Carbene

Structure

  Generally there are two types of carbenes; 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 with the exception of carbenes with nitrogen, oxygen, sulfur atoms, and dihalocarbenes.

Singlet and triplet carbenes are named so because of the electronic spins they possess. Triplet carbenes are electron spin resonance spectroscopy if they can exist long enough without undergoing further reactions. 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 methylene and 102° for singlet methylene (determined by EPR). Triplet carbenes are generally stable in gaseous state while singlet carbenes are often found in aqueous media.

For simple hydrocarbons, triplet carbenes usually have energies 8 kcal/Substituents that can donate electron pairs may stabilize the singlet state by delocalizing the pair into a 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 biradicals. In silico experiments suggest that triplet carbenes can be stabilized with electropositive groups such as trifluorosilyl groups ^[3].

Reactivity

  Singlet and triplet carbenes do not demonstrate the same reactivity. Singlet carbenes generally participate in 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 stereospecific which proves that in this reaction methylene is a singlet.^[4]

Reactivity of a particular carbene depends on the Alkyl carbenes 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 iodine, where the active reagent is believed to be iodomethylzinc iodide. Reagent is complexed by hydroxy groups such that addition commonly happens syn to 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.

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.

Alkylidene carbenes are alluring in that they offer formation of cyclopentene moieties. To generate an alkylidene carbene a ketone can be exposed to diazomethane.

Carbenes and carbene ligands in organometallic chemistry

Carbenes can be stabilized as 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.
• 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, Bamford-Stevens reaction, which gives carbenes in aprotic solvents and carbenium ions in protic solvents.
• Another method is induced elimination of 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.

 

• alkyl and another aryl, the aryl-substituted carbon is usually released as a carbene fragment.
• Thermolysis of alpha-halomercury compounds is another method to generate carbenes.
• copper complexes promote carbene formation.
• Carbenes are intermediates in the Wolff rearrangement

See also

carbenoids

References

1. ^ Organic Chemistry R.T Morrison, R.N Boyd pp 473-478
2. ^ 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.
3. ^ 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
4. ^ Structure of Carbene CH2 Philip S. Skell, Robert C. Woodworth; J. Am. Chem. Soc.; 1956; 78(17); 4496-4497. Abstract