Compounds of the formally divalent carbon atom having two unshared electrons, either paired or unpai — Organic Chemistry Chemistry Question
Persistent Carbenes
Compounds of the formally divalent carbon atom having two unshared electrons, either paired or unpaired, are known as carbenes. Free or metal–coordinated carbenes are often considered as unstable and short-lived intermediates in a number of organic reactions. In the 1950s Ronald Breslow proposed that stable carbenes exist as intermediates in reactions involving vitamin B1, which occur in human body. The first persistent (stable) carbenes were isolated in 1990s, and some representatives are shown below. [VISUAL] Some stable carbenes now find applications in chemistry, as organocatalysts and ligands, as well as in coordination chemistry of metals, and they are available commercially.
Draw Lewis structures for the simplest carbene, CH2, the one in which all the electrons are paired (singlet carbene), and the one where there are two electrons of the same spin (triplet carbene).
Model Answer
Singlet carbene: H2C: with a lone pair in one of the sp2 hybrid orbitals and an empty p-orbital on the carbon atom.
Triplet carbene: H2C. with two unpaired electrons of parallel spins (one in an sp2 hybrid orbital, the other in a p-orbital).
Draw resonance structures for I–IV that would help you to account for their persistence.
Model Answer
Resonance structures of persistent carbenes I, II, III, and IV showing electron donation from the nitrogen/oxygen lone pairs into the empty p-orbital of the carbene carbon, which stabilizes the singlet state. For I and III, this delocalization leads to a 6-electron heteroaromatic structure.
Which other factors may be responsible for the persistence of these species?
Model Answer
Besides electronic effects involving delocalization of pi-electrons between the carbene carbon providing an empty p-orbital and adjacent heteroatoms (which can lead to 6-electron heteroaromatic structures in case of I and III), steric bulk around the carbene carbon atom may also be responsible for diminished reactivity of some persistent carbenes (II–IV).
The triplet carbene CH2 is noticeably more stable than the singlet carbene. In contrast, all the compounds I–IV above are formally derived from the singlet carbene CH2; their triplet analogs are much less stable and have not been isolated. Why?
Model Answer
The electronic effects responsible for stabilization and diminished reactivity of persistent carbenes are most pronounced when the carbene carbon provides an empty p-orbital and when therefore two pi-electrons can be involved in delocalization between the carbene carbon and adjacent heteroatoms. This is only possible in singlet carbenes. In triplet carbenes one of the carbene carbon lone electrons is occupying the carbon atom p-orbital, diminishing the pi-electron delocalization.
Fill in the structures of the missing compounds A–D in the scheme leading to a persistent carbene D:
[VISUAL]
Model Answer
A: N-(2-hydroxyethyl-2,4,6-trimethylaniline (or its hydrobromide salt), formed by alkylation of 2,4,6-trimethylaniline with 2-bromoethanol (BrC2H4OH).
B: N,N'-bis(2,4,6-trimethylphenyl)ethylenediamine, formed from reaction of the activated intermediate (derived from A with I2/PPh3) with 2,4,6-trimethylaniline.
C: 1,3-bis(2,4,6-trimethylphenyl-4,5-dihydroimidazolium salt, formed by reaction of B with triethyl orthoformate (HC(OEt)3) in the presence of formic acid (HCOOH) and PhNH3Cl.
D: 1,3-bis(2,4,6-trimethylphenyl)dihydroimidazol-2-ylidene (saturated N-heterocyclic carbene), formed by deprotonation of C using potassium tert-butoxide (KOC(CH3)3).
(An isomer of C with a different position of the C=N bond can also form; both will produce the same carbene D.)
A reaction very typical in carbene chemistry is carbene dimerization which may be reversible. Write a reaction scheme for dimerization of I.
Model Answer
Reversible dimerization of carbene I (imidazol-2-ylidene) to form a tetraaminoethylene derivative dimer via formation of a carbon-carbon double bond between the carbene carbons of two molecules.