The allylboration of 1-Ar-1H-1,2,3-triazole-4-carbaldehydes with triallylborane was shown by combined DFT computations and experiments (Mikhailov, Gurskii, Kurbangalieva and Gridnev, Int. J. Mol. Sci. 2024) to contain three computational sources of mirror-symmetry breaking — sequestration of one enantiomer in a more stable heterochiral dimer (reservoir mechanism), enantioselective transfer of the second allyl group from a chiral boronate, and autocatalytic regeneration of two same-handed boronate molecules from a borinic ester — yet experiments gave only low, inconsistently handed enantiomeric excesses, and the triallylborane stage is too fast (reacting even at −100 °C) for the racemic intermediate to accumulate before a symmetry-breaking bifurcation. The open problem is to determine the border conditions — the required balance of dimerization equilibria, enantioselective transfer kinetics, regeneration energetics, and reagent reactivity (e.g., less reactive diallylboronate or aminoallylborane reagents) — under which formally non-catalytic allylboration of triazolic aldehydes undergoes Frank-type spontaneous mirror-symmetry breaking amplified to reproducibly measurable enantiomeric excess, and to demonstrate or rigorously refute the phenomenon experimentally, together with a quantitative kinetic-thermodynamic model predicting the observed enantiomeric-excess magnitudes and stochastic handedness statistics. A 2025/2026 follow-up with diallylboronates proposed a mechanism for the random handedness but confirmed that amplification remains unrealized outside the Soai reaction.

Background

Absolute asymmetric synthesis is the formation of an enantiomerically enriched product from achiral precursors without any added chiral source. Its only established molecular example with amplification is the Soai reaction (Soai, Shibata, Morioka and Choji, Nature 1995): diisopropylzinc addition to specifically substituted pyrimidine-5-carbaldehydes gives a chiral pyrimidyl alkanol that catalyzes its own formation, so a tiny stochastic enantiomeric imbalance is exponentially amplified to near-enantiopurity. The classical theoretical template is Frank's 1953 scheme, in which an autocatalytic species that reproduces its own handedness, combined with mutual inhibition of the two enantiomers, makes the racemic state unstable. Kinetic analyses of chiral symmetry breaking in non-equilibrium systems (Kondepudi and Nelson, Phys. Rev. Lett. 1983) show that the amplifying bifurcation requires the initially racemic intermediate to accumulate for a characteristic time before the system reaches the bifurcation point, i.e., sufficiently slow reactions; the structural and mechanistic basis of the Soai amplification has been quantified only recently (Athavale, Simon, Houk and Denmark, Nat. Chem. 2020). The candidate system extends this question to a formally non-catalytic organoboron reaction. Triallylborane adds an allyl group to 1-Ar-1H-1,2,3-triazole-4-carbaldehydes (heterocyclic aldehydes bearing a five-membered azole ring with three adjacent nitrogens), giving a chiral boronate alcoholate 4 in which a new stereocenter is formed at the carbinol carbon and two allyl groups remain bound to boron. DFT computations (density-functional theory at the dispersion-corrected ωB97XD/6-31G(d,p) level with the SMD implicit-solvent continuum model) found three possible symmetry-breaking elements: (i) dimerization of 4 gives homochiral dimers 5 and heterochiral dimers 6, with the heterochiral dimer computed to be thermodynamically more stable, so that binding of the minority enantiomer in dimers enriches the solution in the majority one (the reservoir mechanism, a chemical autocatalysis-related amplification route known from nonlinear effects in asymmetric catalysis); (ii) the second allyl transfer from optically active 4 to another aldehyde molecule is enantioselective, activated and stereoregulated by intramolecular dispersive interactions (weak, non-directional van-der-Waals-type attractions, here CH⋯π contacts) of the 1,2,3-triazole ring; and (iii) the resulting borinic ester 8 reacts with triallylborane to recover two molecules of 4 of the same handedness, lending the whole sequence an autocatalytic character. Experimentally, however, reacting R-enriched boronates with the aldehydes gave products of low optical yield and, surprisingly, both handednesses across replicate runs (seven S and three R outcomes in ten trials for one substrate; six S and four R for another), which the authors could not explain mechanistically. Two obstacles were identified: the computed dimerization and regeneration energetics are substrate-dependent and in some cases too weak or too endergonic for significant amplification, and the primary allylboration by triallylborane is so fast (reacting even at −100 °C) that the racemic intermediate cannot accumulate before the bifurcation point; the authors therefore proposed using less reactive allylboration reagents, accessible by replacing one or two allyl groups with electron-donating OR or NR substituents. Prior work by the same group on triazolic aldehydes with diisopropylzinc (Mikhaylov, Saigitbatalova, Latypova, Kurbangalieva and Gridnev, Symmetry 2023) showed that the outcome — chirality amplification, conservation, or dissipation — depends on the oligomerization state of the catalyst, underscoring how sensitively these multi-equilibrium systems respond to structure.

Problem Statement

Determine the border conditions — the required balance of thermodynamic and kinetic parameters of the multi-stage reaction cycle — under which the allylboration of 1,2,3-triazolic aldehydes with allylboron reagents exhibits genuine spontaneous emergence of macroscopic chirality: stochastic mirror-symmetry breaking at the microscopic level amplified, by the coupled reservoir, enantioselective-transfer, and autocatalytic-regeneration steps, to reproducibly measurable enantiomeric excess in a formally non-catalytic reaction following Frank's scheme, and demonstrate or rigorously refute the phenomenon experimentally. This single objective comprises jointly linked components: (1) identify substrate structures and reaction conditions for which the heterochiral-dimer reservoir and the regeneration of active monomers are thermodynamically and kinetically sufficient for net amplification rather than dissipation of the initial imbalance; (2) determine how the reactivity of the allylboration reagent must be moderated within this balance, so that the primary allylboration is slow enough for the racemic intermediate to accumulate before the bifurcation point while preserving enantioselective allyl transfer and amplification — for which the source study proposed less reactive reagents with electron-donating OR or NR substituents replacing one or two allyl groups (e.g., diallylboronates or aminoallylboranes) as a promising example; (3) establish a quantitative kinetic-thermodynamic model, including the conformational ensembles of the boronates, that predicts the observed enantiomeric-excess magnitudes and the stochastic distribution of product handedness under experimental conditions; and (4) demonstrate or rigorously refute the phenomenon experimentally, with the statistical distribution of handedness across replicate reactions, under controls excluding trace chiral contamination, serving as the criterion of authenticity. All four components are jointly required for a complete resolution: (1)–(3) constitute the modelling sub-goal, so a result covering (1)–(3) without the experimental demonstration or refutation of (4) resolves only that sub-goal and not the task. Components (1) and (2) are not independent — (2) states the reagent-reactivity (rate-balance) facet of the conditions in (1) and must be addressed alongside it. 'Reproducibly measurable enantiomeric excess' means an enantiomeric excess that exceeds the experimental measurement uncertainty, with the handedness varying stochastically across replicate runs; the distribution of handedness over replicate runs, measured under controls excluding trace chiral contamination, is the criterion of authenticity.

References

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