Chemospecific C3- and C2-Olefinations of Isatins by TfOH-Promoted Tandem Aldol-Grob and Semiacetalization-Grob Fragmentations.

A novel method for TfOH-promoted chemospecific C3- and C2-olefinations of isatins is developed, which offers the first examples of Grob fragmentation using isatins and amides as substrates.

Switched Proton Conduction in Metal-Organic Frameworks.

Stimuli-responsive materials can respond to external effects, and proton transport is widespread and plays a key role in living systems, making stimuli-responsive proton transport in artificial materials of particular interest to researchers due to its desirable application prospects. On the basis of the rapid growth of proton-conducting porous metal-organic frameworks (MOFs), switched proton-conducting MOFs have also begun to attract attention. MOFs have advantages in crystallinity, porosity, functionalization, and structural designability, and they can facilitate the fabrication of novel switchable proton conductors and promote an understanding of the comprehensive mechanisms. In this Perspective, we highlight the current progress in the rational design and fabrication of stimuli-responsive proton-conducting MOFs and their applications. The dynamic structural change of proton transfer pathways and the role of trigger molecules are discussed to elucidate the stimuli-responsive mechanisms. Subsequently, we also discuss the challenges and propose new research opportunities for further development.

Hydrogen-Bonded Organic Frameworks: Functionalized Construction Strategy by Nitrogen-Containing Functional Group.

The construction of hydrogen-bonded organic framework materials by intermolecular hydrogen bonding forces has been rapidly developed in the last decade, among which, the strong intermolecular hydrogen bonding and functional binding sites exhibited by nitrogen-containing functional groups have made them favorites for designing organic components to customize functionalized porous materials. This review systematically introduces the types of nitrogen-containing monomers used to prepare porous hydrogen-bonded organic backbones and the principles of their construction, summarizes the design advantages of crystalline materials from an elemental perspective, and presents the applications of such HOFs in the fields of gas adsorption/separation, molecular recognition, plasmonic conductivity, biomedical, and luminescent materials, etc. Finally, the prospects for the development of such materials are discussed and potential directions for future work are analyzed.

Solvent-Assisted Modification to Enhance Proton Conductivity and Water Stability in Metal Phosphonates.

Although proton-conductive metal phosphonates with well-defined structure offer a favorable platform for exploring their structure-property relationship, investigating of the synergic effect of phosphonate groups and functional moieties on proton conduction is rare. In this work, we have synthesized two new copper phosphonates, [Cu(4-cppH)(4,4'-bipy)(H2O)3] (FJU-80) and [Cu(4-cppH)(4,4'-bipy)]·H2O·DMF (FJU-81), by the method of solvent-assisted modification, giving a 1D metal coordination polymer and a 3D metal open framework, respectively. Single-crystal X-ray diffraction shows that FJU-80 is full of hydrogen-bonding sites contributed from the improved synergic effect of phosphonate groups, carboxylate groups, and coordinated water molecules, thereby facilitating continuous hydrogen-bonding networks, whereas FJU-81 only has discrete hydrogen-bonding fragments. Powder X-ray diffraction and impedance analyses confirm that FJU-80 possesses higher water stability as well as improved proton conductivity, indicating that solvent-assisted modification is effective in increasing the hydrogen-bonding sites from phosphonate groups and functional moieties and then realizing facile proton transfer.

Enhancement of Intrinsic Proton Conductivity and Aniline Sensitivity by Introducing Dye Molecules into the MOF Channel.

The encapsulation of dyes into metal-organic frameworks (MOFs) has generated a variety of platforms for luminescence, but little attention has been paid to their application in proton conduction. Here, a cationic MOF {{[In3OL1.5(H2O)3](NO3)}·(DMA)3·(CH3CN)6·(H2O)30} n (FJU-10, H4L = 4,4',4″,4‴-(1,4-phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid, DMA = N, N-dimethylacetamide) was synthesized, and the dye molecule 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS) was further added to the MOF growth solution, but during the reaction, HPTS was nitrated and nitrated HPTS was encapsulated into the FJU-10 to obtain dye@FJU-10. As a result, the intrinsic proton conductivity of dye@FJU-10 is nearly 5 times higher than that of FJU-10 at 90 °C. Dye@FJU-10 exhibits more sensitive fluorescence quenching toward aniline than FJU-10 in DMF solution (the detection limits of FJU-10 and dye@FJU-10 are as low as 0.58 and 0.62 µM, respectively). Here, it is demonstrated for the first time that intrinsic proton conductivity can be effectively improved by encapsulating a nitrated HPTS dye into an MOF.

Metal-Free Method for Direct Synthesis of Functionalized ß-Ketoenamines.

A new method for direct synthesis of ß-ketoenamines was developed by a BF3·OEt2-catalyzed cyclization of 1-iodoalkyne and α-keto acid followed by an amine-mediated ring-opening in one pot. Its metal-free conditions allowed the easy synthesis of those products bearing the transition metal-sensitive functional groups. Its three-component process achieved wide range of functionalized products.

A general two-step one-pot synthesis process of ynones from α-keto acids and 1-iodoalkynes.

A general two-step one-pot synthesis process of ynones was developed by cycloaddition of α-keto acids and 1-iodoalkynes followed by a ring-opening reaction. Its easy conditions and novel mechanism endowed it with two distinctive advantages: iodine-atom bonded to C(sp2) remained intact and α-keto acids became a part of the triple bonds in ynones.

Silver-catalyzed decarboxylative acylation of quinoxalin-2(1H)-ones with α-oxo-carboxylic acids.

A novel silver-catalyzed decarboxylative acylation of α-oxo-carboxylic acids was developed, by which various 3-acyl quinoxalin-2(1H)-ones were synthesized by direct C-H bond acylation of quinoxalin-2(1H)-ones. In this method, α-oxo-carboxylic acids served as efficient acylating reagents to in situ generate the required active acyl radical. Its excellent chemoselectivity allowed the molecular diversity of 3-acyl quinoxalin-2(1H)-ones to be achieved by convenient functionalizations of both N1- and C3-positions.

In-situ Generated and Premade 1-Copper(I) Alkynes in Cycloadditions.

The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) was discovered in 2002, which has become the most remarkable example for "click chemistry" to date. In CuAAC reaction, 1-copper(I) alkyne has been recognized to be a key intermediate. However, many contradictory experimental results for this intermediate were reported in literature. For example, only the in-situ generated 1-copper(I) alkyne was used, while the premade 1-copper(I) alkyne proved to be inefficient under the standard conditions. The kinetic studies indicated that CuAAC reaction had a strict second-order dependence on Cu(I) and the DFT studies demonstrated that 1-copper(I) alkyne intermediate should be a dinuclear copper(I) complex. But these results were inconsistent with the structure of the premade 1-copper(I) alkyne. Although hundreds of structurally different ligands were reported to significantly enhance the efficiency of CuAAC reaction, their functions were assigned to prevent the oxidation and the disproportionation of Cu(I) ion. Based on the investigation of the references and our works, we proposed that the in-situ generated 1-copper(I) alkyne in CuAAC reaction is not identical with the premade 1-copper(I) alkyne. The ligands may play dual roles to activate the 1-copper(I) alkyne by blocking the polymerization of the in-situ formed 1-copper(I) alkynes and dissociating the polymeric structures of the premade 1-copper(I) alkynes. As a result, we first disclosed that carboxylic acids can function as such activators and a novel carboxylic acid-catalyzed CuAAC strategy was developed, which has been proven to be the most convenient and highly efficient CuAAC method to date. Furthermore, highly efficient and regioselective methods for the syntheses of 1,4,5-trisubstituted 1,2,3-triazoles were developed by using the premade 1-copper(I) alkynes as substrates, in which the novel function of the premade 1-copper(I) alkynes as excellent dipolarophiles was first disclosed and applied. In this article, a series of works reported by our group for the in-situ generated and the premade 1-copper(I) alkynes in cycloadditions are reviewed.

Ruthenium-Catalyzed Synthesis of Fused Tricyclic 1H-2,3-Dihydropyrimido[1,2-a]quinolines in One Step.

A novel ruthenium-catalyzed intramolecular cyclization of a nitrile and an azetidine was developed to achieve a one-step synthesis of the fused tricyclic 1H-2,3-dihydropyrimido[1,2-a]quinoline, which is the core skeleton for more than 100 natural pyoverdines and is also responsible for their fluorescence.

Synthesis and micellar mimic properties of bile acid trimers.

Two fan-shaped bile acid trimers have been synthesized via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) 'click chemistry', and their extraction experiments of cresol red sodium (CR) and pyrene were investigated in the polar and non-polar solvents, respectively. The transmission electron microscopy (TEM) results showed that the homogenous hollow capsules formed with the diameter size range of 40-70 nm in a solution of water and acetone. Thus the amphiphilicity of fan-shaped bile acid trimers might be used as the promising candidate in biological and drug delivery applications.