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{{Short description|Organic molecule containing a neutral carbon with two unbound valence electrons}}
{{About|the chemical class|the compound|Methylene (compound)}}
{{distinguishDistinguish|carbine|carbyne}}
 
[[File:Carbene.svg|thumb|60px|[[Methylene (compound)|Methylene]] is the simplest carbene.]]
In [[organic chemistry]], a '''carbene''' is a [[molecule]] containing a neutral [[carbon]] atom with a [[Valence (chemistry)|valence]] of two and two unshared [[valence electron]]s. The general formula is {{chem2|R-(\s:C\sR''':''')-R'}} or {{chem2|R=C'''\dC:'''}} where the R representrepresents [[substituent]]s or hydrogen atoms.▼
 
The term "carbene" may also refer to the specific compound {{chem2|:CH2}}, also called [[methylene radical|methylene]], the parent [[hydride]] from which all other carbene compounds are formally derived.<ref>{{Cite book|title=Molecular Orbitals of Transition Metal Complexes|isbn=978-0-19-853093-0|page=7|last=Hoffmann|first=Roald|author-link=Roald Hoffmann|publisher=Oxford|year=2005}}</ref><ref>{{GoldBookRef|title=carbenes|file=C00806}}</ref>
▲In [[chemistry]], a '''carbene''' is a [[molecule]] containing a neutral [[carbon]] atom with a [[Valence (chemistry)|valence]] of two and two unshared [[valence electron]]s. The general formula is R-(C''':''')-R' or R=C''':''' where the R represent [[substituent]]s or hydrogen atoms.
 
There are two types of carbenes: [[singlet state|singlets]] or [[triplet state|triplets]], depending upon their electronic structure.<ref>{{cite book |last1=Grossman |first1=Robert&nbsp;B. |title=The Art of Writing Reasonable Organic Reaction Mechanisms |publisher=Springer |year=2003 |isbn=0-387-95468-6 |edition=2nd |location=New York |page=84}}</ref> The different classes undergo different reactions.
The term "carbene" may also refer to the specific compound H<sub>2</sub>C''':''', also called [[methylene radical|methylene]], the parent [[hydride]] from which all other carbene compounds are formally derived.<ref>{{Cite book|title=Molecular Orbitals of Transition Metal Complexes|isbn=978-0-19-853093-0|page=7|last=Hoffmann|first=Roald|authorlink=Roald Hoffmann|publisher=Oxford|year=2005}}</ref><ref>{{GoldBookRef|title=carbenes|file=C00806}}</ref> Carbenes are classified as either [[singlet state|singlets]] or [[triplet state|triplets]], depending upon their electronic structure. Most carbenes are very short lived, although [[persistent carbene]]s <ref> For detailed reviews on stable carbenes, see: (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. ''[[Chem. Rev.]]'' '''2000''', ''100'', 39-91. (b) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. ''[[Angew. Chem. Int. Ed.]]'' '''2010''', ''49'', 8810-8849.</ref> are known. One well-studied carbene is [[dichlorocarbene]] Cl<sub>2</sub>C''':''', which can be generated ''[[in situ]]'' from [[chloroform]] and a strong [[base (chemistry)|base]].
 
Most carbenes are extremely reactive and short-lived. A small number (the di[[Halogen|halo]]<nowiki/>carbenes, [[carbon monoxide]],{{Sfn|Grossman|2003|p=35}} and [[carbon monosulfide]]) can be isolated, and can stabilize as [[Coordination complex|metal ligands]], but otherwise cannot be stored in bulk. A rare exception are the [[persistent carbene]]s,<ref>For detailed reviews on stable carbenes, see: (a) {{cite journal | last1 = Bourissou | first1 = D. | last2 = Guerret | first2 = O. | last3 = Gabbai | first3 = F. P. | last4 = Bertrand | first4 = G. | year = 2000 | title = Stable Carbenes| journal = [[Chem. Rev.]] | volume = 100 | issue = 1 | pages = 39–91 | doi = 10.1021/cr940472u | pmid = 11749234 }} (b) {{cite journal | last1 = Melaimi | first1 = M. | last2 = Soleilhavoup | first2 = M. | last3 = Bertrand | first3 = G. | title = Stable cyclic carbenes and related species beyond diaminocarbenes | year = 2010 | journal = [[Angew. Chem. Int. Ed.]] | volume = 49 | issue = 47 | pages = 8810–8849 | doi = 10.1002/anie.201000165 | pmid = 20836099 | pmc = 3130005 }}</ref> which have extensive application in modern [[organometallic chemistry]].
==Structures and bondings==▼
[[Image:Carbenes.svg|thumb|right|Singlet and triplet carbenes]]▼
 
==Generation of carbenes==▼
The two classes of carbenes are [[Diradical|singlet]] and [[diradical|triplet]] carbenes. Singlet carbenes are spin-paired. In the language of [[valence bond theory]], the molecule adopts an sp<sup>2</sup> [[Orbital hybridisation|hybrid structure]]. Triplet carbenes have two unpaired electrons. Most carbenes have a nonlinear triplet ground state, except for those with nitrogen, oxygen, or sulfur atoms, and halides directly bonded to the divalent carbon.
There are two common methods for carbene generation.
 
In [[Alpha elimination|α elimination]], two substituents eliminate from the same carbon atom. This occurs with reagents with no good [[Leaving group|leaving groups]] vicinal to an acidic proton are exposed to strong base; for example, [[phenyllithium]] will abstract [[Hydrohalic acid|HX]] from a [[haloform]] (CHX<sub>3</sub>).{{Sfn|Grossman|2003|pp=84-85}} Such reactions typically require [[phase-transfer catalyst|phase-transfer conditions]].{{Cn|date=January 2024}}
Carbenes are called singlet or triplet depending on the electronic [[spin (physics)|spins]] they possess. Triplet carbenes are [[paramagnetic]] and may be observed by [[electron spin resonance spectroscopy]] if they persist long enough. The total spin of singlet carbenes is zero while that of triplet carbenes is one (in units of <math>\hbar</math>). Bond angles are 125–140° for triplet methylene and 102° for singlet methylene (as determined by [[Electron paramagnetic resonance|EPR]]). Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media.
 
Molecules with no acidic proton can also form carbenes. A [[geminal]] dihalide exposed to [[Organolithium reagent|organolithiums]] can undergo [[metal-halogen exchange]] and then eliminate a [[lithium salt]] to give a carbene, and [[Zinc|zinc metal]] abstracts halogens similarly in the [[Simmons–Smith reaction]].{{Sfn|Grossman|2003|p=85}}
For simple hydrocarbons, triplet carbenes usually have energies 8 [[kilocalorie|kcal]]/[[mole (unit)|mol]] (33 [[kilojoule|kJ]]/mol) lower than singlet carbenes (see also [[Hund's rule of maximum multiplicity]]), thus, in general, triplet is the more stable state (the [[ground state]]) and singlet is the [[excited state]] species. [[Substituent]]s that can donate [[electron pair]]s may stabilize the singlet state by delocalizing the pair into an empty p orbital. If the energy of the singlet state is sufficiently reduced it will actually become the ground state.
:R<sub>2</sub>CBr<sub>2</sub> + BuLi → R<sub>2</sub>CLi(Br) + BuBr▼
No viable strategies exist for triplet stabilization. The carbene called [[9-fluorenylidene]] has been shown to be a rapidly [[chemical equilibrium|equilibrating]] mixture of singlet and triplet states with an approximately 1.1 kcal/mol (4.6 kJ/mol) energy difference.<ref>{{Cite journal| last4 = Kaufmann | first1 = P. B.| last2 = Brauer| last5 = Schuster| last3 = Zupancic | first2 = B. E.| first3 = J. J.| last1 = Grasse| first4 = K. J.| first5 = G. B.| title = Chemical and physical properties of fluorenylidene: equilibration of the singlet and triplet carbenes| journal = Journal of the American Chemical Society| volume = 105| issue = 23| pages = 6833| year = 1983 | doi = 10.1021/ja00361a014}}</ref> It is, however, debatable whether di[[aryl]] carbenes such as the [[fluorene]] carbene are true carbenes because the electrons can delocalize to such an extent that they become in fact [[biradical]]s. ''[[In silico]]'' experiments suggest that triplet carbenes can be [[thermodynamic]]ally stabilized with [[electropositive]] heteroatoms such as in [[silyl]] and [[silyloxy]] carbenes, especially trifluorosilyl carbenes.<ref name="nemirowski">{{cite journal | title=Electronic Stabilization of Ground State Triplet Carbenes | author=Nemirowski, A | author2=Schreiner, P. R. | journal=J. Org. Chem. |date=November 2007 | volume=72 | issue=25 | pages=9533–9540 | doi=10.1021/jo701615x | pmid=17994760}}</ref>▼
:R<sub>2</sub>CLi(Br) → R<sub>2</sub>C + LiBr▼
It remains uncertain if these conditions form truly free carbenes or a metal-carbene complex. Nevertheless, metallocarbenes so formed give the expected organic products.{{Sfn|Grossman|2003|p=85}} In a specialized but instructive case, α-halo[[Mercury (element)|mercury]] compounds can be isolated and separately thermolyzed. The "[[Seyferth reagent]]" releases CCl<sub>2</sub> upon heating:
:C<sub>6</sub>H<sub>5</sub>HgCCl<sub>3</sub> → CCl<sub>2</sub> + C<sub>6</sub>H<sub>5</sub>HgCl▼
 
Separately, carbenes can be produced from an extrusion reaction with a large free energy change. [[Diazirine]]s and [[epoxide]]s photolyze with a tremendous release in [[ring strain]] to carbenes. The former extrude inert [[nitrogen]] gas, but epoxides typically give reactive [[carbonyl]] wastes, and [[asymmetric synthesis|asymmetric]] epoxides can potentially form two different carbenes. Typically, the C-O bond with lesser fractional bond order (fewer double-bond resonance structures) breaks. For example, when one substituent is [[alkyl]] and another [[aryl]], the aryl-substituted carbon is usually released as a carbene fragment.
 
Ring strain is not necessary for a strong thermodynamic driving force. [[Photolysis]], [[heat]], or [[transition metal]] catalysts (typically [[rhodium]] and [[copper]]) decompose [[Diazoalkane|diazoalkanes]] to a carbene and gaseous [[nitrogen]]; this occurs in the [[Bamford–Stevens reaction]] and [[Wolff rearrangement]]. As with the case of metallocarbenes, some reactions of diazoalkanes that formally proceed via carbenes may instead form a [[1,3-Dipolar cycloaddition|[3+2] cycloadduct]] intermediate that extrudes nitrogen. [[image:alkylidene carbene.svg|frame|Alkylidene carbene]]To generate an [[alkylidene]] carbene a ketone can be exposed to [[trimethylsilyl]] [[diazomethane]] and then a strong base.
 
▲==Structures and bondingsbonding==
▲[[Image:Carbenes.svg|thumb|right|Singlet and triplet carbenes]]
The two classes of carbenes are [[Diradical|singlet]] and [[diradical|triplet]] carbenes. Triplet carbenes are [[Diradical|diradicals]] with two unpaired electrons, typically form from reactions that break two [[Sigma bond|σ bonds]] (α elimination and some extrusion reactions), and do not [[Orbital hybridisation|rehybridize]] the carbene atom. Singlet carbenes have a single [[lone pair]], typically form from diazo decompositions, and adopt an [[Trigonal planar molecular geometry|''sp''<sup>2</sup>]] orbital structure.{{Sfn|Grossman|2003|p=84}} Bond angles (as determined by [[Electron paramagnetic resonance|EPR]]) are 125–140° for triplet methylene and 102° for singlet methylene.
 
▲NoMost viablecarbenes strategieshave exista for[[Bent geometry|nonlinear]] triplet stabilizationground state. For simple hydrocarbons, triplet carbenes are usually only 8 [[kilocalorie|kcal]]/[[mole (unit)|mol]] (33 [[kilojoule|kJ]]/mol) more stable than singlet carbenes, comparable to [[Amine#Structure|nitrogen inversion]]. The carbenestabilization calledis in part attributed to [[Hund's rule of maximum multiplicity]]. However, strategies to stabilize triplet carbenes at room temperature are elusive. [[9-fluorenylidene|9-Fluorenylidene]] has been shown to be a rapidly [[chemical equilibrium|equilibrating]] mixture of singlet and triplet states with an approximately 1.1 kcal/mol (4.6 kJ/mol) energy difference, although extensive [[electron delocalization]] into the rings complicates any conclusions drawn from di[[aryl]] carbenes.<ref>{{Cite journal| last4 = Kaufmann | first1 = P. B.| last2 = Brauer| last5 = Schuster| last3 = Zupancic | first2 = B. E.| first3 = J. J.| last1 = Grasse| first4 = K. J.| first5 = G. B.| title = Chemical and physical properties of fluorenylidene: equilibration of the singlet and triplet carbenes| journal = Journal of the American Chemical Society| volume = 105| issue = 23| pages = 6833| year = 1983 | doi = 10.1021/ja00361a014}}</ref> It is,Simulations however, debatable whether di[[aryl]] carbenes such as the [[fluorene]] carbene are true carbenes because the electrons can delocalize to such an extentsuggest that they become in fact [[biradicalelectropositive]]s. ''[[In silico]]'' experiments suggest that triplet carbenesheteroatoms can be [[thermodynamic]]ally stabilizedstabilize withtriplet [[electropositive]] heteroatomscarbenes, such as in [[silyl]] and [[silyloxy]] carbenes, especially {{chem name|trifluorosilyl}} carbenes.<ref name="nemirowski">{{cite journal | title=Electronic Stabilization of Ground State Triplet Carbenes | author=Nemirowski, A. | author2=Schreiner, P. R. | journal=J. Org. Chem. |date=November 2007 | volume=72 | issue=25 | pages=9533–9540 | doi=10.1021/jo701615x | pmid=17994760}}</ref>
 
[[Lewis basic|Lewis-basic]] nitrogen, oxygen, sulphur, or halide [[Substituent|substituents]] bonded to the divalent carbon can [[Ylide|delocalize an electron pair into an empty ''p'' orbital]] to stabilize the singlet state. This phenomenon underlies [[Persistent carbene|persistent carbenes]]' remarkable stability.
 
==Reactivity==
Carbenes behave like very aggressive [[Lewis acids and bases|Lewis acids]]. They can attack [[Lone pair|lone pairs]], but their primary synthetic utility arises from attacks on [[Pi bond|π bonds]], which give cyclopropanes; and on [[Sigma bond|σ bonds]], which cause [[Carbene C−H insertion|carbene insertion]]. Other reactions include rearrangements and dimerizations. A particular carbene's reactivity depends on the [[substituent|substituents]], including any [[metal]]s present.
[[image:singletriplet.png|right|frame|Carbene addition to alkenes]]▼
Singlet and triplet carbenes exhibit divergent reactivity. Singlet carbenes generally participate in [[cheletropic reaction]]s as either [[electrophile]]s or [[nucleophile]]s. Singlet carbenes with unfilled p-orbital should be electrophilic. Triplet carbenes can be considered to be [[free radical|diradicals]], and participate in stepwise radical additions. Triplet carbenes have to go through an [[reactive intermediate|intermediate]] with two unpaired electrons whereas singlet carbene can react in a single [[concerted reaction|concerted]] step.
 
=== Singlet-triplet effects ===
Due to these two modes of reactivity, reactions of singlet methylene are [[stereospecific]] whereas those of triplet methylene are [[stereoselective]]. This difference can be used to probe the nature of a carbene. For example, the reaction of methylene generated from [[photolysis]] of [[diazomethane]] with ''cis''-[[2-butene]] or with ''trans''-[[2-butene]] each give a single diastereomer of the 1,2-dimethylcyclopropane product: ''cis'' from ''cis'' and ''trans'' from ''trans'', which proves that the methylene is a singlet.<ref>{{Cite journal| last1 = Skell | first1 = P. S.| title = Structure of Carbene, Ch2| last2 = Woodworth| journal = Journal of the American Chemical Society| volume = 78| issue = 17| pages = 4496| year = 1956 | doi = 10.1021/ja01598a087 | first2 = R. C.}}</ref> If the methylene were a triplet, one would not expect the product to depend upon the starting alkene geometry, but rather a nearly identical mixture in each case.
▲[[image:singletripletSinglettriplet.pngsvg|right|frame|Carbene addition to alkenes]]
Singlet and triplet carbenes exhibit divergent reactivity.<ref>{{March6th}}</ref>{{Page needed|date=January 2024}}<ref>Contrariwise, {{Harvnb|Grossman|2003|p=85}} states: "The reactivities of carbenes and carbenoids are the same no matter how they are generated." Grossman's analysis is not supported by modern physical organic chemistry texts, and likely refers to rapid equilibration between carbene states following most carbene generation methods.</ref>
 
Triplet carbenes are [[free radical|diradicals]], and participate in stepwise [[Single electron transfer|radical additions]]. Triplet carbene addition necessarily involves (at least one) [[reactive intermediate|intermediate]] with two unpaired electrons.
Reactivity of a particular carbene depends on the [[substituent]] groups. Their reactivity can be affected by [[metal]]s. Some of the reactions carbenes can do are [[Carbene C-H insertion|insertions into C-H bonds]], skeletal rearrangements, and additions to double bonds. Carbenes can be classified as nucleophilic, electrophilic, or ambiphilic. For example, if a substituent is able to donate a pair of electrons, most likely carbene will not be electrophilic. [[Alkyl]] carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.
 
Singlet carbenes can (and do) react as [[electrophile]]s, [[nucleophile]]s, or [[Ambiphile (chemistry)|ambiphiles]].{{Sfn|Grossman|2003|p=35}} Their reactions are typically [[concerted reaction|concerted]] and often [[cheletropic reaction|cheletropic]].{{Cn|date=January 2024}} Singlet carbenes are typically electrophilic,{{Sfn|Grossman|2003|p=35}} unless they have a filled ''p'' orbital, in which case they can react as Lewis bases. The [[Bamford–Stevens reaction]] gives carbenes in [[aprotic solvent]]s and [[Carbenium ion|carbenium ions]] in [[Protic solvent|protic ones]].
 
The different mechanisms imply that singlet carbene additions are [[stereospecific]] but triplet carbene additions [[stereoselective]]. Methylene from [[diazomethane]] [[photolysis]] reacts with either ''cis''- or ''trans''-[[2-butene]] to give a single [[diastereomer]] of [[1,2-Dimethylcyclopropane|1,2-dimethylcyclopropane]]: ''cis'' from ''cis'' and ''trans'' from ''trans''. Thus methylene is a singlet carbene; if it were triplet, the product would not depend on the starting alkene geometry.<ref>{{Cite journal |last1=Skell |first1=P. S. |last2=Woodworth |first2=R. C. |year=1956 |title=Structure of Carbene, Ch2 |journal=Journal of the American Chemical Society |volume=78 |issue=17 |pages=4496 |doi=10.1021/ja01598a087}}</ref>
 
===Cyclopropanation===
{{main|Cyclopropanation}}
[[image:cyclopropanation.pngsvg|rightthumb|frameright|Carbene cyclopropanation]]
Carbenes add to double bonds to form [[Cyclopropane#Cyclopropanes|cyclopropanes]],{{Sfn|Grossman|2003|pp=85-86}} and, in the presence of a copper [[Catalysis|catalyst]], to [[Alkyne|alkynes]] to give [[Cyclopropene#Syntheses of derivatives|cyclopropenes]]. Addition reactions are commonly very fast and [[exothermic]], and carbene generation limits reaction rate.
Carbenes add to double bonds to form [[Cyclopropane#Cyclopropanes|cyclopropanes]]. A concerted mechanism is available for singlet carbenes. Triplet carbenes do not retain [[stereochemistry]] in the product molecule. Addition reactions are commonly very fast and [[exothermic]]. The slow step in most instances is generation of carbene. A well-known reagent employed for alkene-to-cyclopropane reactions is [[Simmons-Smith reagent]]. This reagent is a system of [[copper]], [[zinc]], and [[iodine]], where the active reagent is believed to be [[iodomethylzinc iodide]]. Reagent is complexed by [[Hydroxyl|hydroxy]] groups such that addition commonly happens [[syn addition|syn]] to such group.
 
In [[Simmons–Smith reaction|Simmons-Smith cyclopropanation]], the [[iodomethylzinc iodide]] typically complexes to any [[Allyl alcohol|allylic hydroxy groups]] such that addition is [[syn addition|''syn'']] to the [[hydroxy group]].
 
===C—H insertion===
{{main|Carbene C−H insertion}}
[[image:Carbene_one-step-insertion.svg|thumb|Carbene insertion]]
[[Carbene C-H insertion|Insertions]] are another common type of carbene reactionsreaction,{{Sfn|Grossman|2003|pp=86-87}} a form of [[oxidative addition]]. TheInsertions carbenemay basicallyor interposesmay itselfnot intooccur anin existingsingle bondstep (see above). The orderend of preferenceresult is commonly:that 1.the X–Hcarbene bondsinterposes whereitself Xinto isan existing bond, preferably X–H (X not carbon), 2.else C–H bondor 3.(failing that) a C–C bond. Insertions may[[Alkyl]] orcarbenes mayinsert much more selectively than methylene, which does not occurdifferentiate inbetween singleprimary, stepsecondary, and tertiary C-H bonds.
 
:[[image:carbene intra.svg|left|frame|Carbene intramolecular reaction]]▼
[[Intramolecular]] insertion reactions present new synthetic solutions. Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no [[intermolecular]] insertions are seen. In flexible structures, five-membered ring formation is preferred to six-membered ring formation. Both inter- and intramolecular insertions are amendable to asymmetric induction by choosing chiral ligands on metal centers.
:[[image:Carbene_intermolecular_insertion.svg|left|frame|Carbene intermolecular reaction]]▼
The [[1,2-rearrangement]] produced from intramolecular insertion into a bond adjacent to the carbene center is a nuisance in some reaction schemes, as it consumes the carbene to yield the same effect as a traditional [[elimination reaction]].{{Sfn|Grossman|2003|p=87}} Generally, rigid structures favor [[Intramolecular reaction|intramolecular]] insertions. In flexible structures, five-membered ring formation is preferred to six-membered ring formation. When such insertions are possible, no [[intermolecular]] insertions are seen. Both inter- and intra-molecular insertions admit asymmetric induction from a chiral metal catalyst.
 
=== Electrophilic attack ===
▲:[[image:carbene intra.svg|left|frame|Carbene intramolecular reaction]]
Carbenes can form adducts with nucleophiles, and are a common precursor to various [[1,3-dipole|1,3-dipoles]].{{Sfn|Grossman|2003|p=87}}
{{clear}}
▲:[[image:Carbene_intermolecular_insertion.svg|left|frame|Carbene intermolecular reaction]]
{{clear}}
Alkylidene carbenes are alluring in that they offer formation of [[cyclopentene]] moieties. To generate an alkylidene carbene a ketone can be exposed to [[trimethylsilyl]] [[diazomethane]].
 
:[[image:alkylidene carbene.svg|left|frame|Alkylidene carbene]]
{{clear}}
 
===Carbene dimerization===
{{main|Carbene dimerization}}
[[image:Wanzlick equilibrium lemal Hahn 1999.svg|right|frame|Wanzlick equilibrium]]
Carbenes and [[carbenoid]] precursors can undergo [[dimer (chemistry)|dimerdimerize]]ization reactions to form [[alkene]]s. While thisThis is often, but not always, an unwanted side reaction,; itmetal cancarbene bedimerization employedhas asbeen aused syntheticin toolthe synthesis of polyalkynylethenes and ais directthe metalmajor carbeneindustrial dimerizationroute hasto beenTeflon used(see in{{Slink|2=Industrial theapplications}}). synthesis ofPersistent polyalkynylethenescarbenes equilibrate with their respective dimers, the [[Wanzlick equilibrium]].
 
==Carbene ligandsLigands in organometallic chemistry==▼
Persistent carbenes exist in equilibrium with their respective dimers. This is known as the [[Wanzlick equilibrium]].
In [[organometallic chemistry|organometallic]] species, metal complexes with the formulae L<sub>n</sub>MCRR' are often described as carbene complexes. <ref> For a concise tutorial on the applications of carbene ligands also beyond diaminocarbenes, see {{cite journal | last1 = Munz, | first1 = D. ''| year = 2018 | title = Pushing Electrons—Which Carbene Ligand for Which Application?| journal = [[Organometallics]]'' '''2018''',| volume = ''37'', 275-289| issue = 3 | pages = 275–289 | doi = 10.1021/acs.organomet.7b00720 }}</ref> Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes.{{Cn|date=January 2024|reason=See subsequent cite arguing otherwise}}<ref>Contrariwise, {{Harvnb|Grossman|2003|ps=: "Diazo compounds are converted to singlet carbenes upon gentle warming and to carbenoids by treatment with a Rh(II) or Cu(II) salt such as Rh<sub>2</sub>(OAc)<sub>4</sub> or CuCl<sub>2</sub>. The transition-metal-derived carbenoids, which have a metal&nbsp;–– C double bond, undergo the reactions typical of singlet carbenes. At this point you can think of them as free singlet carbenes, even though they’re not."}}</ref> The [[transition metal carbene complex]]es can be classified according to their reactivity, with the first two classes being the most clearly defined:▼
 
▲==Carbene ligands in organometallic chemistry==
▲In [[organometallic chemistry|organometallic]] species, metal complexes with the formulae L<sub>n</sub>MCRR' are often described as carbene complexes. <ref> For a concise tutorial on the applications of carbene ligands also beyond diaminocarbenes, see Munz, D. ''[[Organometallics]]'' '''2018''', ''37'', 275-289.</ref> Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes. The [[transition metal carbene complex]]es can be classified according to their reactivity, with the first two classes being the most clearly defined:
*[[Fischer carbene]]s, in which the carbene is bonded to a metal that bears an electron-withdrawing group (usually a carbonyl). In such cases the carbenoid carbon is mildly electrophilic.
*[[Schrock carbene]]s, in which the carbene is bonded to a metal that bears an electron-donating group. In such cases the carbenoid carbon is nucleophilic and resembles a Wittig reagent (which are not considered carbene derivatives).
*[[Carbene radical]]s, in which the carbene is bonded to an open-shell metal with the carbene carbon possessing a radical character. Carbene radicals have features of both Fischer and Schrock carbenes, but are typically long-lived reaction intermediates.
*[[Image:Grubbs_catalyst_Gen2.svg|thumb|right|220px|The "second generation" of the [[Grubbs catalyst]]s for [[alkene metathesis]] features an NHC ligand.]][[Persistent carbene|N-Heterocyclic]] (NHC), [[Anthony Joseph Arduengo III|Arduengo]] or [[Wanzlick equilibrium|Wanzlick]] carbenes<ref>For a general review with a focus on applications with diaminocarbenes, see: {{cite journal | last1 = Hopkinson | first1 = M. N. | last2 = Richter | first2 = C. | last3 = Schedler | first3 = M. | last4 = Glorius | first4 = F. | year = 2014 | title = An overview of N-heterocyclic carbenes| journal = [[Nature (journal)|Nature]] | volume = 510 | issue = 7506 | pages = 485–496 | doi = 10.1038/nature13384 | pmid = 24965649 | bibcode = 2014Natur.510..485H | s2cid = 672379 }}</ref> are C-deprotonated imidazolium or dihydroimidazolium salts. They often are deployed as [[Ligand|ancillary ligand]]s in [[organometallic chemistry]]. Such carbenes are usually very strong σ-donor [[spectator ligand]]s, similar to phosphines.<ref>S. P. Nolan "N-Heterocyclic Carbenes in Synthesis" 2006, Wiley-VCH, Weinheim. Print {{ISBN|9783527314003}}. Online {{ISBN|9783527609451}}. {{doi|10.1002/9783527609451}}</ref><ref>{{cite journal | last1 = Marion | first1 = N. | last2 = Diez-Gonzalez | first2 = S. | last3 = Nolan | first3 = S. P. | year = 2007 | title = N-heterocyclic carbenes as organocatalysts | journal = Angew. Chem. Int. Ed. | volume = 46 | issue = 17 | pages = 2988–3000 | doi = 10.1002/anie.200603380 | pmid = 17348057 }}</ref>
[[Image:Grubbs_catalyst_Gen2.svg|thumb|right|220px|The "second generation" of the [[Grubbs catalyst]]s for [[alkene metathesis]] features an NHC ligand.]]
*[[Persistent carbene|N-Heterocyclic carbenes]] (NHCs) <ref>For a general review with a focus on applications with diaminocarbenes, see: Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. ''[[Nature]]'' '''2014''', ''510'', 485-496.</ref> are derived by C-deprotonation imidazolium or dihydroimidazolium salts. They often are deployed as [[Ligand|ancillary ligand]]s in [[organometallic chemistry]]. Such carbenes are [[spectator ligand]]s that are usually very strong sigma donors, often drawing comparisons to phosphines.<ref>S. P. Nolan "N-Heterocyclic Carbenes in Synthesis" 2006, Wiley-VCH, Weinheim. Print {{ISBN|9783527314003}}. Online {{ISBN|9783527609451}}. {{doi|10.1002/9783527609451}}</ref><ref>Marion, N.; Diez-Gonzalez, S.; Nolan, S. P., "N-heterocyclic carbenes as organocatalysts", Angew. Chem. Int. Ed. 2007, volume 46, 2988-3000. {{doi|10.1002/anie.200603380}}</ref> The ligands themselves, especially when they are isolated free of the metal, are sometimes known as [[Anthony Joseph Arduengo III|Arduengo]] or [[Wanzlick equilibrium|Wanzlick]] carbenes.
 
▲==Generation of carbenes==
*A method that is broadly applicable to organic synthesis is induced elimination of [[halides]] from gem-dihalides employing [[organolithium reagent]]s. It remains uncertain if under these conditions free carbenes are formed or metal-carbene complex. Nevertheless, these metallocarbenes (or carbenoids) give the expected organic products.
 
▲:R<sub>2</sub>CBr<sub>2</sub> + BuLi → R<sub>2</sub>CLi(Br) + BuBr
▲:R<sub>2</sub>CLi(Br) → R<sub>2</sub>C + LiBr
 
*For cyclopropanations, zinc is employed in the [[Simmons–Smith reaction]]. In a specialized but instructive case, alpha-halomercury compounds can be isolated and separately thermolyzed. For example, the "Seyferth reagent" releases CCl<sub>2</sub> upon heating.
 
▲:C<sub>6</sub>H<sub>5</sub>HgCCl<sub>3</sub> → CCl<sub>2</sub> + C<sub>6</sub>H<sub>5</sub>HgCl
 
*Most commonly, carbenes are generated from [[diazoalkane]]s, via [[photolysis|photolytic]], thermal, or [[transition metal]]-catalyzed routes. Catalysts typically feature [[rhodium]] and [[copper]]. The [[Bamford-Stevens reaction]] gives carbenes in [[aprotic solvent]]s and carbenium ions in protic solvents.
* Base-induced elimination HX from haloforms (CHX<sub>3</sub>) under [[phase-transfer catalyst|phase-transfer conditions]].
* [[Photolysis]] of [[diazirine]]s and [[epoxide]]s can also be employed. Diazirines are cyclic forms of diazoalkanes. The strain of the small ring makes [[photoexcitation]] easy. Photolysis of epoxides gives [[carbonyl]] compounds as side products. With [[asymmetric synthesis|asymmetric]] epoxides, two different carbonyl compounds can potentially form. The nature of substituents usually favors formation of one over the other. One of the C-O bonds will have a greater double bond character and thus will be stronger and less likely to break. Resonance structures can be drawn to determine which part will contribute more to the formation of carbonyl. When one substituent is [[alkyl]] and another [[aryl]], the aryl-substituted carbon is usually released as a carbene fragment.
* Carbenes are intermediates in the [[Wolff rearrangement]]
 
==Industrial applications==
==Applications of carbenes==
A large -scale application of carbenes is the industrial production of [[tetrafluoroethylene]], the precursor to [[Teflon]]. Tetrafluoroethylene is generated via the intermediacy of [[difluorocarbene]]:<ref name="William">{{Cite book| last1 = Bajzer | first1 = W. X.| year = 2004| chapter = Fluorine Compounds, Organic| title = Kirk-Othmer Encyclopedia of Chemical Technology| publisher = John Wiley & Sons| doi = 10.1002/0471238961.0914201802011026.a01.pub2| isbn = 978-0471238966}}</ref>
: CHClF<sub>2</sub> → CF<sub>2</sub> + HCl
:2 CF<sub>2</sub> → F<sub>2</sub>C=CF<sub>2</sub>
 
The insertion of carbenes into C–H bonds has been exploited widely, e.g. the [[surface functionalization|functionalization]] of polymeric materials<ref>{{Cite journal|lastlast1=Yang|firstfirst1=Peng|last2=Yang|first2=Wantai|date=2013-07-10|title=Surface Chemoselective Phototransformation of C–H Bonds on Organic Polymeric Materials and Related High-Tech Applications|journal=Chemical Reviews|volume=113|issue=7|pages=5547–5594|doi=10.1021/cr300246p|pmid=23614481|issn=0009-2665}}</ref> and electro-curing of [[adhesive]]s.<ref name=":1">{{Cite journal|lastlast1=Ping|firstfirst1=Jianfeng|last2=Gao|first2=Feng|last3=Chen|first3=Jian Lin|last4=Webster|first4=Richard D.|last5=Steele|first5=Terry W. J.|date=2015-08-18|title=Adhesive curing through low-voltage activation|journal=Nature Communications|language=en|volume=6|pages=8050|doi=10.1038/ncomms9050|issn=2041-1723|pmc=4557340|pmid=26282730|bibcode=2015NatCo...6.8050P}}</ref> TheMany applications{{clarify|date=March 2017}}<!-- [[carbene dye]] says something different. Is this sentence only some applications, or only one method? --> rely on synthetic 3-aryl-3-trifluoromethyl[[diazirine]]s,<ref>{{Cite journal|lastlast1=Nakashima|firstfirst1=Hiroyuki|last2=Hashimoto|first2=Makoto|last3=Sadakane|first3=Yutaka|last4=Tomohiro|first4=Takenori|last5=Hatanaka|first5=Yasumaru|date=2006-11-01|title=Simple and Versatile Method for Tagging Phenyldiazirine Photophores|journal=Journal of the American Chemical Society|volume=128|issue=47|pages=15092–15093|doi=10.1021/ja066479y|pmid=17117852|issn=0002-7863}}</ref><ref name=":0">{{Cite journal|lastlast1=Blencowe|firstfirst1=Anton|last2=Hayes|first2=Wayne|date=2005-08-05|title=Development and application of diazirines in biological and synthetic macromolecular systems|journal=Soft Matter|language=en|volume=1|issue=3|pages=178178–205|doi=10.1039/b501989c|pmid=32646075|issn=1744-6848|bibcode=2005SMat....1..178B}}</ref> (a carbene precursor that can be activated by heat,<ref name=":2">{{Cite journal|last=Liu|first=Michael T. H.|date=1982-01-01|title=The thermolysis and photolysis of diazirines|journal=Chemical Society Reviews|language=en|volume=11|issue=2|pages=127|doi=10.1039/cs9821100127|issn=1460-4744}}</ref> light,<ref name=":0" /><ref name=":2" /> or [[voltage]].)<ref>{{Cite journal|lastlast1=Elson|firstfirst1=Clive M.|last2=Liu|first2=Michael T. H.|date=1982-01-01|title=Electrochemical behaviour of diazirines|url=http://xlink.rsc.org/?DOI=c39820000415|journal=Journal of the Chemical Society, Chemical Communications|language=en|issue=7|pages=415415–416|doi=10.1039/c39820000415|issn=0022-4936}}</ref><ref name=":1" /> but there is a whole family of [[Carbene dye|carbene dyes]].
 
==History==
Carbenes had first been postulated by [[Eduard Buchner]] in 1903 in [[cyclopropanation]] studies of [[ethyl diazoacetate]] with toluene.<ref>{{Cite journal| doi = 10.1002/cber.190303603139| title = Diazoessigester und Toluol| year = 1903| last1 = Buchner | first1 = E.| last2 = Feldmann | first2 = L.| journal = Berichte der Deutschen Chemischen Gesellschaft| volume = 36| issue = 3| pages = 3509 | url = https://zenodo.org/record/1426080}}</ref> In 1912 [[Hermann Staudinger]] <ref>{{Cite journal| doi = 10.1002/cber.19120450174| title = Über Reaktionen des Methylens. III. Diazomethan| year = 1912| last1 = Staudinger | first1 = H.| last2 = Kupfer | first2 = O.| journal = Berichte der Deutschen Chemischen Gesellschaft| volume = 45| pages = 501–509 | url = https://zenodo.org/record/1426477}}</ref> also converted alkenes to cyclopropanes with [[diazomethane]] and CH<sub>2</sub> as an intermediate. [[William von Eggers Doering|Doering]] in 1954 demonstrated their synthetic utility with [[dichlorocarbene]] synthetic utility.<ref>{{Cite journal| doi = 10.1021/ja01652a087| year = 1954| last1 = Von E. Doering | first1 = W.| title = The Addition of Dichlorocarbene to Olefins| last2 = Hoffmann | first2 = A. K.| journal = Journal of the American Chemical Society| volume = 76| issue = 23| pages = 6162 }}</ref>
 
==See also==
*[[Transition metal carbene complex]]es, also known as [[carbenoids]]
*[[Atomic carbon]] a single carbon atom with the chemical formula :C:, in effect a twofold carbene. Also has been used to make "true carbenes" in situ.
*[[Foiled carbene]]s derive their stability from proximity of a double bond (i.e. their ability to form conjugated systems).
*[[Carbene analogs]] and [[carbenoids]]
*[[Carbenium ion]]s, protonated carbenes
* [[Ring opening metathesis polymerization]]
 
==References==
{{reflistReflist|33em}}
 
==External links==
*{{CommonscatCommons category-inline|Carbenes}}
 
{{Functional Groups}}
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