N-Heterocyclic Carbenes

Studies on the Effect of Positive and Negative Charges on the 77Se NMR Shifts of Selenones and Selenenyls of N-heterocyclic Carbenes of Imidazolium-4,5-dicarboxylates

1,3-Imidazole-4,5-dicarboxylic acid derivatives were investigated with respect to the properties of the underlying N-heterocyclic carbenes. Their selenium adducts were prepared as diesters, monoesters, and dianionic dicarboxylates. Se-methylation yielded selenenyls (cationic selenoethers). 77Se NMR shifts of selenium adducts are considered a measure of the electronic properties of the underlying N-heterocyclic carbene. Since they are very sensitive to external (solvent, pH, temperature, reference reagent, reference method) and internal parameters (substituent effects, steric effects, nonclassical H-bonding, anisotropy), difficulties can arise in reliably interpreting these values. Our model compounds allow the systematic investigation of the influence of charges on the 77Se NMR shifts since both structural changes and changes of the measurement conditions have been minimized. Depending on the charge, the 77Se NMR shifts cover the range from 42.1 ppm (dianionic dicarboxylate) via 65.0 ppm (monoanionic dicarboxylate) and 92.5 ppm (neutral diester) to 153.8 ppm (cationic selenoether) in DMSO-d6. The signals also show considerable solvent and pH dependence, which was investigated with a selection of NMR solvents (MeOD, CDCl3, CD2Cl2, CD3COOD, CD3CN, acetone-d6, THF-d8, pyridine-d5, toluene-d8, DMSO-d6) and pH values in D2O. Linear relationships were found between the HOMO and LUMO energies and the 77Se NMR shifts, respectively.

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.

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.

Mesoionic N-Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2

An extended class of stable mesoionic N-heterocyclic imines (mNHIs), containing a highly polarized exocyclic imine moiety, were synthesized. The calculated proton affinities (PA) and experimentally determined Tolman electronic parameters (TEPs) reveal that these synthesized mNHIs have the highest basicity and donor ability among NHIs reported so far. The superior nucleophilicity of newly designed mNHIs was utilized in devising a strategy to incorporate CO₂ as a bridging unit under reductive conditions to couple inert primary amides. This strategy was further extended to hetero-couplings between amide and amine using CO₂ . These hitherto unknown catalytic transformations were introduced in the diversification of various biologically active drug molecules under metal-free conditions. The underlying mechanism was explored by performing a series of control experiments, characterizing key intermediates using spectroscopic and crystallographic techniques.

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.

Vanadium complexes with N-heterocyclic vinylidene ligands

The preparation and the structural characterization of vanadium complexes with terminal and bridging N-heterocyclic vinylidene ligands is reported. The synthesis of the complexes was enabled by utilization of diazoolefins as ligand precursors. Structural data and theoretical results show that N-heterocyclic vinylidenes can act as 6e^- donor ligands, leading to strong metal-carbon interactions.

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

Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as π-electron rich anionic N-heterocyclic carbenes. Sulfur and selenium adducts were stabilized as their methyl ethers, and mercury, gold as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo C-C coupling reactions with 1-fluoro-4-iodobenzene under Pd(PPh3 )4 and CuBr catalysis. 77 Se NMR resonance frequencies and 1 JC4-Se as well as 1 JC4-H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N-heterocyclic carbenes. The carbene carbon atom of the sydnone methide anion 3 j resonates at δ=155.2 ppm in 13 C NMR spectroscopy at -40 °C which is extremely shifted upfield in comparison to classical N-heterocyclic carbenes.

Imidazolium-benzimidazolates as convenient sources of donor-functionalised normal and abnormal N-heterocyclic carbenes

Imidazolium-benzimidazolates are readily available conjugated mesomeric betaines, whose carbenic tautomers can form C,N_imine-metal chelates containing amino-functionalised normal or abnormal NHC ligands, depending on the degree of steric congestion. Deprotonation of the amino moiety gives rise to imine-functionalised C,N_amido-metal chelates, whose N_imine atom can be easily utilised for the construction of heteronuclear complexes.

Purine-substituted imidazolium mesomeric betaines and their tautomeric N-heterocyclic carbenes. Formation of a cyclic borane adduct

6-Chloropurine and 2,6-dichloropurine were reacted with N1-substituted imidazoles to give purin-6-yl substituted imidazolium salts, respectively. Deprotonation of the 1-methylimidazolium derivative resulted in the formation of the corresponding stable conjugated mesomeric betaine, whereas the 1-phenyl,- 1-vinyl- and 1-(2-hydroxyethyl) derivatives proved to be unstable. In situ generation of the mesomeric betaines by caesium carbonate in the presence of sulfur and selenium, however, gave the thiones and the selenones of the tautomeric purine-substituted imidazol-2-ylidene, respectively. Its anionic N-heterocyclic carbene was formally trapped by reaction with triethylborane at high temperatures as a cyclic boron adduct which is the first representative of a new heterocyclic ring system. DFT calculations gained insight into the electronic properties of the N-heterocyclic carbenes substituted by π-electron donators. Results of a single crystal X-ray analysis of the boron adduct are presented.

Sulfur, mercury, and boron adducts of sydnone imine derived anionic N-heterocyclic carbenes

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 [(PPh3)Au-S-sydnone imine] and a bis(ligand) mercury(ii) complex. The latter possesses a tetrahedral coordination of the mercury central atom to the sulfur atoms with the N6 nitrogen atoms coordinating as neutral ligands. Water converted the molsidomine anion into ethyl(2-morpholino-2-thioxoacetyl)carbamate. Mercury(ii)chloride and triphenylborane were employed to trap the sydnone imine carbenes as mercury complexes as well as BPh3 adducts.

An enantioselective assembly of naphthopyran via NHC-catalyzed [3 + 3] annulation of bromoenal with β-tetralone

An asymmetric assembly of naphthopyran was realized via the N-heterocyclic carbene (NHC)-catalyzed formal [3 + 3] annulation of bromoenal and β-tetralone. The key advantages of this protocol include ready availability of starting materials, mild reaction conditions, good yields.

Chemistry and spectroscopy of cross-conjugated and pseudo-cross-conjugated quinolinium-ethynyl-benzoate mesomeric betaines

The three isomers 1-methylquinolinium-2-, 3-, and 4-ethynyl(phenyl-4-carboxylates) belong to two distinct types of heterocyclic mesomeric betaines. The quinolinium substituted in position 3 is a cross-conjugated mesomeric betaine (CCMB), whereas the quinolinium derivatives substituted in positions 2 and 4 are members of the class of pseudo-cross-conjugated mesomeric betaines (PCCMBs). While the charges are strictly separated within the common π-electron system of the CCMB according to the canonical formulae, the charges are effectively but not exclusively delocalized in the PCCMBs because cumulenoid resonance forms including electron sextet structures without external octet stabilization can be formed in accordance with the definition of PCCMBs. As a consequence, despite being closely related structures, the three isomers differ in their chemical and spectroscopic behaviors. Thus, on trying to hydrolyze the ester group of the methyl quinolinium-2-ethynyl-benzoate into the corresponding acid by subsequent treatment with sodium hydroxide in methanol and aqueous hydrochloric acid at pH 3, the acetal methyl 1,1-dimethoxy-2-(quinolinium-ylidene)ethyl]benzoate and the corresponding β-enamino carbonyl compound were formed, respectively. The corresponding acids of the 2- and 4-substituted quinolinium-ethynyl-benzoates were obtained by a modified procedure. On deprotonation, the resulting cross-conjugated quinolinium-3-ethynyl-benzoate betaine proved to be stable, whereas the corresponding pseudo-cross-conjugated quinolinium-2- and -4-ethynyl-benzoate betaines decomposed. Frontier orbital profiles were calculated, and IR and Raman spectra of the starting materials were measured and calculated to analyze the differences of CCMBs and PCCMBs of mesomeric betaines possessing triple bonds. A higher contribution of the cumulenoid resonance forms to the overall structure of the PCCMBs was determined.

From betaines to anionic N-heterocyclic carbenes. Borane, gold, rhodium, and nickel complexes starting from an imidazoliumphenolate and its carbene tautomer

The mesomeric betaine imidazolium-1-ylphenolate forms a borane adduct with tris(pentafluorophenyl)borane by coordination with the phenolate oxygen, whereas its NHC tautomer 1-(2-phenol)imidazol-2-ylidene reacts with (triphenylphosphine)gold(I) chloride to give the cationic NHC complex [Au(NHC)2][Cl] by coordination with the carbene carbon atom. The anionic N-heterocyclic carbene 1-(2-phenolate)imidazol-2-ylidene gives the complexes [K][Au(NHC-)2], [Rh(NHC-)3] and [Ni(NHC-)2], respectively. Results of four single crystal analyses are presented.