Transcript Carbenes - Metro State Atheists

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.
R
1
C
R
2
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
py
p
py
p

px
px
Triplet

Singlet
sp-hybridized
sp2-hybridized
Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev., December 22, 1999.100 (1), 39 –92.
Carbene Examples
H
Cl
C
C
H
methylene
R
R
N
Cl
N
R
dichlorocarbene
B
C
R
diaminocarbenes
R
C
B
R
R
R
diborocarbene
R
N
C
N
R
cyclic diaminocarbenes
Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev., December 22, 1999.100 (1), 39 –92.
Carbene Reactivity
• Diverse Reactivity
– Electrophilic
– Nucleophilic
– Ambiphilic
– Radical
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.
Cl
C
Cl
dichlorocarbene
W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.
Dichlorocarbene
Cl
C
Cl
Cl
+
Cl
dichlorocarbene
cyclohexene
1,1-dichloro-2,2-dimethychyclopropane
W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.
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.
H
Cl
Cl
Cl
+
O
-
C
-
+
HO
t-butoxide
trichloromethide ion
C
Cl
Cl
Cl
chloroform
Cl
Cl
t-butyl alcohol
dichlorocarbene
W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.
Dichlorocarbene
Cl
C
Cl
Cl
+
Cl
cyclohexene
dichlorocarbene
1,1-dichloro-2,2-dimethychyclopropane
W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.
Matrix Isolation Spectroscopy
The Astrochemistry Lab at NASA Ames Research Center. http://www.astrochem.org/matrix_isol/MI.html
Matrix Isolation Spectroscopy
The Astrochemistry Lab at NASA Ames Research Center. http://www.astrochem.org/matrix_isol/MI.html
Matrix Isolation Spectroscopy
Types of Matrices
Organic Glass 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
Rare Gas Matrix
• 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
• 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
1
H3C
O
C
H
Cl
-
+
C N N
Cl-
O
H
2
+
C C N N
H3C
O
H
C
C
CH3
H
H
-
3
H
+
N
H
-
O
NH
C
H3C
C
+
N
N
+
C N N
H
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
H
H
-
O
C
C
CH3
heat or uv
+
N
N
O
C
C
+
N
N
CH3
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
Stabilization
Substituent Effects
- Stabilization from Induction
- Stabilization from Resonance
- Stabilization from Steric Factros
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
+
N
N
C
N
N
C
C
N
+
N
Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev., December 22, 1999.100 (1), 39 –92.
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
Ph
Cl
N
Cl
N
Cl
Ph
Bourissou D., Guerret O., Gabbaï F., and Bertrand G. Chem. Rev., December 22, 1999.100 (1), 39 –92.
Diamnocarbene
Ph
Ph
Cl
N
Cl
N
Cl
Ph
N
Ph
Ph
N
N
C
N
N
N
Ph
Ph
Ph
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
• 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-2ylidene.
Anthony J. Arduengo III. Richard L. Harlow, Michael Kline. J. Am. Chem. Soc. 1995, 117, 11027.