Wenkai Zhang

Copper-Catalyzed One-Pot Synthesis of 1,2,4-Triazoles from Nitriles and Hydroxylamine

A simple and efficient copper-catalyzed one-pot synthesis of substituted 1,2,4-triazoles through reactions of two nitriles with hydroxylamine has been developed. The protocol uses simple and readily available nitriles and hydroxylamine hydrochloride as the starting materials and inexpensive Cu(OAc)2 as the catalyst, and the corresponding 1,2,4-triazole derivatives are obtained in moderate to good yields. The reactions include sequential intermolecular addition of hydroxylamine to one nitrile to provide amidoxime, copper-catalyzed treatment of the amidoxime with another nitrile, and intramolecular dehydration/cyclization. This finding provides a new and useful strategy for synthesis of 1,2,4-triazole derivatives.

Supramolecular interactions via hydrogen bonding contributing to citric-acid derived carbon dots with high quantum yield and sensitive photoluminescence

Herein, we report the characterization of highly fluorescent citric-acid derived carbon dots (CACDs) synthesized by hydrothermal treatment of citric acid and diethylenetriamine below 200 °C. After being purified using a gel permeation chromatography cleanup system, the complexity and chemical composition of the CACDs were evaluated by liquid chromatography coupled with high-resolution Fourier transform ion cyclotron resonance mass spectrometry. The fluorophores consisted of five-membered ring fused 2-pyridones identified as the photoluminescence origin. M06-2X density functional calculations, surface tension and morphological studies suggested DETA@5CA serves as the main building block to fabricate supramolecular aggregates. Then we proposed that the dimeric and trimeric fluorophores coupled with DETA@5CA led to “dot” topologies in the CACDs solution under the effect of hydrogen bonding. In aqueous solution, the CACDs exhibited narrowly dispersed optical properties and a high fluorescent quantum yield (∼98%). Moreover, the supramolecular interaction induced CACDs have high sensitivity under various ambient conditions, such as pH, organic solvents and metal ions.

Base-mediated one-pot synthesis of 1,2,4-oxadiazoles from nitriles, aldehydes and hydroxylamine hydrochloride without addition of extra oxidant

A simple base-mediated one-pot synthesis of 3,5-disubstituted 1,2,4-oxadiazoles from nitriles, aldehydes and hydroxylamine hydrochloride has been developed, in which the aldehydes act as both substrates and oxidants. The reactions include three sequential procedures: base-promoted intermolecular addition of hydroxylamine to nitrile to lead to amidoxime, treatment of the amidoxime with an aldehyde to form 4,5-dihydro-1,2,4-oxadiazole, and oxidization of the 4,5-dihydro-1,2,4-oxadiazole by using another aldehyde to afford 1,2,4-oxadiazole. This method represents a direct and simple protocol for the synthesis of 3,5-disubstituted 1,2,4-oxadiazoles.

Photo-cross-linked poly(ether amine) micelles for controlled drug release

In order to improve the stability of micelles and decrease the burst release of loaded drugs, photo-cross-linked micelles were prepared via photodimerization of the coumarin moiety on amphiphilic poly(ether amine) (PEAC). The structures of the obtained monomer and polymers were confirmed by Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-vis), 1H NMR and 13C NMR (Nuclear Magnetic Resonance, NMR). PEAC could self-assemble into micelles by directly dispersing in water with a hydrophobic coumarin core and a hydrophilic poly(ethylene glycol) (PEG) shell. The photo-cross-linking process of the PEAC micelles was monitored by UV-vis spectroscopy. The morphology and size distribution of the micelles was characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Anticancer drug doxorubicin (DOX) was loaded into the micelles during the process of micelle formation. Photo-cross-linked micelles showed slower drug release and cellular uptake in comparison with the uncross-linked micelles. And both DOX-loaded micelles displayed pH-sensitive release behaviours. Moreover, the DOX-loaded photo-cross-linked micelles exhibit comparative anticancer efficacy as free DOX. These results indicated that photo-cross-linked PEAC micelles can be used as potential drug carriers for intelligent drug delivery.

Effect of oxygen functionalities of graphene oxide on polymerization and thermal properties of reactive benzoxazine nanocomposites

Herein, we report the effect of oxygen functionalities of graphene oxide on thermal activated polymerization and thermal properties of reactive benzoxazine nanocomposites. The numbers of oxygen moieties of graphene oxide (GO) are controlled by hydrothermal reduction. The polymerization behavior of benzoxazine monomer (BA-a) is studied by Fourier transform infrared spectroscopy, differential scanning calorimetry and rheological analysis. It is hypothesized that the GO not only exhibits accelerated effect on the polymerization of the BA-a, but also the oxygen moieties such as carboxyl groups of GO interact with the benzoxazine polymers, leading to several orders of magnitude increase in the chemoviscosity and modulus of composite system. Thermal conductivity of poly(BA-a)/GO composite increases from an initial value of ∼0.27 W/mK to 0.47 W/mK as the loading increases from 1 wt% to 6 wt% (enhancement factor up to 176%). Moreover, the nanocomposites display enhanced initial decomposition temperature and char yields as the degree of GO reduction increases.

Base-mediated synthesis of unsymmetrical 1,3,5-triazin-2-amines via three-component reaction of imidates, guanidines and amides or aldehydes

A simple and efficient method for the base-mediated synthesis of unsymmetrical 1,3,5-triazin-2-amines has been developed. The protocol uses readily available imidates, guanidines, and amides or aldehydes as the starting materials, cesium carbonate as the base, no catalyst or additive is required, and the three-component reaction provides diverse 1,3,5-triazin-2-amines in moderate to good yields with tolerance of wide functional groups.

Structure and photoluminescence evolution of nanodots during pyrolysis of citric acid: from molecular nanoclusters to carbogenic nanoparticles

Citric-acid-derived carbon nanodots are increasingly being explored as novel fluorescent nanomaterials due to their strong photoluminescence (PL). However, an accurate picture of the formation of carbon nanodots and an exhaustive structure–property correlation are still lacking. Herein we present a systematic investigation of the formation mechanism of carbon nanodots by following the pyrolysis of a citric acid–diethylenetriamine precursor at different temperatures. The collective nanodots are investigated by dynamic rheological measurements, exhibiting a strong pyrolytic temperature dependence of the viscoelastic properties. Atomic force microscopy, transmission electron microscopy, and Raman spectroscopy reveal that the synthesized “dots” at different pyrolytic temperatures are different in essence, and the transition of their chemical structure from molecular clusters to carbogenic nanoparticles during pyrolysis is highly verified. We find that a molecular fluorophore with intense PL predominates at low temperature (<250 °C), but a newly created quasi-molecular fluorophore with blue-shifted and decreased PL quantum yield predominates at high temperatures (300 °C). Time-resolved photoluminescence spectroscopy suggests that the strong PL suppression at high temperature is due mostly to a dramatic increase in the nonradiative decay rate of the quasi-molecular electronic state.

Nmp-based ionic liquids: Recyclable catalysts for both hetero-Michael addition and Knoevenagel condensation in water

ABSTRACT A series of novel N-methyl piperidine (Nmp)-based ionic liquids with 1,2-propanediol group are synthesized and used as catalysts for both hetero-Michael addition of α,β-unsaturated amides and Knoevenagel condensation at room temperature in water; and all the examined substrates could be transformed into corresponding products in good to excellent yields. Meanwhile IL-catalyzed hetero-Michael addition of α,β-unsaturated amides in water has not been reported in the previous literatures. Additionally, the catalyst is recyclable for the two reactions. This finding provides a green catalyst for both hetero-Michael addition of α,β-unsaturated amides and Knoevenagel condensation in water. GRAPHICAL ABSTRACT

Iron-Mediated Synthesis of Isoxazoles from Alkynes: Using Iron(III) Nitrate as a Nitration and Cyclization Reagent

A simple and direct method for the iron(III) nitrate-mediated synthesis of isoxazoles from alkynes has been developed; both self-coupling and cross-coupling products could be successfully prepared from alkynes. Meanwhile, for the cross-coupling and cyclizing of two different alkynes examined, the iron-mediated system shows a good chemoselectivity for the synthesis of corresponding isoxazoles. In our method, cheap and eco-friendly iron(III) nitrate is used as the nitration and cyclization reagent, and KI is used as the additive; they both play a positive role in this transformation. Furthermore, a different mechanism for the formation of isoxazoles from alkynes has been proposed.

Carbonization temperature controlled thermal conductivity of graphitic carbon nanoparticles and their polymer composites

Carbon materials as versatile fillers have drawn increasing attention in thermal conductive polymer composites, however, the thermal conductivity (TC) regulation of them remains challenging. Herein, the tunable lattice thermal conductivity is reported for glucose derived graphitic carbon nanoparticles (GCPs) and their polymer composites. Both the in-plane (La) and out-of-plane (Lc) coherence lengths of GCPs increase with carbonization temperature in the range of 700 °C to 1300 °C. The intrinsic TC of GCPs film is directly extracted from the dependence of the Raman G peak frequency on the excitation laser power and the first order temperature coefficient. It is found that the in-plane lattice TC increases exponentially with both of the increasing La and decreasing defect concentration. The GCPs are then used as highly processible fillers to fabricate thermoset composites based on reactive benzoxazine (BA-a). The total TC of the poly(BA-a)/GCPs are found increase monotonically from 0.27 W·m-1·K-1 to 0.34 W·m-1·K-1 with the increasing graphitization levels of GCPs, and a clear signature of thermal percolation threshold at 6 vol% GCPs loadings is also observed.Carbon materials as versatile fillers have drawn increasing attention in thermal conductive polymer composites, however, the thermal conductivity (TC) regulation of them remains challenging. Herein, the tunable lattice thermal conductivity is reported for glucose derived graphitic carbon nanoparticles (GCPs) and their polymer composites. Both the in-plane (La) and out-of-plane (Lc) coherence lengths of GCPs increase with carbonization temperature in the range of 700 °C to 1300 °C. The intrinsic TC of GCPs film is directly extracted from the dependence of the Raman G peak frequency on the excitation laser power and the first order temperature coefficient. It is found that the in-plane lattice TC increases exponentially with both of the increasing La and decreasing defect concentration. The GCPs are then used as highly processible fillers to fabricate thermoset composites based on reactive benzoxazine (BA-a). The total TC of the poly(BA-a)/GCPs are found increase monotonically from 0.27 W·m-1·K-1 to 0.34 W·...