Carbene

Source: Wikipedia, the free encyclopedia.
This article is about the chemical class. For the compound, see Methylene (compound).
Not to be confused with carbine or carbyne.
Methylene is the simplest carbene.

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 term "carbene" may also refer to the specific compound :CH2, also called

methylene, the parent hydride from which all other carbene compounds are formally derived.[1][2]

There are two types of carbenes: singlets or triplets, depending upon their electronic structure.[3] The different classes undergo different reactions.

Most carbenes are extremely reactive and short-lived. A small number (the dihalocarbenes, carbon monoxide,[4] and carbon monosulfide) can be isolated, and can stabilize as metal ligands, but otherwise cannot be stored in bulk. A rare exception are the persistent carbenes,[5] which have extensive application in modern organometallic chemistry.

Generation

There are two common methods for carbene generation.

In

haloform (CHX3).[6] Such reactions typically require phase-transfer conditions.[citation needed
]

Molecules with no acidic proton can also form carbenes. A

lithium salt to give a carbene, and zinc metal abstracts halogens similarly in the Simmons–Smith reaction.[7]

R2CBr2 + BuLi → R2CLi(Br) + BuBr
R2CLi(Br) → R2C + 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.[7] In a specialized but instructive case, α-halomercury compounds can be isolated and separately thermolyzed. The "Seyferth reagent" releases CCl2 upon heating:

C6H5HgCCl3 → CCl2 + C6H5HgCl

Separately, carbenes can be produced from an extrusion reaction with a large free energy change.

aryl
, the aryl-substituted carbon is usually released as a carbene fragment.

Ring strain is not necessary for a strong thermodynamic driving force.

trimethylsilyl diazomethane
and then a strong base.

Structures and bonding

Singlet and triplet carbenes

The two classes of carbenes are singlet and triplet carbenes. Triplet carbenes are diradicals with two unpaired electrons, typically form from reactions that break two σ bonds (α elimination and some extrusion reactions), and do not rehybridize the carbene atom. Singlet carbenes have a single lone pair, typically form from diazo decompositions, and adopt an sp2 orbital structure.[8] Bond angles (as determined by EPR) are 125–140° for triplet methylene and 102° for singlet methylene.

Most carbenes have a

silyloxy carbenes, especially trifluorosilyl carbenes.[10]

Lewis-basic nitrogen, oxygen, sulphur, or halide substituents bonded to the divalent carbon can delocalize an electron pair into an empty p orbital to stabilize the singlet state. This phenomenon underlies persistent carbenes
' remarkable stability.

Reactivity

Carbenes behave like very aggressive Lewis acids. They can attack lone pairs, but their primary synthetic utility arises from attacks on π bonds, which give cyclopropanes; and on σ bonds, which cause carbene insertion. Other reactions include rearrangements and dimerizations. A particular carbene's reactivity depends on the substituents, including any metals present.

Singlet-triplet effects

Carbene addition to alkenes

Singlet and triplet carbenes exhibit divergent reactivity.[11][page needed][12]

Triplet carbenes are

radical additions. Triplet carbene addition necessarily involves (at least one) intermediate
with two unpaired electrons.

Singlet carbenes can (and do) react as

aprotic solvents and carbenium ions in protic ones
.

The different mechanisms imply that singlet carbene additions are

2-butene to give a single diastereomer of 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.[13]

Cyclopropanation

Main article: Cyclopropanation
Carbene cyclopropanation

Carbenes add to double bonds to form

exothermic
, and carbene generation limits reaction rate.

In

syn to the hydroxy group
.

C—H insertion

Main article: Carbene C−H insertion
Carbene insertion

Alkyl
carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.

Carbene intramolecular reaction
Carbene intermolecular reaction

The

intermolecular
insertions are seen. Both inter- and intra-molecular insertions admit asymmetric induction from a chiral metal catalyst.

Electrophilic attack

Carbenes can form adducts with nucleophiles, and are a common precursor to various 1,3-dipoles.[16]

Carbene dimerization

Main article: Carbene dimerization
Wanzlick equilibrium

Carbenes and

dimerize to alkenes. This is often, but not always, an unwanted side reaction; metal carbene dimerization has been used in the synthesis of polyalkynylethenes and is the major industrial route to Teflon (see Carbene § Industrial applications). Persistent carbenes equilibrate with their respective dimers, the Wanzlick equilibrium
.

Ligands in organometallic chemistry

In organometallic species, metal complexes with the formulae LnMCRR' are often described as carbene complexes.[17] Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes.[citation needed][18] The transition metal carbene complexes can be classified according to their reactivity, with the first two classes being the most clearly defined:

  • Fischer carbenes, 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 carbenes
    , 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 radicals, 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.
  • Arduengo or Wanzlick carbenes[19] are C-deprotonated imidazolium or dihydroimidazolium salts. They often are deployed as ancillary ligands in organometallic chemistry. Such carbenes are usually very strong σ-donor spectator ligands, similar to phosphines.[20][21]

Industrial applications

A large-scale application of carbenes is the industrial production of

Teflon. Tetrafluoroethylene is generated via the intermediacy of difluorocarbene:[22]

CHClF2 → CF2 + HCl
2 CF2 → F2C=CF2

The insertion of carbenes into C–H bonds has been exploited widely, e.g. the

functionalization of polymeric materials[23] and electro-curing of adhesives.[24] Many applications rely on synthetic 3-aryl-3-trifluoromethyldiazirines[25][26] (a carbene precursor that can be activated by heat,[27] light,[26][27] or voltage)[28][24] but there is a whole family of carbene dyes
.

History

Carbenes had first been postulated by Eduard Buchner in 1903 in cyclopropanation studies of ethyl diazoacetate with toluene.[29] In 1912 Hermann Staudinger[30] also converted alkenes to cyclopropanes with diazomethane and CH2 as an intermediate. Doering in 1954 demonstrated their synthetic utility with dichlorocarbene.[31]

See also

  • Transition metal carbene complexes
  • 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 carbenes derive their stability from proximity of a double bond (i.e. their ability to form conjugated systems).
  • carbenoids
  • Carbenium ions, protonated carbenes
  • Ring opening metathesis polymerization

References

  1. ISBN 978-0-19-853093-0
    .
  2. doi:10.1351/goldbook.C00806
  3. ISBN 0-387-95468-6
    .
  4. ^ a b c Grossman 2003, p. 35.
  5. PMID 20836099
    .
  6. ^ Grossman 2003, pp. 84–85.
  7. ^ a b Grossman 2003, p. 85.
  8. ^ Grossman 2003, p. 84.
  9. doi:10.1021/ja00361a014
    .
  10. PMID 17994760
    .
  11. ISBN 978-0-471-72091-1
  12. ^ Contrariwise, 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.
  13. doi:10.1021/ja01598a087
    .
  14. ^ Grossman 2003, pp. 85–86.
  15. ^ Grossman 2003, pp. 86–87.
  16. ^ a b Grossman 2003, p. 87.
  17. doi:10.1021/acs.organomet.7b00720
    .
  18. ^ Contrariwise, Grossman 2003: "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 Rh2(OAc)4 or CuCl2. The transition-metal-derived carbenoids, which have a metal –– 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."
  19. S2CID 672379
    .
  20. doi:10.1002/9783527609451
  21. PMID 17348057
    .
  22. ISBN 978-0471238966
    .
  23. PMID 23614481
    .
  24. ^
    PMID 26282730
    .
  25. PMID 17117852
    .
  26. ^
    PMID 32646075
    .
  27. ^
    ISSN 1460-4744
    .
  28. ISSN 0022-4936
    .
  29. doi:10.1002/cber.190303603139
    .
  30. doi:10.1002/cber.19120450174
    .
  31. doi:10.1021/ja01652a087
    .

External links

  • Media related to Carbenes at Wikimedia Commons
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