Asymmetric Autocatalysis Research Articles

Achiral amino acid glycine acts as an origin of homochirality in asymmetric autocatalysis.

Chiral crystals of the only achiral proteinogenic α-amino acid, glycine induced the asymmetric autocatalysis with amplification of enantiomeric excess (ee). The P31 crystals of γ-glycine, which display positive Cotton effect (CD) at around 215 nm, mediate the asymmetric autocatalysis to yield (R)-pyrimidyl alkanol with high ee. In contrast, the enantiomorphic P32 crystals, which display negative Cotton effect, afford (S)-alkanol after the significant amplification of ee by asymmetric autocatalysis.

Formation of enantioenriched alkanol with stochastic distribution of enantiomers in the absolute asymmetric synthesis under heterogeneous solid-vapor phase conditions.

Among several theories proposed for the origin of homochirality, absolute asymmetric synthesis is unique because it produces chiral compounds without the intervention of any chiral factor. Here we report on the kinetically controlled heterogeneous solid-vapor phase absolute asymmetric synthesis in conjunction with asymmetric autocatalysis with amplification of chirality. Each reaction, carried out in a test tube, between achiral powder crystals of pyrimidine-5-carbaldehyde and the vapor of diisopropylzinc, is controlled kinetically to afford either (S)- or (R)-pyrimidyl alkanol.

Trajectory of the Discovery of Asymmetric Autocatalysis and the Study on the Origin of Homochirality

Trajectory of the Discovery of Asymmetric Autocatalysis and the Study on the Origin of Homochirality

Proline autocatalysis in the origin of biological enantioenriched chirality.

Biological enantioenriched chirality is a phenomenon that in living organisms, amino acids and carbohydrates typically have the same absolute configuration. Perhaps one of the earliest attempts to delineate the origins of this phenomenon was a theory known as asymmetric autocatalysis, a reaction in which the structures of the chiral catalyst and the product are the same, and in which the chiral product acts as a chiral catalyst for its own production. In theory, this would mean that small asymmetries in the product will propagate rapidly. However, autocatalysis also relies on the cross-inhibition of chiral states, something that would not likely be possible on primordial Earth. But recently, theories on asymmetric autocatalysis have begun to resurface as more recent findings indicate that other mechanisms exist to stabilize the homochiral states. In this study, I propose an autocatalytic cycle, and using density functional theory, prove that (1) it is plausible on primordial Earth, and (2) it propagates arbitrary asymmetries in proline. Thus, facilitating asymmetry in proline and allowing access to a wide variety of asymmetric proline-catalyzed reactions, including those involved in the synthesis of amino acids and carbohydrates from achiral precursors.

Rationalization of Asymmetric Amplification via Autocatalysis Triggered by Isotopically Chiral Molecules

Asymmetric amplification induced in the Soai autocatalytic reaction by chiral initiators that are enantiomeric only by virtue of an isotope, e.g., −CH3 vs CD3–, is examined by spectroscopic, kinetic, and DFT modeling studies to help understand requirements for the emergence of biological homochirality. We find that the initiator inhibits the autocatalytic pathway at the outset of the reaction but ultimately provides the imbalance required for asymmetric amplification. This work provides clues in the ongoing search for prebiotically plausible versions of asymmetric autocatalysis.

Isotope chirality in long-armed multifunctional organosilicon ("Cephalopod") molecules.

Long-armed multifunctional organosilicon molecules display self-replicating and self-perfecting behavior in asymmetric autocatalysis (Soai reaction). Two representatives of this class were studied by statistical methods aiming at determination of probabilities of natural abundance chiral isotopomers. The results, reported here, show an astonishing richness of possibilities of the formation of chiral isotopically substituted derivatives. This feature could serve as a model for the evolution of biological chirality in prebiotic and early biotic stereochemistry.

On the asymmetric autocatalysis of aldol reactions: The case of 4-nitrobenzaldehyde and acetone. A critical appraisal with a focus on theory.

Under neutral conditions, spontaneous mirror symmetry breaking has been occasionally reported for aldol reactions starting from achiral reagents and conditions. Chiral induction might be interpreted in terms of autocatalysis exerted by chiral mono-aldol or bis-aldol products as source of initial enantiomeric excesses, which may account for such experimental observations. We describe here a thorough Density Functional Theory (DFT) study on this complex and otherwise difficult problem, which provides some insights into this phenomenon. The picture adds further rationale to an in-depth analysis by Moyano et al, who showed the isolation and characterization of bis-aldol adducts and their participation in a complex network of reversible steps. However, the lack of enantiodiscrimination (ees vanish rapidly in solution) suggests, according to the present results, a weak association in complexes formed by the catalysts and substrates. The latter would also be consistent with almost flat transition states having similar heights for competitive catalyst-bound transition structures (actually, we were unable to locate them at the level explored). Overall, neither autocatalysis as once conjectured nor mutual inhibition of enantiomers appears to be operating mechanisms. Asymmetric amplification in early stages harnessing unavoidable enantiomeric imbalances in reaction mixtures of chiral products represents a plausible interpretation.

Asymmetric Autoamplification in the Oxidative Kinetic Resolution of Secondary Benzylic Alcohols Catalyzed by Manganese Complexes

AbstractHerein, chiral Mn–aminopyridine complexes have been shown to catalyze the oxidation of alkylarenes to enantiomerically enriched 1‐arylalkanols with hydrogen peroxide. The observed enantiomeric excess values result from the direct enantioselective benzylic C−H hydroxylation, accompanied by stereoconvergent oxidative kinetic resolution of the resulting alcohol. Testing several (S,S)‐bipyrrolidine derived Mn complexes has revealed a novel catalyst (6) that exhibits the best kinetic resolution in the series (krel up to 8.8), along with sufficient reactivity and efficiency (>1000 catalytic turnovers). The mechanistic study of the Mn‐mediated alcohol oxidation witnesses electrophilic active species (ρ=−1.2), with rate‐limiting H abstraction (kH/kD=2.2), followed by oxygen rebound and dehydration of the resulting gem‐diol to form the ketone. Intriguingly, while for the resolution of the relatively bulky 1,2‐diphenylethanol, krel is virtually constant throughout the reaction, for less bulky alcohols, krel increases with increasing conversion, in line with the rising optical purity of the 1‐arylalkanol. The latter participates in the oxidation as an auxiliary ligand, assisting the chiral recognition. This effect is related to the previously described asymmetric autocatalysis and asymmetric autoinduction, but is not identical with either of those, with the differences being discussed. To unambiguously identify this effect, the term asymmetric autoamplification (chiral autoamplification) is proposed.

Advances in Asymmetric Autocatalysis and Related Topics

Advances in Asymmetric Autocatalysis and Related Topics

Unusual reversal of enantioselectivity in the asymmetric autocatalysis of pyrimidyl alkanol triggered by chiral aromatic alkanols and amines.

Temperature dependent inversion of enantioselectivity in asymmetric catalysis is an interesting and somewhat unusual phenomenon. We have observed temperature dependent inversion of enantioselectivity in the asymmetric autocatalysis reaction when triggered by a wide scope of enantioenriched alcohols and amines. The addition reaction of diisopropylzinc to pyrimidine-5-carbaldehyde in the presence of enantiopure alcohols or amines affords the pyrimidyl alkanol product at 0 °C with high ee. However, lowering the reaction temperature to -44 °C affords the opposite enantioselectivity.

Reversal of the sense of enantioselectivity between 1- and 2-aza[6]helicenes used as chiral inducers of asymmetric autocatalysis.

Reversal of the sense of enantioselectivity was observed between 1-aza[6]helicene 2 and 2-aza[6]helicene 3 employed as chiral inducers of asymmetric autocatalysis of pyrimidyl alkanol. In the presence of (P)-(+)-1-aza[6]helicene 2, the reaction of pyrimidine-5-carbaldehyde 1 with diisopropylzinc afforded, in conjunction with asymmetric autocatalysis, (S)-pyrimidyl alkanol 4 with high ee. Surprisingly, the reaction in the presence of (P)-(+)-2-aza[6]helicene 3 gave the opposite enantiomer of (R)-alkanol 4 with high ee. In the same manner, (M)-(-)-2 and (M)-(-)-3 afforded (R)- and (S)-alkanol 4, respectively. The sense of enantioselectivity is controlled not only by the helicity of the azahelicene derivatives but also by the position of the nitrogen atom.

Achiral Inorganic Gypsum Acts as an Origin of Chirality through Its Enantiotopic Surface in Conjunction with Asymmetric Autocatalysis

AbstractAchiral inorganic gypsum (CaSO4⋅2 H2O) triggers the asymmetric autocatalysis of pyrimidyl alkanol on its two‐dimensional enantiotopic faces to give highly enantioenriched alkanol products with absolute configurations corresponding to the respective enantiotopic surfaces. This is the first example of highly enantioselective synthesis on the enantiotopic surface of an achiral mineral.

Achiral Inorganic Gypsum Acts as an Origin of Chirality through Its Enantiotopic Surface in Conjunction with Asymmetric Autocatalysis.

Achiral inorganic gypsum (CaSO4 ⋅2 H2 O) triggers the asymmetric autocatalysis of pyrimidyl alkanol on its two-dimensional enantiotopic faces to give highly enantioenriched alkanol products with absolute configurations corresponding to the respective enantiotopic surfaces. This is the first example of highly enantioselective synthesis on the enantiotopic surface of an achiral mineral.

Point-to-Point Ultra-Remote Asymmetric Control with Flexible Linker.

An ultra-remote intramolecular (point-to-point) asymmetric control through 38 bonds (1,39-asymmetric induction) has been achieved by using the principle of direct supramolecular orientation of catalytic and reactive moieties in asymmetric autocatalysis. We found the highly stereoselective diisopropylzinc addition reaction using designed molecules possessing pyrimidine sites at each terminal of a conformationally flexible simple methylene chain.

Cover Picture: Asymmetric Induction by a Nitrogen 14N/15N Isotopomer in Conjunction with Asymmetric Autocatalysis (Angew. Chem. Int. Ed. 49/2016)

Cover Picture: Asymmetric Induction by a Nitrogen ¹⁴N/¹⁵N Isotopomer in Conjunction with Asymmetric Autocatalysis (Angew. Chem. Int. Ed. 49/2016)

Asymmetric Induction by a Nitrogen 14N/15N Isotopomer in Conjunction with Asymmetric Autocatalysis

AbstractChirality arising from isotope substitution, especially with atoms heavier than the hydrogen isotopes, is usually not considered a source of chirality in a chemical reaction. An N2,N2,N3,N3‐tetramethyl‐2,3‐butanediamine containing nitrogen (14N/15N) isotope chirality was synthesized and it was revealed that this isotopically chiral diamine compound acts as a chiral initiator for asymmetric autocatalysis.

Asymmetric Induction by a Nitrogen 14 N/15 N Isotopomer in Conjunction with Asymmetric Autocatalysis.

Chirality arising from isotope substitution, especially with atoms heavier than the hydrogen isotopes, is usually not considered a source of chirality in a chemical reaction. An N 2 ,N 2 ,N 3 ,N 3‐tetramethyl‐2,3‐butanediamine containing nitrogen (14N/15N) isotope chirality was synthesized and it was revealed that this isotopically chiral diamine compound acts as a chiral initiator for asymmetric autocatalysis.

Determination of the absolute structure of the chiral crystal consisting of achiral dibutylhydroxytoluene and asymmetric autocatalysis triggered by this chiral crystal

Absolute structure is determined of chiral crystal composed of achiral dibutylhydroxytoluene (BHT, 2,6-di-tert-butyl-p-cresol) by single crystal X-ray diffraction and circular dichroism. Left-handed (M-) crystals exhibited a positive Cotton effect at a wavelength of approximately 288nm observed using KBr disk [CD(+)288KBr]. In contrast, right-handed (P-) crystals exhibited a negative Cotton effect at the same wavelength [CD(−)288KBr]. Asymmetric autocatalysis is triggered by the chiral crystals of achiral BHT, i.e., M-(+)-BHT afforded (S)-pyrimidyl alkanol and P-(−)-BHT afforded (R)-pyrimidyl alkanol.

Elucidation of the Structures of Asymmetric Autocatalyst Based on X-ray Crystallography

Abstract Much interest has been paid to the origin of homochirality, such as that of l-amino acids. The proposed origins of chirality have usually induced only low enantiomeric excess. Thus, asymmetric amplification by asymmetric autocatalysis has been invoked. Asymmetric autocatalysis of 5-pyrimidyl alkanol in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde is a unique example of amplification of chirality. Crystallization of possible intermediates, specifically zinc alkoxides of this 5-pyrimidyl alkanol, was investigated to obtain mechanistic insights into this reaction. Single-crystal X-ray crystallography of the formed zinc alkoxide crystals shows the existence of tetramer or higher oligomer structures in this asymmetric autocatalytic system.

Asymmetric Autocatalysis as a Relay for Remote Amplification of Chirality of Target Molecules Used as Triggers.

Nearly racemic target molecules are enantiomerically enriched through an asymmetric autocatalytic relay for a remote amplification of chirality. Target alkynols with very low initial ee act as chiral triggers for asymmetric amplification of the Soai autocatalyst, which in turn enables the formation of the same alkynols with greater enantiomeric purity. Additionally, the stereochemical correlation between the trigger/target and autocatalyst molecules is discussed.