The first example of cobalt-catalyzed oxidative C−H/C−H cross-coupling between two heteroarenes is reported, which exhibits a broad substrate scope and a high tolerance level for sensitive functional groups. When the amount of Co(OAc)₂⋅4 H₂O is reduced from 6.0 to 0.5 mol %, an excellent yield is still obtained at an elevated temperature with a prolonged reaction time. The method can be extended to the reaction between an arene and a heteroarene. It is worth noting that the Ag₂CO₃ oxidant is renewable. Preliminary mechanistic studies by radical trapping experiments, hydrogen/deuterium exchange experiments, kinetic isotope effect, electron paramagnetic resonance (EPR), and high resolution mass spectrometry (HRMS) suggest that a single electron transfer (SET) pathway is operative, which is distinctly different from the dual C−H bond activation pathway that the well-described oxidative C−H/C−H cross-coupling reactions between two heteroarenes typically undergo.

The first example of cobalt-catalyzed oxidative C−H/C−H cross-coupling between two heteroarenes is reported, which exhibits a broad substrate scope and a high tolerance level for sensitive functional groups. When the amount of Co(OAc)₂⋅4 H₂O is reduced from 6.0 to 0.5 mol %, an excellent yield is still obtained at an elevated temperature with a prolonged reaction time. The method can be extended to the reaction between an arene and a heteroarene. It is worth noting that the Ag₂CO₃ oxidant is renewable. Preliminary mechanistic studies by radical trapping experiments, hydrogen/deuterium exchange experiments, kinetic isotope effect, electron paramagnetic resonance (EPR), and high resolution mass spectrometry (HRMS) suggest that a single electron transfer (SET) pathway is operative, which is distinctly different from the dual C−H bond activation pathway that the well-described oxidative C−H/C−H cross-coupling reactions between two heteroarenes typically undergo.

Oxidative transformations of phenols have attracted significant attention of chemists due to their importance in biological process and organic synthesis. In contrast to the relatively well-developed oxygenation and coupling reactions of phenols, the highly efficient and selective oxidative ring cleavage of phenols is under-represented. This work describes a novel CuCl-catalyzed tandem homocoupling/skeletal rearrangement of phenols that realizes the cleavage of the phenol ring by using air or Ag₂CO₃ as the oxidant. Interestingly, simply changing the oxidant to K₂S₂O₈ results in the oxidative coupling/cyclization of phenols to give dibenzofurans. These results set an important precedent of oxidant-controlled catalytic transformations of phenols.

Disclosed herein is a Rh^III-catalyzed chelation-assisted activation of unreactive CH bonds, thus enabling an intermolecular amidation to provide a practical and step-economic route to 2-(pyridin-2-yl)ethanamine derivatives. Substrates with other N-donor groups are also compatible with the amidation. This protocol proceeds at room temperature, has a relatively broad functional-group tolerance and high selectivity, and demonstrates the potential of rhodium(III) in the promotive functionalization of unreactive CH bonds. A rhodacycle having a SbF₆⁻ counterion was identified as a plausible intermediate.

Disclosed herein is a Rh^III-catalyzed chelation-assisted activation of unreactive CH bonds, thus enabling an intermolecular amidation to provide a practical and step-economic route to 2-(pyridin-2-yl)ethanamine derivatives. Substrates with other N-donor groups are also compatible with the amidation. This protocol proceeds at room temperature, has a relatively broad functional-group tolerance and high selectivity, and demonstrates the potential of rhodium(III) in the promotive functionalization of unreactive CH bonds. A rhodacycle having a SbF₆⁻ counterion was identified as a plausible intermediate.

A transition-metal-free formal decarboxylative coupling reaction between α-oxocarboxylates and α-bromoketones to synthesize 1,3-diketone derivatives is presented. In this reaction, a broad scope of substrates can be employed, and neither a metal-based reagent nor an additional base is required. DFT calculations reveal that this reaction proceeds through a coupling followed by decarboxylation mechanism and the α-bromoketone unprecedentedly serves as a nucleophile under neutral conditions. The rate-determining step is an unusual hydrogen-bond-assisted enolate formation by thermolysis.

Mechanofluorochromic materials have great potential for a wide variety of applications such as sensors, memory devices, motion systems, security systems, and so forth. However, only few design principles have been disclosed, which greatly impedes the growth of mechanofluorochromic dyes. Here, a strategy of molecular design for mechanochromic luminescence is reported, based on the cation–anion interaction-directed switching of molecular stacking. On the basis of this strategy, a series of common N-heteroaromatic onium fluorophores such as imidazolium, 1,2,4-triazolium, triazolopyridinium, benzoimidazolium, γ-carbolinium, and pyridinium salts have been designed and proved to have striking reversible mechanofluorochromic behaviors. The simple attachment of a non-fluorescent imidazolium unit to the pyrene scaffold through a flexible carbon chain can even trigger the mechanofluorochromic phenomenon, which gives a consummate interpretation that the cation–anion interaction can be considered as an important general tool to design organic mechanochromic luminescent materials.

A counterion-directed molecular design strategy is described for the spontaneous formation of stable vesicles via readily available imidazolium salts with the EDTA counterion in aqueous media. The counteranion is employed to adjust the balance between hydrophobic and hydrophilic parts, which satisfies the requirement of packing parameter for vesicle formation. The counterion-induced vesicles (CIVs) feature spontaneous formation, simple preparation, and easy availability of surfactants. Importantly, the unusual counterion-induced vesicle-like sphere aggregates can further chelate europium(III) ions to enhance the europium-centered emission in aqueous media and make it viable for an optical pH sensor, which exhibits a linear response in a wide range of pH values from 3 to 11. To the best of our knowledge, this constitutes one of the widest ranges of Eu-based pH sensors reported so far. This design concept has offered a new avenue for the preparation of functional vesicles.

Diversification of the β-carboline skeleton has been demonstrated to assemble a β-carboline library starting from the tetrahydro-β-carboline framework. This strategy affords feasible access to heteroaryl-, aryl-, alkenyl-, or alkynyl-substituted β-carbolines at the C1, C3, or C8 position through three categorically different types of transition-metal-catalyzed CC bond-forming reactions, in the presence of multiple potentially reactive positions. These site-selective functionalizations include; 1) the Cu-catalyzed C1/C3-selective decarboxylative CC and CC_sp coupling of hexahydro-β-carboline-3-carboxylic acid with a CH bond of a heteroarene or terminal alkyne; 2) the chelation-assisted Pd-catalyzed C1/C8-selective CH arylation of hexahydro-β-carboline with aryl boron reagents; and 3) the chelation-assisted Pd-catalyzed C1/C3-selective oxidative CH/CH cross-coupling of β-carboline-N-oxide with arenes, heteroarenes, or alkenes. The saturated structural feature of the hexahydro-β-carboline framework can increase reactivity and control site selectivity. The robustness of these approaches has been demonstrated through the synthesis of hyrtioerectine analogues and perlolyrine. We believe that these strategies could provide inspiration for late-stage diversifications of bioactive core scaffolds.

In sharp contrast to the gold-catalyzed reactions of alkynes/allenes with nucleophiles, gold-catalyzed oxidative cross-couplings and especially CH/CH cross-coupling have been under represented. By taking advantage of the unique redox property and carbophilic π acidity of gold, this work realizes the first gold-catalyzed direct C(sp³)H alkynylation of 1,3-dicarbonyl compounds with terminal alkynes under mild reaction conditions, with subsequent cyclization and in situ oxidative alkynylation. A variety of terminal alkynes including aryl, heteroaryl, alkenyl, alkynyl, alkyl, and cyclopropyl alkynes all successfully participate in the domino reaction. The protocol offers a simple and region-defined approach to 3-alkynyl polysubstituted furans.

An unprecedented catalytic system composed of the Wilkinson catalyst [Rh(PPh₃)₃Cl] and CF₃COOH enabled the highly regioselective cross-coupling of aromatic amines with a variety of heteroarenes through dual CH bond cleavage. This protocol provided a facile and rapid route from readily available substrates to (2-aminophenyl)heteroaryl compounds, which may be conveniently transformed into highly extended π-conjugated heteroacenes. The experimental studies and calculations showed that thianaphtheno[3,2-b]indoles have large HOMO–LUMO energy gaps and low-lying HOMO levels, and could therefore potentially be high-performance organic semiconductors. Herein we report the first use of a rhodium(I) catalyst for oxidative CH/CH coupling reactions. The current innovative catalyst system is much less expensive than [RhCp*Cl₂]₂/AgSbF₆ and could open the door for the application of this approach to other types of CH activation processes.

An unprecedented catalytic system composed of the Wilkinson catalyst [Rh(PPh₃)₃Cl] and CF₃COOH enabled the highly regioselective cross-coupling of aromatic amines with a variety of heteroarenes through dual CH bond cleavage. This protocol provided a facile and rapid route from readily available substrates to (2-aminophenyl)heteroaryl compounds, which may be conveniently transformed into highly extended π-conjugated heteroacenes. The experimental studies and calculations showed that thianaphtheno[3,2-b]indoles have large HOMO–LUMO energy gaps and low-lying HOMO levels, and could therefore potentially be high-performance organic semiconductors. Herein we report the first use of a rhodium(I) catalyst for oxidative CH/CH coupling reactions. The current innovative catalyst system is much less expensive than [RhCp*Cl₂]₂/AgSbF₆ and could open the door for the application of this approach to other types of CH activation processes.

A concise, highly efficient palladium-catalyzed direct C–H (hetero)arylation is developed to modularly assemble a diketopyrrolopyrrole (DTDPP)-based polymer library to screen low-bandgap and near-infrared (NIR) absorbing materials. The DTDPP-based copolymers P1 and P2 with an alternating donor–acceptor–donor–acceptor (D–A–D–A) sequence and the homopolymer P9 exhibit planarity and excellent π-conjugation, which lead to low bandgaps (down to 1.22 eV) as well as strong and broad NIR absorption bands (up to 1000 nm).

The biheteroaryl structural motif is prevalent in polymers, advanced materials, liquid crystals, ligands, molecules of medicinal interest, and natural products. Many types of synthetic transformations have been known for the construction of heteroaryl–heteroaryl linkages. Coupling reactions provide one of the most efficient ways to achieve these biheterocyclic structures. In this review, four types of coupling reactions are discussed: 1) transition-metal-catalyzed coupling reactions of heteroaryl halides or surrogates with heteroarylmetals; 2) direct inter- and intramolecular heteroarylations of CH bonds of heteroarenes with heteroaryl halides or pseudohalides; 3) oxidative CH/CH homo- and cross-couplings of two unpreactivated heteroarenes; and 4) transition-metal-catalyzed decarboxylative cross-coupling reactions between haloheteroarenes or heteroarenes and heteroarenecarboxylic acids. The general purpose of this review is to give an exhaustive and clear picture in heteroaryl–heteroaryl bond formation as well as its application in the synthesis of natural products, pharmaceuticals, catalyst ligands, and materials.

N-Heterocyclic carbenes (NHCs) can serve as very reactive nucleophilic catalysts and exhibit strong basicity. Herein, we initiate a combined experimental and computational investigation of the NHC-catalyzed ring-closing reactions of 4-(2-formylphenoxy)but-2-enoate derivatives 1 to uncover the relationship between the counteranion of an azolium salt, the nucleophilicity and basicity of the carbene species, and the catalytic performance of the carbene species by taking imidazolium salts IPr⋅HX (X=counteranion, IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) as the representative precatalysts. The plausible mechanisms of IPr-mediated ring-closing reactions have been investigated by using DFT calculations. The hydrogen-accepting ability, assigned as the basicity of the counteranion of IPr⋅HX and evaluated by DFT calculations, is correlated with the rate of deprotonation of C2 in IPr⋅HX, which could be monitored by the capture of the free carbene formed in situ with elemental sulfur. The deprotonation of C2 in IPr⋅HX with a more basic anion gives rise to a higher concentration of the free carbene and vice versa. At a relatively low concentration, IPr prefers to show a nucleophilic character to induce the intramolecular Stetter reaction. At a relatively high concentration, IPr primarily acts as a base to afford benzofuran derivatives. These data comprehensively disclose, for the first time, that the counteranions of azolium salts significantly influence not only the catalytic activity, but also possibly the reaction mechanism.

The chiral tris-monodentate imidazolinyl ligands 1 a–c exhibit a strong tendency to form the discrete, helical [2+3] nanocages 3 ([1₂⋅2₃]) with tartaric acids 2. Circular dichroism (CD) spectra and theoretical calculations reveal that supramolecular handedness of capsulelike architectures is determined only by the chirality of the imidazolinyl ligands rather than tartaric acids. The chirality of imidazolinyl ligands is transferred to the helicity of the complexes through the directed hydrogen bonds between the N3 atom of imidazoline rings and the carboxyl of tartaric acids. These hydrogen-bonded nanocages can spontaneously self-assemble into spherical vesicles, during which the hydrogen bonding that arises from the hydroxyl groups of tartaric acids plays a crucial issue. The vesicles formed by [{(S,S,S)-1 a}₂(2^L)₃] (3 a) may further evolve into microspheres that gelate organic solvents after being aged at −20 °C for 24 h, and can also be unprecedentedly transformed to tubular assemblies capable of rigidifying the solvents when subjected to ultrasound irradiation.

We herein present an effective strategy to create water-soluble fluorescent bioimaging dyes by introducing the imidazolium-based ionic liquid (IL) pendants into a fluorescent skeleton. A new type of water-soluble imidazolium-anchored squaraine dye was synthesized accordingly. The relationship between the aggregate of squaraines and their fluorescent cell imaging application was elucidated in detail. Firstly, the aggregation behavior of squaraines in water solutions could be suppressed by varying the alkyl chain attached to the imidazolium unit. Secondly, the capability of cellular uptake and staining of dyes was also dramatically enhanced upon increasing the length of the paraffinic chain. These squaraine dyes displayed an excellent photostability that could permit real-time fluorescence bioimaging experiments to be monitored over a long time period with constant sample irradiation. Additionally, we designed for the first time an Fe^II-ion probe on the basis of an attack of the hydroxyl radical to the four-membered ring of squaraine. The results demonstrated that the imidazolium-anchored squaraines could perform “naked-eye” detection of the Fe²⁺ ion over a wide range of other interfering metals in aqueous media. More surprisingly, this process showed a fluorescence “turn-off” and “-on” response through the regeneration of squaraines in cells.

The encouraging results of preliminary toxicological studies on imidazolium-based ionic liquids provide good opportunities for the development of ionic liquids in biomedical applications. In this work, the polymerized ionic liquid poly[3-butyl-1-vinylimidazolium L-proline salt] has been synthesized as a gene vector. The interaction of poly[3-butyl-1-vinylimidazolium L-proline salt] with DNA was studied by agarose gel electrophoresis. The cell viability was determined through PI (propidium iodine) staining and flow cytometry, showing marginal toxicity toward the cells examined. The transfection efficiency was evaluated through the in vitro transfection experiment. The results indicated that the imidazolium cation had a high binding ability to DNA, and the condensed DNA in the complexes could be effectively protected against enzymatic degradation. Poly[3-butyl-1-vinylimidazolium L-proline salt] could further transfer the reporter gene into the HeLa cell and successfully mediate the gene expression without the aid of additional agent.

A solution to the long-standing challenge of developing a highly effective method for the enantioselective intermolecular benzoin condensation of aromatic aldehydes is described. The chiral bis-bicyclic triazolium salt – 1,3-bis{(S)-5-benzyl-6,8-dihydro-5H-[1,4]oxazino[2,1-c][1,2,4]triazol-2-ium-2-yl}benzene dichloride [(S)-5a-1] is currently the most efficient precatalyst for the asymmetric variant of the benzoin condensation.

A novel type of chiral tris-monodentate imidazolinyl ligands ((S,S,S)-4 and (R,R,R)-4) has been achieved in good yields. The ligands show a strong tendency to induce the generation of the discrete sandwich-shaped M₃L₂ architectures with programmed helicity through the edge-directed complexation with a series of d³–d¹⁰ transition-metal ions, while taking advantage of the steric hindrance of the bulky substituents of the imidazoline rings to avoid the formation of extended metal–organic frameworks (MOFs). In spite of different coordination geometries, monovalent metal ions (e.g. Ag⁺), divalent metal ions (e.g. Pd²⁺, Cu²⁺, Cd²⁺, Zn²⁺, Co²⁺, Mn²⁺, and Ni²⁺), and even trivalent metal ions (e.g. Fe³⁺ and Cr³⁺) exhibit isostructural coordination. Installation of stereocenters fused onto the imidazoline rings results in favored handedness of the self-assemblies through the expression of molecular chirality into supramolecular helicity. In the crystal structures of [M₃{(S,S,S)-4}₂], the self-assembly has to adopt the M form to relax the van der Waals repulsions of the phenyl and isopropyl groups. The replacement of (S,S,S)-4 with (R,R,R)-4 exclusively affords the opposite helicity (P). These results should provide important insights for the design of chiral helical capsule-like assemblies.