chemistry, a carbene is a highly reactive organic moleculecontaining a carbonatom with six valence electrons and having the general formula: R1R2C: (two substituents and two electrons). [Organic Chemistry R.T Morrison, R.N Boyd pp 473-478] There are two kinds of carbenes: singlets and triplets. The carbon atoms of singlets are sp2-hybridised, with one empty p-orbital crossing the plane containing R1, R2, and the free electron pair. Such molecules are generally very short lived, although persistent carbenes are known.
The prototypical carbene is H2C: also called
methylene. One well studied carbene is Cl2C:, or dichlorocarbene, which can be generated in situfrom chloroformand a strong base.
Generally there are two types of carbenes; singlet or triplet carbenes. Singlet carbenes have a pair of electrons and an sp2 hybrid structure. Triplet carbenes have two unpaired electrons. They may be either sp2 hybrid or linear sp hybrid. Most carbenes have a nonlinear triplet ground state, except for those with nitrogen, oxygen, or sulfur atoms, and dihalocarbenes.
Carbenes are called singlet or triplet depending on the electronic spins they possess. Triplet carbenes are
paramagneticand may be observed by electron spin resonance spectroscopyif they persist long enough. The total spin of singlet carbenes is zero while that of triplet carbenes is one (in units of ). Bond angles are 125-140° for triplet methyleneand 102° for singlet methylene (as determined by EPR). Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media.
For simple hydrocarbons, triplet carbenes usually have energies 8 kcal/mol (33 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 statespecies. Substituents that can donate electron pairs 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. No viable strategies exist for triplet stabilization. The carbene called 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. ["Chemical and Physical Properties of Fluorenylidene: Equilibrium of the Singlet and Triplet Carbenes" Peter B. Grasse, Beth-Ellen Brauer, Joseph J. Zupancic, Kenneth J. Kaufmann, Gary B. Schuster; J. Am. Chem. Soc.; 1983; 105; 6833-6845.] . It is however debatable whether diaryl carbenes such as the fluorenecarbene are true carbenes because the electrons can delocalize to such an extent that they become in fact biradicals. In silicoexperiments suggest that triplet carbenes can be stabilized with electropositivegroups such as trifluorosilyl groups "Electronic Stabilization of Ground State Triplet Carbenes" Adelina Nemirowski and Peter R. Schreiner J. Org. Chem.2007, 72, 9533-9540 9533 DOI|10.1021/jo701615x] .
Singlet and triplet carbenes do not demonstrate the same reactivity. Singlet carbenes generally participate in cheletropic reactions as either
electrophiles or nucleophiles. Singlet carbene with its unfilled p-orbital should be electrophilic. Triplet carbenes should be considered to be diradicals, and participate in stepwise radical additions. Triplet carbenes have to go through an intermediate with two unpaired electrons whereas singlet carbene can react in a single concerted step. Addition of singlet carbenes to olefinic double bonds is more stereoselective than that of triplet carbenes. Addition reactions with alkenes can be used to determine whether the singlet or triplet carbene is involved.
Reactions of singlet methylene are stereospecific while those of triplet methylene are not. For instance the reaction of methylene generated from
photolysisof diazomethanewith cis-2-butene and trans-2-butene is stereospecificwhich proves that in this reaction methylene is a singlet. ["Structure of Carbene CH2" Philip S. Skell, Robert C. Woodworth; J. Am. Chem. Soc.; 1956; 78(17); 4496-4497. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1956/78/i17/f_ja01598a087.pdf Abstract] ]
Reactivity of a particular carbene depends on the
substituentgroups, preparation method, reaction conditions such as presence or absence of metals. Some of the reactions carbenes can do are insertions into C-H bonds, skeletal rearrangements, and additions to double bonds. Carbenes can be classified as nucleophilic, electrophilic, or ambiphilic. Reactivity is especially strongly influenced by substituents. For example, if a substituent is able to donate a pair of electrons, most likely carbene will not be electrophilic. Alkylcarbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.Carbenes add to double bonds to form cyclopropanes. A concerted mechanism is available for singlet carbenes. Triplet carbenes do not retain stereochemistryin 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 reagents is a system of copper, zinc, and iodine, where the active reagent is believed to be iodomethylzinc iodide. Reagent is complexed by hydroxygroups such that addition commonly happens synto such group.
Insertions are another common type of carbene reactions. The carbene basically interposes itself into an existing bond. The order of preference is commonly: 1. X-H bonds where X is not carbon 2. C-H bond 3. C-C bond. Insertions may or may not occur in single step.
Intramolecularinsertion reactions present new synthetic solutions. Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no intermolecularinsertions 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.
Alkylidene carbenes are alluring in that they offer formation of
cyclopentenemoieties. To generate an alkylidene carbene a ketone can be exposed to trimethylsilyl diazomethane.
Carbenes and carbene ligands in organometallic chemistry
Carbenes can be stabilized as organometallic species. These
transition metal carbene complexes fall into three categories, with the first two being the most clearly defined:
Fischer carbenes, in which the carbene is tethered to a metal that bears an electron-withdrawing group (usually a carbonyl).
Schrock carbenes, in which the carbene is tethered to a metal that bears an electron-donating group.
Persistent carbenes, also known as stable carbenes or Arduengo carbenes. These include the class of "N"-heterocyclic carbenes (NHCs) and are often are used as ancillary ligands in organometallic chemistry.
Foiled carbenes derive their stability from proximity of a double bond (i.e their ability to form conjugated systems).
Generation of Carbenes
*Most commonly, photolytic, thermal, or
transition metalcatalyzed decomposition of diazoalkanes is used to create carbene molecules. A variation on catalyzed decomposition of diazoalkanes is the Bamford-Stevens reaction, which gives carbenes in aprotic solvents and carbenium ions in protic solvents.
*Another method is induced elimination of
halogenfrom gem-dihalides or HX from CHX3 moiety, employing organolithium reagents (or another strong base). It is not certain that in these reactions actual free carbenes are formed. In some cases there is evidence that completely free carbene is never present. It is likely that instead a metal-carbene complex forms. Nevertheless, these metallocarbenes (or carbenoids) give the expected products.
Photolysisof diazarines and epoxides can also be employed. Diazarines contain 3-membered rings and are cyclic forms or diazoalkanes. The strain of the small ring makes photoexcitationeasy. Photolysis of epoxides gives carbonylcompounds as side products. With 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 alkyland another aryl, the aryl-substituted carbon is usually released as a carbene fragment.
Thermolysisof alpha-halomercury compounds is another method to generate carbenes.
Rhodiumand coppercomplexes promote carbene formation.
* Carbenes are intermediates in the
Transition metal carbene complexalso known as carbenoids
Atomic carbona single carbon atom with chemical formula :C:, in effect a dicarbene. Also has been used to make "true carbenes" in situ.
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