Carbenes are highly reactive intermediates in organic synthesis. These divalent carbon species are generally transient in nature and cannot be isolated. However, they can form stabile metal complexes. Later on, the development of N-heterocyclic carbene (NHC) and other stable carbene led to the application of these carbon (II) donor ligands in the synthesis of complex natural products, transition metal catalysis, organo-catalysis and several other synthetic methodologies. Here in this short review, we will discuss the brief history of the development of carbenes, synthesis of stable carbenes (NHC in particular), and their applications in natural products synthesis transition metal chemistry/organometallics. In addition to synthesis and application, the chapter will consist of a detailed structural analysis of carbenes and exciting photophysics of this class of compounds. Special emphasis will be given to electronic structure. The role of carbene in the development of luminescent NHC transition metal complexes, the tuning of emission properties as well as their active role as photocatalysts in the reduction of CO2 will also be discussed.
- N-heterocyclic carbene
- electronic structure
Carbenes are divalent carbon compounds which are generally highly reactive organic intermediates with six valence electrons having the general formula R2C or R1R2C [1, 2]. Carbenes are classified as either singlets or triplets, depending upon their electronic structure. Most carbenes are very short lived, although persistent carbene are also known.
Carbene generally have either linear (as an extreme case) or bent geometry with sp2 hybridized central carbon atom. These structures are related to electronically different orbital coupled states of central carbon atom in carbene. The orbital coupling between sp-hybrid orbitals and other two energetically degenerated p-orbitals results linear geometry of carbene (Figure 1). In other case, the hybrid sp2 orbitals (δ orbitals) coupled with a p orbital (p-π orbital) promotes bent geometry of carbine (Figure 1).
The arrangement of two nonbonding electrons in carbene is extremely important to reactivity of carbene. The two different electronic states are obtained from different arrangement of two nonbonding electrons in carbene. These electronic states are related to triplet and singlet state of carbene.
1.1 Triplet state of carbene
From two nonbonding electrons, one electron occupies in empty δ orbitals and another electron resides in empty pπ with parallel spin orientation (p: 3B1). If electronic spin orientation is antiparallel, then the carbene is no longer triplet carbene (p: 1B1) (Figure 2).
1.2 Singlet state of carbene
When the two nonbonding electrons occupy as a lone-pair in the emptyδ orbital, then the pπ orbital is being vacant (p: 1A1 state). If these two nonbonding electrons are present in pπ orbital as a lone pair with empty δ-orbital, then 1A1 state is also created with pelectronic configuration. Interestingly, the p(1A1 state) is considered as a more stable state than the p(another 1A1 state) where the lone-pair occupies the pπ orbital (Figure 2).
The stability of singlet state (p(1A1 state)) is explained on the basis of significant energy difference between δ and pπ orbital (>2.0 eV).1 Singlet carbenes show the amphiphilic behavior (nucleophilic and electrophilic character) due to the presence of a sp2 hybridized lone-pair and of a vacant p-orbital.
Initially, carbene were so reactive that they were only considered as reaction intermediates or transition states. They could not be isolated and were only indirectly studied, often by trapping them in the presence of suitable reagents. However, now carbene can be stabilized and isolated by forming complexes with transition metals. They act as ligands for organometallic complexes. Two types of carbene-metal complex are known and they are Fischer and Schrock-type complexes (Figure 3).
The Fischer carbene, which were first described in 1960s, form complexes with low valent or lower oxidation state of metal and are versatile reagents for organic synthesis due to presence of electrophilic carbon center (Figure 4). The Schrock-type compounds (first reported in the early 1970s) play an important role in olefin metathesis due to present of nucleophilic carbon center (Figure 5).
2. Types of N-heterocyclic carbene (NHC) ligands
The structure of carbene depends upon the nature of ligands.
2.1 4-Membered NHC
Grubbs et al. was the first group to develop the 4-membered NHC  (Figure 6). It was found that for the isolation of carbene carbon steric shielding was very much important. The 2, 6-diisopropyl-substituted constituents led to the successful isolation of the free NHC [3, 4]. The vibrational, ν(CO) values of the corresponding Rhodium dicarbonyl complex (ν(CO) in toluene: 2080 and 1988 cm−1) show that it’s σ-electron donating properties is slightly less than the dihydroimidazol-2-ylidene analogue .
2.2 5-Membered NHC
5-membered ring systems were most and widely reported NHC carbene so far [6, 7, 8, 9, 10, 11]. This is due to the fact that 5-membered ring system is sterically more stable and hence provide the extra stability to the NHC as well as improving its catalytic properties [8, 9]. Some of the scaffolds used for the preparation of 5-membered NHC were imidazole-2-ylidenes Imes (
IBIox system of NHC ligands have been well explored and readily derived from bioxazolines (Figure 8) (
all of them have a rigid tricyclic backbone
the presence of the substituents R1 and R2 on the peripheral rings. These groups shield the carbene carbon, and can cause the metal’s coordination sphere either to expand or to contract.
A salient feature of these carbenes (NHC ligands) is that steric bulkiness of the ligands can be modified according to the uses without affecting the electronic character of carbene which is an ideal criterion for selection of ligands (Figure 9). It is an important and a unique property for such monodentate ligands as compared to monodentate phosphines where increasing the size of the phosphine ligands can affect both their steric and electronic properties.
N-heterocyclic carbene based on Benzimidazolium (Figure 8)
Weiss et al. was the first person to develop and introduce the Bipyridocarbene (
The structures of some other interesting carbene ligands (
2.3 6- and 7-Membered NHC
6 or 7 membered ring carbenes of N-heterocyclic such as 1,3-disubstituted pyrimidin- 2-ylidenes
Borazines, also known as “inorganic benzene” and isoelectronic with benzene are excellent scaffolds for highly stable heterocycles. When the borane moiety is “exchanged” with an iso-electronic carbene moiety one can obtain NHC
The first synthesis of a 7-membered NHC ligand was reported by Stahl et al. very recently [29, 30]. Even though NHC
2.4 Bi- and multi-dentate NHC
In addition to these monodentate ligands, several multi-dentate ligands have been synthesized and used for various applications. The rigid bidentate benzimidazole-based N-heterocyclic carbene was used in the synthesis of conjugated organometallic polymers which show interesting electronic and mechanical properties . Another application of such bidentate NHC was the formation of stable chelate complexes. One such palladium-NHC complex was used in the catalytic conversion of methane to methanol . The stability of the complexes is a pre-requirement for such applications as the reaction takes place in an acidic medium (trifluoroacetic acid) at high temperatures (80°C) in the presence of strong oxidizing agents like potassium peroxodisulfate.
Similar stable metal-chelate complexes were reported using tri- and tetra-dentate ligands. Iron (III) and chromium (III) form complexes of the structure [M(L29)2]+ with the tripodal tricarbene ligand
On the other hand, the development of macrocyclic ligands was found to be challenging. Hahn et al. successfully synthesized tetracarbene ligands having crown ether topology in a template-controlled synthetic approach.Initially, a transition metal complex with four unsubstituted benzimidazol-derived NHC
While the first report on the synthesis of carbene dates back to the 1920s, N-Hetero carbene (NHC) was discovered, from independent research works by Ofele , Wanzlick , and Lappert  in 1960s. A (phosphino)-(silyl) carbene was the first stable carbene to be synthesized which led to the tremendous fluorishment in carbene chemistry (Figure 12: left) [36, 38, 39]. After long time from discovery of carbene, Arduengo et al. reported for the first time the isolation of metal-free N-heterocyclic carbene in 1991 (Figure 12: right) [40, 41, 42, 43, 44].
After the synthesis and isolation of stable free NHC, carbene chemistry has attracted much more attention in recent times and scientists began to look for new NHC ligands and the synthesis of stable NHC. Prior to this carbene were thought to be highly reactive to be isolated and thus it limited the studies on carbene. But the stability, isolation of NHC and the ease of synthesis from cheap and easily available precursors such as imidazolium salts, made the field advance rapidly in the last three decades. The stability of Arduengo-type free carbene owes to the presence of two heteroatoms in the molecule. The inductive effect of these heteroatoms stabilize the carbene . The chemistry, structure, and propertiesof these “classical” heterocyclic carbene have been reviewed elaborately [46, 47]. The diverse applications of NHC have prompted the design and development of novel NHC structures. However, NHC are obtained generally from their suitable precursors. Thus, the facile, diverse synthesis of the NHC precursors is of great importance in order to get NHCs of various designs. Depending on NHC precursors their synthesis can be divided into the following major categories (Figure 13):
Deprotonation of azolium salts
Elimination reactions from imidazolines
Desulfurization of imidazol- and benzamidazol-2-thiones
Two of the most commonly used NHC scaffolds are imidazolin-2-ylidene and its saturated version, imidazolidin-2-ylidene. In both the cases, the two nitrogen atoms are substituted with alkyl and aryl groups either in a symmetrical or in an asymmetrical way . One of the easiest and widely used methods for the preparation of NHCs is the deprotonation of imidazolium or imidazolinium salts with strong bases such as sodium hydride , potassium tert-butoxide , or potassium bis(trimethylsilyl)amide .
The azolium salts can be also be prepared by the following routes namely,
3.1 N-alkylation of heterocycles
The first approach is simple and straightforward; the successive alkylation of the nitrogen atom in these heterocycles generates the quaternary N atoms which are excellent NHC precursors. The various methodologies for synthesis of imidazole, oxazole, thiazole, and other five membered heterocyclic rings have been reviewed extensively [48, 49, 50, 51, 52, 53].
3.2 Symmetrical synthesis of imidazolinium salts as NHC precursors
Symmetrical synthesis of imidazolinium salts as NHC precursors can be achieved by various methods. One such example is shown in Figure 14  following condensation reduction route. This is widely applicable for a variety of primary amines [55, 56, 57, 58, 59, 60, 61, 62, 63, 64].
3.3 Unsymmetrical synthesis of imidazolinium salts as NHC precursors
Unsymmetrical synthesis of imidazolinium salts as NHC precursors similarly have been prepared following different methods. The uses of oxalyl chloride or derivatives give more flexibility leading to the synthesis of unsymmetrical imidazoliniums. Mol and co-workers synthesized the mixed adamantyl/mesityl N-heterocyclic precursor 1-(1-adamantyl)-3- mesityl-imidazolinium chloride. At first, oxalyl chloride was reacted with mesityl amine to afford the intermediate acyl chloride which on successive reaction with another amine afforded the desired compound (Figure 15) [65, 66]. A small library of imidazolinium derivatives were preparedhaving groups with various steric or electronic properties using this method [67, 68, 69, 70, 71, 72].
Similarly, the unsaturated azolinium salts have been prepared by (a) alkylation of the nitrogen atom of imidazolinium, (b) symmetric and (c) unsymmetrical synthesis of such salts.
Paraformaldehyde has been extensively used for the synthesis of both unsaturated and saturated NHC precursors as shown in Figure 16 . There are several other strategies for the preparation of such azolium salts as NHC precursors which have been nicely reviewed . The deprotonation method of generating carbene from imidazolium salts using a strong base is generally performed in-situ so that the air sensitive free or ligated carbene is not isolated. But sometimes, the use of such a strong base and harsh reaction conditions leads to unwanted side-reactions. Thus to avoid those unnecessary complications, elimination of hydrogen atoms from imidazolinium salts are undertaken to provide NHC carbene.
Alternatively, less common routes involve reduction of thiourea derivatives, pyrolysis of an NHC − volatile compound adduct or release of NHC.
3.3.1 NHC·CO2 zwitterions
The carbon dioxide adducts of NHCs can be prepared easily by passing carbon dioxide gas into a free carbene solution, and then evaporating the solvent. They are comparatively air stable and can be stored for a long time. Imidazol(in)ium-2-carboxylate derivatives act as an excellent NHC precursor for the synthesis of NHC-transition metal complexes by releasing carbon dioxide during thermolysis. The steric effect on N substituent and other electronic, steric factors affecting the stability of such precursors has been also studied . Later on, imidazol(in)ium hydrogen carbonates have been shown as another excellent source of NHCs when they lose H2CO3 upon heating [75, 76, 77, 78, 79].
3.3.2 NHC − metal adducts
Another important NHC precursor is silver (I) complexes of NHC. There have been several types of such complexes like: Imidazolin-2-ylidene involving imidazole ring with substituents at the nitrogen atoms, Benzimidazol-2-ylidenes having a benzene ring fused with the imidazole moiety, imidazolidin-2-ylidenes-and related heterocycles. The first Ag (I)–NHC adduct was reported by Arduengo in the early 90s by the reaction of Ag(I) salt with a free NHC . Later, the uses of Ag(OAc) and Ag2O as silver base were reported to synthesize various Ag(I)–NHCs. These silver bases are used for the deprotonation of azolium salts, and generation of Ag(I)–NHCs in situ. These Ag(I)-NHC complexes easily decompose under thermolysis to provide the free carbenes for various applications. One limitation to this approach is the use of silver metal in stoichiometric amounts. The different synthetic routes as well the structural diversity and the applications of such precursors have been well established in the carbene literature .
3.3.3 Other NHC precursors
In addition to the above mentioned adducts there are reports of similar complexes of NHC namely the chloroform and pentafluorobenzene adducts of 1,3-dimesitylimidazolidin-2-ylidene (SIMes). They are stable at room temperature and afford the corresponding NHC on thermolysis [82, 83]. One such example is SIMes(H)(O-t-Bu) which can produce the corresponding NHC at room temperature. The alcohol adducts of triazolin-5-ylidene and imidazolidin-2-ylidene also proved to be excellent NHC precursors [84, 85] (Figure 17).
4. Photophysical studies of N-heterocyclic carbene
New efficient light-emitting materials related to iridium(III) and platinum(II) complexes have attracted research area and wide range of applications in OLED and WOLED technologies. The suitable ligand based iridium(III) and platinum(II) complexes allow tailoring the emission properties for specific application in organic light emitting devices (OLEDs) and white organic light emitting devices (WOLEDs).
In particular, extensive investigations have been carried out on iridium(III) and platinum(II) complexes as triplet emitters in OLEDs. In OLEDs, significant progress has been achieved for making highly efficient and stable green and red emitters. But further advancement in recent progress of solid state full-colored OLED displays and WOLEDs appliances is also required in the research area of blue and white light emitting iridium(III) and platinum(II) complexes.
4.1 Photophysical studies of N-heterocyclic carbene platinum(II) complexes
Here N-heterocyclic carbene platinum(II) complexes are selected to investigated their photophysical properties (Figure 18). Selected N-heterocyclic carbene platinum(II) complexes have shown distinct absorption bands in 325–405 nm region with higher extinction coefficients (order of 103 or 104). These complexes are known as either blue, bluish green, or green emitters depending on emission bands within the 430–530 nm region with large Stokes shifts.
The [PtII(C^N^C)Cl][PF6] complex shows strong absorption bands at 272 nm and moderately intense bands at364 nm with higher extinction coefficients (ε ∼ 103–104) in acetonitrile (Figure 19). The high-energy intense absorption band (277–291 nm) is assigned as π → π* transitions (IL: Intra ligand) of the C ≡ CR and C^N^C pincer ligands, whereas the low-energy absorption band (band (383–471 nm)) is observed due to presence of the dπ(Pt) → π* (C^N^C)] transitions, and [π(C ≡ CR) → π* (C^N^C)] transition, considered as metal-to-ligand charge-transfer (MLCT), ligand-to-ligand charge-transfer (LLCT), mixed with the π → π* transitions (IL) of the C^N^C pincer ligands [75, 76, 77, 78, 79].
A blue-shifted absorption band (383 nm) of Complex
The MLCT/LLCT absorption band is sensitive towards polarity of the solvents and shows a blue shifted absorption band from DCM (382 nm) to ACN (376 nm). A negative solvatochromism is observed in Pt (II)–polypyridine [82, 83], and Pt (II)–bzimpy complexes [76, 79] because of decreasing dipole moment during electronic transition.
Non-emissive nature of [PtII(C^N^C)Cl][PF6] complex in ACN is can be explained on the basis of low-energy d-d ligand field (LF) states, and effective quenched 3MLCT/3IL state . In contrast to the [PtII(C^N^C)Cl][PF6] complex, the tridentate pyridine-based N-heterocyclic carbene ligand based alkynylplatinum(II) complexes 1–5 (Figure 18) exhibit strong luminescence in solution with gaussian shaped emission bands (range: 497–631 nm) (Figure 20). Only alkynylplatinum(II) complex 6 (Figure 18) shows non-emissive character in solution. Interestingly, all alkynylplatinum(II) complexes 1–6 (Figure 18) have shown emissive character at low temperature in solid state and glass matrices at 77 K.
The large Stokes shifts and lifetimes in the microsecond are originated from triplet energy state. The emission bands are appeared from an predominantly 3MLCT excited state of [dπ(Pt) → π* (C^N^C)] transition, along with 3LLCT [π(C ≡ CR) → π*(C^N^C)] transition (Figure 20). Moreover, CT band and emission band of these metal complexes is altered depending on the nature of the substituted phenyl ring of alkynyl ligands in solution.
The intense luminescence from green to yellow and high PL quantum yield of alkynylplatinum(II) complexes 1–5 can be readily achieved by alternation of alkynyl ligands. The electron-rich moiety quenches the luminescence from 3MLCT excited state via photoinduced electron transfer (PET) process is responsible for non-emissive property of complex 6 in solution [82, 83]. Depending upon increasing the polarity of the solvents, excited state (3MLCT/3LLCT) is lesser stabilized compared to its ground state, leading to a blue shift of absorption spectra in solution, shows negative solvatochromism for alkynylplatinum(II) complex 2. A red shift of the emission band state (559–640 nm) has also been observed for complexes 2–4 in solid at room temperature. The low-energy emission band (559–640 nm) is originated from triplet states (due to presence of metal to-ligand charge transfer (MMLCT) character) of alkynylplatinum(II) complexes 2–4 due to presence of significant contribution from Pt···Pt interaction in solid state .
4.2 N-heterocyclic carbene Ir (III) complexes and their applications to deep-blue phosphorescent organic light-emitting diodes
Absorption and emission spectra of N-heterocyclic carbene Ir (III) complexes 1–3 (Figure 21) are measured in DCM (Figure 22). The absorption band at around 320 nm is due to overlap of the π → π* transition of triazolate chelate, the benzyl (carbene) and pyridyl (triazolate fragment) and considered as LLCT transition. Furthermore, spin-orbit coupling is enhanced by iridium and plays significant role on triplet absorption cross section.
N-heterocyclic carbene Ir (III) complexes 1–3 (Figure 21) show emission band at 461, 460, and 458 nm, respectively in DCM. The weak phosphorescence intensity of complex 1 is observed at 392 nm and also indicated by its low quantum efficiency (QE) (only 5.0 × 10−4). It is observed that the fluorescence quantum yield of complex 2 and 3, is much higher than that of complex 1. The radiative lifetimes of N-heterocyclic carbene Ir (III) complexes 1–3 confirm their phosphorescent nature. The nonradiative decay rate constants (knr) are found to be 1.2 × 109, 3.5 × 106 and 7.0 × 105 s−1 with large differences in quantum efficiency for complex 1–3 respectively.
The carbene chemistry became more popular for their applications as organocatalysts [87, 88] and transition metal catalysts in the synthesis of complex molecules [89, 90]. The N-heterocyclic carbene (NHCs) is widely used in organometallic chemistry during the last few years . A brief summary of the applications include:
5.1 Oxidation reactions catalyzed by NHC-metal complexes
They can be employed for various types of oxidation reactions like (a) Opppenauer-Type Alcohol Oxidation  where smaller R groups show catalytic activity, (b) Palladium-Catalyzed Aerobic Alcohol Oxidation  or (c) Wacker-Type Oxidation  as shown in Figure 23.
In addition, NHC metal complexes have been extensively used for
5.2 Palladium catalyzed reactions
Another important application has been the α-arylation of carbonyl compounds at moderate temperature and short reaction time . The same strategy has been applied for esters and amides [60, 97] (Figure 26).
Besides these, NHC act as great ligands for Pd catalyzed various coupling reactions like Heck reaction , Negishi reaction , Sonagashira reaction , Suzuki-Miyaura reaction , Stille coupling , and Buchwald-Hartwig reaction .
5.3 NHC Complexes in Olefin Metathesis
After the development of Grubbs I catalyst several modifications were carried out to develop more efficient catalysts. In this regard, NHC-Ruthenium complex was synthesized for RCM . But the initial design did not show marked difference in activity compared to Grubbs I catalyst, Later on, some combination catalysts were developed which showed greater activity and selectivity [106, 107, 108, 109]. Nowadays, these second-generation Grubbs’ olefin metathesis catalysts are widely used for metathesis reactions (Figure 28).
5.4 NHC as ligands in asymmetric synthesis
There are different approaches for inducing chirality by NHC ligands. One of the ways is N-substituents containing centers of chirality. But initially this method did not show great stereoselectivity  but later on the development of bidentate ligands enhanced the enantioselectivity. Grubbs et al. developed another method wherein, NHC ligands had the chiral elements within the N-heterocycles . These ligands showed great selectivity. Another interesting method involves an element of chirality like axial chirality or planer chirality to make the NHC ligands as stereo directing ligands for asymmetric synthesis [112, 113] (Figure 29).
Carbene has been all along an important reactive intermediate in organic chemistry but the development of N-Heterocyclic carbene has revolutionized the field of organic synthesis. NHC owing to their ease and flexibility of synthesis, their interesting structural properties and in particular their stability have received a great deal of attention within the last few years. There are several strategies for the development of stable and versatile NHC ligands. We have highlighted in this book chapter some general synthetic strategies for the synthesis of some interesting NHC. Along with the discussion on the different strategies adopted in the synthesis of NHC, we focused primarily on some of the salient features of their structures and photophysical properties. Finally, we focus briefly on the various uses of NHC in organic synthesis. Over the years, NHCs have been extensively studied for several applications in organic transformations. Herein, we have very briefly touched upon some of those reactions.
The recent advances in the design of novel ligands, development of interesting structures of NHC enable their exciting potential applications in organic synthesis in the future.
Bourissou D, Guerret O, Gabbaï F, Bertrand G. Stable carbenes. Chemical Reviews. 1999; 100:39-92
Hahn FE, Jahnke MC. Heterocyclic carbenes: Synthesis and coordination chemistry. Angewandte Chemie, International Edition. 2008; 47:3122-3172
Despagnet-Ayoub E, Grubbs R. A stable four-membered N-heterocyclic carbene. Journal of the American Chemical Society. 2004; 126:10198-10199
Despagnet-Ayoub E, Ritter T. N-heterocyclic carbenes as ligands for olefin metathesis catalysts. N-Heterocyclic Carbenes in Transition Metal Catalysis. 2006; 21:193-218
Altenhoff G, Goddard R, Lehmann C, Glorius F. An N-heterocyclic carbene ligand with flexible steric bulk allows suzuki cross-coupling of sterically hindered aryl chlorides at room temperature. Angewandte Chemie, International Edition. 2003; 42:3690-3693
Altenhoff G, Goddard R, Lehmann C, Glorius F. Sterically demanding, bioxazoline-derived N-heterocyclic carbene ligands with restricted flexibility for catalysis. Journal of the American Chemical Society. 2004; 126:15195-15201
Burstein C, Lehmann C, Glorius F. Imidazo [1,5-A] pyridine-3-ylidenes—pyridine derived N-heterocyclic carbene ligands. Tetrahedron. 2005; 61:6207-6217
Alcarazo M, Roseblade S, Cowley A, Fernández R, Brown J, Lassaletta J. Imidazo [1,5-A] pyridine:a versatile architecture for stable N-heterocyclic carbenes. Journal of the American Chemical Society. 2005; 127:3290-3291
Lavallo V, Canac Y, Präsang C, Donnadieu B, Bertrand G. Stable cyclic (Alkyl) (amino) carbenes as rigid or flexible, bulky, electron-rich ligands for transition-metal catalysts: A quaternary carbon atom makes the difference. Angewandte Chemie, International Edition. 2005; 44:5705-5709
Lavallo V, Canac Y, DeHope A, Donnadieu B, Bertrand G. A rigid cyclic (Alkyl) (amino) carbene ligand leads to isolation of low-coordinate transition-metal complexes. Angewandte Chemie, International Edition. 2005; 44:7236-7239
Yamashita M, Goto K, Kawashima T. Fixation of both O2 and Co2 from air by a crystalline palladium complex bearingn-heterocyclic carbene ligands. Journal of the American Chemical Society. 2005; 127:7294-7295
Raubenheimer H, Lindeque L, Cronje S. Synthesis and characterization of neutral and cationic diamino carbene complexes of gold (I). Journal of Organometallic Chemistry. 1996; 511:177-184
Köcher C, Herrmann W. Heterocyclic carbenes. One-pot synthesis of rhodium and iridium carbene complexes. Journal of Organometallic Chemistry. 1997; 532:261-265
Hahn F, Wittenbecher L, Boese R, Bläser D. N, N′-Bis(2,2-Dimethylpropyl) Benzimidazolin-2-Ylidene: A stable nucleophilic carbene derived from Benzimidazole. Chemistry—A European Journal. 1999; 5:1931-1935
Hahn F, Wittenbecher L, Le Van D, Fröhlich R. Evidence for an equilibrium between an N-heterocyclic carbene and its dimer in solution. Angewandte Chemie, International Edition. 2000; 39:541-544
Tan K, Bergman R, Ellman J. Intermediacy of an N-heterocyclic carbene complex in the catalytic C − H activation of a substituted benzimidazole. Journal of the American Chemical Society. 2002; 124:3202-3203
Metallinos C, Barrett F, Chaytor J, Heska M. A benzimidazole-based N-heterocyclic carbene derived from 1,10-phenanthroline. Organic Letters. 2004; 6:3641-3644
Hadei N, Kantchev E, O'Brie C, Organ M. Electronic nature of N-heterocyclic carbene ligands: Effect on the suzuki reaction. Organic Letters. 2005; 7:1991-1994
O'Brien C, Kantchev E, Chass G, Hadei N, Hopkinson A, Organ M, et al. Towards the rational design of palladium-N-heterocyclic carbene catalysts by a combined experimental and computational approach. Tetrahedron. 2005; 61:9723-9735
Weiss R, Reichel S, Handke M, Hampel F. Generation and trapping reactions of a formal 1:1 complex between singlet carbon and 2,2′-bipyridine. Angewandte Chemie, International Edition. 1998; 37:344-347
Weiss R, Reichel S. Novel urea derivatives as two-step redox systems. European Journal of Inorganic Chemistry. 2000; 2000:1935-1939
Nonnenmacher M, Kunz D, Rominger F, Oeser T. First examples of dipyrido [1,2-C:2′,1′-E] imidazolin-7-ylidenes serving as NHC-ligands: Synthesis, properties and structural features of their chromium and tungsten pentacarbonyl complexes. Journal of Organometallic Chemistry. 2005; 690:5647-5653
Nonnenmacher M, Kunz D, Rominger F, Oeser T. X-ray crystal structures of 10Π- and 14Π-electron pyrido-annelated N-heterocyclic carbenes. Chemical Communications. 2006; 13:1378
Alder RW, Blake ME, Bortolotti C, Bufali S, Butts CP, Linehan E, et al. Complexation of stable carbenes with alkali metals. Chemical Communications. 1999; 3:241-242
Guillen F, Winn C, Alexakis A. Enantioselective copper-catalyzed conjugate addition using chiral diaminocarbene ligands. Tetrahedron: Asymmetry. 2001; 12:2083-2086
Mayr M, Wurst K, Ongania K, Buchmeiser M. 1,3-dialkyl- and 1,3-diaryl-3,4,5,6-tetrahydropyrimidin-2-ylidene rhodium(I) and palladium(II) complexes: Synthesis, structure, and reactivity. Chemistry—A European Journal. 2004; 10:1256-1266
Yun J, Marinez E, Grubbs R. A new ruthenium-based olefin metathesis catalyst coordinated with 1,3-dimesityl-1,4,5,6-tetrahydropyrimidin-2-ylidene: Synthesis, X-ray structure, and reactivity. Organometallics. 2004; 23:4172-4173
Bazinet P, Yap G, Richeson D. Constructing a stable carbene with a novel topology and electronic framework. Journal of the American Chemical Society. 2003; 125:13314-13315
Präsang C, Donnadieu B, Bertrand G. Stable planar six-Π-electron six-membered N-heterocyclic carbenes with tunable electronic properties. Journal of the American Chemical Society. 2005; 127:10182-10183
Scarborough C, Grady M, Guzei I, Gandhi B, Bunel E, Stahl S. Pdii complexes possessing a seven-membered N-heterocyclic carbene ligand. Angewandte Chemie, International Edition. 2005; 44:5269-5272
Kastrup C, Oldfield S, Rzepa H. An Ab initio computational study of monodentate palladium ligand complexes with möbius-aromatic chiral character. Journal of the Chemical Society, Dalton Transactions. 2002; 12:2421
Boydston A, Williams K, Bielawski C. A modular approach to main-chain organometallic polymers. Journal of the American Chemical Society. 2005; 127:12496-12497
Muehlhofer M, Strassner T, Herrmann W. New catalyst systems for the catalytic conversion of methane into methanol. Angewandte Chemie, International Edition. 2002; 41:1745-1747
Fränkel R, Kernbach U, Bakola-Christianopoulou M, Plaia U, Suter M, Ponikwar W, et al. Homoleptic carbene complexes. Journal of Organometallic Chemistry. 2001; 617-618:530-545
Ofele KJ. A new transition-metal carbon complexe 1, 3-dimethyl −4-imidazolinylid-2-ene pentacarbonylchromium. Journal of Organometallic Chemistry. 1968; 12:42-43
Wanzlick H-W, Schönherr H-J. Direct synthesis of a mercury salt-carbene complex. Angewandte Chemie, International Edition. 1968; 7:141-142
Cardin DJ, Cetinkaya B, Lappert MF, Manojlović-Muir L, Muir KW. An electron-rich olefin as a source of co-ordinated carbene; synthesis of trans-Ptcl2 [C(Nphch2)2] Pet3. Journal of the Chemical Society [Section] D: Chemical Communications. 1971; 5:400-401
Igau A, Grutzmacher H, Baceiredo A, Bertrand G. Analogous. Alpha., Alpha.'-Bis-Carbenoid, triply bonded species: Synthesis of a stable lambda 3-phosphino carbene-.lambda.5-phosphaacetylene. Journal of the American Chemical Society. 1988; 110:6463-6466
Igau A, Baceiredo A, Trinquier G, Bertrand G. [Bis (Diisopropylamino) Phosphino] trimethylsilyl carbene: A stable nucleophilic carbene. Angewandte Chemie, International Edition in English. 1989; 28:621-622
Arduengo A, Dias H, Harlow R, Kline M. Electronic stabilization of nucleophilic carbenes. Journal of the American Chemical Society. 1992; 114:5530-5534
Schuster O, Yang L, Raubenheimer H, Albrecht M. Beyond conventional N-heterocyclic carbenes: Abnormal, remote, and other classes of NHC ligands with reduced heteroatom stabilization. Chemical Reviews. 2009; 109:3445-3478
Crabtree R. Abnormal, mesoionic and remote N-heterocyclic carbene complexes. Coordination Chemistry Reviews. 2013; 257:755-766
Huang D, Zhao P, Astruc D. Catalysis by 1, 2, 3-triazole- and related transition-metal complexes. Coordination Chemistry Reviews. 2014; 272:145-165
Vivancos Á, Segarra C, Albrecht M. Mesoionic and related less heteroatom-stabilized N-heterocyclic carbene complexes: Synthesis, catalysis, and other applications. Chemical Reviews. 2018; 118:9493-9586
Arduengo A, Harlow R, Kline M. A stable crystalline carbene. Journal of the American Chemical Society. 1991; 113:361-363
Edworthy I, Rodden M, Mungur S, Davis K, Blake A, Wilson C, et al. Silver alkoxide and amino N-heterocyclic carbenes; syntheses and crystal structures. Journal of Organometallic Chemistry. 2005; 690:5710-5719
Nolan SP, editor. N-heterocyclic carbenes in synthesis. Weinheim, Germany: Wiley-VCH; 2006
Ishii T, Kakeno Y, Nagao K, Ohmiya H. N-heterocyclic carbene-catalyzed decarboxylative alkylation of aldehydes. Journal of American Chemical Society. 2019; 141(9):3854-3858
Bellotti P, Koy M, Hopkinson MN. Recent advances in the chemistry and applications of N-heterocyclic carbenes. Nature Review Chemistry. 2021; 5:711-725
Kamijo S, Yamamoto Y. Recent progress in the catalytic synthesis of imidazoles. Chemistry: An Asian Journal. 2007; 2:568-578
Boulton AJ, Katritzky AR. Advances in Heterocyclic Chemistry. Vol. 27. New York: Academic Press; 1980. p. 241
Neier R, Grimmett MR, Bellus D, Houben J, Weyl T. Science of Synthesis. Stuttgart: Thieme; 2002. p. 325
Grimmett MR. Comprehensive heterocyclic chemistry: The structure, reactions, synthesis and uses of heterocyclic compounds. In: Katritzky AR, Rees CW, editors. Vol. 5. Oxford: Pergamon; 1984. p. 345
Jahnke MC, Hahn FE. Introduction to N-heterocyclic carbenes: Synthesis and stereolectronic parameters in N-heterocyclic carbenes: From laboratory curiosities to efficient synthetic tools. Catalysis Series. 2016; 2:1-45
Leuthäußer S, Schwarz D, Plenio H. Tuning the electronic properties of N‐heterocyclic carbenes. Chemistry—A European Journal. 2007; 13:7195
Rosadiuk K, Bohle D. Anhydrous dinitrogen trioxide solutions for brønsted acid free nitrous acid chemistry. European Journal of Inorganic Chemistry. 2017; 46:5461-5465
Fleckenstein C, Roy S, Leuthäußer S, Plenio H. Sulfonated N-heterocyclic carbenes for suzuki coupling in water. Chemical Communication. 2007; 27:2870-2872
Kantchev E, Ying J. Practical one-pot, three-component synthesis of n-heterocyclic carbene (NHC) ligated palladacycles derived from N,N-dimethylbenzylamine. Organometallics. 2008; 28(1):289-299
Fliedel C, Maisse-François A, Bellemin-Laponnaz S. Palladium(II) complexes of a Bis-2-aminobiphenyl N-heterocyclic carbene: Synthesis, structural studies and catalytic activity. Inorganica Chimica Acta. 2007; 360(1):143-148
Lee S, Hartwig J. Improved catalysts for the palladium-catalyzed synthesis of oxindoles by amide Α-arylation. rate acceleration, use of aryl chloride substrates, and a new carbene ligand for asymmetric transformations. The Journal of Organic Chemistry. 2001; 66(10):3402-3415
Jia Y, Hillgren J, Watson E, Marsden S, Kündig E. Chiral N-Heterocyclic carbene ligands for asymmetric catalytic oxindole synthesis. Chemical Communications. 2008; 34:4040-4042
Kündig E, Seidel T, Jia Y, Bernardinelli G. Bulky chiral carbene ligands and their application in the palladium-catalyzed asymmetric intramolecular Α-arylation of amides. Angewandte Chemie International Edition. 2007; 46(44):8484-8487
Sauvage X, Demonceau A, Delaude L. Homobimetallic Ruthenium–N-Heterocyclic Carbene Complexes for Olefin Metathesis. Metathesis Chemistry. 2007; 243:91-109
Delaude L, Szypa M, Demonceau A, Noels AF. In situ generated ruthenium catalysts bearing N-heterocyclic carbene ligands for the ring-opening metathesis polymerization of cyclooctene. Advanced Synthesis and Catalysis. 2002; 344:749-756
Waltman A, Grubbs R. A new class of chelating N-heterocyclic carbene ligands and their complexes with palladium. Organometallics. 2004; 23(13):3105-3107
Vougioukalakis GC, Grubbs RH. Ruthenium olefin metathesis catalysts bearing an N-Fluorophenyl-N-Mesityl-substituted unsymmetrical N-Heterocyclic carbene. Organometallics. 2007; 26(9):2469-2472
Ritter T, Day M, Grubbs R. Rate acceleration in olefin metathesis through a fluorine−Ruthenium interaction. Journal of the American Chemical Society. 2006; 128(36):11768-11769
Winkelmann O, Linder D, Lacour J, Näther C, Lüning U. Chiral concave imidazolinium salts as precursors to chiral concaven-heterocyclic carbenes. European Journal of Organic Chemistry. 2007; 22:3687-3697
Xu G, Gilbertson S. Development of building blocks for the synthesis of N-Heterocyclic carbene ligands. Organic Letters. 2005; 7(21):4605-4608
Makino T, Masu H, Katagiri K, Yamasaki R, Azumaya I, Saito S. Synthesis, structure, and solvent‐induced spontaneous homochiral assembly of bidentate Bis(N,N′‐Diaryl‐N‐Heterocyclic Carbene)‐palladium complexes. European Journal of Inorganic Chemistry. 2008; 31:4861-4865
Kuriyama M, Shimazawa R, Shirai R. Design and synthesis of thioether-imidazolium chlorides as efficient ligands for palladium-catalyzed suzuki–miyaura coupling of aryl bromides with arylboronic acids. Tetrahedron. 2007; 63(38):9393-9400
Clavier H, Coutable L, Guillemin JC, Mauduit M. New bidentate alkoxy-NHC ligands for enantioselective copper-catalysed conjugate addition. Tetrahedron: Asymmetry. 2005; 16(5):921-924
Bildstein B, Malaun M, Kopacka H, Wurst K, Mitterb€ock M, Ongania K-H, et al. N,N‘-Diferrocenyl-N-Heterocyclic carbenes and their derivatives. Organometallics. 1999; 18(21):4325-4336
Van Ausdall B, Glass J, Wiggins K, Aarif A, Louie J. A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates. The Journal of Organic Chemistry. 2009; 74(20):7935-7942
Bailey J, Hill M, Marsh R, Miskowski V, Schaefer W, Gray H. Electronic spectroscopy of chloro(terpyridine)platinum(II). Inorganic Chemistry. 1995; 34:4591-4599
Tam AY-Y, Lam WH, Wong KM-C, Zhu N, Yam VW-W. Luminescent alkynylplatinum(II) complexes of 2,6-Bis(N-Alkylbenzimidazol-2′-Yl)pyridine-type ligands with ready tunability of the nature of the emissive states by solvent and electronic property modulation. Chemistry—A European Journal. 2008; 14:4562-4576
Chan C-W, Cheng L-K, Che CM. Luminescent donor-acceptor platinum(Ii) complexes. Coordination Chemistry Reviews. 1994; 132:87-97
Lu W, Chui S, Ng K, Che C. A submicrometer wire-to-wheel metamorphism of hybrid tridentate cyclometalated platinum(II) complexes. Angewandte Chemie. 2008; 120:4644-4648
Tam AY-Y, Wong KM, Yam VW-W. Unusual luminescence enhancement of metallogels of alkynylplatinum(II) 2, 6-bis(N-alkylbenzimidazol-2′-Yl)pyridine complexes upon a gel-to-sol phase transition at elevated temperatures. Journal of the American Chemical Society. 2009; 131:6253-6260
Arduengo A, Dias H, Calabrese J, Davidson F. Homoleptic carbene-silver(I) and carbene-copper(I) complexes. Organometallics. 1993; 12(9):3405-3409
Lin I, Vasam C. Preparation and application of N-heterocyclic carbene complexes of Ag(I). Coordination Chemistry Reviews. 2007; 251(5-6):642-670
Wong KM, Tang W-S, Lu X-X, Zhu N, Yam VW-W. Functionalized platinum(II) terpyridyl alkynyl complexes as colorimetric and luminescence Ph sensors. Inorganic Chemistry. 2005; 44:1492-1498
Leung SY-L, Tam AY-Y, Tao C-H, Chow HS, Yam VW-W. Single-turn helix–Coil strands stabilized by metal···metal and Π–Π interactions of the alkynylplatinum(II) terpyridyl moieties in meta-phenylene ethynylene foldamers. Journal of the American Chemical Society. 2011; 134:1047-1056
Yersin H, Donges D. Low-lying electronic states and photophysical properties of organometallic Pd(II) and Pt(II) compounds. Modern research trends presented in detailed case studies. In: Yersin H, editor. Transition Metal and Rare Earth Compounds. Excited States, Transitions and Interactions. Vol. II. Berlin: Springer-Verlag; 2001. p. 81
Herber RH, Croft M, Coyer MJ, Bilash B, Sahiner A. Origin of polychromism of cis square-planar platinum(II) complexes: Comparison of two forms of [Pt (2, 2’-Bpy)(Cl2)]. Inorganic Chemistry. 1994; 33:2422-2426
Lee C, Sabiah S, Wang J, Hwang W, Lin I. Water-induced changes of photoluminescence of a pincer-type n-heterocyclic carbene platinum(II) complex. Organometallics. 2010; 29:286-289
Enders D, Niemeier O, Henseler A. Organocatalysis by N-heterocyclic carbenes. Chemical Reviews. 2007; 107:5606-5655
Flanigan DM, Romanov-Michailidis F, White NA, Rovis T. Organocatalytic reactions enabled by N-heterocyclic carbenes. Chemical Reviews. 2015; 115:9307-9387
Díez-González S, Marion N, Nolan S. N-heterocyclic carbenes in late transition metal catalysis. Chemical Reviews. 2009; 109:3612-3676
Hahn FE. Introduction: Carbene chemistry. Chemical Reviews. 2018; 118:9455-9456
Hanasaka F, Fujita K, Yamaguchi R. Cp*Ir complexes bearing N-heterocyclic carbene ligands as effective catalysts for oppenauer-type oxidation of alcohols. Organometallics. 2004; 23(7):1490-1492
Schultz M, Hamilton S, Jensen D, Sigman M. Development and comparison of the substrate scope of Pd-catalysts for the aerobic oxidation of alcohols. The Journal of Organic Chemistry. 2004; 70(9):3343-3352
Muñiz K. Palladium-carbene catalysts for aerobic, intramolecular wacker-type cyclisation reactions. Advanced Synthesis and Catalysis. 2004; 346(12):1425-1428
Poyatos M, Mata J, Falomir E, Crabtree R, Peris E. New ruthenium(II) CNC-pincer bis(carbene) complexes: Synthesis and catalytic activity. Organometallics. 2003; 22(5):1110-1114
Sato Y, Yoshino T, Mori M. Pd-catalyzed allylic substitution using nucleophilic N-heterocyclic carbene as a ligand. Organic Letters. 2002; 5(1):31-33
Navarro O, Marion N, Scott N, González J, Amoroso D, Bell A, et al. Synthesis of novel (NHC)Pd (Acac)Cl complexes (acac = acetylacetonate) and their activity in cross-coupling reactions. Tetrahedron. 2005; 61(41):9716-9722
Lee S, Beare N, Hartwig J. Palladium-catalyzed Α-arylation of esters and protected amino acids. Journal of the American Chemical Society. 2001; 123(34):8410-8411
Herrmann W, Elison M, Fischer J, Köcher C, Artus G. Metal complexes of N-heterocyclic carbenes—A new structural principle for catalysts in homogeneous catalysis. Angwandte Chemie International Edition. 1995; 34(21):2371-2374
Hadei N, Kantchev E, O'Brie C, Organ M. The first negishi cross-coupling reaction of two alkyl centers utilizing a Pd—N-heterocyclic carbene (NHC) catalyst. Organic Letters. 2005; 7(17):3805-3807
Batey R, Shen M, Lough A. Carbamoyl-substituted N-heterocyclic carbene complexes of palladium(II): Application to sonogashira cross-coupling reactions. Organic Letters. 2002; 4(9):1411-1414
Walker S, Barder T, Martinelli J, Buchwald S. A rationally designed universal catalyst for Suzuki–Miyaura coupling processes. Angewandte Chemie, International Edition. 2004; 43(14):1871-1876
Weskamp T, Böhm V, Herrmann W. Combining N-heterocyclic carbenes and phosphines: Improved palladium(II) catalysts for aryl coupling reactions. Journal of Organometallic Chemistry. 1999; 585(2):348-352
Viciu M, Kelly R, Stevens E, Naud F, Studer M, Nolan S. Synthesis, characterization, and catalytic activity of N-heterocyclic carbene (NHC) palladacycle complexes. Organic Letters. 2003; 5(9):1479-1482
Ackermann L. General and efficient indole syntheses based on catalytic amination reactions. Organic Letters. 2005; 7(3):439-442
Herrmann W. N-heterocyclic carbenes: A new concept in organometallic catalysis. Angewandte Chemie, International Edition. 2002; 41(8):1290-1309
Huang J, Stevens E, Nolan S, Petersen J. Olefin metathesis-active ruthenium complexes bearing a nucleophilic carbene ligand. Journal of the American Chemical Society. 1999; 121(12):2674-2678
Weskamp T, Kohl F, Hieringer W, Gleich D, Herrmann W. Highly active ruthenium catalysts for olefin metathesis: The synergy of N-heterocyclic carbenes and coordinatively labile ligands. Angewandte Chemie, International Edition. 1999; 38(16):2416-2419
Ackermann L, Fürstner A, Weskamp T, Kohl F, Herrmann W. Ruthenium carbene complexes with imidazolin-2-ylidene ligands allow the formation of tetrasubstituted cycloalkenes by RCM. Tetrahedron Letters. 1999; 40(26):4787-4790
Scholl M, Ding S, Lee C, Grubbs R. Synthesis and activity of a new generation of ruthenium-based olefin metathesis catalysts coordinated with 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligands. Organic Letters. 1999; 1(6):953-956
Herrmann W, Goossen L, Köcher C, Artus G. Chiral heterocylic carbenes in asymmetric homogeneous catalysis. Angewandte Chemie, International Edition. 1996; 35(2324):2805-2807
Seiders T, Ward D, Grubbs R. Enantioselective ruthenium-catalyzed ring-closing metathesis. Organic Letters. 2001; 3(20):3225-3228
Clyne D, Jin J, Genest E, Gallucci J, RajanBabu T. First chelated chiral N-heterocyclic bis-carbene complexes. Organic Letters. 2000; 2(8):1125-1128
Broggini D, Togni A. Synthesis and structure of an enantiomerically pure C2 symmetric ferrocenyl carbene. Helvetica Chimica Acta. 2002; 85(8):2518-2522
van den Berg HJJ. Molecular Catalysis. 1943; 51:1-12
Sheehan J, Hunneman D. Homogeneous asymmetric catalysis. Journal of the American Chemical Society. 1966; 88(15):3666-3667
Ramanjaneyulu BT, Mahesh S, Anand RV. N-Heterocyclic Carbene Catalyzed Highly Chemoselective Intermolecular Crossed Acyloin Condensation of Aromatic Aldehydes with Trifluoroacetaldehyde Ethyl Hemiacetal. Organic Letters. 2015; 17(1):6-9
Tagaki W, Tamura Y, Yano Y. Asymmetric benzoin condensation catalyzed by optically active thiazolium salts in micellar two-phase media. Bulletin of the Chemical Society of Japan. 1980; 53(2):478-480
Knight R, Leeper F. Synthesis of and asymmetric induction by chiral bicyclic thiazolium salts. Tetrahedron Letters. 1997; 38(20):3611-3614
Dvorak CA, Rawal VH. Catalysis of benzoin condensation by conformationally-restricted chiral bicyclic thiazolium salts. Tetrahedron Letters. 1998; 39(19):2925-2928