1. Fischer and Schrock Carbenes: A Brief OverviewOr
Fischer and Schrock Carbenes: A Brief Overview
Alison Brown & Nathan Buehler
University of Utah

2. Free Carbenes: A Brief Primer
A carbene is any neutral carbon species which contains a non-bonding valance pair of electrons
- The simplest example of a carbene is methylene
- Traditional organic chemistry requires photolysis or thermolysis for carbene formation (specifically methylene)
- Carbenes exist in either the singlet or triplet state
Both singlet and triplet carbenes are highly reactive, reacting with all C—H and C=C bonds
Singlet state carbenes have spin-paired electrons, and can be thought of as having Zwitter-ionic character
As a result, the singlet state is highly reactive
Triplet state carbenes are considered to be in the ground state, yet still highly reactive
Chemical behavior similar to radical chemistry
Wiki Page:
Other Helpful References:

3. Historical Metal-Carbene Complexes
Early Examples
The first transition metal stabilized carbene was developed by Fischer in 1964
The next major variety of carbene was developed by Schrock in 1974
Overview:
Mechanistic Implications:
The Fischer carbenes are electrophillic due to the formally empty pi orbital
In contrast, Schrock carbenes are nucleophillic in nature, and behave as strong bases. ‬
To explain the differences in reactivity one should consider FMO theory, transition metal character, and available back-bonding. Schrock carbenes have a empty metal-like LUMO and a filled ligand-like HOMO, while in comparison the Fischer carbenes have filled metal based HOMO and a ligand based LUMO.  As a result Schrock carbenes experience little backbonding and exhibit significant ionic character due to the early transition metal character.  Fischer carbenes have a low lying metal orbital and a high lying empty ligand orbital, and this arrangement allows for particularly strong backbonding.
Wiki Page:
Other References: Schrock Paper, Fischer

4. Modern Metal-Carbene Complexes
Overview
Very recent work has focused on the use of Au as a catalyst capable of performing enantio-selective reactions with carbene intermediates.
Other interesting work by de Bruin involves radical Co chemistry to generate a carbene intermediate
Examples
Wiki Page:
Other References, Chem. Sci., 2013, 4, , Acc. Chem. Res., ASAP., Chem. Eur. J., 2013, 19, , J. Phys. Chem. A, 2013, 117, ,

5. Modern Metal-Carbene Complexes
Example Mechanism
Wiki Page:
Other References, Chem. Sci., 2013, 4, , Acc. Chem. Res., ASAP., Chem. Eur. J., 2013, 19, , J. Phys. Chem. A, 2013, 117, ,

6. Modern Metal-Carbene Complexes
Future Directions
Generation of the ‘simplest carbene’, BeCH2.
Computational studies indicate that formation of BeCH2 is thermodynamically favorable but that mechanistic or kinetic reasons prevent BeCH2 from being made or persisting.
Wiki Page:
Other References, Chem. Sci., 2013, 4, , Acc. Chem. Res., ASAP., Chem. Eur. J., 2013, 19, , J. Phys. Chem. A, 2013, 117, ,

7. Problems
References
Acc. Chem. Res., ASAP., Acc. Chem. Res.,ASAP.,

8. Contributed by: Alison Brown & Nathan Buehler (Undergraduates)
University of Utah, 2013