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Fischer and Schrock Carbenes: A Brief Overview

Alison Brown & Nathan BuehlerUniversity of Utah

Or

Free Carbenes: A Brief Primer

Wiki Page: http://en.wikipedia.org/wiki/Carbene Other Helpful References: http://chemistry.caltech.edu/courses/ch242/L20.pdf,

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 characterAs 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

Historical Metal-Carbene Complexes

Wiki Page: http://en.wikipedia.org/wiki/Transition_metal_carbene_complex Other References: Schrock Paper, Fischer

Overview: Early ExamplesThe first transition metal stabilized carbene was developed by Fischer in 1964

The next major variety of carbene was developed by Schrock in 1974

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.

Modern Metal-Carbene Complexes

Wiki Page: http://en.wikipedia.org/wiki/Transition_metal_carbene_complexOther References, Chem. Sci., 2013, 4, 1053-1058., Acc. Chem. Res., ASAP., Chem. Eur. J., 2013, 19, 12953-12958., J. Phys. Chem. A, 2013, 117,9266-9273.,

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

Modern Metal-Carbene Complexes

Wiki Page: http://en.wikipedia.org/wiki/Transition_metal_carbene_complexOther References, Chem. Sci., 2013, 4, 1053-1058., Acc. Chem. Res., ASAP., Chem. Eur. J., 2013, 19, 12953-12958., J. Phys. Chem. A, 2013, 117,9266-9273.,

Example Mechanism

N N

NNCo

N N

NNCo

N2

R2R1

R1R2

N

N N

NNCo

R1R2

N N

NNCo

C

O

R2R1

O

R1

R2n

N2 CO

N N

NNCo

N

N

R2R1

N

N N

NNCo

R1R2

Modern Metal-Carbene Complexes

Wiki Page: http://en.wikipedia.org/wiki/Transition_metal_carbene_complexOther References, Chem. Sci., 2013, 4, 1053-1058., Acc. Chem. Res., ASAP., Chem. Eur. J., 2013, 19, 12953-12958., J. Phys. Chem. A, 2013, 117,9266-9273.,

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.

Problems

ReferencesAcc. Chem. Res., ASAP., Acc. Chem. Res.,ASAP.,

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Contributed by: Alison Brown & Nathan Buehler (Undergraduates)

University of Utah, 2013