The neutral carbene exists in either a singlet or triplet state:
The stable singlet carbenes are more important, as they considered extremely useful as catalysts when complexed to a transition metal.
NHCs are by far the largest group with hundreds of publications, so review articles tend to focus on these.
We can see the singlet-triplet gaps of the five families (kcal/mol):
This suggests that NHCs are mostly likely to produce singlets, which might explain why they are so popular with researchers.
σ-electron withdrawing groups favor the singlet state, σ donating groups favor triplet state. But the most important factor in stablizing singlets is the presence of a π-donor (an element or a pi bond) next to the carbene carbon. Bulky substituents can also offer kinetic stability and force the molecule into a shape which encourages π-donation onto the carbene carbon. The more full the empty carbene p-orbital is, the higher the singlet-triplet gap, the more stable the carbene.
The carbene also gains stability by linking its side atoms into a 3-5 membered ring, like in the examples above. First, because the shape of the ring can force s-character into the carbene lone-pair orbital. Second because it forces π-orbitals of π-donors to align with the empty carbene p orbital, increasing overlap/donation.
The first stable NHCs was made in 1991 by Arduengo. Who grew crystals of this thing, now known as Arduengo's Carbene:
Those side groups are called adamantane groups. They tend to be shorthanded as "Ad". This molecule has all the stabilization attributes mentioned above.
The most well known NHC is the 2nd generation Grubbs catalyst used for olefin metathesis, which won him the nobel prize in 2005.
Another feature of NHCs is that some are used as organic catalysts in their own right, without being complexed to a metal.
Going through the other four classes, stable thiazolylidenes came in 1997. The inferior singlet-triplet gap is from the inferior pi-donating capacity of sulfur compared to nitrogen. They tend to require very bulky side groups. They have some utility in metal complexes, but no highly effective thiazolylidene-based catalysts have been reported.
The first stable P-heterocyclic carbenes was isolated in 2005:
Mes* = 2,4,6-tri-(t-butyl)phenyl.
Only a few other PHCs are known. They all require enormous groups to be stable.
Also in 2005 were the first cyclic (alkyl) (amino) carbenes. These also have low orbital splitting, partly from having only one heteroatom next to the carbene carbon, and partly from the sigma donating character of the sp³ carbon. The orbital splitting of CAACs and PHCs are similar, suggesting that one animo group has about the same effect as two phospino groups with bulky substituents.
The adjacent carbon has to be quaternary to prevent a 1,3 hydride shift.
CAACs are being heavily researched and arguably have the most potential uses, since the sp³ carbon can produce sigma-donor effects on the carbene carbon more powerful than all the other cyclic carbenes. The sp³ carbon also has more potential for steric bulk protecting the carbene atom.
The relatively empty p orbital in CAACs also give this class some stranger reactivity and unique side-reactions. They form stable aminoketenes when exposed to carbon monoxide, and split H2 and NH3 under normal laboratory conditions.
We have the final class produced in 2006, Cyclopropenylidenes. People were surprised to see a stable carbene without even one pi-donating heteroatom adjacent to the carbene carbon. Only one has ever been isolated:
The strained flat shape of the ring puts the empty p orbital of the carbene planar with the double bond, so both adjacent carbons act as pi donors. This is enhanced by the isopropylamino groups, for they can donate their lone pairs into the pi* antibonding orbital, which has the right symmetry to overlap with the empty carbene p orbital. Finally the carbene lone pair is forced into a sp2 shape, which enhances the sigma-pi split even further.
Nice post, well documented and very interesting! About thiazolylidene, you could add the Benzoin condensation catalyzed by thiamine, as an example of thiazolylidene-based catalysts.
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