Inorganic chemistry, acarbene is amolecule containing a neutralcarbon atom with avalence of two and two unsharedvalence electrons. The general formula isR−:C−R' orR=C: where the R representssubstituents or hydrogen atoms.
The term "carbene" may also refer to the specific compound:CH2, also calledmethylene, the parenthydride from which all other carbene compounds are formally derived.[1][2]
There are two types of carbenes:singlets ortriplets, 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] andcarbon monosulfide) can be isolated, and can stabilize asmetal ligands, but otherwise cannot be stored in bulk. A rare exception are thepersistent carbenes,[5] which have extensive application in modernorganometallic chemistry.
There are two common methods for carbene generation: α-elimination and small-molecule extrusion.
Inα elimination, two substituents eliminate from the same carbon atom. Α-elimination typically occurs when strong bases act on acidic protons with no good vicinalleaving groups. For example,phenyllithium will abstractHX from ahaloform (CHX3).[6] Such reactions often requirephase-transfer conditions.[citation needed]
Molecules with no acidic proton can still be induced to α-eliminate. Ageminal dihalide exposed toorganolithiums can undergometal-halogen exchange and then eliminate alithium salt:
Zinc metal abstracts halogens similarly in theSimmons–Smith reaction.[7]
Mercuric and organomercury halides (except fluorides) can stably store a wide variety carbenes as the α-halomercury adduct until a mild thermolysis.[citation needed] For example, the "Seyferth reagent" releases CCl2 upon heating:
It remains uncertain which (if any) of such metallated reagents form truly free carbenes, instead of areactive metal-carbene complex. Nevertheless, reactions with such metallocarbenes generally give the same organic products as with other carbene sources.[7]
Separately, carbenes can be produced from an extrusion reaction with a large free energy change.Diazirines andepoxides photolyze with a tremendous release inring strain to carbenes, the former to inertnitrogen gas. Epoxides typically give reactivecarbonyl 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 isalkyl and anotheraryl, the aryl-substituted carbon is usually released as a carbene fragment.
Ring strain is not necessary for a strong thermodynamic driving force.Photolysis,heat, ortransition metal catalysts (typicallyrhodium andcopper) decomposediazoalkanes to a carbene and gaseousnitrogen; such are theBamford–Stevens reaction andWolff rearrangement. As with metallocarbenes, some reactions of diazoalkanes that formally proceed via carbenes may instead form a[3+2] cycloadduct intermediate that extrudes nitrogen.
To generate analkylidene carbene a ketone can be exposed totrimethylsilyldiazomethane and then a strong base:
The two classes of carbenes aresinglet andtriplet carbenes. Triplet carbenes arediradicals with two unpaired electrons, typically form from reactions that break twoσ bonds (α elimination and some extrusion reactions), and do notrehybridize the carbene atom. Singlet carbenes have a singlelone pair, typically form from diazo decompositions, and adopt ansp2 orbital structure.[8] Bond angles (as determined byEPR) are 125–140° for triplet methylene and 102° for singlet methylene.
Most carbenes have anonlinear triplet ground state. For simple hydrocarbons, triplet carbenes are usually only 8kcal/mol (33kJ/mol) more stable than singlet carbenes, comparable tonitrogen inversion. The stabilization is in part attributed toHund's rule of maximum multiplicity. Only few persistent triplet carbenes are known,[9] either stabilized by a metal substituent such as lead, palladium or platinum (metalla-carbenes),[10][11] or two aryl groups for efficient resonance stabilization.[12]9-Fluorenylidene has been shown to be a rapidlyequilibrating mixture of singlet and triplet states with an approximately 1.1 kcal/mol (4.6 kJ/mol) energy difference, although extensiveelectron delocalization into the rings complicates any conclusions drawn from diaryl carbenes.[13] Simulations suggest thatelectropositive heteroatoms canthermodynamically stabilize triplet carbenes, such as insilyl andsilyloxy carbenes, especially trifluorosilyl carbenes.[14]
Lewis-basic nitrogen, oxygen, sulphur, or halidesubstituents bonded to the divalent carbon candelocalize an electron pair into an emptyp orbital to stabilize the singlet state. This phenomenon underliespersistent carbenes' remarkable stability.
At a very high level of generality, carbenes behave like aggressiveLewis acids. They can attacklone pairs, but their primary synthetic utility arises from attacks onπ bonds, which give cyclopropanes; and onσ bonds, which causecarbene insertion. Other reactions include rearrangements and dimerizations. A particular carbene's reactivity depends on thesubstituents, including anymetals present.
Singlet and triplet carbenes exhibit divergent reactivity.[15][page needed][16]
Triplet carbenes arediradicals, and participate in stepwiseradical additions. Triplet carbene addition necessarily involves (at least one)intermediate with two unpaired electrons.
Singlet carbenes can (and do) react aselectrophiles,nucleophiles, orambiphiles.[4] Their reactions are typicallyconcerted and oftencheletropic.[citation needed] Singlet carbenes are typically electrophilic,[4] unless they have a filledp orbital, in which case they can react as Lewis bases. TheBamford–Stevens reaction gives carbenes inaprotic solvents andcarbenium ions inprotic ones.
The different mechanisms imply that singlet carbene additions arestereospecific but triplet carbene additionsstereoselective. Methylene fromdiazomethanephotolysis reacts with eithercis- ortrans-2-butene to give a singlediastereomer of1,2-dimethylcyclopropane:cis fromcis andtrans fromtrans. Thus methylene is a singlet carbene; if it were triplet, the product would not depend on the starting alkene geometry.[17]
Carbenes add to double bonds to formcyclopropanes,[18] and, in the presence of a coppercatalyst, toalkynes to givecyclopropenes. Addition reactions are commonly very fast andexothermic, and carbene generation limits reaction rate.
InSimmons-Smith cyclopropanation, theiodomethylzinc iodide typically complexes to anyallylic hydroxy groups such that addition issyn to thehydroxy group.
Insertions are another common type of carbene reaction,[19] a form ofoxidative 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.
The1,2-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 traditionalelimination reaction.[20] Generally, rigid structures favorintramolecular insertions. In flexible structures, five-membered ring formation is preferred to six-membered ring formation. When such insertions are possible, nointermolecular insertions are seen. Both inter- and intra-molecular insertions admit asymmetric induction from a chiral metal catalyst.
Carbenes can form adducts with nucleophiles, and are a common precursor to various1,3-dipoles.[20]
Carbenes andcarbenoid precursors candimerize toalkenes. 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 (seeCarbene § Industrial applications). Persistent carbenes equilibrate with their respective dimers, theWanzlick equilibrium.
Inorganometallic species, metal complexes with the formulae LnMCRR' are often described as carbene complexes.[21] Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes.[citation needed][22] Thetransition metal carbene complexes can be classified according to their reactivity, with the first two classes being the most clearly defined:
A large-scale application of carbenes is the industrial production oftetrafluoroethylene, the precursor toTeflon. Tetrafluoroethylene is generated via the intermediacy ofdifluorocarbene:[26]
The insertion of carbenes into C–H bonds has been exploited widely, e.g. thefunctionalization of polymeric materials[27] and electro-curing ofadhesives.[28] Many applications rely on synthetic 3-aryl-3-trifluoromethyldiazirines[29][30] (a carbene precursor that can be activated by heat,[31] light,[30][31] orvoltage)[32][28] but there is a whole family ofcarbene dyes.
Carbenes had first been postulated byEduard Buchner in 1903 incyclopropanation studies ofethyl diazoacetate with toluene.[33] In 1912Hermann Staudinger[34] also converted alkenes to cyclopropanes withdiazomethane and CH2 as an intermediate.Doering in 1954 demonstrated their synthetic utility withdichlorocarbene.[35]
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