Carbenes
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Carbenes. Synthesis, Isolation, and Characterization. By Chalmer Wren. What is a Carbene?. Carbenes are a family of organic molecules composed of a neutral divalent carbon atom with a sextet of electrons and two substituents. .

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Carbenes

Carbenes

Synthesis, Isolation, and Characterization

By Chalmer Wren


What is a carbene

What is a Carbene?

  • Carbenes are a family of organic molecules composed of a neutral divalent carbon atom with a sextet of electrons and two substituents.

Peter R. Schreiner, Hans Peter Reisenauer, Frank C. Pickard IV, Andrew C. Simmonett, Wesley D. Allen, Mátyus & Attila G. Császár. Nature. June 12, 2008. 453, 906-909.


Singlets and triplets

Singlets and Triplets

Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev.,December 22, 1999.100 (1), 39 –92.


Carbene examples

Carbene Examples

Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev.,December 22, 1999.100 (1), 39 –92.


Carbene reactivity

Carbene Reactivity

  • Diverse Reactivity

    • Electrophilic

    • Nucleophilic

    • Ambiphilic

    • Radical


Do they exist

Do They Exist?

  • In 1954, Doering and Hoffman established dichlorocarbene as an intermediate in the hydrolysis of chloroform under basic conditions.

  • Dichlorocarbene was not observed directly, but mechanistically inferred.

W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.


Dichlorocarbene

Dichlorocarbene

W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.


Dichlorocarbene1

Dichlorocarbene

tert-butoxide first removes a proton from chloroform, resulting in the formation of t-butyl alcohol and the conjugate base of chloroform. The conjugate base of chloroform, a trichloromethide anion, forms chloride ion and dichlorocarbene.

W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.


Dichlorocarbene2

Dichlorocarbene

W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.


Matrix isolation spectroscopy

Matrix Isolation Spectroscopy

The Astrochemistry Lab at NASA Ames Research Center. http://www.astrochem.org/matrix_isol/MI.html


Matrix isolation spectroscopy1

Matrix Isolation Spectroscopy

The Astrochemistry Lab at NASA Ames Research Center. http://www.astrochem.org/matrix_isol/MI.html


Matrix isolation spectroscopy2

Matrix Isolation Spectroscopy

Types of Matrices

Organic Glass Matrix

Rare Gas Matrix

  • Composed of an inert organic colvent.

  • High reactivity of carbenes limits the use of all but the most inert organic solvents and, in for the more reactive carbenes, organic glass matrices can’t be used at all

  • Useful for isolating and characterizing highly unstable carbenes.

Whittle, E.; Dows, D.A.; Pimentel, G.C. J. Chem. Phys. 1954, 22, 1943.


Matrix isolation spectroscopy for a carbene

Matrix Isolation Spectroscopy for a Carbene

  • Usually, a photochemical precursor, i.e, one that forms a carbene following exposure to light, is isolated in the solid matrix. The precursor can then be prompted to react and form the carbene directly in the matrix.


Synthesis of photochemical precursor

Synthesis of Photochemical Precursor

3

1

2

Nick Greeves, Alex Lawrenson, Kirsty Barnes. (2007) University of Liverpool. ChemTube3D – Interactive 3D Organic Reaction Mechanism. Carbenes. http://osxs.ch.liv.ac.uk/~ng/external/Carbenes-PhotolysisDiazomethaneCarbene.html (accessed November 9, 2008).


Formation of carbene

Formation of Carbene

Nick Greeves, Alex Lawrenson, Kirsty Barnes. (2007) University of Liverpool. ChemTube3D – Interactive 3D Organic Reaction Mechanism. Carbenes. http://osxs.ch.liv.ac.uk/~ng/external/Carbenes-PhotolysisDiazomethaneCarbene.html (accessed November 9, 2008).


Carbene stabilization

Carbene Stabilization


Stabilization

Stabilization

  • Substituent Effects

  • Stabilization from Induction

  • Stabilization from Resonance

  • Stabilization from Steric Factros


Carbene stabilization1

Carbene Stabilization

  • The stability of singlet carbenes is largely dictated by substituents associated with it.

  • The stability of carbenes can be increased using amino substituents. This is because the adjacent nitrogen atoms decrease the electron deficiency of pπ orbital by donating electron density through resonance, while stabilizing the lone pair of the s orbital of carbon by inductively withdrawing electron density.

Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev.,December 22, 1999.100 (1), 39 –92.


Diamnocarbene

Diamnocarbene

Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev.,December 22, 1999.100 (1), 39 –92.


Diamnocarbene1

Diamnocarbene

  • Realizing the potential for amino groups to stabilize carbenes, Wanzlick attempted to synthesize a stable aminocarbene in 1962 by the thermal elimination of chloroform

Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev.,December 22, 1999.100 (1), 39 –92.


Diamnocarbene2

Diamnocarbene

The formation of the diaminocarbene could not be confirmed

Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev.,December 22, 1999.100 (1), 39 –92.


Diamniocarbene

Diamniocarbene

  • Despite Wanzlick’s failure, the isolation of stable aminocarbenes was eventually achieved.

  • In 1990, Arduengo, Harlow, and Kline reported the synthesis, structure, and characterization of the first stable crystalline carbene; 1,3-di-1-adamantylimidazol-2-ylidene.

Anthony J. Arduengo III. Richard L. Harlow, Michael Kline. J. Am. Chem. Soc. 1995, 117, 11027.


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