Crystallization-Driven Asymmetric Helical Assembly of Conjugated Block Copolymers and the Aggregation Induced White-light Emission and Circularly Polarized Luminescence

Controlling the self-assembly morphology of π-conjugated block copolymer is of great interesting. Herein, amphiphilic poly(3-hexylthiophene)-block-poly(phenyl isocyanide)s (P3HT-b-PPI) copolymers composed of π-conjugated P3HT and optically active helical PPI segments were readily prepared. Taking advantage of the crystallizable nature of P3HT and the chirality of the helical PPI segment, crystallization-driven asymmetric self-assembly (CDASA) of the block copolymers lead to the formation of single-handed helical nanofibers with controlled length, narrow dispersity, and well-defined helicity. During the self-assembly process, the chirality of helical PPI was transferred to the supramolecular assemblies, giving the helical assemblies large optical activity. The single-handed helical assemblies of the block copolymers exhibited interesting white-light emission and circularly polarized luminescence (CPL). The handedness and dissymmetric factor of the induced CPL can be finely tuned through the variation on the helicity and length of the helical nanofibers.     

X-ray Crystallographic Observation of Chiral Transformations within a Metal–Peptide Pore

Porous metal complexes enable single-crystal X-ray crystallographic observation of included guests or reaction intermediates through simple soaking with the guests/substrates. Previous studies on this technique have often encountered difficulties in the observation of chiral structures because the host frameworks had no chirality. We synthesized a new metal–peptide porous complex through a folding-and-assembly strategy and utilized the chiral pore for trapping chiral guests. Chiral alcohols and ketones were successfully included within the pore. Crystallographic analyses clearly revealed not only their chemical structures but also chiral transformation events within the pore such as fixed conformations or an unstable hemiacetal formation.     

An unprecedented transformation mode in aluminium oxazoline-amido-phenolate complexes

This paper describes the effect of solvent-induced synthetic routes of aluminium pendant oxazoline-amido-phenolate complexes. Treatment of ligand precursor L with AlMe₃ in a 1:1 ratio in diethyl ether yielded the four-coordinated complex (LAlMe)₂. Reaction of ligand precursor L with AlMe₃ in a 1:2 ratio in hexane generated the four-coordinated complex L(AlMe₂)₂. A novel transformation mode occurred from L(AlMe₂)₂ to (LAlMe)₂ when using diethyl ether or tetrahydrofuran as solvent. A density functional theory computational study also supports a plausible mechanism. All results were supported by spectroscopic data and in agreement with single-crystal X-ray diffraction structural analysis.     

Stereoselective [4+2]-Cycloaddition with Chiral Alkenylboranes

A method for the stereoselective [4+2]-cycloaddition of alkenylboranes and dienes is presented. This transformation was accomplished through the introduction of a new strategy that involves the use of chiral N-protonated alkenyl oxazaborolidines as dieneophiles. The reaction leads to the formation of products that can be readily derivatized to more complex structural motifs through stereospecific transformations of the C−B bond such as oxidation and homologation. Detailed computation evaluation of the reaction has uncovered a surprising role of the counterion on stereoselectivity.     

Transition Metal-Catalyzed Enantioselective C−H Functionalization via Chiral Transient Directing Group Strategies

Transition metal-catalyzed enantioselective functionalization of C−H bond, the most abundant functionality in organic molecules, has emerged as an expedient synthetic approach to streamline the synthesis of complex chiral molecules. Despite significant progress, traditional directing group-enabled strategies require additional steps for the installation and removal of directing groups from the target molecule. The recently developed asymmetric C−H functionalization using chiral transient directing groups (cTDGs) offers a promising alternative that can circumvent this obstacle and therefore simplify the process. In this Minireview, we briefly discuss the advent and recent advances of this emerging concept, with an emphasis on discussing the creation of various stereogenic centers and the developments of cTDGs. Applications in natural product synthesis and ligand derivatizations are also discussed. We hope this Minireview will highlight the great potential of this strategy and help to inspire further endeavors.     

Face-Directed Assembly of Molecular Cubes: In Situ Substitution of a Predetermined Concave Cluster

Systematic design and self-assembly of metal–organic polyhedra with predictable configurations has been a long-standing challenge in crystal engineering. Herein a concave polyoxovanadate cluster, [V₆O₆(OCH₃)₉(SO₄)₄]⁵⁻, which can be generated in situ under specific reaction conditions, is reported. Based on this cluster, a potential trivalent molecular building block, [V₆O₆(OCH₃)₉(SO₄)(CO₂)₃]²⁻, can be obtained by the bridging-ligand-substitution strategy and it possesses appropriate angle information for the design of molecular cubes. Utilizing the face-directed assembly of the trivalent molecular building block and a diverse set of tetratopic carboxylate linkers, a series of metal–organic cubes ( VMOC-1 – VMOC-5 ) with the same topology but different functionalities and dimensions were designed and constructed. An inclusion study using VMOC-3 shows that they are potential molecular receptors for selective capture of size-matching polycyclic aromatic hydrocarbon guest molecules.     

Environmentally Compatible Access to α-Trifluoromethylseleno-Enones

α-Trifluoromethylselenolated enones constitute valuable building-blocks for further synthesis of innovative fluorinated compounds. Herein, we described an easy access to such compounds in green conditions through a Morita-Baylis-Hillman like reaction. These conditions have also been extended to higher fluorinated homologs.     

Diazoacetonitrile (N2CHCN): A Long Forgotten but Valuable Reagent for Organic Synthesis

Diazoacetonitrile (N₂CHCN) is a small reactive diazoalkane. It has been synthesized for the first time already in 1898 by Theodor Curtius, however, did not gain much recognition in organic synthesis until recently. Only in 2015, after introduction of in situ and flow protocols for the safe generation of diazoacetonitrile, it started gaining popularity. In this minireview, the synthetic properties and applications of this valuable reagent are discussed.     

In Situ Metal-Assisted Ligand Modification Induces Mn4 Cluster-to-Cluster Transformation: A Crystallography,Mass Spectrometry,and DFT Study

Dehydration of (S,S)-1,2-bis(1H-benzo[d]imidazol-2-yl)ethane-1,2-diol (H₄L) to (Z)-1,2-bis(1H-benzo[d]imidazol-2-yl)ethenol) (H₃L′) was found to be metal-assisted, occurs under solvothermal conditions (H₂O/CH₃OH), and leads to [Mn^II₄(H₃L)₄Cl₂]Cl₂ ⋅ 5 H₂O ⋅ 5 CH₃OH ( Mn₄L₄ ) and [Mn^II₄(H₂L′)₆(μ₃-OH)]Cl ⋅ 4 CH₃OH ⋅ H₂O ( Mn₄L′₆ ), respectively. Their structures were determined by single-crystal XRD. Extensive ESI-MS studies on solutions and solids of the reaction led to the proposal consisting of an initial stepwise assembly of Mn₄L₄ from the reactants via [MnL] and [Mn₂L₂] below 80 °C, and then disassembly to [MnL] and [MnL₂] followed by ligand modification before reassembly to Mn₄L′₆ via [MnL′], [MnL′₂], and [Mn₂L′₃] with increasing solvothermal temperature up to 140 °C. Identification of intermediates [Mn₄L_xL′_6−x] (x=5, 4, 3, 2, 1) in the process further suggested an assembly/disassembly/in situ reaction/reassembly transformation mechanism. These results not only reveal that multiple phase transformations are possible even though they were not realized in the crystalline state, but also help to better understand the complex transformation process between coordination clusters during “black-box” reactions.     

On the Effect of Secondary Nucleation on Deracemization through Temperature Cycles

Herein, the pivotal role of secondary nucleation in a crystallization-enhanced deracemization process is reported. During this process, complete and rapid deracemization of chiral conglomerate crystals of an isoindolinone is attained through fast microwave-assisted temperature cycling. A parametric study of the main factors that affect the occurrence of secondary nucleation in this process, namely agitation rate, suspension density, and solute supersaturation, confirms that an enhanced stereoselective secondary nucleation rate maximizes the deracemization rate. Analysis of the system during a single temperature cycle showed that, although stereoselective particle production during the crystallization stage leads to enantiomeric enrichment, undesired kinetic dissolution of smaller particles of the preferred enantiomer occurs during the dissolution step. Therefore, secondary nucleation is crucial for the enhancement of deracemization through temperature cycles and as such should be considered in further design and optimization of this process, as well as in other temperature cycling processes commonly applied in particle engineering.