In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared . The general formula is or where the R represents or hydrogen atoms.
The term "carbene" may also refer to the specific compound , also called methylene, the parent hydride from which all other carbene compounds are formally derived.
There are two types of carbenes: singlet state or triplet state, depending upon their electronic structure.
Most carbenes are extremely reactive and short-lived. A small number (the diHalogencarbenes, carbon monoxide, 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,[For detailed reviews on stable carbenes, see: (a) (b) ] which have extensive application in modern organometallic chemistry.
Generation
There are two common methods for carbene generation: α-elimination and small-molecule extrusion.
Α-Elimination
In α elimination, two substituents eliminate from the same carbon atom. Α-elimination typically occurs when strong bases act on acidic protons with no good vicinal
Leaving group. For example,
phenyllithium will abstract
Hydrohalic acid from a
haloform (CHX
3). Such reactions often require phase-transfer conditions.
Molecules with no acidic proton can still be induced to α-eliminate. A geminal dihalide exposed to organolithiums can undergo metal-halogen exchange and then eliminate a lithium salt:
- R2CBr2 + BuLi → R2CLi(Br) + BuBr
- R2CLi(Br) → R2C + LiBr
Zinc abstracts halogens similarly in the Simmons–Smith reaction.
Mercuric and organomercury halides (except fluorides) can stably store a wide variety carbenes as the α-halomercury adduct until a mild thermolysis. For example, the "Seyferth reagent" releases CCl2 upon heating:
- C6H5HgCCl3 → CCl2 + C6H5HgCl
It remains uncertain which (if any) of such metallated reagents form truly free carbenes, instead of a reactive metal-carbene complex. Nevertheless, reactions with such metallocarbenes generally give the same organic products as with other carbene sources.
Small-molecule extrusion
Separately, carbenes can be produced from an extrusion reaction with a large free energy change.
and
photolyze with a tremendous release in
ring strain to carbenes, the former to inert
nitrogen gas. Epoxides typically give reactive
carbonyl wastes, and 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 to a carbene and gaseous nitrogen; such are the Bamford–Stevens reaction and Wolff rearrangement. As with metallocarbenes, some reactions of diazoalkanes that formally proceed via carbenes may instead form a [1,3-Dipolar cycloadduct]] intermediate that extrudes nitrogen.
To generate an alkylidene carbene a ketone can be exposed to trimethylsilyldiazomethane and then a strong base:
Structures and bonding
The two classes of carbenes are
Diradical and
diradical carbenes. Triplet carbenes are
Diradical with two unpaired electrons, typically form from reactions that break two
Sigma bond (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. Bond angles (as determined by EPR) are 125–140° for triplet methylene and 102° for singlet methylene.
Most carbenes have a Bent geometry triplet ground state. For simple hydrocarbons, triplet carbenes are usually only 8 kilocalorie/mol (33 kilojoule/mol) more stable than singlet carbenes, comparable to nitrogen inversion. The stabilization is in part attributed to Hund's rule of maximum multiplicity. However, strategies to stabilize triplet carbenes at room temperature are elusive. 9-Fluorenylidene has been shown to be a rapidly 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 diaryl carbenes.
Lewis basic nitrogen, oxygen, sulphur, or halide Substituent bonded to the divalent carbon can Ylide to stabilize the singlet state. This phenomenon underlies persistent carbenes' remarkable stability.
Reactivity
Carbenes behave like very aggressive Lewis acids. They can attack
Lone pair, but their primary synthetic utility arises from attacks on
Pi bond, which give cyclopropanes; and on
Sigma bond, which cause carbene insertion. Other reactions include rearrangements and dimerizations. A particular carbene's reactivity depends on the
substituent, including any
present.
Singlet-triplet effects
Singlet and triplet carbenes exhibit divergent reactivity.
[Contrariwise, 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.]
Triplet carbenes are free radical, and participate in stepwise radical additions. Triplet carbene addition necessarily involves (at least one) intermediate with two unpaired electrons.
Singlet carbenes can (and do) react as , , or ambiphiles. Their reactions are typically concerted and often cheletropic. Singlet carbenes are typically electrophilic, unless they have a filled p orbital, in which case they can react as Lewis bases. The Bamford–Stevens reaction gives carbenes in and Carbenium ion in Protic solvent.
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: 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.
Cyclopropanation
Carbenes add to double bonds to form cyclopropanes, and, in the presence of a copper
Catalysis, to
Alkyne to give cyclopropenes. Addition reactions are commonly very fast and
exothermic, and carbene generation limits reaction rate.
In Simmons-Smith cyclopropanation, the iodomethylzinc iodide typically complexes to any Allyl alcohol such that addition is syn addition to the hydroxy group.
C—H insertion
Insertions are another common type of carbene reaction, a form of oxidative addition. Insertions may or may not occur in single step (see above). The end result is that the carbene interposes itself into an existing bond, preferably X–H (X not carbon), else C–H or (failing that) a C–C bond.
Alkyl carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.
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. Generally, rigid structures favor 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
Carbenes can form adducts with nucleophiles, and are a common precursor to various 1,3-dipoles.
Carbene dimerization
Carbenes and
carbenoid precursors can dimerize to
. 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 ). Persistent carbenes equilibrate with their respective dimers, the Wanzlick equilibrium.
Ligands in organometallic chemistry
In organometallic species, metal complexes with the formulae L
nMCRR' are often described as carbene complexes.
[For a concise tutorial on the applications of carbene ligands also beyond diaminocarbenes, see ] Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes.
[Contrariwise, ] The transition metal carbene complexes can be classified according to their reactivity, with the first two classes being the most clearly defined:
-
, 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.
-
, 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).
-
, 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.
-
for alkene metathesis features an NHC ligand.]]N-Heterocyclic (NHC), Arduengo or Wanzlick carbenes
[For a general review with a focus on applications with diaminocarbenes, see: ] are C-deprotonated imidazolium or dihydroimidazolium salts. They often are deployed as Ligand in organometallic chemistry. Such carbenes are usually very strong σ-donor , similar to phosphines.[S. P. Nolan "N-Heterocyclic Carbenes in Synthesis" 2006, Wiley-VCH, Weinheim. Print . Online . ]
Industrial applications
A large-scale application of carbenes is the industrial production of tetrafluoroethylene, the precursor to
Teflon. Tetrafluoroethylene is generated via the intermediacy of
difluorocarbene:
- 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
History
Carbenes had first been postulated by
Eduard Buchner in 1903 in
cyclopropanation studies of ethyl diazoacetate with toluene.
In 1912 Hermann Staudinger
also converted alkenes to cyclopropanes with
diazomethane and CH
2 as an intermediate. Doering in 1954 demonstrated their synthetic utility with
dichlorocarbene.
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.
-
derive their stability from proximity of a double bond (i.e. their ability to form conjugated systems).
-
Carbene analogs and carbenoids
-
, protonated carbenes
-
Ring opening metathesis polymerization
External links
