Triazol-5-ylidene

Last updated
Triazol-5-ylidene constitutional isomers Triazol-5-ylidene isomers.png
Triazol-5-ylidene constitutional isomers

The triazol-5-ylidenes are a group of persistent carbenes which includes the 1,2,4-triazol-5-ylidene system and the 1,2,3-triazol-5-ylidene system. As opposed to the now ubiquitous NHC (N-heterocyclic carbene) systems based on imidazole rings, these carbenes are structured from triazole rings. 1,2,4-triazol-5-ylidene can be thought of as an analog member of the NHC family, with an extra nitrogen in the ring, while 1,2,3-triazol-5-ylidene is better thought of as a mesoionic carbene (MIC). [1] Both isomers of this group of carbenes benefit from enhanced stability, with certain examples exhibiting greater thermal stability, and others extended shelf life. [1] [2]

Contents

The 1,2,4-triazol-5-ylidene system is of special historic interest, as this system contains the first known instance of a characterized NHC, a compound colloquially known as Nitron, which was first isolated in 1905. [3] This compound was first proposed as an analytical reagent for the gravimetric analysis of moieties commonly found in explosives. [3] [4] Nitron's properties as an NHC, however, were not reported and utilized until 2011. [5]

Another member from this group of carbenes is of particular interest due to its robust stability up to temperatures of 150 °C in the absence of air or oxygen. It was first reported in 1995 by Dieter Enders and coworkers and has since become known as the "Ender's carbene. [2] " This particular reagent bears the notable distinction of being the first commercially available carbene. [4] [6]

History and Synthesis

Nitron

Proposed structure of Nitron c. 1917 Nitron proposed structure c. 1917.png
Proposed structure of Nitron c. 1917

Cope and Barab reported in 1917 that Nitron had been first synthesized as early as 1905 by Max Busch, who published extensively on its use as an analytical reagent for gravimetric analysis. [3] This molecule's potential for carbene-like reactivity would not be recognized until Färber et al. from the University of Kassel published a paper in 2011 showcasing its potential as a carbenic species. This group demonstrated that Nitron reacts as a nucleophilic carbene. [5]

The mesoionic structure of Nitron and the structure of its NHC-type carbenic tautomer Nitron carbenic tautomer.png
The mesoionic structure of Nitron and the structure of its NHC-type carbenic tautomer

Reaction with elemental Sulfur in THF afforded a triazolinethione derivative. This formation of a C=S double bond is characteristic of nucleophilic carbenes, often referred to as a "trapping" reaction. With addition of CS2 in THF, a betainic dithiocarboxylate was synthesized, with its crystal structure fully characterized and its 13CNMR and IR spectra corresponding well with typical NHC analogues. The Rhodium complexes that the group synthesized showed that Nitron acts as a moderate donor ligand, as a reduced CO stretching frequency in the product was confirmed by IR analysis when compared to the starting material, indicating that the significant back-donation into the metal center had occurred, as would be expected from a nucleophilic carbene. [5] Nitron has gained relatively little attention in the literature since this discovery of its carbene reactivity, although a few investigations [7] have been undertaken to determine how its reactivity compares to the more rigorously tested and more commonly used carbene ligands.

Reactions utilized by Farber et al. (2011) showing nucleophilic carbene-like reactivity. Nitron reactions.png
Reactions utilized by Färber et al. (2011) showing nucleophilic carbene-like reactivity.

Enders Carbene

The University of Kassel group cited their interest in generating new, cheaper-to-produce carbenes because, at the time, the commercially available carbenes "exceed[ed] several hundred US$ per gram. [5] These commercially available carbenes had been in development since the late 1960s. Chemists were trying to make these carbenic species more stable at higher temperatures and exist free in solution without needing to form coordination compounds. Hans-Werner Wanzlick, Guy Bertrand, and Anthony Arduengo were pioneers in the development of these types of persistent carbenes, not exclusively working with the triazol-5-ylidenes. [8] [9] [10]

Synthesis of the "Enders carbene" reported in 1995 starting from benzoyl chloride. Enders carbene synthesis.png
Synthesis of the "Enders carbene" reported in 1995 starting from benzoyl chloride.

Dieter Enders' group developed a carbene in 1995 that was stable enough to be commercially distributed. Starting with benzoyl chloride, they formed a triazolium perchlorate salt over 5 steps. They reacted this triazolium salt with sodium methoxide in methanol, and then carried out a thermal α-elimination of methanol at 80 °C and under low pressure conditions to form the Enders carbene. While all carbenes are very sensitive to oxygen and air and typically decompose readily when exposed to high temperatures. Enders showed that his new carbene was stable up to 150 °C in the absence of air and oxygen. [2] [11] [12] These advances in carbene stability helped to make the commercialization of these reagents a reality. Enders carbene would become the first commercially available carbene. [6] These carbenes, however, were still expensive, as noted by Färber et al. [5] Following this commercialization and dissemination, many analogues of the 1,2,4-triazol-5-ylidene system have been reported and utilized, most often as transition metal coordination compounds. [13] [14] [15] The enders Carbene itself proved to be a powerful catalyst for the conversion of formaldehyde to glycolaldehyde in the "formoin reaction. [16] [17] "

1,2,3-triazol-5-ylidene

The chemistry of the 1,2,3-triazol-5-ylidene system is a much more recently developed field. This system is based on the 1,2,3-triazole ring and had been indicated to have "non negligible lifetimes" in solution as early as 1975. [18] In 2008, 1,2,3-triazolium iodide salts were observed to react with transition metals to form metal-ligand complexes. [19] In 2010, however, Guy Bertrand's group reported the first crystalline carbene of this class, synthesized via a copper-catalyzed azide–alkyne cycloaddition (click reaction) of 2,6-diisopropylphenyl azide and phenylacetylene. Bertrand's group reported high stability and shelf life for this compound. [20] Since then, many coordination compounds have been reported based on this system—most notably, compounds which are active organocatalysts. [21] [22]

Bertrand's synthesis of free 1,2,3-triazol-5-ylidenes Bertrand's synthesis of free 1,2,3-triazol-5-ylidenes.png
Bertrand's synthesis of free 1,2,3-triazol-5-ylidenes

Reactivity

Enhanced Stability

Arduengo postulated that the stability of NHC-type carbenes arose from accumulation of electron density around the carbene center, hindering addition reactions from opportunistic nucleophiles. Arduengo concluded that the overall stability of these NHC's resulted from kinetic factors. He stated that "the isolation of a stable carbene is dependent upon the ability of the carbene to exist in a deep local minimum on the potential energy surfaces. It is not important what other minima might also exist on the potential surfaces so long as these minima are not kinetically accessible under ambient conditions likewise. [23] " Enders, in a similar manner, referring to the stability of the "enders carbene", posited that the "2p-2p interactions between the carbene carbon atom and the adjacent nitrogen atoms play a significant role in the stabilization of [the molecule]," due to their observation that these N-C bond lengths are considerably shorter than would be expected from single bonds. [2] When comparing their own assessment to Arduengo's rationale for stability, Enders et al. acknowledged in their 1995 paper that "Neither our crystallographic nor our theoretical results permit us to judge the significance of these factors for the stability of the system examined in this work. [2] " The combination of strong lone pair donation from the two Nitrogens to the carbene center and the Nitrogens' sigma withdrawing effects are the primary rationales for the stability of these systems. [24]

Wanzlick Equilibrium

MIC's do not dimerize MIC's do not dimerize.png
MIC's do not dimerize

The 1,2,3-triazol-5-ylidene system demonstrates fascinating reactivity, particularly with respect to the typical dimerization pathways for NHC's. Guy Bertrand notes "the Wanzlick equilibrium pathway for classical carbenes is disfavored [for these MIC's]. [20] " The Wanzlick equilibrium describes a typical dimerization for Arduengo type carbenes (NHC's). [25] Due to their apparent reluctance to participate in this dimerization pathway, carbenes based on the 1,2,3-triazol-5-ylidene system have vastly extended shelf lives. [1] These systems still require significant kinetic stabilization to be stable in solution. [1]

Reactions

Enders reported that the Enders carbene exhibits typical Lewis basicity, as it readily adds to Lewis acids like BH3∙THF, giving the triazoline-borane adduct. [11] In the same paper, Enders reports many other types of nucleophilic carbene reactions that are not exclusive to this system. The enders carbene undergoes insertion reactions, addition reactions, and cycloadditions in a similar manner to many other NHC systems. [11]

Enders carbene reaction with BH3[?]THF Enders carbene reaction with BH3[?]THF.png
Enders carbene reaction with BH3∙THF
Sn2' selective Grignard-allylic substitution reaction catalyzed by 1,2,3-triazol-5-ylidene magnesium complexes generated in situ. Sn2' selective Grignard-allylic substitution reaction catalyzed by 1,2,3-triazol-5-ylidene magnesium complexes generated in situ.png
Sn2’ selective Grignard-allylic substitution reaction catalyzed by 1,2,3-triazol-5-ylidene magnesium complexes generated in situ.

Both triazol-5-ylidene systems prove to be excellent organocatalysts. [1] [16] One such catalytic use of these carbenes is an allylic substitution Grignard reaction reported in 2013. The catalytic use of a triazolium salt generates a 1,2,3-triazol-5-ylidene magnesium complex in situ, which, due to its significant Lewis basicity, can back donate to the magnesium center and push the Schlenk equilibrium towards alkyl magnesium products. The Lewis basicity of the catalyst also promotes Sn2' selectivity for this specific reaction. [22] The 1,2,3-triazol-5-ylidene ligand has also been shown to work well with catalytic ruthenium systems promoting olefin metathesis reactions. [21] Other reported catalytic processes facilitated by compounds bearing these MIC ligands include: hydrohydrazination of alkynes, reductive formylation of amines with carbon dioxide and diphenylsilane, hydrogenation and dehydrogenation of N-heteroarenes in water, cycloisomerization of enynes, asymmetric Suzuki−Miyaura cross-coupling reactions, and water oxidation (WO) reactions. [1]

Regarding the 1,2,4-triazol-5-ylidene system, many of its reported coordination compounds are with transition metals, which are usually generated in similar fashion to the analogous imidazole-based NHC ligand-metal systems. [13] [14] [15] One such catalytic use of this system coupled to a transition metal was described in 2010, where the authors used a Gold (I) complex as a regioselective catalyst for the hydroamination of alkynes. [14]

Triazaborole System

Synthesis of 1,2,4,3-Triazaborol-3-yl-Lithium. Synthesis of 1,2,4,3-Triazaborol-3-yl-Lithium.png
Synthesis of 1,2,4,3-Triazaborol-3-yl-Lithium.

A substituted analogue of the 1,2,4-triazol-5-ylidene system was synthesized in 2016, with a boron atom replacing the carbenic carbon. [26] The synthesized triazaborole-metal system showed interesting reactivity toward CO and isonitriles. The authors also reported that reactions with this triazaborole ring yielded some exceptionally rare boron-metal bonds, such as B-Sb and B-Bi. [26] The structures of these triazaborole rings are stabilized by the interaction between the empty P orbital on the Boron and the lone pairs on the flanking Nitrogens. The aryl groups also provide good kinetic stabilization to the system. Insertion reactions of CO to 1,2,4,3-Triazaborol-3-yl-Lithium yielded reactive carbene species, which the authors utilized as a starting material to generate a 1,2-diboranylethene adduct.

Proposed reaction pathway in the synthesis of the 1,2-diboranylethene adduct. Proposed reaction pathway in the synthesis of the 1,2-diboranylethene adduct.png
Proposed reaction pathway in the synthesis of the 1,2-diboranylethene adduct.

See also

Related Research Articles

In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R−:C−R' or R=C: where the R represents substituents or hydrogen atoms.

The Sonogashira reaction is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It employs a palladium catalyst as well as copper co-catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or vinyl halide.

Grubbs catalysts are a series of transition metal carbene complexes used as catalysts for olefin metathesis. They are named after Robert H. Grubbs, the chemist who supervised their synthesis. Several generations of the catalyst have also been developed. Grubbs catalysts tolerate many functional groups in the alkene substrates, are air-tolerant, and are compatible with a wide range of solvents. For these reasons, Grubbs catalysts have become popular in synthetic organic chemistry. Grubbs, together with Richard R. Schrock and Yves Chauvin, won the Nobel Prize in Chemistry in recognition of their contributions to the development of olefin metathesis.

A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (−CR3) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.

A triazole is a heterocyclic compound featuring a five-membered ring of two carbon atoms and three nitrogen atoms with molecular formula C2H3N3. Triazoles exhibit substantial isomerism, depending on the positioning of the nitrogen atoms within the ring.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole.

<span class="mw-page-title-main">Tebbe's reagent</span> Chemical compound

Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylidenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. It is a red solid that is pyrophoric in the air, and thus is typically handled with air-free techniques. It was originally synthesized by Fred Tebbe at DuPont Central Research.

<span class="mw-page-title-main">Wolff rearrangement</span>

The Wolff rearrangement is a reaction in organic chemistry in which an α-diazocarbonyl compound is converted into a ketene by loss of dinitrogen with accompanying 1,2-rearrangement. The Wolff rearrangement yields a ketene as an intermediate product, which can undergo nucleophilic attack with weakly acidic nucleophiles such as water, alcohols, and amines, to generate carboxylic acid derivatives or undergo [2+2] cycloaddition reactions to form four-membered rings. The mechanism of the Wolff rearrangement has been the subject of debate since its first use. No single mechanism sufficiently describes the reaction, and there are often competing concerted and carbene-mediated pathways; for simplicity, only the textbook, concerted mechanism is shown below. The reaction was discovered by Ludwig Wolff in 1902. The Wolff rearrangement has great synthetic utility due to the accessibility of α-diazocarbonyl compounds, variety of reactions from the ketene intermediate, and stereochemical retention of the migrating group. However, the Wolff rearrangement has limitations due to the highly reactive nature of α-diazocarbonyl compounds, which can undergo a variety of competing reactions.

IMes is an abbreviation for an organic compound that is a common ligand in organometallic chemistry. It is an N-heterocyclic carbene (NHC). The compound, a white solid, is often not isolated but instead is generated upon attachment to the metal centre.

<span class="mw-page-title-main">Germylene</span> Class of germanium (II) compounds

Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.

<span class="mw-page-title-main">Anthony Joseph Arduengo III</span> American chemist

Anthony Joseph Arduengo III is Professor of the Practice at the Georgia Institute of Technology, Saxon Professor Emeritus of Chemistry at the University of Alabama, adjunct professor at the Institute for Inorganic Chemistry of Braunschweig University of Technology in Germany, and co-founder of the StanCE coalition for sustainable chemistry based on woody biomass. He is notable for his work on chemical compounds with unusual valency, especially in the field of stable carbene research.

Guy Bertrand, born on July 17, 1952, at Limoges is a chemistry professor at the University of California, San Diego.

<span class="mw-page-title-main">Dihydroimidazol-2-ylidene</span> Chemical compound

Dihydroimidazol-2-ylidene is a hypothetical organic compound with formula C3H6N2. It would be a heterocyclic compound, formally derived from imidazolidine with two hydrogen atoms removed from carbon number 2, leaving two vacant chemical bonds — which makes it a carbene.

<span class="mw-page-title-main">PEPPSI</span> Group of chemical compounds

PEPPSI is an abbreviation for pyridine-enhanced precatalyst preparation stabilization and initiation. It refers to a family of commercially available palladium catalysts developed around 2005 by Prof. Michael G. Organ and co-workers at York University, which can accelerate various carbon-carbon and carbon-heteroatom bond forming cross-coupling reactions. In comparison to many alternative palladium catalysts, Pd-PEPPSI-type complexes are stable to air and moisture and are relatively easy to synthesize and handle.

In chemistry, mesoionic carbenes (MICs) are a type of reactive intermediate that are related to N-heterocyclic carbenes (NHCs); thus, MICs are also referred to as abnormal N-heterocyclic carbenes (aNHCs) or remote N-heterocyclic carbenes (rNHCs). Unlike simple NHCs, the canonical resonance structures of these carbenes are mesoionic: an MIC cannot be drawn without adding additional charges to some of the atoms.

<span class="mw-page-title-main">Palladium–NHC complex</span>

In organometallic chemistry, palladium-NHC complexes are a family of organopalladium compounds in which palladium forms a coordination complex with N-heterocyclic carbenes (NHCs). They have been investigated for applications in homogeneous catalysis, particularly cross-coupling reactions.

<span class="mw-page-title-main">Transition metal NHC complex</span>

In coordination chemistry, a transition metal NHC complex is a metal complex containing one or more N-heterocyclic carbene ligands. Such compounds are the subject of much research, in part because of prospective applications in homogeneous catalysis. One such success is the second generation Grubbs catalyst.

<span class="mw-page-title-main">Borylene</span>

A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.

Coinage metal N-heterocyclic carbene (NHC) complexes refer to transition metal complexes incorporating at least one coinage metal center (M = Cu, Ag, Au) ligated by at least one NHC-type persistent carbene. A variety of such complexes have been synthesized through deprotonation of the appropriate imidazolium precursor and metalation by the appropriate metal source, producing MI, MII, or MIII NHC complexes. While the general form can be represented as (R2N)2C:–M (R = various alkyl or aryl groups), the exact nature of the bond between NHC and M has been investigated extensively through computational modeling and experimental probes. These results indicate that the M-NHC bond consists mostly of electrostatic attractive interactions, with some covalent bond character arising from NHC to M σ donation and minor M to NHC π back-donation. Coinage metal NHC complexes show effective activity as catalysts for various organic transformations functionalizing C-H and C-C bonds, and as antimicrobial and anticancer agents in medicinal chemistry.

<i>N</i>-Heterocyclic carbene boryl anion Isoelectronic structure

An N-heterocyclic carbene boryl anion is an isoelectronic structure of an N-heterocyclic carbene (NHC), where the carbene carbon is replaced with a boron atom that has a -1 charge. NHC boryl anions have a planar geometry, and the boron atom is considered to be sp2-hybridized. They serve as extremely strong bases, as they are very nucleophilic. They also have a very strong trans influence, due to the σ-donation coming from the boron atom. NHC boryl anions have stronger electron-releasing character when compared to normal NHCs. These characteristics make NHC boryl anions key ligands in many applications, such as polycyclic aromatic hydrocarbons, and more commonly low oxidation state main group element bonding.

References

  1. 1 2 3 4 5 6 Guisado-Barrios, G.; Soleilhavoup, M.; Bertrand, G. (2018). "1H-1,2,3-Triazol-5-ylidenes: Readily Available Mesoionic Carbenes". Accounts of Chemical Research. 51 (12): 3236–3244. doi:10.1021/acs.accounts.8b00480. PMID   30417642. S2CID   53282584.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  2. 1 2 3 4 5 Enders, D.; Breuer, K.; Raabe, G.; Runsink, J. Teles J.H.; Melder, J.P.; Ebel, K.; Brode, S. (1995). "Preparation, Structure, and Reactivity of 1,3,4-Triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene, a New Stable Carbene". Angew. Chem. Int. Ed. Engl. 34 (9): 1021–1023. doi:10.1002/anie.199510211.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. 1 2 3 Cope, W.C.; Barab, J. (1917). ""Nitron" as a Gravimetric Reagent for the Analysis of Substances Used in Explosives". J. Am. Chem. Soc. 39 (3): 504–514. doi:10.1021/ja02248a020.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. 1 2 Hitzel, S.; Färber, C.; Bruhn, C.; Siemeling, U. (2014). "Reactions of [RuCl2(PPh3)3] with Nitron and with the "Enders Carbene": Access to Ruthenium(III) NHC Complexes". Organometallics. 33 (1): 425–428. doi:10.1021/om401058e.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. 1 2 3 4 5 Färber, C.; Leibold, M.; Bruhn, C.; Maurer, M.; Siemeling, U. (2012). "Nitron: a stable N-heterocyclic carbene that has been commercially available for more than a century". Chem. Commun. 48 (2): 227–229. doi:10.1039/C1CC16460K. PMID   22068309.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. 1 2 Enders, D.; Balensiefer, T. (2004). "Nucleophilic Carbenes in Asymmetric Organocatalysis". Acc. Chem. Res. 37 (8): 534–541. doi:10.1021/ar030050j. PMID   15311952.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ↑ Thie, C.; Bruhn, C.; Leibold, M.; Siemeling, U. (2017). "Coinage Metal Complexes of the Carbenic Tautomer of a Conjugated Mesomeric Betaine Akin to Nitron". Molecules. 22 (7): 1133. doi: 10.3390/molecules22071133 . PMC   6152182 . PMID   28686202.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ↑ Wanzlick, H. W.; Schönherr, H. J. (1968). "Direct Synthesis of a Mercury Salt-Carbene Complex". Angewandte Chemie International Edition in English. 7 (2): 141–142. doi:10.1002/anie.196801412.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ↑ Igau, A.; Baceiredo, A.; Trinquier, G.; Bertrand, G. (1989). "[Bis(diisopropylamino)phosphino]trimethylsilylcarbene: A Stable Nucleophilic Carbene". Angewandte Chemie International Edition in English. 28 (5): 621–622. doi:10.1002/anie.198906211.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ↑ Arduengo, A.; Harlow, R.; Kline, M. (1991). "A stable crystalline carbene". J. Am. Chem. Soc. 113 (1): 361–363. doi:10.1021/ja00001a054.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. 1 2 3 Enders, D.; Breuer, K.; Runsink, J.; Teles, J. H. (1996). "Chemical Reactions of the Stable Carbene 1,3,4-Triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene". Liebigs Ann. 1996 (12): 2019–2028. doi:10.1002/jlac.199619961212.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ↑ Enders, D.; Breuer, K.; Kallfass, U.; Balensiefer, T. (2003). "Preparation and Application of 1,3,4-Triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene, A Stable Carbene". Synthesis. 8 (8): 1292–1295. doi:10.1055/s-2003-39409.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. 1 2 Marchenko, A. P.; Koidan, H. N.; Zarudnitskii, E. V.; Hurieva, A. N.; Kirilchuk, A. A.; Yurchenko, A. A.; Biffis, A.; Kostyuk, A. N. (2012). "Stable N-Phosphorylated 1,2,4-Triazol-5-ylidenes: Novel Ligands for Metal Complexes". Organometallics. 31 (23): 8257–8264. doi:10.1021/om300872g.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. 1 2 3 Dash, C.; Shaikh, M. M.; Butcher, R. J.; Ghosh, P. (2010). "Highly Convenient Regioselective Intermolecular Hydroamination of Alkynes Yielding Ketimines Catalyzed by Gold(I) Complexes of 1,2,4-triazole Based N-heterocyclic Carbenes". Inorg. Chem. 49 (11): 4972–4983. doi:10.1021/ic100087d. PMID   20429537.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. 1 2 Korotkikh, N.; Rayenko, G.; Shvaika, O. P.; Pekhtereva, T. M.; Cowley, A. H.; Jones, J. N.; Macdonald, C. L. B. (2003). "Synthesis of 1,2,4-Triazol-5-ylidenes and Their Interaction with Acetonitrile and Chalcogens". J. Org. Chem. 68 (14): 5762–5765. doi:10.1021/jo034234n. PMID   12839479.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. 1 2 Enders, D.; Niemeier, O.; Henseler, A. (2007). "Organocatalysis by N-Heterocyclic Carbenes". Chem. Rev. 107 (12): 5606–5655. doi:10.1021/cr068372z. PMID   17956132.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ↑ Teles, J. H.; Melder, J. P.; Ebel, K.; Schneider, R.; Gehrer, E.; Harder, W.; Brode, S.; Enders, D.; Breuer, K.; Raabe, G. (1996). "The Chemistry of Stable Carbenes. Part 2. Benzoin-type condensations of formaldehyde catalyzed by stable carbenes". Helv. Chim. Acta. 79 (1): 61–83. doi:10.1002/hlca.19960790108.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  18. ↑ Begtrup, M. (1975). "Azolium anions and their reaction with electrophilic reagents". J. Chem. Soc. Chem. Commun. (9): 334–335. doi:10.1039/C39750000334.
  19. ↑ Mathew, P.; Neels, A.; Albrecht, M. (2008). "1,2,3-Triazolylidenes as Versatile Abnormal Carbene Ligands for Late Transition Metals". J. Am. Chem. Soc. 130 (41): 13534–13535. doi:10.1021/ja805781s. PMID   18785741.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  20. 1 2 Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G. (2010). "Crystalline 1H-1,2,3-triazol-5-ylidenes: new stable mesoionic carbenes (MICs)". Angew. Chem. Int. Ed. 49 (28): 4759–4762. doi:10.1002/anie.201001864. PMC   3131155 . PMID   20509134.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  21. 1 2 Bouffard, J.; Kietz, B. K.; Tonner, R.; Guisado-Barrios, G.; Frenking, G.; Grubbs, R. H.; Bertrand, G. (2011). "Synthesis of Highly Stable 1,3-Diaryl-1H-1,2,3-triazol-5-ylidenes and Their Applications in Ruthenium-Catalyzed Olefin Metathesis". Organometallics. 30 (9): 2617–2627. doi:10.1021/om200272m. PMC   3092707 . PMID   21572542.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. 1 2 Nomura, R.; Tsuchiya, Y.; Ishikawa, H.; Okamoto, S. (2013). "Grignard allylic substitution reaction catalyzed by 1,2,3-triazol-5-ylidene magnesium complexes". Tetrahedron Letters. 54 (11): 1360–1363. doi:10.1016/j.tetlet.2012.12.124.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  23. ↑ Arduengo, A.; Dias, H. V. R.; Dixon, D. A.; Harlow, R. L.; Klooster, W. T.; Koetzle, T.F. (1994). "Electron Distribution in a Stable Carbene". J. Am. Chem. Soc. 116 (15): 6812–6822. doi:10.1021/ja00094a040.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  24. ↑ Fèvre, M.; Pinaud, J.; Gnanou, Yves.; Vignolle, J.; Taton, D. (2013). "N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis". Chem. Soc. Rev. 42 (5): 2142–2172. doi:10.1039/c2cs35383k. PMID   23288304.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  25. ↑ Alder, R. W.; Blake, M. E.; Chaker, L.; Harvey, J. N.; Paolini, F.; Schütz, J. (2004). "When and How Do Diaminocarbenes Dimerize?". Angewandte Chemie International Edition. 43 (44): 5896–5911. doi: 10.1002/anie.200400654 . PMID   15457494.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  26. 1 2 Lu, W.; Hu, H.; Li, Y. Ganguly, R.; Kinjo, R. (2016). "Isolation of 1,2,4,3-Triazaborol-3-yl-metal (Li, Mg, Al, Au, Zn, Sb, Bi) Derivatives and Reactivity toward CO and Isonitriles". J. Am. Chem. Soc. 138 (20): 6650–6661. doi:10.1021/jacs.6b03432. PMID   27135617.{{cite journal}}: CS1 maint: multiple names: authors list (link)
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.