimental kinetic data by inverse treatment shows that the formation of MEC is more favorable than that of the pure caprylate micelles (M) and occurs at a critical concentration that is smaller than the cmc associated with the formation of M. We demonstrate, in contrast to previous claims, that classical micellar catalysis is not involved in the overall reaction process, but the observed nonlinear kinetics is a consequence of the dynamics of the MEC-mediated phase-transfer reaction.

The autoxidation of tetralin is treated as a model reaction system to define the applicability of stereospecific autocatalysis. This concept, predicting a spontaneous amplification of enantiomeric excess generated by an autocatalytic chemical reaction, is used in several theoretical models as an explanation for the origin of natural optical activity. The reaction system investigated obeys the basic criteria of these models: a chiral intermediate (tetralin hydroperoxide) is produced from an achiral substrate (tetralin) via an autocatalytic pathway where the feedback mechanism is expected to generate a state of broken chiral symmetry. In order to test the amplification capacity of this reaction a computer analysis of the kinetic scheme is performed. This simulation is derived from the known kinetic scheme of autoxidation and is validated by fitting the experimentally observed data of hydroperoxide evolution. Calculations show that this model allows powerful amplification of enantiomeric excess and a transient amplification of the optical rotation. It is also demonstrated that the model system exhibits pronounced sensitivity toward any loss of absolute configuration of the involved chiral species. Since an amplification effect results exclusively at a high degree of stereoselectivity, it is concluded that stereospecific autocatalysis is possible in systems which show template reactions, crystallization, or colloidal effects. © 1993 Wiley-Liss, Inc.

The addition of diisopropylzinc to prochiral pyrimidine carbaldehydes (Soai reaction) is the onlyknown example of spontaneous asymmetric synthesis in organic chemistry. It serves as a model systemfor the spontaneous occurrence of chiral asymmetry from achiral initial conditions. This review describesthe possible kinetic origin of specific experimental features of this reaction. It is shown that generickinetic models, including enantioselective autocatalysis and mutual inhibition between the enantiomers,are adequate to describe a variety of the astonishing properties of the Soai reaction. Namely, theseare the unprecedented strong chiral amplification, extreme sensitivity to the presence of very small amountsof chiral initiator, mirror-symmetry breaking when starting from achiral conditions, and the reversal ofenantioselectivity of chiral catalysts by addition of achiral additives. The described kinetic approachallows a closer insight into the nonlinear dynamics of the Soai reaction. It also reveals that a numberof open questions concerning the detailed reaction mechanism are still to be solved and that further experimentalstudies are required.

Kinetic modeling using nonlinear differential equations is proposed to analyze the spontaneous generation of enantiomeric excess in the autocatalytic addition of diisopropylzinc to prochiral pyrimidine carbaldehydes (Soai reaction). Our approach reproduces experimentally observed giant chiral amplification from an initial enantiomeric excess of –6% to >60%, high sensitivity and positive response to the presence of minute amounts of chiral initiator at concentrations –14M, and spontaneous absolute asymmetric synthesis from achiral starting conditions. From our numerical simulations using kinetic schemes derived from the Frank model, including stereospecific autocatalysis and mutual inhibition, we have shown that it is possible to reproduce the mirror-symmetry-breaking behavior of the Soai reaction under batch conditions leading to a bimodal enantiomeric product distribution. Mirror-symmetry breaking was found to be resistant to a loss of stereoselectivity up to 30%. While the mutual inhibition between enantiomers seems to originate from the presence of dimerization equilibria, the exact nature of the autocatalytic stereoselective process still remains to be revealed. From the kinetic viewpoint, simple autocatalysis involving monomers as the catalytic species is consistent with all reported experimental effects of the Soai reaction.