Role of the isopropyl group in asymmetric autocatalytic zinc alkylations

One Soai reaction, two mechanisms?

For over 25 years the chemistry community has puzzled over the mechanism of the Soai reaction, a fascinating chemical process which achieves chiral symmetry breaking by combining autocatalysis with asymmetric amplification. In 2020, the groups of Denmark and Trapp each made a proposal, based on extensive experimental work, on what is the catalytic species there: either a tetrameric product alkoxide aggregate ("SMS tetramer") or a product-substrate dimer ("hemiacetal"). These models seemingly oppose and exclude each other; however, they might also be both valid since the studies were conducted on different substrates which are not necessarily equivalent. This is shown in this Viewpoint by an in-depth comparison of the two studies and of data from earlier reports, which opens up to a discussion on this scenario's far-reaching implications on the fundamental understanding of asymmetry-amplifying autocatalysis.

Circular dichroism spectroscopy of catalyst preequilibrium in asymmetric autocatalysis of pyrimidyl alkanol

Mechanistic understanding of the asymmetric autocatalysis of pyrimidyl alkanol is a highly attractive and challenging topic due to its unique feature of amplification of enantiomeric excess. Circular dichroism spectroscopic analysis of this reaction allows monitoring of the structual changes of possible catalyst precursors in the solution state and shows characteristic temperature and solvent dependence. TD-DFT calculations suggest that these spectral changes are induced by a dimer-tetramer equilibrium of zinc alkoxides.

Symmetry Breaking and Autocatalytic Amplification in Soai Reaction Confined within UiO-MOFs under Heterogenous Conditions

Symmetry breaking is observed in the Soai reaction in a confinement environment provided by zirconium‐based UiO‐MOFs used as crystalline sponges. Subsequent reaction of encapsulated Soai aldehyde with Zn(i‐Pr)₂ vapour promoted absolute asymmetric synthesis of the corresponding alkanol. ATR‐IR and NMR confirm integration of aldehyde into the porous material, and a similar localization of newly formed chiral alkanol after reaction. Despite the confinement, the Soai reaction exhibits significant activity and autocatalytic amplification. Comparative catalytic studies with various UiO‐MOFs indicate different outcomes in terms of enantiomeric excess, handedness distribution of the product and reaction rate, when compared to pristine solid Soai aldehyde, while the crystalline MOF remains highly stable to action of Zn(iPr)₂ vapour. This is an unprecedented example of absolute asymmetric synthesis using MOFs.

In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis

Chemical reactions that lead to a spontaneous symmetry breaking or amplification of the enantiomeric excess are of fundamental interest in explaining the formation of a homochiral world. An outstanding example is Soai's asymmetric autocatalysis, in which small enantiomeric excesses of the added product alcohol are amplified in the reaction of diisopropylzinc and pyrimidine-5-carbaldehydes. The exact mechanism is still in dispute due to complex reaction equilibria and elusive intermediates. In situ high-resolution mass spectrometric measurements, detailed kinetic analyses and doping with in situ reacting reaction mixtures show the transient formation of hemiacetal complexes, which can establish an autocatalytic cycle. We propose a mechanism that explains the autocatalytic amplification involving these hemiacetal complexes. Comprehensive kinetic experiments and modelling of the hemiacetal formation and the Soai reaction allow the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Experimental structural data give insights into the privileged properties of the pyrimidyl units and the formation of diastereomeric structures leading to an efficient amplification of even minimal enantiomeric excesses, respectively.

Structural Contributions to Autocatalysis and Asymmetric Amplification in the Soai Reaction

Diisopropylzinc alkylation of pyrimidine aldehydes-the Soai reaction, with its astonishing attribute of amplifying asymmetric autocatalysis-occupies a unique position in organic chemistry and stands as an eminent challenge for mechanistic elucidation. A new paradigm of "mixed catalyst-substrate" experiments with pyrimidine and pyridine systems allows a disconnection of catalysis from autocatalysis, providing insights into the role played by reactant and alkoxide structure. The alkynyl substituent favorably tunes catalyst solubility, aggregation, and conformation while modulating substrate reactivity and selectivity. The alkyl groups and the heteroaromatic core play further complementary roles in catalyst aggregation and substrate binding. In the study of these structure-activity relationships, novel pyridine substrates demonstrating amplifying autocatalysis were identified. Comparison of three autocatalytic systems representing a continuum of nitrogen Lewis basicity strength suggests how the strength of N-Zn binding events is a predominant contributor toward the rate of autocatalytic progression.

Demystifying the asymmetry-amplifying, autocatalytic behaviour of the Soai reaction through structural, mechanistic and computational studies

The Soai reaction has profoundly impacted chemists' perspective of autocatalysis, chiral symmetry breaking, absolute asymmetric synthesis and its role in the origin of biological homochirality. Here we describe the unprecedented observation of asymmetry-amplifying autocatalysis in the alkylation of 5-(trimethylsilylethynyl)pyridine-3-carbaldehyde using diisopropylzinc. Kinetic studies with a surrogate substrate and spectroscopic analysis of a series of zinc alkoxides that incorporate specific structural mutations reveal a 'pyridine-assisted cube escape'. The new tetrameric cluster functions as a catalyst that activates the substrate through a two-point binding mode and poises a coordinated diisopropylzinc moiety for alkyl group transfer. Transition-state models leading to both the homochiral and heterochiral products were validated by density functional theory calculations. Moreover, experimental and computational analysis of the heterochiral complex provides a definitive explanation for the nonlinear behaviour of this system. Our deconstruction of the Soai system reveals the structural logic for autocatalyst evolution, function and substrate compatibility-a central mechanistic aspect of this iconic transformation.

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.

By-design enantioselective self-amplification based on non-covalent product-catalyst interactions

The synthesis of enantiomerically pure compounds is of great importance in pharmaceuticals, fragrances and biological applications, and functions as a key to many processes in nature. Asymmetric catalysis using enantiomerically pure catalysts represents an efficient synthetic method to achieve this goal. The enantiomeric excess of the reaction product correlates with the enantiomeric purity of the catalysts, except for nonlinear behaviour, therefore the use of stereochemically flexible catalysts seems to complicate the control of stereoselectivity. Self-amplifying catalytic reactions are attractive, but a general rational design is highly challenging. Here we show that product interaction with chiral recognition sites attached to structurally flexible phoshoramidite-type catalysts can sense the chirality and induce enantioselectivity in the catalyst. Structural flexibility along with sensing of the chirality of the product molecules results in a rapid increase of enantioselectivity of the dynamic catalysts (Δe.e. of up to 76%) and a shift out of equilibrium. In contrast to stereodynamic catalysts controlled with cleavable chiral auxiliaries, the enantioselectivity does not decrease.

Zebra reaction or the recipe for the synthesis of heterodimeric zinc complexes

A series of asymmetric heterodimeric zinc complexes have been synthesized in a direct reaction between conformationally flexible chiral/achiral homodimers. The cooperative activity of steric factors and coordination codes resulted in an intriguing chiral self-sorting process. Herein, we are reporting our recent exploration of the first example of such a type of reaction.

Crystal Structure of the Isopropylzinc Alkoxide of Pyrimidyl Alkanol: Mechanistic Insights for Asymmetric Autocatalysis with Amplification of Enantiomeric Excess

Asymmetric amplification during self-replication is a key feature that is used to explain the origin of homochirality. Asymmetric autocatalysis of pyrimidyl alkanol in the asymmetric addition of diisopropylzinc to pyrimidine-5-carbaldehyde is a unique example of this phenomenon. Crystallization of zinc alkoxides of this 5-pyrimidyl alkanol and single-crystal X-ray diffraction analysis of the alkoxide crystals reveal the existence of tetramer or higher oligomer structures in this asymmetric autocatalytic system.

Amplification of enantiomeric excess, mirror-image symmetry breaking and kinetic proofreading in Soai reaction models with different oligomeric orders

A comprehensive kinetic analysis of three prototypical autocatalytic cycle models based on the absolute asymmetric Soai reaction is presented. The three models, which can give rise to amplification of enantiomeric excess and mirror-image symmetry breaking, vary by their monomeric, dimeric or trimeric order of the assumed catalytic species. Our numerical approach considered the entire chiral combinatorics of the diastereomeric interactions in the models as well as the multiplicity of coupled reversible reactions without applying fast equilibration or quasi-steady state approximations. For the simplest monomeric model, an extensive range of parameters was explored employing a random grid parameter scanning method that revealed the influence of the parameter values on the product distribution, the reaction-time, the attenuation or amplification of enantiomeric excess as well as on the presence or absence of mirror-image symmetry breaking. A symmetry breaking test was imposed on the three models showing that an increase in the catalytic oligomer size from one to three leads to a higher tolerance to poorer chiral recognition between the diastereoisomers and identifies the greater impact of the diastereoisomeric energy difference over an imperfect stereoselectivity in the catalytic step. This robustness is understood as a particular case of so-called kinetic proofreading in asymmetric autocatalysis.

A concise summary of experimental facts about the Soai reaction

The Soai reaction amplifies small enantiomeric excesses in a spectacular manner. Being known for 20 years, it has drawn the attention of many scientists in different fields as it is to date the only chemical reaction offering the chance to study the phenomenon of asymmetric autocatalysis in conjunction with high amplification of enantiomeric excess (ee). This mini-review comprises an introduction to the discovery of asymmetric autocatalysis with amplification of ee and a concise summary of published experimental results showing which starting materials and reaction parameters play an important role in this reaction and which influences are understood. It is addressed especially to scientists entering the field of the Soai reaction to get a quick overview of important aspects.

Putting the mechanism of the Soai reaction to the test: DFT study of the role of aldehyde and dialkylzinc structure

Previous DFT calculations provided support to the proposal that the Soai reaction involves a mechanism in which dimer catalysts serve as templates for the reaction of two molecules of dialkylzinc with two molecules of aldehyde so as to reproduce themselves (ref 11). Here it is shown that, from the point of view of formal kinetics, this mechanism can be reduced to a general model, dubbed the extended dimer model, that has the Blackmond-Brown dimer model as a particular case. Depending on the interplay of kinetic constants, the extended dimer model can give rise to either chiral amplification or depletion. Calculations of the kinetic constants at the M05-2X/6-31G(d) level of theory were carried out in order to theoretically evaluate the effect of the second aza group in the six-membered aromatic ring of the aldehydic substrate and the effect of dialkylzinc structure. Predictions of chiral amplification or depletion are in striking agreement with experimental data thus lending support to the proposed mechanism.

Unusual inverse temperature dependence on reaction rate in the asymmetric autocatalytic alkylation of pyrimidyl aldehydes

Observations of an intriguing inverse temperature dependence on reaction rate and a profound induction period in the Soai autocatalytic reaction are reported along with detailed kinetic and NMR investigations of the product alkoxide at low temperatures, leading to the suggestion that the active catalyst is derived in situ from the tetrameric ground state.

Mechanism of the asymmetric autocatalytic Soai reaction studied by density functional theory

The mechanism of the Soai reaction has been thoroughly investigated at the M05-2X/6-31G(d) level of theory, by considering ten energetically distinct paths. The study indicates the fully enantioselective catalytic cycle of the homochiral dimers to be the dominant mechanism. Two other catalytic cycles are shown to both be important for correct understanding of the Soai reaction. These are the catalytic cycle of the heterochiral dimer and the non-enantioselective catalytic cycle of the homochiral dimers. The former has been proved to be not really competitive with the principal cycle, as required for the Soai reaction to manifest chiral amplification, whereas the latter, which is only slightly competitive with the principal one, nicely explains the experimental enantioselectivity observed in the reaction of 2-methylpyrimidine-5-carbaldehyde. The study has also evidenced the inadequacy of the B3LYP functional for mechanistic investigations of the Soai reaction.

Asymmetric autocatalysis induced by chiral crystals of achiral tetraphenylethylenes

The achiral hydrocarbon tetraphenylethylene crystallizes in enantiomorphous forms (chiral space group: P2(1)) to afford right- and left-handed hemihedral crystals, which can be recognized by solid-state circular dichroism spectroscopic analysis. Chiral organic crystals of tetraphenylethylene mediated enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde to give, in conjunction with asymmetric autocatalysis with amplification of chirality, almost enantiomerically pure (S)- and (R)-5-pyrimidyl alkanols whose absolute configurations were controlled efficiently by the crystalline chirality of the tetraphenylethylene substrate. Tetrakis(p-chlorophenyl)ethylene and tetrakis(p-bromophenyl)ethylene also show chirality in the crystalline state, which can also act as a chiral substrate and induce enantioselectivity of diisopropylzinc addition to pyrimidine-5-carbaldehyde in asymmetric autocatalysis to give enantiomerically enriched 5-pyrimidyl alkanols with the absolute configuration correlated with that of the chiral crystals. Highly enantioselective synthesis has been achieved using chiral crystals composed of achiral hydrocarbons, tetraphenylethylenes, as chiral inducers. This chemical system enables significant amplification of the amount of chirality using spontaneously formed chiral crystals of achiral organic compounds as the seed for the chirality of asymmetric autocatalysis.

Amplification of chirality and enantioselectivity in the asymmetric autocatalytic Soai reaction

From a kinetic analysis of the "dimer model", which is the most prominent mechanism of the Soai reaction, an equation is derived predicting the amplification of enantiomeric excess as a function of initial conditions. The role played by the enantioselectivity of the catalyst-product is also taken into account. Comparison with experimental data obtained at 0 degrees C by Soai et al. shows that the predicted enantiomeric excesses are lower than the experimental values by up to four orders of magnitude, and thus revision of the dimer model in the low-temperature regime is warranted. A kinetic analysis including the formation of tetramers is presented that fits the data at 0 degrees C and indicates that 2:2 heterochiral tetramers are more stable than homochiral and 3:1 heterochiral tetramers. A DFT study on diastereomers of barrel-like tetramers indeed shows higher stability of 2:2 heterochiral tetramers and thus lends support to the above kinetic analysis.

Systematic studies using 2-(1-adamantylethynyl)pyrimidine-5-carbaldehyde as a starting material in Soai's asymmetric autocatalysis

Herein, we present a new substrate for the Soai reaction, which has an adamantylethynyl residue (1 g) and exhibits asymmetric autocatalysis, yielding products with enantiomeric excesses above 99%. For the first time, all reactions were performed on a parallel synthesizer system to ensure identical reaction conditions. A detailed systematic study of reaction parameters was performed and we report the highest enhancements of enantiomeric excess reported so far in the Soai reaction in one reaction cycle (7.2-->94.1% ee or 3.1-->92.1% ee). Our results led to a set of reaction parameters that yield reproducible results. Therefore, our new starting material 1 g is suitable for systematic and mechanistic studies on this remarkable reaction. A series of experiments designed to quantify the amplification of enantiomeric excess demonstrated that the reaction can be used in principle as a tool for the detection of low enantiomeric excesses: under definite conditions, an unknown low enantiomeric excess (0.1-7%) was amplified to a detectable one. A back calculation to the original value offers a new method for the determination of small enantiomeric excesses.

Asymmetric autocatalysis induced by meteoritic amino acids with hydrogen isotope chirality

Achiral meteoritic amino acids, glycine and alpha-methylalanine, with hydrogen isotope (D/H) chirality, acted as the source of chirality in asymmetric autocatalysis with amplification of ee to afford highly enantioenriched 5-pyrimidyl alkanols.