J. Am. Chem. Soc. 1999, 121, 8891-8897
8891
Insertion of Difluorovinylidene into Hydrogen and Methane
Carsten Ko1tting and Wolfram Sander*
Contribution from the Lehrstuhl fu¨r Organische Chemie II der Ruhr-UniVersita¨t,
D-44780 Bochum, Germany
ReceiVed March 29, 1999. ReVised Manuscript ReceiVed July 15, 1999
Abstract: Insertion reactions of difluorovinylidene 1b into H2, CH4, and CD4 have been observed in argon
matrixes at 20-40 K. These reactions are controlled by diffusion of trapped species rather than by activation
barriers, indicating that the activation barriers are very small (<1 kcal/mol) or absent. In contrast, no insertion
was observed into CF4 or C2H4. With C2H4, addition to the double bond to give (difluoromethylene)cyclopropane
(5) is the only pathway. This was rationalized by ab initio (MP2/6-31G(d,p)) and DFT (B3LYP/6-311++G-
(d,p)) calculations, which predict activation barriers of essentially zero for the H2 and CH4 insertion, but 40
and 3 kcal/mol for the insertion into CF4 and C2H4, respectively.
Introduction
The insertions of singlet carbenes into CH bonds are concerted
reactions. The activation energy for the insertion of (singlet)
methylene into methane was calculated to be 9.1 kcal/mol at
the SCF/6-31G(d) level of theory, but this barrier is reduced to
zero if electron correlation is considered at the MP3/6-31G(d)
level of theory.13 For typical singlet ground-state carbenes
bearing electronegative substituents (halogen, OR, NR2) a
reduced electrophilicity and substantial barriers for insertions
are expected, and thus these reactions are generally not observed
in low-temperature matrixes. One of the few examples is the
rearrangement of tert-butylchlorocarbene to 2,2-dimethyl-1-
chlorocyclopropane in argon at 11 K, an intramolecular CH
insertion.14 High-level ab initio calculations (CCSD(T)) predict
activation barriers of 8 and 33 kcal/mol for the insertion of
HFC: and F2C: into H2, respectively.15 Substitution of the
hydrogen atoms in H2C: by fluorine leads to an increase of the
nucleophilicity of these carbenes, and since the primary step in
the carbene insertion is an electrophilic attack of the carbene
center on the XH bond, the reactivity is drastically reduced.
This also explains why the highly stable nucleophilic carbenes
of Arduengo type16 are completely unreactive toward CH bonds.
The insertion into XH bonds (X ) C, Si, O, etc.) is highly
characteristic of carbenes and widely used in synthetic chem-
istry. Both singlet and triplet carbenes show insertion reactions,
although the mechanisms are different. Intermolecular CH
insertions were systematically studied in low-temperature
organic glasses by Tomioka et al.1-7 and Platz et al.8-11 The
conclusion of these and many other studies is that triplet
carbenes at low temperature react via an abstraction recombina-
tion mechanism with radical pairs as intermediates. Typical
activation barriers for CH insertions of triplet carbenes are
several kcal/mol (for example, for the reaction of diphenylcar-
bene with toluene in the solid state a barrier of 6.9 kcal/mol
was determined).11 Since in several cases the insertions were
still observed at 77 K and curved Arrhenius plots were found,
quantum chemical tunneling was suggested to be important
under these conditions.11 A problem of the investigation of
thermal carbene reactions in organic glasses is secondary
photolysis of the long-lived carbenes during photolysis of the
precursor, which leads to the photochemical rather than the
thermal products (e.g., CH insertion instead of OH insertion in
polycrystalline alcohols).12
(1) Tomioka, H.; Izawa, Y. J. Am. Chem. Soc. 1977, 99, 6128.
(2) Tomioka, H. J. Am. Chem. Soc. 1979, 101, 256.
(3) Tomioka, H.; Itoh, M.; Yamakawa, S.; Izawa, Y. J. Chem. Soc.,
Perkin Trans. 2 1980, 603.
(4) Tomioka, H.; Suzuki, S.; Izawa, Y. J. Am. Chem. Soc. 1982, 104,
3156.
EA
EA
(5) Tomioka, H.; Ozaki, Y.; Izawa, Y. Chem. Lett. 1982, 843.
(6) Tomioka, H. Reactions of carbenes in solidified organic molecules
at low temperature. Res. Chem. Intermed. 1994, 20, 605.
(7) Tomioka, H.; Kawasaki, H.; Kobayashi, N.; Hirai, K. J. Am. Chem.
Soc. 1995, 117, 4483.
(8) (a) Zayas, J.; Platz, M. S. J. Am. Chem. Soc. 1985, 107, 7065. (b)
Zayas, J.; Platz, M. S. Tetrahedron Lett. 1985, 26, 2853. (c) Savino, T. G.;
Kanakarajan, K.; Platz, M. S. J. Org. Chem. 1986, 51, 1305.
(9) Platz, M. S.; Senthilnathan, V. P.; Wright, B. B.; McCurdy, C. W. J.
J. Am. Chem. Soc. 1982, 104, 6494.
Vinylidenes 1 are another class of ground-state singlet
carbenes, and for the parent vinylidene 1a the activation energy
for the insertion into H2 was calculated to be almost 13 kcal/
mol using high-level ab initio calculations, again predicting a
low reactivity.17 Substitution of the hydrogen atoms in 1a by
fluorine atoms drastically increases the electron affinity EA, in
(10) Doyle, M. P.; Taunton, J.; Oon, S.-M.; Liu, M. T. H.; Soundararajan,
N.; Platz, M. S.; Jackson, J. E. Tetrahedron Lett. 1988, 29, 5863.
(11) Platz, M. S. In The Chemistry, Kinetics, and Mechanisms of Triplet
Carbene Processes in Low-Temperature Glasses and Solids; Platz, M. S.,
Ed.; Plenum: New York, 1990; pp 143-211.
(13) Gordon, M. S.; Gano, D. R. J. Am. Chem. Soc. 1984, 106, 5421.
(14) Zuev, P.; Sheridan, R. S. J. Am. Chem. Soc. 1994, 116, 4123.
(15) Ignatyev, I. S.; Schaefer, H. F. J. Am. Chem. Soc. 1997, 119, 12306.
(16) Arduengo, A. J.; Goerlich, J. R.; Krafczyk, R.; Marshall, W. J.
Angew. Chem., Int. Ed. Engl. 1998, 37, 1963.
(17) Jensen, J. H.; Morokuma, K.; Gordon, M. S. J. Chem. Phys. 1994,
100, 1981.
(12) Leyva, E.; Barcus, R. L.; Platz, M. S. J. Am. Chem. Soc. 1986,
108, 7786.
10.1021/ja991003i CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/09/1999