Search

Fluorescence-based amine-reactive dyes are highly valuable for the sensing of amines and the labeling of biomolecules. Although it would be highly desirable, large changes in emission spectra and intensity seldom accompany the conjugation of known amine-reactive dyes to their target molecules. On the contrary, amide bond formation between amines and the pentafluorophenyl (2-PFP) and succinimidyl (2-NHS) esters of meso-carboxyBODIPY results in significant changes in emission maxima (Δλ: 70-100 nm) and intensity (up to 3000-fold), enabling the fast (down to 5 min) and selective fluorogenic detection and labeling of amines, amino acids, and proteins. This approach further benefits from the demonstrated versatility and high reliability of activated ester chemistry, and background hydrolysis is negligible. The large "turn-on" response is a testament of the extreme sensitivity of meso-carboxyBODIPYs to the minimal changes in electronic properties that distinguish esters from amides. Applications to the detection of food spoilage, staining of proteins on electrophoretic gels or in living cells, and the expedited synthesis of organelle-specific fluorescence microscope imaging agents are further demonstrated.

Controlling the nanoscale morphology of conducting polymer/nanoparticle hybrid films is a highly desired but challenging task. Here, we report that such functional hybrid films with unprecedented structural order can be formed through the self-assembly of conjugated block copolymers and CdSe quantum dots at the air–water interface. The one-step assembly of quantum dots and block copolymers composed of polythiophene and polyethylene glycol (P3HT-b-PEG) at the fluidic interface generated a highly ordered assembly structure of P3HT nanowires and one-dimensional quantum dot arrays. Structure analyses revealed a unique self-assembly behavior and size dependency, which are distinct from the conventional self-assembly of coil-type polymers on solid substrates. Interestingly, hydrophobic quantum dots reside at the interface between P3HT and PEG domains without disrupting the P3HT packing structure, which is advantageous for the optoelectronic properties. Furthermore, large particles bridge the P3HT nanowires at b...

We report the design of an enhanced PeT-based fluorogenic phosgene probe 1-CN comprising a BODIPY fluorescence reporter, and a catechol quencher at the meso-position acting as the phosgene reaction site. The fluorescent catechol-derived probe 1-CN specifically reacts with phosgene to result in a cyclic carbonate ester. The fast (

Fluorescence-based amine-reactive dyes are highly valuable for the sensing of amines and the labeling of biomolecules. Although it would be highly desirable, large changes in emission spectra and intensity seldom accompany the conjugation of known amine-reactive dyes to their target molecules. On the contrary, amide bond formation between amines and the pentafluorophenyl (

N-Heterocyclic carbene (NHC) boranes undergo oxidative hydrolysis to give imidazolium salts with excellent kinetic selectivity for HOCl over other reactive oxygen species (ROS), including peroxides and peroxynitrite. Selectivity for HOCl results from the electrophilic oxidation mechanism of NHC boranes, which stands in contrast to the nucleophilic oxidation mechanism of arylboronic acids with ROS. The change in polarity that accompanies the conversion of NHC boranes to imidazolium salts can control the formation of emissive excimers, forming the basis for the design of the first fluorescence probe for ROS based on the oxidation of B-H bonds. Two-photon microscope (TPM) ratiometric imaging of HOCl in living cells and tissues is demonstrated.

The meso-oxime-substituted-1,3,5,7-tetramethyl BODIPY (1-oxime) was developed into a colorimetric and fluorogenic probe to selectively detect and quantify phosgene. The fast (

Here, we demonstrate that the self-assembly of poly(3-hexylthiophene) (P3HT) at the air–water interface can lead to free-standing films of densely packed P3HT nanowires. Interfacial self-assembly on various liquid subphases, such as water, diethylene glycol, and glycerol, indicates that the viscosity of the subphase is an important factor for the formation of well-ordered nanostructures. The thin-film morphology is also sensitive to the concentration of P3HT, its molecular weight (MW), and the presence of oxidative defects. The densely packed nanowire films can be easily transferred to solid substrates for device applications. The ultrathin films of P3HT prepared by the interfacial assembly showed significantly higher hole mobility (∼3.6 × 10–2 cm2/V s) in a field-effect transistor than comparably thin spin-cast films. This work demonstrates that the air–liquid interfacial assembly is not limited to amphiphilic polymers and can, under optimized conditions, be applied to fabricate ultrathin films of widely...

Summary H/D exchange at formyl groups is the most direct approach for the synthesis of deuterated aldehydes. Platinum-group metal complexes have been employed to catalyze this transformation, with significant substrate scope limitations. Although N-heterocyclic carbenes can also activate the C–H bond of aldehydes through the formation of Breslow intermediates, benzoin condensation and other C–C-bond-forming pathways have so far outpaced synthetically useful H/D exchange. Investigation of the reaction of aldehydes with 1,2,3-triazolylidenes has revealed the reversible formation of Breslow intermediates and the inhibition of the condensation steps in methanol solvent. 1,2,3-Triazolylidenes catalyze the H/D exchange of aryl, alkenyl, and alkyl aldehydes in high yields and deuterium incorporation levels using deuterated methanol as an affordable D source. The unique properties of these mesoionic carbenes (MICs) enable a streamlined preparation of deuterated synthetic intermediates and pharmacophores that are highly valuable as mechanistic and metabolic probes.

The naphthoimidazolium borane 4 is shown to be a selective probe for HOCl over other reactive oxygen species. Unlike other boronate–reactive oxygen species (ROS) fluorogenic probes that are oxidized by HOCl through a nucleophilic borono-Dakin oxidation mechanism, probe 4 is distinguished by its electrophilic oxidation mechanism involving B–H bond cleavage. Two-photon microscopy experiments in living cells and tissues with the probe 4 demonstrate the monitoring of endogenous HOCl generation and changes in HOCl concentrations generated in the endoplasmic reticulum during oxidative stress situations.

Despite the common use of quinones as oxidizing agents in N-heterocyclic carbene (NHC)-based organocatalysis and transition-metal catalysis, the direct reactivity of quinones with NHCs remains underexplored. In this paper, we report the reactivity of NHCs with common p- and o-quinones, uncovering three unreported reactions involving contractions of the quinone ring that lead to push-pull furanolactone chromophores, NHC fulvalenes, and α-acylimidazolium cyclopentenone derivatives. These experiments also provide a rationale for the superior compatibility of tetra- tert-alkylated diphenoquinones in NHC-based oxidative transformations.

Recent advances in transition metal-catalyzed C–H bond functionalization have profoundly impacted synthetic strategy. Since organic substrates typically contain several chemically distinct C–H bonds, controlling the regioselectivity of C–H bond functionalization is imperative to harness its full potential. Moreover, the ability to alter reaction pathways to selectively functionalize different C–H bonds in a substrate represents a greater opportunity and challenge. The choice of catalysts, ligands, solvents, and even more subtle variations of the reaction conditions have been shown to allow the formation of regioisomeric C–H functionalization products starting from the same precursors. This review describes recent advances in transition metal-catalyzed divergent C–H bond functionalization that highlight its potential in organic synthesis.

A rhodium(II)-catalyzed denitrogenative coupling of N-sulfonyl-1,2,3-triazoles with ambiphilic β-enamino esters affords 2,5-dihydro-1H-imidazoles (3-imidazolines) with broad functional group tolerance. Mechanistic studies using a deuterium-labeled triazole suggest that the reaction proceeds in a cascade through the N-H insertion of an α-imino rhodium-carbene, followed by enamine-imine tautomerization and conjugate addition. Moreover, the reaction proceeds with high diastereoselectivity for α-substituted β-enamino esters (R(3) = Me, Ph) to give a single diastereomer.

The Pd-catalyzed oxidative olefin amination of the zinc bromide complex intermediate, formed by the sequential reaction of nitriles with a Reformatsky reagent and 1-alkynes, affords pyrrole derivatives in good to excellent yields. This tandem protocol provides a simple, efficient, and atom- and pot-economical way to quickly build polysubstituted NH-pyrroles starting from readily available reagents in a regiocontrolled manner with a broad substrate scope and high functional group tolerance. In contrast, the Pd-catalyzed oxidative olefin amination of an isolated α-vinyl-β-enamino ester did not proceed effectively, but the reaction efficiency can be restored by addition of n-BuZnBr or Zn(OAc)2, indicating the crucial role of the zinc complex in this transformation. The synthetic utility of this protocol is exemplified by the rapid synthesis of pyrrolophenanthrenes and pyranopyrrolones through selective Pd- and Cu-catalyzed C-C and C-O bond-forming reactions.

A CF3-BODIPY forms strongly luminescent aggregates, which contrasts with the quenched condensed-phase photophysics that are typical for BODIPY dyes. Examination of the luminescent properties of these aggregates – narrow red-shifted absorption and emission bands, minimal Stokes shift and increased fluorescence rate constants – and of the solid-state packing of the dye establish the CF3-BODIPY as the first structurally characterized genuine BODIPY J-aggregate.

A rhodium(II)-catalyzed coupling of 1-sulfonyl-1,2,3-triazoles, prepared from 1-alkynes and sulfonyl azides, with Morita–Baylis–Hillman (MBH) adducts afforded highly functionalized α-methylene-δ-oxo-γ-amino esters in excellent yields with broad functional group tolerance. This transformation can also be successfully accomplished as a multicomponent all-in-one-pot reaction of 1-alkynes, sulfonyl azides and MBH adducts in the presence of Cu(I) and Rh(II) catalysts.

The fluorogenic probe o-OH is able to detect and quantify organophosphorus nerve agent mimics in solution and in the vapor phase following immobilization on a solid substrate, making the system a suitable candidate for the field detection of chemical warfare agents. Detection is achieved by the suppression of internal rotation upon phosphorylation of a reactive phenolate, resulting in a large fluorescence “turn-on” response.

During the last two decades, N-heterocyclic carbenes (NHCs), such as A (Scheme 1), have played a prominent role as ligands for transition-metal catalysts.[1] Their popularity is mainly due to their strong σ-donor properties and the robustness of the corresponding complexes. These two features result from the presence of the electropositive carbon center and the strength of the carbon–metal bond. Therefore, other types of carbon-based L ligands are highly desirable. The simplest method for the preparation of metal complexes featuring a given L ligand is by ligand substitution at the metal center; however, the availability of stable compounds with a lone pair of electrons at a carbon center is very limited.[2] It has recently been shown that mesoionic carbenes (MICs)[3–5] B–D can be isolated as free species.[6] In contrast to “normal carbenes”, no obvious dimerization pathway can be foreseen for MICs. Consequently, a variety of these unusual carbenes should in principle be available without the need for kinetic protection. No derivatives of the 1,3-dithiol-2-ylidene F are known owing to their dimerization into derivatives of tetrathiafulvalene G.[7] Herein, we report our attempts to prepare a free MIC isomer by carbenes of type F, namely, a 1,3-dithiol-5-ylidene E. We show that this compound is unstable owing to spontaneous ring opening to form the corresponding ethynylcarbamodithioate. Importantly, the latter reacts with a variety of metals to give 1,3-dithiol-5-ylidene–metal complexes and therefore is a ligand equivalent of E. Scheme 1 Structural framework of classical NHCs (A), types of MIC that have been isolated previously (B–D), the targeted 1,3-dithiol-5-ylidenes (E), and their unknown 1,3-dithiol-2-ylidene isomers (F), which dimerize to tetrathiafulvalenes of type G. In analogy with the classical synthetic route used to prepare NHCs and MICs, we chose the readily available dithiolium tetrafluoroborate salt 1a as a precursor (Scheme 2).[8] Deprotonation with potassium bis(trimethylsilyl) amide proceeded cleanly, as shown by the disappearance of the signal for the dithiolium-ring proton in the 1H NMR spectrum. The 13C NMR spectrum displayed a signal at δ = 81.5 ppm: significantly further upfield than those observed for other MICs (B: δ = 200 ppm, C: δ = 115 ppm, D: δ = 200 ppm).[6] However, the 13C NMR chemical shift for carbenes is unpredictable[2b] (ranging from δ = 77 ppm[9] to δ = 326 ppm[10]). Therefore, in the hope of confirming quickly the MIC structure of the product, we added trifluoromethanesulfonic acid. We were pleased to observe the quantitative formation of the dithiolium triflate salt 1b (Figure 1).[11] However, when single crystals of the deprotonation product of dithiolium salt 1a were obtained, an X-ray diffraction study revealed that it was not the expected cyclic 1,3-dithiol-5-ylidene 3, but the acyclic ethynylcarbamodithioate 2 (see the Supporting Information). The true identity of 2 rationalizes the 13C NMR spectroscopic data; furthermore, the infrared spectrum shows a band at 2160 cm−1 characteristic of a C≡C triple bond. The formation of 2 is reminiscent of the ring-opening reaction observed in the deprotonation of isoxazolium[12] and isothiazolium salts.[13] Monitoring of the addition of potassium bis(trimethylsilyl)amide to 1a by NMR spectroscopy showed, even at −60 °C, the instantaneous formation of 2. Note that the deprotonation/ring-opening process might be concerted and therefore does not necessarily imply the transient formation of MIC 3. Figure 1 Molecular structures of 1b (top left), 5 (top right), 6 (bottom left), and 9 (bottom right) in the solid state (hydrogen atoms are omitted for clarity). Scheme 2 Deprotonation of the dithiolium salt 1a did not enable the isolation of MIC 3, but led to ethynylcarbamodithioate 2. The addition of trifluoromethanesulfonic acid to 2 induced ring closure to afford the dithiolium salt 1b. Tipp = 2,4,6-triisopropylphenyl, ... The proton-induced cyclization of 2 into 1 prompted us to study the reactivity of the ethynylcarbamodithioate 2 with gold(I) complexes, which are well-known alkynophilic π acids.[14] We were particularly interested in the apparent suitability of compound 2 as a precursor for the formation of stable vinyl–gold complexes [(R1R2C=CR3)AuL][15] (Scheme 3), which are still rare, although they are believed to be key intermediates in gold-catalyzed alkyne activation.[16] The reaction of 2 with (tetrahydrothiophene)gold chloride in THF proceeded cleanly, but did not afford the expected complex 4, in which the heterocycle acts as an X ligand as in vinyl–gold complexes. Instead, the MIC–gold(I) complex 5 was isolated in 68% yield. The 13C NMR spectrum of 5 showed a signal at δ = 146.9 ppm: a chemical shift comparable to that observed for the [(MIC B)AuCl] complex (δ = 153.7 ppm)[6a] and at significantly higher field than those of vinyl–gold complexes (δ = 178–199 ppm).[14] Similarly, an X-ray diffraction study of 5 (Figure 1) revealed that the gold–carbon bond distance (1.978(4)A) is similar to that found in [(MIC B)AuCl] (1.98A)[6a] and [(NHC)AuCl] (1.94–2.00A),[17] and slightly shorter than that in vinyl–gold complexes (2.04–2.06A).[15]. Scheme 3 The gold-induced cyclization of 2 did not afford the expected vinyl–gold complex 4 but the 1,3-dithiol-5-ylidene complex 5. THT = tetrahydrothiophene. These results show that with a gold(I) complex, ethynylcarbamodithioate 2 acts as a ligand equivalent of 1,3-dithiol-5-ylidene 3. To test the scope of this finding, we treated compound 2 with the less electrophilic complexes [{PdCl(allyl)}2] and [{RuCl2 (p-cym)}2] (Scheme 4). MIC complexes 6 (Figure 1) and 7 were isolated in 69 and 83%yield, respectively. To evaluate the donor properties of the 1,3-dithiol-5-ylidene ligand 3, we prepared the corresponding rhodium(I) dicarbonyl chloride complex 9 (Figure 1) by the addition of half an equivalent of [{RhCl-(cod)}2] to 2, followed by treatment with excess carbon monoxide. The CO vibration frequencies for 9 (νav = 2030.8 cm−1) indicate that 3 is a stronger electron donor than classical NHCs (νav = 2039–2041 cm−1)[18] and cyclic (alkyl)(amino)carbenes (CAACs; νav = 2036 cm−1),[19] but is weaker than other MICs (νav=2016–2025 cm−1).[2a] Scheme 4 The MIC–palladium, ruthenium, and rhodium complexes 6–9 were readily prepared. Thus, acyclic ethynylcarbamodithioate 2 is a ligand equivalent of MIC 3. p-cym = para-cymene, cod = 1,5-cyclooctadiene. The 1,3-dithiol-5-ylidene–metal complexes reported herein are thermally robust (m.p. = 272 (5), 219 (6), 217 (7), 194 (8), 186 °C (9)) and not air-sensitive. The precursor, namely, the acyclic ethynylcarbamodithioate 2, can be prepared in gram-scale quantities within a day and is stable for several weeks in the solid state under an inert atmosphere and in solution for up to 1 h at 140°C. These results suggest that the variety of isolable free mesoionic carbenes will be limited by their propensity to undergo ring-opening reactions. However, the reverse process, triggered by transition metals, should be of broad applicability. Since many different analogues of ethynylcarbamodithioate 2 (R-C≡C-X-C(Y)R′, in which X and Y are heteroatoms with a lone pair of electrons) can readily be prepared, numerous MIC complexes will be available. The catalytic study of metal complexes supported by MIC 3 is a subject of current investigations in our laboratory.

Precursor polymers prepared from 1,4-dialkoxy-1,4-diarylcyclohexane monomers are converted to the conjugated poly(arylene dienylene)s during the course of an acid-catalyzed deprotection−elimination reaction. These materials are employed as combined active materials and photoresists in the surface patterning of luminescent conjugated polymers.

A Pd(II) catalyst for the hydroarylation of fullerene with boronic acids is presented. Treatment of C60 with an arylboronic acid in the presence of a catalytic amount of Pd(2-PyCH=NPh)(OCOC6F5)2 in H2O/1,2-Cl2C6H4 at room temperature furnishes the hydroarylation product (Ar-C60-H) in good yield with high selectivity. This complex possesses high catalytic activity paired with bench stability in the solid state.

Iptycene-type quinoxaline and thienopyrazine monomers were successfully synthesized via a condensation between 10-dihydro-9,10-ethanoanthracene-11,12-dione and the corresponding diamines. Copolymers based on fluorene and three iptycene monomers were prepared via Suzuki coupling reaction, and they exhibited good solubility in appropriate organic solvents. These copolymers are fluorescent in both solution and the solid state, emitting blue, greenish-blue, and red color due to the different electronic properties of the iptycene comonomers. The difference in their absorption and emission spectra was attributed to the donor−acceptor charge transfer interactions and/or polymer backbone conformational changes induced by steric effects. Moreover, the spectroscopic data clearly demonstrated the insulating effect of iptycene units, which prevented the aggregation of the polymer chains and the formation of excimers in the solid state.

Heterocyclic monomers based on 2,1,3-benzooxadiazole and 2,1,3-benzothiadiazole bearing solubilizing side chains have been synthesized in high yields over three steps from readily available starting materials. The monomers are efficiently cross-coupled with diynes and bis(boronates) to afford high molecular weight luminescent poly(arylene ethynylene)s and polyfluorenes that exhibit red-shifted absorption and emission maxima, greater solubility, and reduced aggregation.

A new fluorescent turn-on probe (3) for the selective sensing and bioimaging of thiols is reported. In aqueous buffer solutions at physiological pH, thiols cleave the 2,4-dinitrobenzenesulfonyl group to release the red-emissive donor-acceptor fluorophore (4). The probe displays excellent immunity to interference from nitrogen and oxygen nucleophiles and the imaging of thiols in living cells is demonstrated.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives bearing varied substituents at the meso position (i.e., CF3 , CH3 , COOR, CHO, CN, Cl, iPr) were synthesized to elucidate the structure-property relationships that give rise to emissive J-aggregates. Several new BODIPY derivatives can be added to the previously reported 1,3,5,7-tetramethyl-8-trifluoromethyl derivative to the list of those forming J-aggregates, in addition to other dyes that are emissive in the solid state without forming J-aggregates.

The design and implementation of tandem reactions provides organic chemists with numerous challenges, in particular that of undesired cross-reactivity between substrates. Among organometallics, the use of organozinc reagents in tandem reactions provides several advantages as a result of their broad functional group tolerance and compatibility with transition metals. This review highlights prominent examples of recent advances in tandem reactions with organozinc reagents that illustrate their potential in organic synthesis.

The rhodium(II)-catalyzed denitrogenative coupling of N-sulfonyl-1,2,3-triazoles with various enamino esters affords dihydroimidazoles with a broad functional group tolerance.

The sequence proceeds via Zn complexes of type (IV) and affords the target compounds in a regioselective manner.

The design and implementation of tandem reactions provides organic chemists with numerous challenges, in particular that of undesired cross-reactivity between substrates. Among organometallics, the use of organozinc reagents in tandem reactions provides several advantages as a result of their broad functional group tolerance and compatibility with transition metals. This review highlights prominent examples of recent advances in tandem reactions with organozinc reagents that illustrate their potential in organic synthesis.

Soluble anthryl-based conjugated poly( phenylene ethynylene)s (PPEs) have been synthesized using palladium-catalyzed Sonogashira-Hagihara cross-coupling polymerization reactions. Molecular weights up to 3.5x10(4) g center dot mol(-1) were obtained, making them suitable for spectroscopic solution characterizations and thin film processing. The selective reactivity of these polymers as multidienes has been successfully demonstrated with strong dienophiles. Diels-Alder reactions proceed cleanly to completion with unhindered dienophiles such as N-alkylated maleimide derivatives. TGA analysis revealed thermal retro-Diels-Alder reactions at modest temperatures around 210 degrees C. Compared with their parent polymers, the cycloadduct polymers exhibited dramatic hypsochromic shifts of their emission and absorption maxima up to 80nm along with a considerable quantum yield enhancement. These original anthryl-based polymers appear attractive as reactive conjugated materials whose optical properties can easily be tuned with quantitative Diels-Alder reactions.

Conjugated polymers often display a decrease of fluorescence efficiency upon aggregation due in large part to enhanced interpolymer interactions that produce weakly emissive species generally described as having excimer-like character. We have found that poly(phenylene ethynylene)s with fused pendant [2.2.2] ring structures having alkene bridges substituted with two ester groups function to give highly emissive, broad, and red-shifted emission spectra in the solid state. To best understand the origin of this new solid-state emissive species, we have performed photophysical studies of a series of different materials in solution, spin-coated thin films, solid solutions, and Langmuir films. We conclude that the new, red-shifted, emissive species originate from excimers produced by interchain interactions being mediated by the particular [2.2.2] ring system employed. The ability to design structures that can reliably produce highly emissive conjugated polymer excimers offers new opportunities in the emission ...

A family of organic chromophores that, like pyrene, forms emissive excimers is reported. Their chemical and photophysical properties are superior to pyrene for the design of chemo- and biosensors. Unlike hydrophobic pyrene, which requires excitation by cell-damaging UV irradiation, these polar dyes absorb strongly in the visible range, and their excimers emit brightly in the red to far-red region of the electromagnetic spectrum. The intensity of the emission signal is greatly increased upon formation of a preassociated dimer that is triggered upon aggregation or crystallization. In demonstration of the potential of this new family of excimer-forming dyes, a probe that is capable of detecting label-free DNA in water down to 10 pM and also doubles as a visualization agent for DNA in gel electrophoresis is reported.

A zinc bromide complex, formed by the sequential reaction of nitriles with a Reformatsky reagent and terminal alkynes, is used as an intermediate for divergent palladium-catalyzed reactions. The reaction pathway of the intermediate is precisely controlled by the choice of the reaction solvent or the palladium catalyst to quickly form four different aromatic scaffolds--arylamines, aminoindenes, pyrroles, and quinolines--starting from readily available nitriles.

We developed a colorimetric and fluorescent turn-on carbon dioxide sensor that relies on a polydiacetylene, PDA-1, functionalized with amines and imidazolium groups. The pendant amines react with CO2 under basic conditions to form carbamoate anions, which partially neutralize the polymer’s positive charges, inducing a phase transition. PDA-1 allows for the selective sensing of CO2 with high sensitivity, down to atmospheric concentrations. Naked-eye detection of CO2 is accomplished either in water solutions of PDA-1 or in the solid state with electrospun coatings of PDA-1 nanofibers.

A super pyrazolate: deprotonation of the flanking hydrogen of metal complexes of mesoionic carbenes (MICs) offers a simple and general route for the preparation of bimetallic complexes of a 1,2-dihapto anionic dicarbene ligand that is isoelectronic with widely used pyrazolate ligands, while conferring greater electron donation and stronger M-L bonds.

The formal cycloaddition between 1,3-diaza-2-azoniaallene salts and alkynes or alkyne equivalents provides an efficient synthesis of 1,3-diaryl-1H-1,2,3-triazolium salts, the direct precursors of 1,2,3-triazol-5-ylidenes. These N,N-diarylated mesoionic carbenes (MICs) exhibit enhanced stability in comparison to their alkylated counterparts. Experimental and computational results confirm that these MICs act as strongly electron-donating ligands. Their increased stability allows for the preparation of ruthenium olefin metathesis catalysts that are efficient in both ring-opening and ring-closing reactions.

A review is presented of synthetic methods for the preparation of biaryls by the rhodium-catalyzed C-H bond arylation of arenes with aryl halides (C-H/ C-X couplings), arylmetal reagents (C-H/C-M couplings) and arenes (C-H/C-H couplings), with an emphasis on postulated mechanisms and their implications on reactivity, selectivity and substrate scope.

Direct metalation of bis(1,2,3-triazolium) salts affords mononuclear rhodium(I) complexes, which feature a 1,4-bidentate bis(1,2,3-triazol-5-ylidene) (i-bitz) ligand. The topology of the ligand is similar to that of 2,2′-bipyridines (bpy) and their congeners, as well as bis(1,2,4-triazol-5-ylidenes) (bitz). As the former, but in contrast to the latter, the free i-bitz can be isolated, which paves the way for various applications.

A review is presented of synthetic methods for the preparation of biaryls by the rhodium-catalyzed C–H bond arylation of arenes with aryl halides (C–H/C–X couplings), arylmetal reagents (C–H/C–M couplings) and arenes (C–H/C–H couplings), with an emphasis on postulated mechanisms and their implications on reactivity, selectivity and substrate scope.

In 2001, Crabtree and co-workers first reported complex A, which features an imidazole ring bound at the C5 position (III), and not at C2 as commonly observed.[7] More recently, Huynh and co-workers[8] and Albrecht and co-workers[9a] showed that pyrazolium and 1,2,3-triazolium salts can serve as precursors to metal complexes of type B and C, which feature pyrazolin-4-ylidenes IV and 1,2,3-triazol-5-ylidenes V as the ligand, respectively. As a consequence of their lineage, these have also been referred to as N-heterocyclic carbenes (NHCs). However, as no reasonable canonical resonance forms containing a carbene can be drawn for free ligands III–V without additional charges (see V′), these ligands have been described as abnormal or remote carbenes (aNHCs or rNHCs, respectively).[10] As they are, in fact, mesoionic compounds,[11] we suggest naming this family of compounds mesoionic carbenes (MICs). There have been no reported dimerizations of MICs III and IV, which suggests that the Wanzlick equilibrium pathway for classical carbenes is disfavored;[12] this observation should lead to relaxed steric requirements for their isolation. Moreover, experimental and theoretical data suggest that MICs III–V are even stronger electron-donating species than NHCs I and II, which opens up interesting perspectives for their applications.[10] Our recent success in the isolation of a free imidazol-5-ylidene III[13] and pyrazolin-4-ylidenes IV (cyclic bent allenes),[14,15] prompted us to investigate the possibility of preparing new types of stable neutral compounds that feature a lone pair of electrons on the carbon atom.[16] Preliminary calculations (B3LYP, 6–311G(d,p); for details, see the Supporting Information) predicted that the parent MIC V is located at an energy minimum, about 32 kcalmol−1 above the regioisomeric parent 1,2,4-triazol-5-ylidene II. Furthermore, parent V is predicted to exhibit an appreciably large singlet–triplet band gap (56 kcalmol−1), which is a good predictor of carbene stability and thus of possible isolation. Herein, we report the preparation, isolation, and characterization of two free 1,2,3-triazol-5-ylidenes of type V. By analogy with the synthetic route used for preparing NHCs and the related species III and IV, 1,2,3-triazolium salts (2a,b) were targeted as precursors for the desired 1,2,3-triazol-5-ylidenes (Va,b). A sterically hindered flanking aryl substituent (2,6-diisopropylphenyl, Dipp) was selected to provide kinetic stabilization to the ensuing free ligand. 1,2,3-Triazole 1 was obtained in 83% yield from the copper-catalyzed azide–alkyne cycloaddition (CuAAC, click chemistry) of 2,6-diisopropylphenyl azide and phenylacetylene.[17] The one-pot conversion of aniline into the desired aryl azide, followed in situ by CuAAC as reported by Moses and co-workers[18] was found to be especially convenient for the synthesis of 1. Alkylation of 1 with methyl or isopropyl trifluoromethanesulfonate afforded the corresponding tri-azolium salts in moderate to excellent yields (2a and 2b, respectively; Scheme 2). Scheme 2 Synthesis of the free 1,2,3-triazol-5-ylidenes Va,b. Potassium bases have been identified as the reagents of choice for the depronation of carbene precursors, as they avoid the formation of stable carbene–alkali-metal adducts that are commonly encountered when lithium bases are used.[12,13,14a,19] Gratifyingly, triazolium salts 2a,b were cleanly deprotonated with either potassium bis(trimethylsilyl)amide or potassium tert-butoxide in ethereal solvents to afford the corresponding MICs Va and Vb in 55 and 39% yield, respectively. Deprotonation was evidenced by the disappearance of the triazolium CH signal in their 1H NMR spectra (2a: δ =8.62 ppm; 2b: δ =8.85 ppm) and the appearance of a signal at low field in the 13C NMR spectrum (Va: δ = 202.1 ppm; Vb: δ =198.3 ppm). The structure of Va was unambiguously confirmed by X-ray crystallography (Figure 1).[20] In the solid state, Va contains a planar heterocycle, characterized by bond lengths that are intermediate between those of single and double bonds; both of these features are indicative of electronic delocalization. Upon deprotonation, the C5 carbon bond angle becomes more acute (2a: 106°; Va: 100°), which is consistent with an increased s character in the σ lone pair orbital of Va compared to the C–H bonding orbital of the precursor 2a. This is in agreement with the generally observed trend for carbenes and their conjugate acids.[5] Figure 1 Molecular views (thermal ellipsoids set at 50 % probability) of 2a (top) and Va (bottom) in the solid state. For clarity, counter ions, solvent molecules, and H atoms are omitted, except for the ring hydrogen of 2 a. Selected bond lengths [A] ... In the solid state, with the exclusion of oxygen and moisture, free 1,2,3-triazol-5-ylidene Va (m.p. 50–52°C decomp.) remained stable for several days at −30°C and for a few hours at room temperature. By contrast, Vb (m.p. 110–112°C) was significantly more stable, showing no sign of decomposition after three days at room temperature in the solid state. Upon heating in a benzene solution for 12 hours at 50°C, Va decomposed to give, among other products, triazole 3 (Scheme 3; for details, see the Supporting Information). We surmise that the latter product results from a nucleophilic attack of the carbon lone pair of Va on the methyl group of a second molecule of Va, giving rise to heterocycles 4 and 5, which react together to afford the observed product 3. This apparent rearrangement is reminiscent of that recently observed in the formation of imidazol-2-ylidenes of type I from imidazol-5-ylidenes of type III that contain an electro-philic Y group.[21] In agreement with this hypothesis, MIC Vb, which contains the less-electrophilic isopropyl group at the N3 position, appears much more robust with respect to this decomposition pathway. Scheme 3 Degradation of free 1,2,3-triazol-5-ylidene Va, and analogy with the rearrangement of III into I. To evaluate the donor properties of 1,2,3-triazol-5-yli-denes, the [(Va)Ir(CO)2Cl] complex was prepared by addition of Va to [{Ir(cod)Cl}2] (cod = 1,5-cyclooctadiene), followed by treatment with an excess of carbon monoxide. The CO vibration frequencies (ν =2061 and 1977 cm−1; νavg = 2019 cm−1) are in line with those of the analogous iridium complex, previously reported by Albrecht and co-workers (νavg = 2021 cm−1),[9a] and are indicative of donor properties that are superior to those of NHCs I and II (νavg = 2022–2031 cm−1),[22] but inferior to those of MICs III (νavg = 2003–2006 cm−1)[23] and IV (νavg = 2002 cm−1).[14b] Free 1H-1,2,3-triazol-5-ylidenes, as exemplified by compounds Va,b, possess an ensemble of properties that portend to their utility. The synthesis of their precursors is short and efficient, from readily available starting materials, yet is modular and thus amenable to a wide variety of potential analogues. As with other mesoionic carbenes III and IV, the dimerization of MICs of type V has not been observed; therefore, the preparation of comparatively unhindered MICs is predicted to be viable. Their donor properties are greater than those of NHCs of type I and II, but they are nonetheless available by deprotonation using mild bases (e.g. alkoxides), thus signaling their potential for applications, such as nucleophilic organocatalysis. Free triazolylidenes V complement the rapidly growing numbers of neutral carbon-based κ1C ligands that are now available. We predict that many other classes of MICs, that are derived from a variety of heteroaromatic scaffolds, can be isolated. This endeavor is currently the object of ongoing efforts in our laboratory.

A Ni(cod)2/IPr catalyst promotes the intermolecular 1,2-addition of arylboronate esters to unactivated aldehydes and ketones. Diaryl, alkyl aryl, and dialkyl ketones show good reactivity under mild reaction conditions (≤80 °C, nonpolar solvents, no strong base or acid additives). A dramatic ligand effect favors either carbonyl addition (IPr) or C−OR cross-coupling (PCy3) with aryl ether substrates. A Ni(0)/Ni(II) catalytic cycle initiated by the oxidative cyclization of the carbonyl substrate is proposed.

A Pd(II) catalyst for the hydroarylation of fullerene with boronic acids is presented. Treatment of C60 with an arylboronic acid in the presence of a catalytic amount of Pd(2-PyCH═NPh)(OCOC6F5)2 in H2O/1,2-Cl2C6H4 at room temperature furnishes the hydroarylation product (Ar−C60−H) in good yield with high selectivity. This complex possesses high catalytic activity paired with bench stability in the solid state.

A review is presented of synthetic methods for the preparation of biaryls by the rhodium-catalyzed C–H bond arylation of arenes with aryl halides (C–H/C–X couplings), arylmetal reagents (C–H/C–M couplings) and arenes (C–H/C–H couplings), with an emphasis on postulated mechanisms and their implications on reactivity, selectivity and substrate scope.

The synthesis of new heterocyclic oligo(phenylene) analogues based on soluble, nonaggregating 1,2-diazines is reported. Improved palladium-catalyzed reductive coupling methods were developed to allow for the preparation of large quantities of iptycene-derived bipyridazines and biphthalazines, and the controlled synthesis of well-defined oligomers up to sexipyridazine. Crystallographic, spectroscopic, and computational evidence indicate that in these analogues, hindrance at the ortho position is relaxed relative to poly(phenylenes). The resulting building blocks are promising for incorporation in conjugated electronics materials, and as new iptycene-derived ligands for transition metals.

The synthesis of a ruthenium complex containing an N-heterocylic carbene (NHC) and a mesoionic carbene (MIC) is described wherein addition of a Bronsted acid results in protonolysis of the Ru–MIC bond to generate an extremely active metathesis catalyst. Mechanistic studies implicated a rate-determining protonation step in the generation of the metathesis-active species. The activity of the NHC/MIC catalyst was found to exceed those of current commercial ruthenium catalysts.