Asymmetric Strecker Reaction Arising from the Molecular Orientation of an Achiral Imine at the Single‐Crystal Face: Enantioenriched l‐ and d‐Amino Acids

Strecker synthesis has long been considered one of the prebiotic reactions for the synthesis of α‐amino acids. However, the correlation between the origin of chirality and highly enantioenriched α‐amino acids through this method remains a puzzle. In the reaction, it may be conceivable that the handedness of amino acids has been determined at the formation stage of the chiral intermediate α‐aminonitrile, that is, the enantioselective addition of hydrogen cyanide to an imine. Herein, an enantiotopic crystal surface of an achiral imine acted as an origin of chirality for the enantioselective formation of α‐aminonitriles by the addition of HCN. In conjunction with the amplification of the enantiomeric excess and multiplication of enantioenriched aminonitrile, a large amount of near enantiopure α‐amino acids, with the l ‐ and d ‐handedness corresponding to the molecular orientation of the imine, is reported.     

Spontaneous Symmetry Breaking in the Formation of Supramolecular Polymers: Implications for the Origin of Biological Homochirality

Aqueous solutions of the achiral, monomeric, nucleobase mimics (2,4,6‐triaminopyrimidine, TAP, and a cyanuric acid derivative, CyCo6) spontaneously assemble into macroscopic homochiral domains of supramolecular polymers. These assemblies exhibit a high degree of chiral amplification. Addition of a small quantity of one handedness of a chiral derivative of CyCo6 generates exclusively homochiral structures. This system exhibits the highest reported degree of chiral amplification for dynamic helical polymers or supramolecular helices. Significantly, homochiral polymers comprised of hexameric rosettes with structural features that resemble nucleic acids are formed from mixtures of cyanuric acid (Cy) and ribonucleotides (l‐, d ‐pTARC) that arise spontaneously from the reaction of TAP with the sugars. These findings support the hypothesis that nucleic acid homochirality was a result of symmetry breaking at the supramolecular polymer level.     

An In‐tether Chiral Center Modulates the Helicity,Cell Permeability,and Target Binding Affinity of a Peptide

The addition of a precisely positioned chiral center in the tether of a constrained peptide is reported, yielding two separable peptide diastereomers with significantly different helicity, as supported by circular dichroism (CD) and NMR spectroscopy. Single crystal X‐ray diffraction analysis suggests that the absolute configuration of the in‐tether chiral center in helical form is R, which is in agreement with theoretical simulations. The relationship between the secondary structure of the short peptides and their biochemical/biophysical properties remains elusive, largely because of the lack of proper controls. The present strategy provides the only method for investigating the influence of solely conformational differences upon the biochemical/biophysical properties of peptides. The significant differences in permeability and target binding affinity between the peptide diastereomers demonstrate the importance of helical conformation.     

Turning Regioselectivity into Stereoselectivity: Efficient Dual Resolution of P‐Stereogenic Phosphine Oxides through Bifurcation of the Reaction Pathway of a Common Intermediate

Synthetic routes that provide facile access to either enantiomeric form of a target compound are particularly valuable. The crystallization‐free dual resolution of phosphine oxides that gives highly enantioenriched materials (up to 94 % ee) in excellent yields is reported. Both enantiomeric oxides have been prepared from a single intermediate, (R_P)‐alkoxyphosphonium chloride, which is formed in the course of a selective dynamic kinetic resolution using a single enantiomer of menthol as the chiral auxiliary. The origin of the dual stereoselectivity lies in bifurcation of the reaction pathway of this intermediate, which works as a stereochemical railroad switch. Under controlled conditions, Arbuzov‐type collapse of this intermediate proceeds through C O bond fission with retention of the configuration at the phosphorus center. Conversely, alkaline hydrolysis of the P O bond leads to the opposite S_P enantiomer.     

Space‐Confined Seeded Growth of Cu Nanorods with Strong Surface Plasmon Resonance for Photothermal Actuation

Herein, we show that copper nanostructures, if made anisotropic, can exhibit strong surface plasmon resonance comparable to that of gold and silver counterparts in the near‐infrared spectrum. Further, we demonstrate that a robust confined seeded growth strategy allows the production of high‐quality samples with excellent control over their size, morphology, and plasmon resonance frequency. As an example, copper nanorods (CuNRs) are successfully grown in a limited space of preformed rod‐shaped polymer nanocapsules, thereby avoiding the complex nucleation kinetics involved in the conventional synthesis. The method is unique in that it enables the flexible control and fine‐tuning of the aspect ratio and the plasmonic resonance. We also show the high efficiency and stability of the as‐synthesized CuNRs in photothermal conversion and demonstrate their incorporation into nanocomposite polymer films that can be used as active components for constructing light‐responsive actuators and microrobots.     

Low‐Temperature Anharmonicity in Cesium Chloride (CsCl)

Anharmonic lattice vibrations govern heat transfer in materials, and anharmonicity is commonly assumed to be dominant at high temperature. The textbook cubic ionic defect‐free crystal CsCl is shown to have an unexplained low thermal conductivity at room temperature (ca. 1 W/(m K)), which increases to around 13 W/(m K) at 25 K. Through high‐resolution X‐ray diffraction it is unexpectedly shown that the Cs atomic displacement parameter becomes anharmonic at 20 K.     

Isolable Silylene Ligands Can Boost Efficiencies and Selectivities in Metal‐Mediated Catalysis

A significant number of isolable silylenes are currently known. They have quickly developed from laboratory curiosities to useful ligands in metal‐mediated homogeneous catalysis. This includes their utilization in various catalytic transformations, such as C?C cross‐coupling, cyclotrimerization, hydroformylation, borylation, deuteration, hydrosilylation, amination, hydrogenation, and transfer semi‐hydrogenation reactions. Recent studies suggest that the silylene ligands surpass the steering properties of their phosphine and N‐heterocyclic carbene (NHC) analogues and provide excellent chemo‐, regio‐, and stereoselectivites. Mechanistic studies suggest that their promoted performance of metal‐mediated catalytic transformations results from a strong σ‐donor character along with cooperative effects of their Si^II centers. This Minireview covers the most recent advances in the field.     

Synthetic Nanosheets of Natural van der Waals Heterostructures

Creation of new van der Waals heterostructures by stacking different two dimensional (2D) crystals on top of each other in a chosen sequence is the next challenge after the discovery of graphene, mono/few layer of h ‐BN, and transition‐metal dichalcogenides. However, chemical syntheses of van der Waals heterostructures are rarer than the physical preparation techniques. Herein, we demonstrate the kinetic stabilization of 2D ultrathin heterostructure (ca. 1.13–2.35 nm thick) nanosheets of layered intergrowth SnBi₂Te₄, SnBi₄Te₇, and SnBi₆Te₁₀, which belong to the Sn_m Bi_2n Te_{3n +m} homologous series, by a simple solution based synthesis. Few‐layer nanosheets exhibit ultralow lattice thermal conductivity (κ _lat) of 0.3–0.5 W m⁻¹ K⁻¹ and semiconducting electron‐transport properties with high carrier mobility.     

Supramolecular Graft Copolymerization of a Polyester by Guest‐Selective Encapsulation of a Self‐Assembled Capsule

Repeating guest units of polyesters poly‐(R )‐ 2 were selectively encapsulated by capsule 1 (BF₄)₄ to produce supramolecular graft polymers. The encapsulation of the guest units was confirmed by ¹H NMR spectroscopy. The graft polymer structures were confirmed by the increase in the hydrodynamic radii and the solution viscosities of the polyesters upon complexation of the capsule. After the capsule was formed, atomic force microscopy showed extension of the polyester chains. The introduction of the graft chains onto poly‐(R )‐ 2 resulted in the main chain of the polymer having an M ‐helical morphology. The complexation of copolymers poly‐[(R )‐ 2 ‐co ‐(S )‐ 2 ] by the capsule gave rise to the unique chiral amplification known as the majority‐rules effect.     

Template Removal via Boudouard Equilibrium Allows for Synthesis of Mesostructured Molybdenum Compounds

Oxidative thermal removal of the polymeric templates is not trivial for molybdenum oxides and hampers mesostructuring of this material. At ambient oxygen fugacity, Mo^VI is the thermodynamically stable oxidation state and sublimation of MoO₃ leads to a quick loss of the mesostructure through Oswald ripening. Taking advantage of the Boudouard equilibrium allows to fix the oxygen fugacity at a level where non‐volatile MoO_2−x is stable while carbonaceous material may be oxidized by CO₂. Mesostructured MoO_2−x can be chemically converted into MoO₃ or MoN under retention of the mesostructure.     

Chiral Amplification in Nature: Studying Cell‐Extracted Chiral Carotenoid Microcrystals via the Resonance Raman Optical Activity of Model Systems

Carotenoid microcrystals, extracted from cells of carrot roots and consisting of 95 % of achiral β‐carotene, exhibit a very intense chiroptical (ECD and ROA) signal. The preferential chirality of crystalline aggregates that consist mostly of achiral building blocks is a newly observed phenomenon in nature, and may be related to asymmetric information transfer from the chiral seeds (small amount of α‐carotene or lutein) present in carrot cells. To confirm this hypothesis, we synthesized several model aggregates from various achiral and chiral carotenoids. Because of the sergeant‐and‐soldier behavior, a small number of chiral sergeants (α‐carotene or astaxanthin) force the achiral soldier molecules (β‐ or 11,11′‐[D₂]‐β‐carotene) to jointly form supramolecular assemblies of induced chirality. The chiral amplification observed in these model systems confirmed that chiral microcrystals appearing in nature might consist predominantly of achiral building blocks and their supramolecular chirality might result from the co‐crystallization of chiral and achiral analogues.