1.1.7.1 Carbenes and Their Classification on the Basis of the Spin States of Electrons
Carbenes are electrically neutral reaction intermediates containing a bivalent carbon atom in which the C atom is covalently bonded to two other atoms or groups and has two other valence electrons distributed between the two nonbonding orbitals, i.e., carbenes are neutral species containing a carbon with only six valence electrons. The general formula of carbenes is :CR2. The simplest member :CH2, often called methylene or simply carbene, is the parent. Carbenes are named by mentioning the substitutes attached to the divalent carbon, e.g., :CBr2, dibromocarbene; :CHPh, phenylcarbene, etc. However, when the divalent carbon of a carbene is a part of a ring or an unsaturation, suffix -ylidine is used.
For example:
Depending upon the spin states of the nonbonding electrons, carbenes are divided into two classes. If the two nonbonding electrons remain spin paired, it is called a singlet carbene, while the carbene having unpaired spins of electrons is referred to as a triplet carbene.
1.1.7.2 Structure and Stability of Carbenes
The C atom in a singlet carbene is sp2-hybridized (trigonal hybridization). Two of these three sp2 hybrid orbitals are utlized in forming covalent bonds, while the third hybrid orbital contains the unshared pair of electrons. There remains a vacant p orbital. A singlet carbene, therefore, resembles a carbocation and it appears to have a bent shape (bond angle 100–110∞). Although a few triplet carbenes are linear (sp-hybridized), most of them are bent species (bond angle 130–150∞) where the carbon is sp2-hybridized, and the two nonbonding electrons occupy an sp2 orbital and an unhybridized p orbital with parallel spin.
Carbenes are generally more stable (about 40 kJ mol–1) as triplets because the energy to be gained by bringing the electron in the p orbital down into the sp2 orbital is insufficient to overcome the repulsion that exists between two electrons in a single orbital. The triplet state of carbene is, therefore, expected to be the ground state.
It is to be noted that the dihalocarbenes are singlet in the ground state, i.e., they are more stable than the corresponding triplets because they are considerably stabilized by resonance involving unshared p electrons of the halogen atom with the vacant p orbital on carbon. Because of similar size of the overlapping orbital of fluorine and carbon (very effective 2p–2p overlap), the resonance interaction is much more effective in :CF2 and so, singlet variety of :CF2 is much more stable than its triplet variety.
1.1.7.3 Equilibrium Between the Two Spin States and Reactivity
All carbenes have the potential to exist in either the singlet state or the triplet sate (the two species remain in equilibrium), so what we mean when we say that :CH2 is a triplet carbene, the triplet state for this carbene is more stable (lower in energy) than the singlet state.
Even though there is more triplet carbene present at equilibrium, its reactions cannot compete with those of the small amount of the much more reactive singlet carbene and this is because the activation barriers surrounding the less stable singlet carbene is substantially lower than those surrounding the more stable triplet carbene. It is for this reason, most of the chemistry of carbene comes from the singlet, i.e., reactions of the singlet are the ones that are normally observed.
1.1.7.4 Generation of Carbenes
Carbenes can be generated from diazoalkanes, ketenes, ylides, epoxides and cyclopropane derivatives by thermal or photochemical decomposition and by a-elimination involving formation of carbanions.
(a) From diazoalkanes: The photolysis or thermolysis of diazoalkanes in aprotic solvents provides the most common root to carbenes. For example:
Methylene (:CH2) is formed when diazomethane is subjected to photolysis or thermolysis.
2 2— 2 2
or h :
aprotic solvent
[CH N N: C H N N] CH N
Diazomethane
≈ ≈ D n
== == @ ´@ ∫∫ ổổổổổổổặ +
(b) From ketenes: Ketenes on pyrolysis or photolysis loose carbon monoxide to form carbenes. For example:
D n l== ã
== == 2 == == ổổổổổổổổổor h ( 280nm)ặ +
2 2 2 :
CH C O or Ph C C O :CH or Ph C CO
Diphenylketene
Ketene Methylene Diphenylcarbene
(c) From ylides: Carbenes can be generated by photolysis or thermolysis of sulphur, phosphrus and nitrogen ylides. For example:
— n
≈ ổổổặ +
2 2
h :
Me S CHCOPh CHCOPh Me S
Benzoylcarbene A sulphur ylide
@
—
≈ ổổặD +
3 : 3
Ph P CH Ph CHPh Ph P
Phenylcarbene A phosphorus ylide
@
—
≈ D
ổổặ +
3 2 : 2 3
Me N CH CH Me N
Methylene A nitrogen ylide
@
(d) From epoxides: Phenyl substituted carbenes are formed from phenyl substituted oxiranes or epoxides by photolysis. For example:
(e) From cyclopropane derivatives: Photolysis or thermolysis of cyclopropane derivatives leads to the formation of carbenes. For example:
(f) From gem-dichloro alkanes: When gem-dichloroalkanes are treated with metallic lithium, carbenes are generated. For example:
(g) By a-elimination reactions: Carbenes can be generated by a-elimination reactions in which both the atoms/groups are lost from the same carbon. For example:
1.1.7.5 Stereospecific addition of a singlet carbene and nonstereospcific addi- tion of triplet carbene to C==C bond to give cyclopropanes
The addition of a singlet carbene to an alkene (an electrophilic addition) is a concerted process (the two s bonds are formed simultaneously) as the unshared electrons of the carbene are in favourable spin for ready bond formation. In this process, the original stereochemical relationship of the groups on the alkene is preserved, i.e., the addition is stereospecific and syn. It thus follows that singlet carbene reacts with cis-2-butene to yield a cis- cyclopropane and reacts with trans-2-butane to yield a trans- cyclopropane.
A triplet carbene, which is diradical in nature, adds to the carbon–carbon double bond of cis-2-butene, for example, by a free radical two-step pathway involving addition followed by recombination. As the two unpaired electrons in a triplet carbene have parallel spins, there is spin restriction on the simultaneous formation of two s bonds. In the first step of this reaction, one of the unpaired electrons forms a s bond with that p-electron which has the opposite spin and as a result, a diradical is formed. The diradical with two unpaired electrons having parallel spin (a triplet intermediate) undergoes spin-flipping by some appropriate collisions and then form the other s bond to give a cis-cyclopropane. Since spin-flipping is relatively slow on the time scale of molecular rotation, a free rotation about the C — C bond takes place during this time. Spin-flipping followed by ring closure then produces a trans-cyclopropane. The reaction is, therefore, nonstereospecific (stereoselective).
Trans-2-butene, by a similar process, produces a mixture of a cis- and a trans-cyclopropane
1.1.7.6 Nucleophilic Carbenes
Because of electron deficiency (incomplete octet of carbon), carbenes are normally electrophilic. However, there are some carbenes in which the singlet structures are stabilized by incorporating the vacant p orbital into some delocalized electronic system and as a consequence of resonance interaction, their electrophilic character is suppressed and nucleophilic character is developed. Such carbenes are called nucleophilic carbenes.
For example:
(i)
(ii)
1.1.7.7 Carbenoids and the Simmons–Smith Reaction
Although dihalocarbenes react with alkenes to give cyclopropanes in good yield, this is not usually the case of methylene, :CH2, the simplest carbene. The reaction of :CH2 with alkene often produces a complex mixture of products and therefore, the reaction cannot be reliably used for cyclopropane synthesis. A useful cyclopropane synthesis was developed by H.E. Simmons and R. D. Smith in 1959.
When alkenes are treated with methylene iodide (CH2I2) in the presence of zinc-copper couple (zinc dust that has been activated with an impurity of copper), cyclopropanes are obtained, this reaction is called the Simmons–Smith reaction. For example:
The active reagent in this reaction is the a-haloorganometallic compound (a compound with halogen and a metal on the same carbon) ICH2ZnI obtained by the following reaction similar to the formation of a Grignard reagent.
— —
2 2 2
CH I Zn(Cu) I CH ZnI
Simmons Smith reagent (a carbenoid) + ổổặ
-
This kind of reagent is termed a carbenoid because it it not a free carbene but has carbenelike reactivity.