Nonlinear Effects in Asymmetric Catalysis

Guillaneux, Denis; Zhao, Shu-Hai; Samuel, O.; Rainford, David; Kagan, Henri B.

doi:10.1021/ja00100a004

Cited by 385 publications

(325 citation statements)

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“…Kagan and coworkers (27,28) introduced the concept of dimer catalysis to explain amplification or depletion of ee during enantioselective catalysis with partially resolved chiral auxiliaries. We examined this possibility by replacing the monomer-catalyzed steps 3 and 4 by the following dimer-catalyzed steps:…”

Section: Modelmentioning

confidence: 99%

Mirror-symmetry breaking in the Soai reaction: A kinetic understanding

Islas

Lavabre

Grévy

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

127

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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 <10 ؊6 % to >60%, high sensitivity and positive response to the presence of minute amounts of chiral initiator at concentrations <10 ؊14 M, 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.alkylzinc addition ͉ autocatalysis ͉ chirality A symmetric synthesis usually requires the intervention of chiral chemical reagents or catalysts. Few examples are known in which the generation of enantiomerically enriched products occurs from achiral precursors without the involvement of such auxiliaries. These cases, known as absolute asymmetric synthesis (1, 2), have been mostly observed in the combination of spontaneous resolution and enantioselective catalysis (3) as well as by the influence of external chiral factors such as circularly polarized light (4) or vortex motion (5). Some cases were reported in which no obvious chiral inductor was used and, nevertheless, high enantiomeric excess (ee) has been obtained systematically. These examples display the signature of mirrorsymmetry breaking (6), i.e., a process in which a small random ee is greatly amplified, whereas the chirality sign remains unpredictable for each individual experiment. The generation of small random ee occurs practically in any chiral system for statistical reasons alone in which the ee is inversely proportional to the square root of the number of molecules, ee ϰ n Ϫ1/2 (7). However, this value remains negligible as long as a large number of molecules is involved, so an amplification mechanism is needed to increase such minute ee to a macroscopic level.Mirror-symmetry breaking in chemical systems is typically associated to autocatalytic kinetics, i.e., a feedback mechanism in which one or several reaction products directly increase the overall rate of the chemical reaction. Hence in the specific case of chiral autocatalysis, the enantiomeric product could act as an asymmetric catalyst of its proper forma...

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Section: Modelmentioning

confidence: 99%

Mirror-symmetry breaking in the Soai reaction: A kinetic understanding

Islas

Lavabre

Grévy

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

127

View full text Add to dashboard Cite

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“…section.) 1,2 For the sake of simplicity, at this stage, it is assumed that 4 the reaction of (achiral) substrate (S) with C R yields only P R , and with C S only P S , i.e. 100% enantioselectivity; also, C R and C S do not act in concert.…”

Section: Resultsmentioning

confidence: 99%

“…[1][2][3] However, these models end up implying that all of the catalyst is converted to the complexes. This is because the starting substrates, reactants and (uncomplexed) catalyst do not appear in the final rate equations that are employed to arrive at the product e.e.…”

Section: A Critical Assessment Of Previous Approachesmentioning

confidence: 99%

“…[1][2][3][4][5][6] NLEAC is predicated on the reasonable assumption that in a reaction that is catalysed by a chiral catalyst, the e.e. of the product cannot be greater than the e.e.…”

Section: Introductionmentioning

confidence: 99%

“…of ~ 25%. [1][2][3][4][5][6] This indicates a relatively enhanced sensitivity of the product e.e. towards changes in the catalyst e.e., in the low catalyst e.e.…”

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confidence: 99%

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Non-linear Effects in Asymmetric Catalysis: Whys and Wherefores

Chandrasekhar

2012

Nature Precedings

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It is argued that the titled non-linear effects (NLE) may arise whenever the order of the reaction in the chiral catalyst in greater than 1. In a fundamental departure from previous approaches, this is mathematically elaborated for the second order case.(NLE may also be observed if the chiral catalyst forms non-reacting dimers in a competing equilibrium; practically, however, this implies the in situ resolution of the catalyst.) The amplification of enantiomeric excess by NLE implies a relative (although modest) reduction in the entropy of mixing. The consequent increase in free energy apparently indicates a non-equilibrium process. It is suggested, based on arguments involving the chemical potential, that kinetically-controlled reactions lead to a state of "quasi-equilibrium": in this, although overall equilibrium is attained, the product-spread is far from equilibrium. Thus, both the linear and NLE cases of chiral catalysis represent departures from equilibrium (which requires that the product e.e. = 0). Interesting similarities exist with models of non-equilibrium systems, the NLE cases apparently being analogs of open systems just after the bifurcation point has been crossed. 2Nature Precedings :

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