Sydnone Methides-A Forgotten Class of Mesoionic Compounds for the Generation of Anionic N-Heterocyclic Carbenes

Anionic N-Heterocyclic Carbenes from Mesoionic Imidazolium-4-pyrrolides: The Influence of Substituents, Solvents, and Charge on their 77Se NMR Chemical Shifts

Mesoionic compounds are the starting material for the synthesis of unique anionic N-heterocyclic carbenes. Herein, mesoionic imidazolium pyrrolides synthesized from pyrrole-2-carbaldehyde via various N-alkyl-4-pyrroyl-imidazoles are described. These were converted into nine new 4-(pyrrol-2-yl)-substituted imidazolium salts and transformed into the mesoionic title compounds using an anion exchange resin. The DFT-calculated (B3LYP/6-311++G**) CREF values indicate a great potential for the formation of anionic N-heterocyclic carbenes by deprotonation, which were generated and reacted with selenium to obtain selenoureas. The 77Se NMR shifts investigated under systematic variation of conditions are dependent on the substitution pattern (ΔδSe = 133 ppm) and the steric demand of the substituents. Solvent dependencies of the 77Se NMR shifts were investigated applying toluene-d8, THF-d8, CDCl3, CD2Cl2, pyridine-d5, acetone-d6, DMSO-d6, CD3CN, AcOD, and MeOD. The influences of the referencing method on the 77Se shifts using external or internal Me2Se or Ph2Se2 and solvent can add up to ΔδSe = ca. 80 ppm. In addition, we observed a temperature dependence of both the selenoureas and the reference reagent Ph2Se2 as well as a 77Se shift difference of the analyte caused by interaction with internally added Ph2Se2. The negative charge of deprotonated selenoureas shifts the values by an additional -20 ppm.

Highly Sensitive Switchable Sensors for Hydroxide on Glass Surfaces Based on Isoquinolinium-Quinolinium-substituted Acetylenes

Bi-substituted acetylenes with a quinolinium and an isoquinolinium substituent are described, which reversibly form intensely colored adducts with O-nucleophiles and thus enable the detection of >0,5 ppm hydroxide on the surfaces of various glasses. Acids reconstitute the colorless bi-substituted acetylenes. The quinolinium and isoquinolinium rings are bound via their 2-, 3-, 4- and 1-, 3-, 4-positions to the triple bond, respectively. The choice of substitution sites of the hetarenium rings enables the design of mixed conjugated/cross-conjugated π-electron systems. Depending on the combination of binding sites, the frontier orbital profile, the triple bond polarization, the fluorescence behaviour, and the sensitivity to hydroxide differs.

Lithium aluminate flotation by pH- and light-switchable collectors based on the natural product punicine

Derivatives of the natural product punicine [1-(2',5'-dihydroxyphenyl)pyridinium chloride] were developed as switchable collectors for the flotation of lithium-containing engineered artifical minerals (EnAMs). These EnAMs are e.g. formed by pyrometallurgical processing of end-of-life lithium-ion batteries. Depending on the pH value and the lighting conditions, punicines exist in water as cations, two different electrostatically neutral mesomeric betaines, anionic tripoles, radical cations or radical anions. The radical species form by photochemically induced disproportionation reactions. We prepared punicine derivatives introducing alkyl chains in the pyridinium moiety (4-methyl, 4-ethyl, 4-octyl and 4-undecanyl) to install hydrophobic groups and examined the recovery rates of the flotation of lithium aluminate (LiAlO2). We varied the lighting conditions (darkness, daylight, LED irradiation at λ = 390-400 nm) and the pH value, the collector's and frother's concentration, and the flotation time. With our collectors, recovery rates of lithium aluminate up to 90% were accomplished when the flotation was conducted in Hallimond tubes exposed to daylight at pH 11 in water.

Synthesis of sydnonimines from sydnones and their use for bioorthogonal release of isocyanates in cells

In this article, we report the synthesis of sydnonimines from sydnones and their use as dipoles for fast click-and-release reactions. The process relies on nucleophilic aromatic substitution of aliphatic and aromatic amines with triflated sydnones. This new methodology allowed the preparation of functionalised sydnonimine probes that are otherwise difficult to prepare. These probes were then used to release a drug and a fluorescent aromatic isocyanate inside living cells.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs)

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

Aldiminium and 1,2,3-triazolium dithiocarboxylate zwitterions derived from cyclic (alkyl)(amino) and mesoionic carbenes

The synthesis of zwitterionic dithiocarboxylate adducts was achieved by deprotonating various aldiminium or 1,2,3-triazolium salts with a strong base, followed by the nucleophilic addition of the in situ-generated cyclic (alkyl)(amino) or mesoionic carbenes (CAACs or MICs) onto carbon disulfide. Nine novel compounds were isolated and fully characterized by 1H and 13C NMR, FTIR, and HRMS techniques. Moreover, the molecular structures of two CAAC·CS2 and two MIC·CS2 betaines were determined by X-ray diffraction analysis. The analytical data recorded for all these adducts were compared with those reported previously for related NHC·CS2 betaines derived from imidazolinium or (benz)imidazolium salts. Due to the absence of electronic communication between the CS2 unit and the orthogonal heterocycle, all the CAAC·CS2, MIC·CS2, and NHC·CS2 zwitterions displayed similar electronic properties and featured the same bite angle. Yet, their steric properties are liable to ample modifications by varying the exact nature of their cationic heterocycle and its substituents.

Nonlinear Refraction, Excited State Absorption, and Multiphoton Absorption of 1,3-Thiazolium-5-thiolate Mesoionic Compound

We synthesized the mesoionic compound 2-(4-chlorophenyl)-3-methyl-4-(4-methylphenyl)-1,3-thiazole-5-thiolate and measured its refractive and absorptive nonlinear optical response in different temporal and spectral regimes. The experiments were performed by using the Z-scan technique with two pulsed light sources: the second harmonic (at 532 nm) of a mode-locked and Q-switched Nd-YAG laser (100 ps, 10 Hz) and a Ti: Sapphire laser system (100 fs, 1 kHz) operating at 800 nm. The observation and characterization of nonlinear refraction, two- and three-photon absorption, and excited state absorption of the mesoionic compound dissolved in dimethyl sulfoxide, in different concentrations, are presented and discussed with basis on the population redistribution in a three-energy-level model that allows the determination of the parameters which characterize the nonlinear response.

Polyether-tethered imidazole-2-thiones, imidazole-2-selenones and imidazolium salts as collectors for the flotation of lithium aluminate and spodumene

Imidazolium salts were prepared which possess 2-ethoxyethyl pivalate or 2-(2-ethoxyethoxy)ethyl pivalate groups as amphiphilic side chains with oxygen donors as well as n-butyl substituents as hydrophobic groups. The N-heterocyclic carbenes of the salts, characterized by 7Li and 13C NMR spectroscopy as well as by Rh and Ir complex formation, were used as starting materials for the preparation of the corresponding imidazole-2-thiones and imidazole-2-selenones. Flotation experiments in Hallimond tubes under variation of the air flow, pH, concentration and flotation time were performed. The title compounds proved to be suitable collectors for the flotation of lithium aluminate and spodumene for lithium recovery. Recovery rates up to 88.9% were obtained when the imidazole-2-thione was used as collector.

Pyrazoles in the Intersection of Mesomeric Betaines and N-Heterocyclic Carbenes: Formation of NHC Selenium Adducts of Pyrazolium-4-aminides

Starting from 4-nitropyrazole, eight mesoionic pyrazolium-4-aminides were prepared by a six-step reaction sequence. The deprotonation of 1,2-disubstituted 4-amido-1H-pyrazolium salts by an anion exchange resin in its hydroxide form is the final step of the synthesis. A tautomeric equilibrium between the mesoionic compounds (pyrazolium-4-aminides) and N-heterocyclic carbenes (pyrazol-3-ylidenes) can be formulated; however, the NHC tautomers were not detected by means of NMR spectroscopy in polar aprotic solvents such as DMSO-d 6 or MeCN-d 3. Apart from tautomerism, anionic N-heterocyclic carbenes can be formulated as a result of a deprotonation of the mesoionic compounds­. Trapping reactions were performed with selenium, which resulted in the formation of pyrazole-3-selenones. Methylation at the selenium atom gave the corresponding 3-(methylselanyl)-4-amido-1H-pyrazolium salts, which were deprotonated to give new mesomeric betaines, 3-(methylselanyl)-1H-pyrazolium-4-aminides as unique compounds­. DFT-calculations as well as 77Se NMR spectroscopic measurements were carried out.

The first example of unusual reversible nucleophilic addition to 2-(5-aryl-2-oxo-3H-pyrrol-3-ylidene)malononitriles – a new tool for the creation of thermosensitive molecular switches

A new thermochromic system based on an unusual nucleophilic addition to pyrrol-3-ylidenemalononitrile derivatives was discovered.