Asymmetric Catalytic Activity Research Articles

Enantioselective Synthesis of Homochiral Hierarchical Nd8Fe3-Oxo Cluster from Racemic Nd9Fe2-Oxo Cluster.

Enantioselective synthesis of homochiral rare earth clusters is still a great challenge. In this work, we developed an efficient "cluster to cluster" approach, that is, a pair of enantiomerical R/S-{Nd8Fe3}-oxo clusters were successfully obtained from the presynthesized racemic {Nd9Fe2}-oxo cluster. R/S-hydrobenzoin ligands trigger the transformation of the pristine clusters by an SN2-like mechanism. Compared to the pristine cluster with an achiral core, the new cluster exhibits hierarchical chirality, from ligand chirality to interface chirality, then to helix chirality, and finally to supramolecular double helix chirality. The spectral experiments monitored the transformation and confirmed distinctly structure-related optical activity. The enantiomeric pure cluster also exhibits a potential asymmetric catalytic activity.

Controlled synthesis of chiral carbon dots with high asymmetric catalytic properties and reproducibility for direct aldol reactions: The impact of functional groups of carbon sources

The controlled synthesis of carbon dots (CDs) has been a key scientific problem in the field of catalysis. In this study, chiral CDs from different carbon sources were prepared using the hydrothermal method with d-proline as the chiral source. The effects of the carbon source skeleton (conjugated and unconjugated) and functional groups (hydroxyl, carboxyl, and amino) on its structure, reproducibility, and asymmetric catalytic performance in Aldol condensation were investigated. It was observed that hydroxyl groups in the carbon source are more favorable for enhancing the asymmetric catalytic activity and reproducibility of CDs compared to amino and carboxyl groups. The mechanism for the difference in reproducibility and asymmetric catalytic properties is investigated, which is ascribed to the effect of bonding (Directional factor or Random factor) between active sites and carbon cores, as well as the effect of carbonization (Random factor). The directional factor contributes to high reproducibility and asymmetric catalytic activity.

A Simple Screening and Optimization Bioprocess for Long-Chain Peptide Catalysts Applied to Asymmetric Aldol Reaction.

Peptides have demonstrated their efficacy as catalysts in asymmetric aldol reactions. But the constraints inherent in chemical synthesis have imposed limitations on the viability of long-chain peptide catalysts. A noticeable dearth of tools has impeded the swift and effective screening of peptide catalysts using biological methods. To address this, we introduce a straightforward bioprocess for the screening of peptide catalysts for asymmetric aldol reactions. We synthesized several peptides through this method and obtained a 15-amino acid peptide. This peptide exhibited asymmetric aldol catalytic activity, achieving 77% ee in DMSO solvent and 63% ee with over an 80.8% yield in DMSO mixed with a pH 9.0 buffer solution. The successful application of our innovative approach not only represents an advancement but also paves the way for currently unexplored research avenues.

Asymmetric catalysis within chiral zeolitic nanospaces: Chiral host-guest match in GTM-3 zeolite

In this work, the asymmetric catalytic activity of the recently discovered GTM-3 chiral zeolite is systematically explored for the ring-opening of trans-stilbene oxide using alcohols of different size as nucleophiles. Catalytic results show a volcano-type curve with a clear maximum enantioselective activity for a specific linear alcohol size of 4C (1-butanol); this is perfectly reproduced in opposite sign for GTM-3 catalysts obtained with the antipode of the structure-directing agent. Such trend evidences that a proper chiral host-guest size match must be established between the reactants and/or transition states and the chiral confined space provided by the zeolite framework in order for the zeolite catalyst to manifest its chirality. Moreover, secondary alcohols, bulkier in size, give higher enantioselectivities than their linear counterparts. On the other hand, variation of the reaction conditions enables to maximize the enantioselectivity of the reaction: in particular, variation of the concentration of the epoxide displays a strong influence, notably improving the enantiomeric excess obtained when the concentration of the epoxide is reduced. Modification of the reaction temperature also influences the enantiomeric excess, and especially, the selectivity to the different products. Our results suggest that if a proper chiral host-guest match is established within the confined chiral nanospace of the zeolite framework, then the enantioselectivity as well as the selectivity to the chiral products is maximized, which indeed can be manipulated through variations of the reaction conditions, leading to unprecedented enantiomeric excess values up to 51%.

Axially Chiral Borinic Acid Catalysts: Design, Synthesis, and Application in Alkylative Desymmetrization of 1,2-Diols.

A novel chiral borinic acid (CBA), an organocatalyst possessing a binaphthyl skeleton, was designed and synthesized. The synthesis of CBA was achieved with a 72% yield in four steps starting with optically pure 1,1'-bi-2-naphthol. The asymmetric catalytic activity was investigated in the desymmetrization of meso-1,2-diol.

Chirality Analysis of Complex Microparticles using Deep Learning on Realistic Sets of Microscopy Images

Nanoscale chirality is an actively growing research field spurred by the giant chiroptical activity, enantioselective biological activity, and asymmetric catalytic activity of chiral nanostructures. Compared to chiral molecules, the handedness of chiral nano- and microstructures can be directly established via electron microscopy, which can be utilized for the automatic analysis of chiral nanostructures and prediction of their properties. However, chirality in complex materials may have multiple geometric forms and scales. Computational identification of chirality from electron microscopy images rather than optical measurements is convenient but is fundamentally challenging, too, because (1) image features differentiating left- and right-handed particles can be ambiguous and (2) three-dimensional structure essential for chirality is 'flattened' into two-dimensional projections. Here, we show that deep learning algorithms can identify twisted bowtie-shaped microparticles with nearly 100% accuracy and classify them as left- and right-handed with as high as 99% accuracy. Importantly, such accuracy was achieved with as few as 30 original electron microscopy images of bowties. Furthermore, after training on bowtie particles with complex nanostructured features, the model can recognize other chiral shapes with different geometries without retraining for their specific chiral geometry with 93% accuracy, indicating the true learning abilities of the employed neural networks. These findings indicate that our algorithm trained on a practically feasible set of experimental data enables automated analysis of microscopy data for the accelerated discovery of chiral particles and their complex systems for multiple applications.

One-step hydrothermal synthesis of chiral carbon dots with high asymmetric catalytic activity for an enantioselective direct aldol reaction.

Chiral carbon dots are prepared by a simple and one-step hydrothermal reaction at 180 °C for 2 h using citric acid and d-proline as precursors, which show high asymmetric catalytic activity for enantioselective direct aldol condensation. This work provides a hint for the simple preparation of heterogeneous chiral catalysts.

Step-by-Step Assembly of 2D Confined Chiral Space Endowing Achiral Clusters with Asymmetric Catalytic Activity for Epoxidation of Allylic Alcohols.

Endowing achiral polyoxometalates (POMs) with asymmetric catalytic properties is always an intriguing but challenging topic because of their high catalytic activities yet highly symmetrical molecular structures. In this work, a novel strategy was proposed to fabricate a series of two-dimensional chiral POM catalysts. Following the steps of exfoliation, covalent modification, and reassembly, the achiral POMs were orderly confined into the chiral interstitial domains of chiral ionic liquid (CIL)-modified layered double hydroxide materials with a decreased molecular symmetry. The chirality of POM molecules was induced by the l- or d-pyrrolidine-type CILs, and their asymmetric catalytic activity was enhanced by the confinement effect. Compared with the reported chiral POM-based catalysts [e.g., 8 turnover frequency (TOF) and 79% enantiomeric excess (ee) for chiral POM-based metal-organic frameworks], the constructed chiral POM catalysts showed a significantly higher TOF and enantioselectivity (up to 240 TOF and 93% ee) for the asymmetric epoxidation of allylic alcohols. The facilitated mass transfer in the IL channels and the increased binding efficiency between the chiral catalytic sites and reactants render this strategy highly promising for constructing efficient chiral catalysts from the catalytically active while achiral building blocks.

Helicity Induction and Its Static Memory of Poly(biphenylylacetylene)s Bearing Pyridine N‐Oxide Groups and Their Use as Asymmetric Organocatalysts

ABSTRACTNovel poly(biphenylylacetylene) derivatives carrying different types of pyridine N‐oxide units with a bulky or less‐bulky substituent at a different position as the functional pendant groups (poly‐2a and poly‐2b) were synthesized by the rhodium‐catalyzed polymerization of the corresponding monomers. The influence of the steric environment around the catalytically active pyridine N‐oxide sites on the helicity induction and its static memory as well as the asymmetric catalytic activities of the resulting helical polymers with a macromolecular helicity memory was investigated. The polyacetylenes formed an excess one‐handed helical conformation upon noncovalent interactions with optically active alcohols and the induced macromolecular helicities of the polyacetylenes were efficiently memorized after the removal of the chiral inducers. Poly‐2b with the macromolecular helicity memory showed an enantioselectivity for the catalytic asymmetric allylation of benzaldehydes, producing optically active allyl alcohols, although their enantioselectivities were low. On the other hand, poly‐2a exhibited a negligible catalytic activity probably due to the bulky substituent at the o‐position of the pyridine N‐oxide residues, while poly‐2a underwent a unique helix‐inversion with the increasing concentration of chiral alcohols and the opposite helicity of poly‐2a was further successfully memorized. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2481–2490

Design of Phosphinic Acid Catalysts with the Closest Stereogenicity at the α-Position: Synthesis and Application of α-Stereogenic Perfluoroalkyl Phosphinic Acid Catalysts.

Chiral C2-symmetric phosphinic acids were designed based on sterically demanding and helical chiral perfluoroalkyl groups at the closest α-position advancing asymmetric reaction environment and catalytic activity. The perfluoroalkyl catalysts, [(CF3)2F2] and [(C2F5)2F2] phosphinic acids, were synthesized via a stereoselective addition/cyclization sequence of methyl phosphinate and deoxofluorination. These new classes of Brønsted acid catalysts were applied to an asymmetric Friedel-Crafts reaction to give up to 89% yield and 82% R-enantioselectivity, which is higher than those obtained with the parent phosphoric acid (42% and 55.5% S).

Solvothermal Syntheses, Crystal Structures and Catalytic Properties of Four Chiral Compounds Built from (R)‐4‐(4‐(1‐Carboxyethoxy)penoxy) Benzoic Acids

AbstractA series of chiral metal−organic compounds [Ag(cpfa)(phen)] (1), [Ag(cpna)(phen)] (2), [Cu(cpna)(phen)]•2H2O (3), [Cd2(cpna)2(phen)2]n (4) (H2cpfa=(R)‐4‐(4‐(1‐carboxyethoxyphenoxy)phenoxy‐3‐fluorobenzoic acid; H2cpna=(R)‐4‐(4‐(1‐carboxyethoxy)phenoxy)‐3‐nitrobenzoic acid; phen=1,10‐phenanthroline) has been successfully obtained via solvothermal reactions and characterized by IR spectra, elemental analyses, thermo gravimetric analyzer (TGA), solid‐state circular dichroism (CD) and powder X‐ray diffraction (PXRD). Compounds 1–4 all crystallize in chiral space groups and their CD spectra exhibit obvious positive or negative Cotton effects, indicating their homochirality. Rich hydrogen bonds based on carboxylic groups and π‐π interactions between adjacent phen molecules are present in the four compounds. It is also found that the pH value and reactant ratio have an impact on the growth of the compounds. In addition, their asymmetric catalytic activities for Henry reaction have also been investigated.

Synthesis and Catalytic Applications of a Triptycene-Based Monophosphine Ligand for Palladium-Mediated Organic Transformations.

1-Methoxy-8-(diphenylphosphino)triptycene (1), featuring high structural rigidity and steric bulkiness around the phosphine functionality, was synthesized as a new chiral monophosphine ligand for transition metal-catalyzed reactions. In the presence of 5–10 mol ppm (i.e., 0.0005–0.001 mol %) Pd(OAc)2 and 1 (2 equiv for Pd), Suzuki–Miyaura cross-coupling reactions of aryl bromides and arylboronic acids proceeded effectively under mild atmospheric conditions to give the corresponding biaryl compounds in a high yield. The single-crystal X-ray analysis of a Pd(II) complex of 1 revealed its coordination structure, in which two homochiral molecules form a dimer, suggesting that triptycene could provide a chiral environment for asymmetric organic transformations. In fact, optically active 1 obtained by optical resolution showed good enantioselectivity in the palladium-catalyzed asymmetric hydrosilylation of styrene, which represents, for the first time, the asymmetric catalytic activity of triptycene-based monophosphine ligands.

Design of Novel Hydrogen-Bonding Donor Organocatalysts and Their Application to Asymmetric Direct Aldol Reaction

Asymmetric catalytic activities of various organocatalysts bearing double hydrogen-bonding donor units showing different pK a values were examined for direct aldol reactions of cyclohexanone with aromatic aldehydes. Organocatalyst with motif exhibiting the highest acidity resulted in the corresponding aldol products with the highest enantioselectivity. A good correlation has been observed between the asymmetric catalytic activity for direct aldol reactions and acidities of double hydrogen-bonding donating units.

Microscopic and continuum descriptions of Janus motor fluid flow fields.

Active media, whose constituents are able to move autonomously, display novel features that differ from those of equilibrium systems. In addition to naturally occurring active systems such as populations of swimming bacteria, active systems of synthetic self-propelled nanomotors have been developed. These synthetic systems are interesting because of their potential applications in a variety of fields. Janus particles, synthetic motors of spherical geometry with one hemisphere that catalyses the conversion of fuel to product and one non-catalytic hemisphere, can propel themselves in solution by self-diffusiophoresis. In this mechanism, the concentration gradient generated by the asymmetric catalytic activity leads to a force on the motor that induces fluid flows in the surrounding medium. These fluid flows are studied in detail through microscopic simulations of Janus motor motion and continuum theory. It is shown that continuum theory is able to capture many, but not all, features of the dynamics of the Janus motor and the velocity fields of the fluid.This article is part of the themed issue 'Multiscale modelling at the physics-chemistry-biology interface'.

Air-Stable Chiral Primary Phosphines: A Gateway to MOP Ligands with Previously Inaccessible Stereoelectronic Profiles

The air-stable chiral primary phosphines 1a,b facilitate the synthesis of previously inaccessible or hard-to-access chiral MOP-type ligands 2a,b–5a,b, which can be prepared in one-pot reactions. These derivatives have been prepared to allow for a unique comparison of their differing structural and electronic profiles, determined here by a number of experimental and theoretical studies. Phosphiranes 2a,b and phosphonites 5a,b are electron-poor compounds, with the former possessing exceptional thermal stability. Conversely, the dimethylarylphosphines 3a,b and bis(dimethylamino)arylphosphines 4a,b are good electron donors, and, in contrast to earlier reports, the dialkylarylphosphines were found to be remarkably air-stable. The ligands were coordinated to platinum(II), and the weak trans-influence of the highly strained phosphiranes 2a,b was revealed both in solution and in the solid state. The steric parameters of the ligands were investigated by the allyl rotation of their methallylpalladium(II) complexes, which showed subtle differences in exchange rates. Aryl side-on coordination of the MOP-backbone to palladium(II) was observed for complexes with a non-coordinating counterion and structurally analyzed in the case of ligand 4b. The asymmetric induction and catalytic activity of 2a,b–5a,b were tested in the hydrosilylation of styrene as well as the allylic alkylation of (rac)-(E)-1,3-diphenylallyl acetate. Major differences in reactivity were related back to the electronic parameters of the ligands.

Chemistry in motion: tiny synthetic motors.

CONSPECTUS: Diffusion is the principal transport mechanism that controls the motion of solute molecules and other species in solution; however, the random walk process that underlies diffusion is slow and often nonspecific. Although diffusion is an essential mechanism for transport in the biological realm, biological systems have devised more efficient transport mechanisms using molecular motors. Most biological motors utilize some form of chemical energy derived from their surroundings to induce conformational changes in order to carry out specific functions. These small molecular motors operate in the presence of strong thermal fluctuations and in the regime of low Reynolds numbers, where viscous forces dominate inertial forces. Thus, their dynamical behavior is fundamentally different from that of macroscopic motors, and different mechanisms are responsible for the production of useful mechanical motion. There is no reason why our interest should be confined to the small motors that occur naturally in biological systems. Recently, micron and nanoscale motors that use chemical energy to produce directed motion by a number of different mechanisms have been made in the laboratory. These small synthetic motors also experience strong thermal fluctuations and operate in regimes where viscous forces dominate. Potentially, these motors could be directed to perform different transport tasks, analogous to those of biological motors, for both in vivo and in vitro applications. Although some synthetic motors execute conformational changes to effect motion, the majority do not, and, instead, they use other mechanisms to convert chemical energy into directed motion. In this Account, we describe how synthetic motors that operate by self-diffusiophoresis make use of a self-generated concentration gradient to drive motor motion. A description of propulsion by self-diffusiophoresis is presented for Janus particle motors comprising catalytic and noncatalytic faces. The properties of the dynamics of chemically powered motors are illustrated by presenting the results of particle-based simulations of sphere-dimer motors constructed from linked catalytic and noncatalytic spheres. The geometries of both Janus and sphere-dimer motors with asymmetric catalytic activity support the formation of concentration gradients around the motors. Because directed motion can occur only when the system is not in equilibrium, the nature of the environment and the role it plays in motor dynamics are described. Rotational Brownian motion also acts to limit directed motion, and it has especially strong effects for very small motors. We address the following question: how small can motors be and still exhibit effects due to propulsion, even if only to enhance diffusion? Synthetic motors have the potential to transform the manner in which chemical dynamical processes are carried out for a wide range of applications.

Ångström-scale chemically powered motors

Like their larger micron-scale counterparts, Ångström-scale chemically self-propelled motors use asymmetric catalytic activity to produce self-generated concentration gradients that lead to directed motion. Unlike their micron-scale counterparts, the sizes of Ångström-scale motors are comparable to the solvent molecules in which they move, they are dominated by fluctuations, and they operate on very different time scales. These new features are studied using molecular dynamics simulations of small sphere dimer motors. We show that the ballistic regime is dominated by the thermal speed but the diffusion coefficients of these motors are orders of magnitude larger than inactive dimers. Such small motors may find applications in nano-confined systems or perhaps eventually in the cell.

ChemInform Abstract: Asymmetric Synthesis and Catalytic Activity of 3‐Methyl‐β‐proline in Enantioselective anti‐Mannich‐Type Reactions.

AbstractChiral β‐prolines are synthesized and used as organocatalysts in the anti‐selective Mannich‐type reaction of ketones/aldehydes with ethyl glyoxylate imine.

Asymmetric Synthesis and Catalytic Activity of 3-Methyl-β-proline in Enantioselective anti-Mannich-type Reactions

Enantiomerically pure 3-methyl-β-proline was synthesized using an asymmetric phase-transfer-catalyzed alkylation of a cyanopropanoate to establish the all-carbon stereogenic center. The catalytic activity of 3-methyl-β-proline in the Mannich-type reaction between a glyoxylate imine and ketones/aldehydes was subsequently investigated. The catalyst was designed and found to be more soluble in nonpolar organic solvents relative to the unsubstituted β-proline catalyst, and as a result allowed for added flexibility during optimization efforts. This work culminated in the development of a highly anti-diastereo- and enantioselective process employing low catalyst loading.

Chiral Schiff base Ligated Dioxomolybdenum (VI) Complexes and their Asymmetric Catalytic Properties in the Epoxidation of Styrene

Two series of Schiff base dioxomolybdenum (VI) complexes of the general formula [MoO2(L)(MeOH)] (L = D-glucosamine-derived Schiff base) have been synthesised, characterised and their asymmetric catalytic activities evaluated through the epoxidation of styrene. The molybdenum complexes with acetyl-protected D-glucosamine Schiff base ligands exhibited higher enanatioselectivity than those with unprotected sugar ligands. Protecting D-glucosamine by acetyl groups is an effective way to improve asymmetric catalytic epoxidation activities.