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We report a simple, iterative strategy for the synthesis of [7]helicenes starting from substituted 1,4-xylene building blocks. In the first step, we take advantage of the deprotonatable methyl groups to achieve ethano-bridged dimers. These are oxidatively coupled (without using metal-containing catalysts or light) using a hypervalent iodine reagent. Both steps are repeated to obtain the respective σ/π-helicenes. The degree of saturation can be controlled thermally during the oxidative coupling.

Suzuki–Miyaura reactions with Pd(PPh3)4 have been carried out using lithium N-phenylsydnone-4-carboxylate as additive, which gave best yields at pH 5.7 in a mixture of acetic acid, water, and sodium carbonate. Reaction parameters such as the Pd source, the solvent, reaction time and temperature, acid, base and carboxylate have been varied and some representative examples of the Suzuki–Miyaura reaction have been examined.

We report the improved direct access to red-emitting BOIMPY fluorophores (λ ≈ 600 nm) via a simple one-pot approach. Our method starts from easily available benzimidazole-2-carboxylic acids and not only greatly improves the overall yields but also saves both costly reagents and time. In addition, the method facilitates the synthesis of novel unsymmetrical BOIMPY motifs. Therefore, these BOIMPY scaffolds derived from the BODIPY core become more accessible for applications as fluorophores at the appealing red edge of the visible spectrum.

The catalyst system consisting of lithium N-phenylsydnone-4-carboxylate/Pd(PPh3)4 has proven to be an effective catalyst for the Suzuki–Miyaura reaction of 2,4-dinitrochlorobenzene with boronic acids in acetic acid at pH 5.7, whereas the N-phenylsydnone carbene palladium complex [sydPd(PPh3)2Br] required pH 8.0.

Sydnone imines were deprotonated with lithium bis(trimethylsilyl)amide at the C4 position to give the corresponding sydnone imine anions as lithium adducts. These can be represented as lithium stabilized anionic N-heterocyclic carbenes. Treatment with diisopropyl azodicarboxylate (DIAD) gave the corresponding C4 adducts, i.e. 4-hydrazinyl-sydnone imines, which form tautomers in solution. Reductive 1,3-dipolar cycloadditions of the sydnone imine anions with tetracyanoethylene (TCNE) resulted in the formation of pyrazoles, the mechanism of formation of which differs from known reactions. Reaction of the anion derived from the 2-methoxyphenyl sydnone imine with N,N-diisopropylcarbodiimide gave a ring-cleaved bisiminonitrile. Structure elucidations were accomplished by NMR spectroscopy and by four single crystal X-ray analyses. (C) 2019 Elsevier Ltd. All rights reserved.

The anion of 2-ethynyl-1-methylpyridinium salts, which polymerized rapidly, can be represented as the mesomeric betaine 1-methylpyridinium-2-acetylide or as the cumulene-type structure 1-methylpyridin-2-allenylidene. Palladium complexes have been prepared starting from 2-ethynylpyridine and Pd(PPh 3 ) 4 or PdCl 2 (PPh 3 ) 2 followed by methylation of the resulting complexes to the title compounds. Results of calculations and spectroscopic measurements are discussed with respect to the zwitterionic or allenylidene resonance forms.

3-Ethynylquinoline was subjected to a Sonogashira-Hagihara reaction with methyl 2-, 3-, and 4-bromobenzoates, respectively, and then N-methylated to give 3-[((methoxycarbonyl)phenyl)ethynyl]-1-methylquinolinium salts (two X-ray analyses). On saponification of the 3- and 4-substituted benzoates, the mesomeric betaines 3- and 4-[(1-methylquinolinium-3-yl)ethynyl]benzoates were formed. By contrast, the 2-benzoate derivative gave either the corresponding (1-oxo-1H-isochromen-3-yl)quinolinium derivative or the mesomeric betaine 2-(1-methylquinolinium-3-yl)-1,3-dioxo-2,3-dihydro-1H-inden-2-ide depending on the reaction conditions. A DFT calculation predicts a transoid conformation of the acetylene bond in the intermediate 2-[(1-methylquinolinium-3-yl)ethynyl]benzoate which is due to a strong hydrogen bond between the carboxylate group and 2H of the quinolinium ring, in addition to a 1,5-interaction between the carboxylate group and the CC triple bond. The bond angles of the transoid CC triple bond were calculated to be 211.6° and -175.1° in vacuo. The corresponding linear triple bond is 50.4 kJ/mol less stable in vacuo according to the calculation, and the N-heterocyclic carbene quinoline-2-ylidene is not formed as a tautomer.

The natural product punicine (Punica granatum) exists in two tautomeric forms, the cross-conjugated mesomeric betaine 1-(pyridinium-1-yl)-2-hydroxy-phenyl-5-olate and the conjugated mesomeric betaine 1-(pyridinium-1-yl)-5-hydroxy-phenyl-2-olate. Punicine as well as its picoline derivatives reacted with tris(pentafluorophenyl)borane exclusively at the 2'-olate group to form zwitterionic borates. Correspondingly, the 5'-dehydroxy derivate of punicine, the conjugated heterocyclic mesomeric betaine 1-(pyridinium-1-yl)-phenyl-2-olate and its picoline derivatives also gave borates, whereas analogous reactions of the cross-conjugated isomer 2'-dehydroxypunicine [1-(pyridinium-1-yl)-phenyl-3-olatel did not result in the formation of stable adducts. (C) 2020 Elsevier Ltd. All rights reserved.

The sydnone imines (5-benzoylimino)-3-(2-methoxyphenyl)-sydnone imine and molsidomine were deprotonated at C4 to give sydnone imine anions which can be represented as anionic N-heterocyclic carbenes, respectively. Trapping reactions with sulfur gave unstable sydnone imine sulfides which were stabilized by the formation of methyl thioethers, methyl sulfoxides, gold complexes [(PPh

Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates of mesoionic compounds to generate anionic N-heterocyclic carbenes. Lithium sydnone-4-carboxylates were therefore prepared via 4-bromosydnones by halogen-lithium exchange with nBuLi and subsequent treatment with carbon dioxide. Protonation gave the corresponding sydnone-4-carboxylic acids. Thermogravimetric measurements in addition to temperature dependent IR spectroscopy proved the decarboxylation of lithium sydnone-4-carboxylates and formation of the corresponding sydnone anions which can be represented as anionic N-heterocyclic carbenes. In DMSO-d6 solution, water favors the decarboxylation. Calculations have been performed to elucidate the mechanism of the decarboxylation in the absence and presence of water. (C) 2018 Elsevier Ltd. All rights reserved.

Deprotonated sydnones, which can be represented as anionic N-heterocyclic carbenes, were prepared as Li adducts and compared with deprotonated O-ethylsydnones (5-ethoxy-1,2,3-oxadiazol-4-ylidenes) which belong to the class of abnormal NHCs. The Pd complexes of the sydnone anions (X-ray analysis) as well as of the O-ethylsydnone carbenes proved to be efficient catalysts in aryl couplings of thiophenes.

We present a Lewis acid catalyzed nucleophilic ring-opening of donor-acceptor cyclopropanes and -butanes by sydnones, utilizing their respective 1,3- and 1,4-reactivity. The same conditions can be applied for the addition of sydnones to Michael acceptors. We propose a Friedel-Crafts like mechanism. The reaction provides a rare, low-temperature, transition metal-free, and functional group tolerant protocol for the late-stage functionalization of these mesoionic compounds of emerging importance in catalysis and bio-orthogonal chemistry.