Dynamic Nonlinear Effects in Asymmetric Catalysis

Low Coordination State RhI -Complex as High Performance Catalyst for Asymmetric Intramolecular Cyclopropanation: Construction of penta-Substituted Cyclopropanes

A simple, broad-scope rhodium(I)/chiral diene catalytic system for challenging asymmetric intramolecular cyclopropanation of various tri-substituted allylic diazoacetates was successfully developed. The low coordination state Rh^I -complex exhibits an extraordinarily high degree of tolerance to the variation in the extent of substitution of the allyl double bond, thus allowing the efficient construction of a wide range of penta-substituted, fused-ring cyclopropanes bearing three contiguous stereogenic centers, including two quaternary carbon stereocenters, in a highly enantioselective manner with ease at catalyst loading as low as 0.1 mol %. The stereoinduction mode of this Rh^I -carbene-directed asymmetric intramolecular cyclopropanation was investigated by DFT calculations, indicating that π-π stacking interactions between the aromatic rings of chiral diene ligand and diazo substrate play a key role in the control of the reaction enantioselectivity.

Molecular Understanding and Practical In Silico Catalyst Design in Computational Organocatalysis and Phase Transfer Catalysis-Challenges and Opportunities

Through the lens of organocatalysis and phase transfer catalysis, we will examine the key components to calculate or predict catalysis-performance metrics, such as turnover frequency and measurement of stereoselectivity, via computational chemistry. The state-of-the-art tools available to calculate potential energy and, consequently, free energy, together with their caveats, will be discussed via examples from the literature. Through various examples from organocatalysis and phase transfer catalysis, we will highlight the challenges related to the mechanism, transition state theory, and solvation involved in translating calculated barriers to the turnover frequency or a metric of stereoselectivity. Examples in the literature that validated their theoretical models will be showcased. Lastly, the relevance and opportunity afforded by machine learning will be discussed.

Turning Enantiomeric Relationships into Diastereomeric Ones: Self-Resolving α-Ureidophosphonates and Their Organocatalytic Enantioselective Synthesis

Controlling chiral recognition and chiral information transfer has major implications in areas ranging from drug design and asymmetric catalysis to supra- and macromolecular chemistry. Especially intriguing are phenomena associated with chiral self-recognition. The design of systems that show self-induced recognition of enantiomers, i.e., involving homochiral versus heterochiral dimers, is particularly challenging. Here, we report the chiral self-recognition of α-ureidophosphonates and its application as both a powerful analytical tool for enantiomeric ratio determination by NMR and as a convenient way to increase their enantiomeric purity by simple achiral column chromatography or fractional precipitation. A combination of NMR, X-ray, and DFT studies indicates that the formation of homo- and heterochiral dimers involving self-complementary intermolecular hydrogen bonds is responsible for their self-resolving properties. It is also shown that these often unnoticed chiral recognition phenomena can facilitate the stereochemical analysis during the development of new asymmetric transformations. As a proof of concept, the enantioselective organocatalytic hydrophosphonylation of alkylidene ureas toward self-resolving α-ureidophosphonates is presented, which also led us to the discovery of the largest family of self-resolving compounds reported to date.

Possible chemical and physical scenarios towards biological homochirality

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

Data Science Meets Physical Organic Chemistry

ConspectusAt the heart of synthetic chemistry is the holy grail of predictable catalyst design. In particular, researchers involved in reaction development in asymmetric catalysis have pursued a variety of strategies toward this goal. This is driven by both the pragmatic need to achieve high selectivities and the inability to readily identify why a certain catalyst is effective for a given reaction. While empiricism and intuition have dominated the field of asymmetric catalysis since its inception, enantioselectivity offers a mechanistically rich platform to interrogate catalyst-structure response patterns that explain the performance of a particular catalyst or substrate.In the early stages of an asymmetric reaction development campaign, the overarching mechanism of the reaction, catalyst speciation, the turnover limiting step, and many other details are unknown or posited based on related reactions. Considering the unclear details leading to a successful reaction, initial enantioselectivity data are often used to intuitively guide the ultimate direction of optimization. However, if the conditions of the Curtin-Hammett principle are satisfied, then measured enantioselectivity can be directly connected to the ensemble of diastereomeric transition states (TSs) that lead to the enantiomeric products, and the associated free energy difference between competing TSs (ΔΔG⧧ = -RT ln[(S)/(R)], where (S) and (R) represent the concentrations of the enantiomeric products). We, and others, speculated that this important piece of information can be leveraged to guide reaction optimization in a quantitative way.Although traditional linear free energy relationships (LFERs), such as Hammett plots, have been used to illuminate important mechanistic features, we sought to develop data science derived tools to expand the power of LFERs in order to describe complex reactions frequently encountered in modern asymmetric catalysis. Specifically, we investigated whether enantioselectivity data from a reaction can be quantitatively connected to the attributes of reaction components, such as catalyst and substrate structural features, to harness data for asymmetric catalyst design.In this context, we developed a workflow to relate computationally derived features of reaction components to enantioselectivity using data science tools. The mathematical representation of molecules can incorporate many aspects of a transformation, such as molecular features from substrate, product, catalyst, and proposed transition states. Statistical models relating these features to reaction outputs can be used for various tasks, such as performance prediction of untested molecules. Perhaps most importantly, statistical models can guide the generation of mechanistic hypotheses that are embedded within complex patterns of reaction responses. Overall, merging traditional physical organic experiments with statistical modeling techniques creates a feedback loop that enables both evaluation of multiple mechanistic hypotheses and future catalyst design. In this Account, we highlight the evolution and application of this approach in the context of a collaborative program based on chiral phosphoric acid catalysts (CPAs) in asymmetric catalysis.

Silica Jar-with-Lid as Chemo-Enzymatic Nano-Compartment for Enantioselective Synthesis inside Living Cells

Nanodevices, harvesting the power of synthetic catalysts and enzymes to perform enantioselective synthesis inside cell, have never been reported. Here, we synthesized round bottom jar-like silica nanostructures (SiJARs) with a chemo-responsive metal-silicate lid. This was isolated as an intermediate structure during highly controlled solid-state nanocrystal-conversion at the arc-section of silica shell. Different catalytic noble metals (Pt, Pd, Ru) were selectively modified on the lid-section through galvanic reactions. And, lid aperture-opening was regulated by mild acidic conditions or intracellular environment which accommodated the metal nanocrystals and enzymes, and in turn created an open-mouth nanoreactor. Distinct from the free enzymes, SiJARs performed asymmetric aldol reactions with high activity and enantioselectivity (yield >99 %, ee=95 %) and also functioned as the artificial catalytic organelles inside living cells. This work bridges the enormous potential of sophisticated nanocrystal-conversion chemistry and advanced platforms for new-to-nature catalysis.

Mechanistic Insight into the Stereoselective Cationic Polymerization of Vinyl Ethers

The control of the tacticity of synthetic polymers enables the realization of emergent physical properties from readily available starting materials. While stereodefined polymers derived from nonpolar vinyl monomers can be efficiently prepared using early transition metal catalysts, general methods for the stereoselective polymerization of polar vinyl monomers remain underdeveloped. We recently demonstrated asymmetric ion pairing catalysis as an effective approach to achieve stereoselective cationic polymerization of vinyl ethers. Herein, we provide a deeper understanding of stereoselective ion-pairing polymerization through comprehensive experimental and computational studies. These findings demonstrate the importance of ligand deceleration effects for the identification of reaction conditions that enhance stereoselectivity, which was supported by computational studies that identified the solution-state catalyst structure. An evaluation of monomer substrates with systematic variations in steric parameters and functional group identities established key structure-reactivity relationships for stereoselective homo- and copolymerization. Expansion of the monomer scope to include enantioenriched vinyl ethers enabled the preparation of an isotactic poly(vinyl ether) with the highest stereoselectivity (95.1% ± 0.1 meso diads) reported to date, which occurred when monomer and catalyst stereochemistry were fully matched under a triple diastereocontrol model. The more complete understanding of stereoselective cationic polymerization reported herein offers a foundation for the design of improved catalytic systems and for the translation of isotactic poly(vinyl ether)s to applied areas.

Chemical Mechanisms of Prebiotic Chirality Amplification

This review article surveys the recent experimental findings that suggest alternative chemical models of directed chirality amplification at the early, prebiotic Earth. It is believed that the chirality amplification step followed the initial emergence of small enantiomeric imbalance and preceded (as a necessary condition) the occurrence of homochiral biopolymers, assembled from enantiomerically pure building blocks. This work focuses on the chemical nature of possible mechanisms of primordial chirality enhancement, without going into detail of the preceding and subsequent phases of origination of biological homochirality and life on Earth. These mechanisms are discussed through the prism of integrity of biological natural selection and chemical kinetic selection.

Possible Physical Basis of Mirror Symmetry Effect in Racemic Mixtures of Enantiomers: From Wallach’s Rule, Nonlinear Effects, B–Z DNA Transition, and Similar Phenomena to Mirror Symmetry Effects of Chiral Objects

Effects associated with mirror symmetry may be underlying for a number of phenomena in chemistry and physics. Increase in the density and melting point of the 50%L/50%D collection of enantiomers of a different sign (Wallach’s rule) is probably based on a physical effect of the mirror image. The catalytic activity of metal complexes with racemic ligands differs from the corresponding complexes with enantiomers as well (nonlinear effect). A similar difference in the physical properties of enantiomers and racemate underlies L/D inversion points of linear helical macromolecules, helical nanocrystals of magnetite and boron nitride etc., B–Z DNA transition and phenomenon of mirror neurons may have a similar nature. Here we propose an explanation of the Wallach effect along with some similar chemical, physical, and biological phenomena related to mirror image.

Chiral Autoamplification Meets Dynamic Chirality Control to Suggest Nonautocatalytic Chemical Model of Prebiotic Chirality Amplification

Oxidative kinetic resolution of 1-phenylethanol in the presence of manganese complexes, bearing conformationally nonrigid achiral bis-amine-bis-pyridine ligands, in the absence of any exogenous chiral additives, is reported. The only driving force for the chiral discrimination is the small initial enantiomeric imbalance of the scalemic (nonracemic) substrate: the latter dynamically controls the chirality of the catalyst, serving itself as the chiral auxiliary. In effect, the ee of 1-phenylethanol increases monotonously over the reaction course. This dynamic control of catalyst chirality by the substrate has been unprecedented; a consistent kinetic model for this process is presented. The reported catalyzed substrate self-enantioenrichment mechanism is discussed in relation to the problem of prebiotic chirality amplification.