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

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

Diverse Reactivity Electrophilic Nucleophilic Ambiphilic Radical 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. W. von E. Doering, A. Kentaro Hoffman. J. Am. Chem. Soc. 1954, 76, 6162.

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

Dichlorocarbene 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 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 can’t be used at all 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 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 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

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

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 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.