Normal N-heterocyclic Carbene Research Articles

Ruthenium(II) complexes bearing chelating Carboxylate-anchored normal and abnormal Carbenes: Synthesis, characterizations and catalytic applications

Functionalized abnormal/mesoionic NHC (NHC = N-heterocyclic carbene) complexes have been much less investigated in contrast to their normal NHC counterparts. In this work, a series of [Ccarbene^Ocarboxylate]- and [Ccarbene^C’phenyl]-type Ru(II) complexes bearing normal and abnormal NHCs have been synthesized. Their spectroscopic features, solid-state structure and aquation reactivity have also been reported. All the as-synthesized complexes have served as precatalysts to catalyze intramolecular or intermolecular carboxylic acid-to-alkyne addition reactions. It was found that the catalytic behavior of abnormal NHC complexes is superior to that of the normal NHC analogs. An anti-Markovnikov addition reactivity, leading to the formation of an E-type enol ester as the major product, was observed. A few stoichiometric experiments have also been carried out to provide preliminary insight into the catalytic mechanism.

Bifunctional N-Heterocyclic Carbenes Derived from l-Pyroglutamic Acid and Their Applications in Enantioselective Organocatalysis.

In nature, enzymes are a powerful medium for the construction of enantiomerically pure chemicals, which always inspires synthetic chemists to explore new catalysts to imitate the enzyme machinery for asymmetric transformations. Vitamin B1, a bifunctional thiazolium N-heterocyclic carbene (NHC) precursor, is the coenzyme for transketolase. In the past two decades, a series of chiral NHCs, including monocyclic, bicyclic, tetracyclic, and even bridged ones, have been synthesized and successfully utilized as efficient organocatalysts for a wide variety of asymmetric organic reactions. The utility of bifunctional catalysts can enhance catalytic activity and improve stereochemical control through their synchronous activation of both reaction partners. However, the NHCs possessing multiple activation sites are far less developed.This Account gives an overview of our research on the design, development, and applications of bifunctional NHCs in organocatalysis. We synthesized a series of l-pyroglutamic acid-derived bifunctional NHCs bearing a free hydroxyl group which can interact with carbonyl or imino groups via hydrogen-bonding. Further studies revealed that these bifunctional catalysts worked well for a variety of reactions. We have developed bifunctional NHC-catalyzed aza-benzoin reactions, [2 + 2], [2 + 3], and [2 + 4] cycloadditions of ketenes, [3 + 2] and [3 + 4] annulations of enals, and aza-MBH and Rauhut-Currier reactions of Michael acceptors. In addition to these reactions via nucleophilic Breslow intermediates, enolates, homoenolates, and zwitterionic azolium intermediates, the bifunctional NHC-catalyzed [3 + 3] annulation via 1,3-biselectrophilic α,β-unsaturated acyl azolium intermediates was also developed.In these reactions, bifunctional NHCs showed amazing effects compared to normal nonbifunctional NHCs. In some cases, the bifunctional NHCs facilitated reactions which did not work under normal NHC catalysis, possibly due to additional activation via H-bonding. More interestingly, the bifunctional NHCs could not only improve but also switch the enantioselectivity to get products with opposite stereochemistry through H-bond controlled stereochemical directing. Furthermore, the reaction mode could be totally changed from [3 + 2] to [3 + 4] annulation to give kinetically favored products when bifunctional NHCs were employed. In future, the applications of bifunctional NHCs in other challenging reactions, such as asymmetric reactions with carbon-carbon unsaturated bonds, and the reactions involving alkyl or heteroatom radicals will be the major focus in our group.

Essential Role of Ancillary Ligand in Color Tuning and Quantum Efficiency of Ir(III) Complexes with N-Heterocyclic or Mesoionic Carbene Ligand: A Comparative Quantum Chemical Study.

Tuning photoluminescence properties is of prime importance for designing efficient light emitting diode (LED) materials. Here, we perform a computational study on the effect of normal N-heterocyclic carbene (NHC) and abnormal mesoionic carbene (MIC) ligands on the photoluminescence properties of some Ir(III) complexes, which are very promising LED materials. We find MIC as the privileged ligand in designing triplet emitters. The strong σ-donating and moderate π-accepting properties of MIC render a lower access to the nonemissive triplet metal-centered state (3MC), resulting in lowering the nonradiative rate constant ( knr) and correspondingly achieving higher quantum efficiency. We also demonstrate that the judicial choice of ancillary ligand can improve the efficiency of these materials even further. This quantum chemical investigation focuses on the importance of MIC as cyclometalating ligand and the substantial effects of ancillary ligands in controlling the color tuning and quantum efficiency for optoelectronic applications.

ChemInform Abstract: Recent Advances in Neutral and Anionic N‐Heterocyclic Carbene‐Betaine Interconversions. Synthesis, Characterization, and Applications

AbstractReview: 147 refs.

Reaching for two new stable ambiphilic quinoline-derivedN-heterocyclic carbenes at DFT level

A detailed investigation on the thermodynamic and kinetic stability of four carbenic tautomers of quinoline 1, including quinoline-2-ylidene 2, quinoline-3-ylidene 3, quinoline-4-ylidene 4, and 3,4-dihydroquinoline-4-ylidene 5, reveals that singlet planar six-membered ring N-heterocyclic carbenes (NHCs) 2 and 4 have less stability than Arduengo type NHC but seems to have enough conceivably for reaching at B3LYP/aug-cc-pVTZ//B3LYP/6–31+G* and B3LYP/6–311++G**//B3LYP/6–31+G* levels. All these six-membered NHCs are extremely ambiphilic with the more nucleophilic and electrophilic characters compared to the Arduengo type one. The aromaticity of singlet 2 and 4 is a significant contributor to their stability which is confirmed through their Nucleus-independent chemical shift(1)zz values. Finally, among 2–5, the normal NHC 2 is thermodynamically preferred but the remote NHC 4 is kinetically proffered over the other isomeric carbenes. The effects of different N- or C-substituted NHCs of 2 are studied using appropriate isodesmic reactions. The trimethylsilyl substituent exhibits slightly larger carbene stabilization in quinoline-derived NHCs than the pyridine analogue. © 2014 Wiley Periodicals, Inc.

Room-temperature hydrohydrazination of terminal alkynes catalyzed by saturated abnormal N-heterocyclic carbene-gold(I) complexes.

A number of saturated abnormal N-heterocyclic carbene (NHC) complexes of gold, in combination with KBAr(F) 4 as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at room temperature, which was not possible to date with gold catalysts. The reaction can be applied to a number of substituted arylalkynes. With alkylalkynes the yields are low. The saturated abnormal NHC ligands are resistant to isomerization to the saturated normal NHC coordination mode under basic reaction conditions. Under acidic conditions, a simple protonation at the nitrogen atom not neighboring the carbene center was observed and unambiguously characterized by an X-ray crystal-structure analysis. Computational studies confirm that such an isomerization would be highly exothermic, the observed kinetic stability probably results from the need to shift two protons in such a process.

Facile base-free in situ generation and palladation of mesoionic and normal N-heterocyclic carbenes at ambient conditions.

Synthesis of normal and mesoionic N-heterocyclic carbene Pd(II) complexes in near quantitative yields from the corresponding NHC precursor salts under base free and ambient conditions is reported. A plausible mechanism involving charge-assisted hydrogen bonded intermediates is proposed.

Changes in ligating abilities of the singlet and triplet states of normal, abnormal and remote N-heterocyclic carbenes depending on their aromaticities

Quantum chemical calculations at B3LYP/aug-cc-pVTZ level about singlet N-heterocyclic carbene (NHC) ligands, imidazol-2-ylidene, imidazol-4-ylidene, pyrazol-3-ylidene and pyrazol-4-ylidene, and their protonated analogues show that they are considerably aromatic except for pyrazol-3-ylidene. This result is experimentally verified by approximately five thousand NHC transition metal complexes retrieved from the Cambridge Structural Database (CSD). CSD search discloses that NHCs can participate in π-stacking interactions, albeit scarce. Geometry-based HOMA and electronic aromaticity index FLU rather than NICS provide a satisfactory description of the bonding situations in NHC ligands. Singlet state of the normal NHC has electron-deficient aromaticity as compared to those of the abnormal and remote NHCs. Depending on the transition from the singlet to triplet state, NHCs become electron-deficient ligands except for remote NHC. Computational studies regarding electronic nature of free NHC ligands show that the π-electronic population of the formally vacant pπ orbital on the carbene atoms in abnormal and remote NHC is occurred as a result of the aromaticity of NHCs, not as a result of the direct electron donation from LP-orbitals of N atoms to carbene atom according to putative push-pull effect used in understanding the electronic stabilization of normal NHC. Increase in the aromaticity raises σ-donating ability of both imidazol- and pyrazol-based NHC ligands. Free abnormal and remote NHC ligands have higher σ-donation ability than normal NHC ligands. The lack of σ-donating ability of normal NHC is compensated by its relatively high π-accepting ability, whereas π-back donation abilities of abnormal and remote NHCs are prohibited by their almost fully occupied π-orbitals. Aromaticities of the triplet NHC ligands are higher than that of the lowest-lying triplet state of benzene. Increase in the aromaticity of NHC ligands decreases van der Waals shortening in TM-NHC bonds mainly due to diminishing dative character of these bonds.

Recent advances in neutral and anionic N-heterocyclic carbene – betaine interconversions. Synthesis, characterization, and applications

Some mesoionic compounds, i.e. five-membered representatives of the class of conjugated mesomeric betaines (CMB), are in equilibrium with their tautomeric normal N-heterocyclic carbenes (nNHC). In addition, anionic N-heterocyclic carbenes, generated by deprotonation of mesoionic compounds, have been described. The first examples of conversions of crossconjugated mesomeric betaines (CCMB), 6-oxopyrimidinium-4-olates, into normal Nheterocyclic carbenes have been reported as well. CCMB such as imidazolium-4-carboxylate and pyrazolium-4-carboxylate can decarboxylate to form abnormal (aNHC) or remote N-heterocyclic carbenes (rNHC). Most conversions of betaines into N-heterocyclic carbenes start from pseudocross-conjugated mesomeric betaines (PCCMB) which can be regarded as heterocumulene adducts of nNHC. Thus, decarboxylations of imidazolium-2-carboxylates, 1,2,4-triazole-3carboxylates, pyrazolium-3-carboxylates or indazolium-3-carboxylates yield N-heterocyclic carbenes which have been used in catalysis, complex chemistry, heterocyclic synthesis, and organocatalysis.

PEPPSI‐Type Palladium Complexes Containing Basic 1,2,3‐Triazolylidene Ligands and Their Role in Suzuki–Miyaura Catalysis

A series of PEPPSI-type palladium(II) complexes was synthesized that contain 3-chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki-Miyaura cross-coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50 °C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol-2-ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N-heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system.

Lewis base mediated dismutation of trichlorosilane

An abnormal N-heterocyclic carbene (aNHC) has been used as a Lewis base to initiate dismutation of trichlorosilane. This report presents the reactivity differences of a normal N-heterocyclic carbene (NHC) versus aNHC with heavier group 14 elements. Three novel compounds (NHC)(2)·SiCl(2)H(2) (2), aNHC·SiCl(2)H(2) (3), and aNHC·GeCl(2) (4) have been synthesized and characterized by single crystal X-ray analysis, solid-state NMR and DFT calculations.

Stabilization of cyclic and acyclic carbon(0) compounds by differential coordination of heterocyclic carbenes: a theoretical assessment

Recently, donor stabilized divalent carbon(0) compounds have undergone intense experimental and theoretical investigation due to their strong electron rich character. In this Article, some new cyclic and acyclic carbon(0) compounds stabilized by differential coordination modes (such as abnormal, remote and a mixture of both) of N-heterocyclic carbenes are studied theoretically. The cyclic carbon(0) compounds proposed in this study are unusual in the sense that they contain a five membered ring consisting of only carbon atoms with a central carbon atom in the formal oxidation state of zero. All these compounds are found to be very strong nucleophiles which might have wide implications in catalysis. Calculation of first proton affinities of these molecules reveal that they are better σ donors than the carbon(0) compound supported by normal N-heterocyclic carbenes. Quantum chemical calculations indicate that these molecules possess very high donor-acceptor L → C bond strengths and are thermodynamically stable. Calculation of the bond dissociation energies for the complexation of one and two molecules of AuCl indicates the possible isolation of their gem dimetalated derivatives.

Abnormal N-Heterocyclic Carbene Promoted Suzuki−Miyaura Coupling Reaction: A Comparative Study

A C2-protected imidazolium salt that generates an abnormal N-heterocyclic carbene (NHC) was applied for the palladium-catalyzed Suzuki−Miyaura coupling reactions. The abnormal NHC precursor promoted the reactions while suppressing homocoupling of aryl boronic acid, which was observed with the corresponding normal NHC precursor. The related well-defined abnormal NHC-based palladium complexes also showed better activity than the normal NHC-based analogues, exhibiting faster reaction rates especially at the initial stage of the reaction.

Rhodium and Iridium Complexes of Abnormal N-Heterocyclic Carbenes Derived from Imidazo[1,2-a]pyridine

Rhodium and iridium complexes of a new type of abnormal N-heterocyclic carbenes (NHCs) derived from imidazo[1,2-a]pyridiniums have been prepared via silver transmetalation, where metalation can be directed to either the C-2 or the C-3 position of imidazo[1,2-a]pyridine ring provided that the other position is appropriately blocked. The donating abilities of these new NHC ligands have been assessed from the average CO stretching frequencies of their corresponding iridium dicarbonyl complexes. By varying the N-alkyl groups or fusing the pyridine moiety with an aromatic ring, electronic effects of abnormal NHCs of this type can be readily tuned to match the extremely donating imidazole-derived abnormal NHCs or the relatively less donating normal NHCs (imidazolin-2-ylidenes). Both VT NMR studies and CO stretching frequency values strongly support the π-accepting ability of an abnormal NHC derived from imidazo[1,2-a]quinoline.

Abnormal and Normal N-Heterocyclic Carbene Osmium Polyhydride Complexes Obtained by Direct Metalation of Imidazolium Salts

The complex OsH6(PiPr3)2 (1) is a basic precursor which promotes the direct metalation of imidazolium salts. Thus, it reacts with 1-mesityl-3-methylimidazolium tetraphenylborate and 1-mesityl-3-ethylimidazolium tetraphenylborate to give the abnormal NHC derivatives [OsH5(1-mesityl-3-methylimidazol-4-ylidene)(PiPr3)2]BPh4 (2) and [OsH5(1-mesityl-3-ethylimidazol-4-ylidene)(PiPr3)2]BPh4 (3), respectively. Treatment of 2 and 3 with NaH produces the deprotonation of the metal center and the formation of the tetrahydride derivatives OsH4(1-mesityl-3-methylimidazol-4-ylidene)(PiPr3)2 (4) and OsH4(1-mesityl-3-ethylimidazol-4-ylidene)(PiPr3)2 (5). In contrast to the case for the mesityl-substituted imidazolium salts, the reaction with 1-benzyl-3-methylimidazolium tetraphenylborate leads to the normal NHC complex [OsH5(1-benzyl-3-methylimidazol-2-ylidene)(PiPr3)2]BPh4 (6). The deprotonation of 6 with NaH affords OsH4(1-benzyl-3-methylimidazol-2-ylidene)(PiPr3)2 (7). Complexes 2 and 4 have been characterized by X-ra...

An Anion-Dependent Switch in Selectivity Results from a Change of C−H Activation Mechanism in the Reaction of an Imidazolium Salt with IrH5(PPh3)2

Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir(III) to give Ir(V) with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir(III), not Ir(V), because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).