Copper-Catalyzed Directed Hydroindolation/Annulation Sequence of Alkynes with Indoles via Copper Carbenes.

Described herein is an efficient copper-catalyzed tandem alkyne indolylcupration-initiated 1,2-indole migration/6π-electrocyclic reaction of allene-ynamides with indoles by the in situ-generated metal carbenes. This method allows the efficient synthesis of valuable indole-fused spirobenzo[f]indole-cyclohexanes with high regio- and stereoselectivity. In addition, this reaction affords rapid access to the functionalized spirobenzo[f]indole-cyclohexanes in the absence of indoles by a presumable 5-exo-dig cyclization/Friedel-Crafts alkylation via copper-containing all-carbon 1,4-dipoles.

Copper(I)-Catalyzed Indolyl Ynamide Oxidation/Dearomatization: Divergent and Regioselective Synthesis of Valuable Indoline Scaffolds.

A highly convenient copper(I)-catalyzed oxidation-initiated cyclopropanation of indolyl ynamide for the rapid construction of indole-fused cyclopropane-lactams is described, which represents, to the best of our knowledge, the first non-noble-metal-catalyzed indolyl ynamide oxidation/dearomatization by the in situ generated α-oxo copper carbenes. Compared to hydrazone and diazo, the use of alkynes as carbene precursors allows cyclopropanation to occur under a safe and convenient pathway. Moreover, this transformation can lead to the divergent synthesis of pentacyclic spiroindolines involving the reversal of ynamide regioselectivity by engineering substrate structures.

Recent advances in transition-metal-catalyzed Büchner reaction of alkynes.

Medium-sized ring-containing organic molecules, especially seven-membered rings, are significant structural motifs. However, such frameworks are considered difficult structures to access owing to entropic effects and transannular interactions. Compared to the construction of five and six-membered rings, the synthesis of seven-membered rings can be more challenging through traditional cyclization pathways. Büchner reactions are particularly attractive and efficient synthetic strategies to construct functionalized seven-membered ring products from the benzenoid double bond with carbene. In recent years, the field of transition-metal-catalyzed Büchner ring expansion reactions of alkynes has experienced a speedy development and a diverse array of efficient synthetic procedures have been disclosed under mild experimental conditions, as the synthesis of synthetically challenging seven-membered rings is easily achieved. In this review, we will focus on the recent progress in the transition-metal-catalyzed Büchner reaction of alkynes and the mechanistic rationale is depicted where possible, with the reactions being sorted according to the type of catalyst.

Bioinspired humidity-responsive liquid crystalline materials: from adaptive soft actuators to visualized sensors and detectors.

Inspired by nature, humidity-responsive materials and devices have attracted significant interest from scientists in multiple disciplines, ranging from chemistry, physics and materials science to biomimetics. Owing to their superiorities, including harmless stimulus and untethered control, humidity-driven materials have been widely investigated for application in soft robots, smart sensors and detectors, biomimetic devices and anticounterfeiting labels. Especially, humidity-responsive liquid crystalline materials are particularly appealing due to the combination of programmable and adaptive liquid crystal matrix and humidity-controllability, enabling the fabrication of advanced self-adaptive robots and visualized sensors. In this review, we summarize the recent progress in humidity-driven liquid crystalline materials. First, a brief introduction of liquid crystal materials, including liquid crystalline polymers, cholesteric liquid crystals, blue-phase liquid crystals and cholesteric cellulose nanocrystals is provided. Subsequently, the mechanisms of humidity-responsiveness are presented, followed by the diverse strategies for the fabrication of humidity-responsive liquid crystalline materials. The applications of humidity-driven devices will be presented ranging from soft actuators to visualized sensors and detectors. Finally, we provide an outlook on the development of humidity-driven liquid crystalline materials.

Regioselective access to polycyclic N-heterocycles via homogeneous copper-catalyzed cascade cyclization of allenynes.

Polycyclic N-heterocycles are important structural motifs commonly found in bioactive compounds, however, their selective construction via the cyclization of allenynes remains challenging yet highly desirable. Here we show a homogeneous copper-catalyzed hetero Diels-Alder (HDA) reaction of allenynes with cis-diazenes (PTAD, 4-phenyl-1,2,4-triazoline-3,5-dione), allowing the practical and efficient synthesis of a diverse array of valuable polycyclic N-heterocycles. A temperature-controlled and stereocontrolled chemoselectivity of the reaction was observed, leading to the chemodivergent synthesis of tetracyclic pyrrolidines, pentacyclic triazepanes and tricyclic pyrrolidines. Compared with related Au-catalyzed cyclization of allenynes, this copper catalysis achieves cyclization of allenynes terminating in C-N bond formation via the HDA reaction.

Copper-catalyzed alkyne oxidation/Büchner-type ring-expansion to access benzo[6,7]azepino[2,3-b]quinolines and pyridine-based diones.

General access to highly valuable seven-membered rings via Büchner-type reaction remains a formidable challenge. Here we report a Cu-catalyzed intermolecular oxidation of alkynes using N-oxides as oxidants, which enables expedient preparation of valuable benzo[6,7]azepino[2,3-b]quinolines and pyridine-based diones. Importantly, in contrast to the well-established gold-catalyzed intermolecular alkyne oxidation, the dissociated pyridine or quinoline partner could be further utilized to construct N-heterocycles in this system and the reaction most likely proceeds by a Büchner-type ring expansion pathway. A mechanistic rationale for this cascade cyclization is supported by DFT calculations.

Reversible Near-Infrared Fluorescent Probe for Rapid Sensing Sulfur Dioxide and Formaldehyde: Recognition and Photoactivation Mechanism and Applications in Bioimaging and Encryption Ink.

A novel near-infrared (NIR) fluorescent Probe 1 was successfully developed for the reversible detection of sulfur dioxide derivatives and formaldehyde. The purple solution of Probe 1 faded to colorless in 1.8 s with the addition of HSO3-. Meanwhile, its fluorescence signal disappeared instantaneously with a 39 nM detection limit. The probe exhibited excellent selectivity toward HSO3- over other potential interfering agents. Then, its absorption and fluorescence bands were able to effectively recover in response to formaldehyde. Remarkably, this reverse process was able to accelerate 84 times under UV light in 122 s and achieved a recovery rate of 98% by UV light, the photoactivation mechanism was fully determined by HRMS and theoretical calculation. Furthermore, we demonstrated that Probe 1 was successfully applied for the detection of sulfur dioxide derivatives and formaldehyde in living cells and data encryption.

Preparation of transmittance-switchable composites from a coexistent system of polymer-dispersed and polymer-stabilized cholesteric liquid crystals.

Polymer-dispersed and polymer-stabilized liquid crystals (PD&PSLCs) are the coexistent systems of PDLCs and PSLCs, in which a mesogenic polymer network forms in the LC droplets to construct the PSLC system and the combination of the LC domains and the isotropic polymer matrix makes up the PDLC system. In this work, a PD&PS cholesteric LC (PD&PSCLC) system, where the isotropic polymer walls make up the PDCLC microstructure and the liquid crystalline polymer network in the CLC droplets constructs the PSCLC system, is fabricated successfully. The CLCs are composed of a negative dielectric anisotropy nematic LC and a chiral dopant. Owing to the stabilizing effect of the liquid crystalline polymer network, the planar texture of the CLCs can be retained after the preparation process, and the PD&PSCLC sample is in a large-transmittance state. The action of a low-frequency electric field induces the disordered texture of the CLCs and the light-scattering (low-transmittance) state because of the electro-hydrodynamic effect and dynamic scattering effects. After the removal of the low-frequency electric field, the stabilizing effect of the liquid crystalline polymer network induces the CLC molecules to return to the homogeneous orientation state, and the sample also re-obtains the large transmittance. This work may provide a facile approach to fabricating polymer/CLC composites with switchable transmittance for reverse-mode smart windows.

Gold-catalyzed cycloadditions of allenes via metal carbenes.

In recent years, gold-catalyzed cycloadditions of allenes, especially those involving a gold carbene intermediate, have received significant interest, as they avoid the utilization of potentially hazardous and inaccessible diazo compounds as starting materials for carbene generation. Cycloaddition reactions consisting of the uncomplicated addition of two or more unsaturated functional groups are one of the most efficient synthetic methodologies for the rapid assembly of carbo- and heterocyclic structures from simple acyclic precursors. In this review, we introduce an overview of the advances in the gold-catalyzed cycloaddition of allenes via a metal carbene intermediate and categorize these reactions according to the reaction types of the cycloadditions.

Silicon nanostructure-doped polymer/nematic liquid crystal composites for low voltage-driven smart windows.

In this work, two silicon nanostructures were doped into polymer/nematic liquid crystal composites to regulate the electric-optical performance. Commercial SiO2nanoparticles and synthesized thiol polyhedral oligomeric silsesquioxane (POSS-SH) were chosen as the dopants to afford the silicon nanostructures. SiO2nanoparticles were physically dispersed in the composites and the nanostructure from POSS-SH was implanted into the polymer matrix of the composites via photoinduced thiol-ene crosslinking. Scanning electron microscopy results indicated that the implantation of POSS microstructure into the polymer matrix was conducive to obtaining the uniform porous polymer microstructures in the composites while the introduction of SiO2nanoparticles led to the loose and heterogeneous polymer morphologies. The electric-optical performance test results also demonstrated that the electric-optical performance regulation effect of POSS microstructure was more obvious than that of SiO2nanoparticles. The driving voltage was reduced by almost 80% if the concentration of POSS-SH in the composite was nearly 8 wt% and the sample could be completely driven by the electric field whose voltage was lower than the safe voltage for continuous contact (24 V). This work could provide a creative approach for the regulation of electric-optical performance for polymer/nematic liquid crystal composites and the fabrication of low voltage-driven PDLC films for smart windows.

Gold-catalyzed oxidative cyclization of amide-alkynes: access to functionalized γ-lactams.

An efficient gold-catalyzed oxidative cyclization of amide-alkynes is developed. A series of functionalized γ-lactams are easily accessed by employing this strategy. The tandem reaction proceeds through alkyne oxidation, carbene/alkyne metathesis, and donor/donor carbene oxidation.

Recent progress towards the transition-metal-catalyzed Nazarov cyclization of alkynes via metal carbenes.

In recent years, transition-metal-catalyzed tandem cyclization reactions of alkynes, especially those involving a metal carbene intermediate, have received worthwhile interest, as this type of reaction does not require the use of risky and potentially explosive diazo compounds as starting materials for carbene generation. A significant and general strategy for the stereospecific synthesis of 5-membered cycles is Nazarov cyclization based on the 4π-conrotatory electrocyclization of a conjugated pentadienyl cation to afford a cyclopentenyl cation. In this review, we introduce an overview of recent advances in the transition-metal-catalyzed Nazarov cyclization of alkynes via a metal carbene intermediate, and categorize these reactions according to the structure of the metal carbene. Our aim is to accelerate advancements in this enchanting area of research.

Rapid Degradation of Nitrochlorobenzene by Activated Persulfate Oxidation With Biochar Supported Nanoscaled Zero Valent Iron.

Although pesticide intermediates are a kind of typical toxic pollutant in contaminated sites, the remediation of these contaminants in groundwater and soils is of limited concern. In the present study we investigated the performance of a novel heterogeneous oxidation system, biochar supported nanoscaled-zero valent iron (nZVI/BC) activated persulfate (PS), in the oxidative degradation of nitrochlorobenzene (NCB), a typical pesticide intermediate. Peanut shell based nZVI/BC was prepared and used as the PS activator. The degradation kinetics of m-, p-, and o-NCB isomers in the aqueous phase were investigated. The effects of BC/nZVI composition (Fe/BC mass ratio), the amount of BC/nZVI and PS, and initial contaminant concentration on NCB removal were also examined. Results suggest that over 90% removals of three NCB isomers could be obtained by the nZVI/BC activated PS system at initial NCB concentration of 10 mg L-1. The combination of nZVI/BC composite and PS showed superior performance to PS alone. The optimal treatment condition was supposed as the Fe: BC ratio of 1:1, Fe amount of 6 mmol L-1, and the mole ratio of Fe to perfulfate of 1:1.

Copper(I)-Catalyzed Enyne Oxidation/Cyclopropanation: Divergent and Enantioselective Synthesis of Cyclopropanes.

An efficient copper(I)-catalyzed enyne oxidation/cyclopropanation for the modular synthesis of cyclopropane derivatives is described, which represents the first non-noble metal-catalyzed enynes oxidation/cyclopropanation by the in situ generated α-oxo copper carbenes. This protocol allows the assembly of valuable cyclopropane-γ-lactams in generally good to excellent yields with excellent diastereoselectivity. More significantly, the enantioselective version of enyne oxidation/cyclopropanation has been disclosed with chiral copper catalysts.

Cu(I)-Catalyzed Oxidative Cyclization of Enynamides: Regioselective Access to Cyclopentadiene Frameworks and 2-Aminofurans.

An efficient Cu(I)-catalyzed oxidative cyclization of alkynyl-tethered enynamides for the construction of fused bicyclic cyclopentadiene derivatives is disclosed. The cascade proceeds through alkyne oxidation, carbene/alkyne metathesis, and formal (3 + 2) cycloaddition. Employing aryl-tethered enynamides as starting materials, substituted 2-aminofurans can be exclusively formed.

Recent advances in catalytic asymmetric intermolecular oxidation of alkynes.

In recent years, gold-catalyzed intermolecular alkyne oxidation by an N-oxide oxidant, which presumably involves a gold carbenoid intermediate, has attracted increasing attention because it circumvents the employment of hazardous and potentially explosive diazocarbonyl compounds as starting materials for carbene generation. More importantly, the development of a catalytic enantioselective version of the reaction can help achieve an array of asymmetric synthesis processes modified from various racemic transformations of the gold carbenoid intermediate. In this review, we will present an overview of these recent advances in the asymmetric transformations by utilizing chiral ligands, chiral N,N'-dioxides and chiral substrates, aiming to facilitate progress in this fascinating field of research.

Symmetries in hard polygon systems determine plastic colloidal crystal mesophases in two dimensions.

Orientational ordering is a necessary step in the crystallization of molecules and anisotropic colloids. Plastic crystals, which are possible mesophases between the fluid and fully ordered crystal, are translationally ordered but exhibit no long range orientational order. Here, we study the two-dimensional phase behavior of hard regular polygons with edge number n = 3-12. This family of particles provides a model system to isolate the effect of shape and symmetry on the existence of plastic crystal phases. We show that the symmetry group of the particle, G, and the symmetry group of the local environment in the crystal, H, together determine plastic colloidal crystal phase behavior in two dimensions. If G contains completely the symmetry elements of H, then a plastic crystal phase is absent. If G and H share some but not all nontrivial symmetry elements, then a plastic crystal phase exists with preferred particle orientations that recover the absent symmetry elements of the crystal; we call this phase the discrete plastic crystal phase. If G and H share no nontrivial symmetry elements, then a plastic crystal phase exists without preferred orientations, which we call an indiscrete plastic crystal.

Zinc-catalyzed reaction of isoxazoles with thioynol ethers involving an unprecedented 1,2-sulfur migration.

A novel zinc-catalyzed reaction of isoxazoles with thioynol ethers involving an unprecedented 1,2-sulfur migration has been developed, which represents the first example of a non-noble metal-catalyzed reaction between isoxazoles and alkynes. This method allows the facile and atom-economical synthesis of a range of valuable ß-keto enamides. Moreover, the computational study provides further evidence for the feasibility of the proposed reaction mechanism.

Divergent synthesis of N-heterocycles via controllable cyclization of azido-diynes catalyzed by copper and gold.

Gold-catalyzed intermolecular alkyne oxidation by an N-O bond oxidant has proven to be a powerful method in organic synthesis during the past decade, because this approach would enable readily available alkynes as precursors in generating α-oxo gold carbenes. Among those, gold-catalyzed oxidative cyclization of dialkynes has received particular attention as this chemistry offers great potential to build structurally complex cyclic molecules. However, these alkyne oxidations have been mostly limited to noble metal catalysts, and, to our knowledge, non-noble metal-catalyzed reactions such as diyne oxidations have not been reported. Herein, we disclose a copper-catalyzed oxidative diyne cyclization, allowing the facile synthesis of a wide range of valuable pyrrolo[3,4-c]quinolin-1-ones. Interestingly, by employing the same starting materials, the gold-catalyzed cascade cyclization leads to the divergent formation of synthetically useful pyrrolo[2,3-b]indoles. Furthermore, the proposed mechanistic rationale for these cascade reactions is strongly supported by both control experiments and theoretical calculations.

Highly Site Selective Formal [5+2] and [4+2] Annulations of Isoxazoles with Heterosubstituted Alkynes by Platinum Catalysis: Rapid Access to Functionalized 1,3-Oxazepines and 2,5-Dihydropyridines.

Platinum-catalyzed formal [5+2] and [4+2] annulations of isoxazoles with heterosubstituted alkynes enabled the atom-economical synthesis of valuable 1,3-oxazepines and 2,5-dihydropyridines, respectively. Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α-imino platinum carbene intermediates.