Carbenes

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Carbenes

• Synthesis, Isolation, and Characterization

By Chalmer Wren
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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.
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Singlets and Triplets
Bourissou D., Guerret O., Gabbaï F., and Bertrand
G. Chem. Rev., December 22, 1999.100 (1), 39 92.
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Carbene Examples
Bourissou D., Guerret O., Gabbaï F., and Bertrand
G. Chem. Rev., December 22, 1999.100 (1), 39 92.
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Carbene Reactivity

• Diverse Reactivity
• Electrophilic
• Nucleophilic
• Ambiphilic
• Radical

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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.
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Dichlorocarbene
W. von E. Doering, A. Kentaro Hoffman. J. Am.
Chem. Soc. 1954, 76, 6162.
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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.
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Dichlorocarbene
W. von E. Doering, A. Kentaro Hoffman. J. Am.
Chem. Soc. 1954, 76, 6162.
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Matrix Isolation Spectroscopy
The Astrochemistry Lab at NASA Ames Research
Center. http//www.astrochem.org/matrix_isol/MI.ht
ml
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Matrix Isolation Spectroscopy
The Astrochemistry Lab at NASA Ames Research
Center. http//www.astrochem.org/matrix_isol/MI.ht
ml
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Matrix Isolation Spectroscopy
Types of Matrices
Organic Glass Matrix
Rare Gas Matrix

• Useful for isolating and characterizing highly
  unstable carbenes.

• 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 cant be used at all

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

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Synthesis of Photochemical Precursor
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1
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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/Ca
rbenes-PhotolysisDiazomethaneCarbene.html
(accessed November 9, 2008).
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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/Ca
rbenes-PhotolysisDiazomethaneCarbene.html
(accessed November 9, 2008).
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Carbene Stabilization
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Stabilization

• Substituent Effects
• Stabilization from Induction
• Stabilization from Resonance
• Stabilization from Steric Factros

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

• Despite Wanzlicks 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.