7150
J. Am. Chem. Soc. 1999, 121, 7150-7151
Communications to the Editor
An Unusual Cleavage of an Energetic Carbene
Guopin Xu, Tsong-Ming Chang, Jinglan Zhou,
Michael L. McKee,* and Philip B. Shevlin*
excess energy in the C-atom reaction is responsible for the
observed cleavage.
Department of Chemistry, Auburn UniVersity
Auburn UniVersity, Alabama 36849-5310
ReceiVed January 21, 1999
ReVised Manuscript ReceiVed June 9, 1999
Transformation of a carbene from a divalent to a tetravalent
species is generally a highly exothermic process for which
1
numerous pathways have been observed. Particularly interesting
examples are two-bond â cleavages, such as that of cyclopropyl-
2
methylenes 1 to an alkene and an alkyne and the cleavage of
To investigate the energy surfaces connecting 2b with the
observed products, we have carried out a computational study in
which geometries were optimized and energies calculated at the
B3LYP/6-311+G(d)+ZPC level.7 Table 1 shows energies of
relevant species relative to the ground state of 2b. Since
deoxygenation of carbonyl compounds by carbon occurs along a
3
2
,5-dioxacyclopentylidene 2a to CO
2
and an alkene. In the latter
provides a thermodynamic driving
reaction, the stability of CO
2
,8
9
singlet energy surface, we have focused our calculations on
singlet species. Carbene 2b was found to have a singlet ground
state with an S-T splitting of 8.7 kcal/mol. Not surprisingly, the
most favorable reaction of 2b was H migration to 4 which has a
barrier of only 5.7 kcal/mol and is exothermic by 63.0 kcal/mol.
Several other intramolecular reactions including ring contraction
force. We now report that the parent cyclopentylidene, 2b, will
undergo an analogous cleavage when generated with sufficient
excess energy.
q
The deoxygenation of carbonyl compounds by atomic carbon,
which is generally exothermic by over 100 kcal/mol, is a
convenient route to carbenes which possess excess energy,2 and
the carbon atom deoxygenation of cyclopentanone, 3, is expected
to generate highly energetic 2b. Co-condensation of arc-generated
to methylenecyclobutane 9 (∆H ) 51.0 kcal/mol) and â C-H
q
insertion to give bicyclo[2.1.0]pentane 10 (∆H ) 27.5 kcal/mol)
c,4
were calculated to have high barriers and seem unlikely to play
a role in the chemistry of 2b.
In examining the energy surface leading from 2b to 5 and 6,
a reaction calculated to be exothermic by 23.2 kcal/mol, it is
immediately obvious that a concerted cleavage preserving the C2V
symmetry of 2b would lead to a planar allene and thus be a high-
energy process. Indeed, such a structure can be located lying 56.1
kcal/mol in energy above 2b with two negative eigenvectors. A
similar problem does not occur in the concerted cleavage of 2a
5
carbon with 3 at 77 K leads to cyclopentene, 4, allene, 5, and
ethylene, 6, in a 4:1:1 ratio (eq 1). These results raise the
possibility that the high exothermicity of the deoxygenation
generates 2b with enough energy to cleave to 5 and 6 in
competition with rearrangement to 4. While it is conceivable that
the cleavage products arise from chemically activated 4, none of
the reported thermal or photochemical decompositions of 4 show
this type of fragmentation.6
in which a calculated barrier of 10 ( 1 kcal/mol has been
3d,10
reported.
Since a careful search of the closed-shell surface
Since we observe that generation of 2b from diazo compound
by pyrolysis of tosylhydrazone lithium salt 8 at 180 °C gives
as the only detectable carbene product, it seems likely that the
connecting 2b with 5 and 6 fails to reveal a low-energy concerted
transition state, we have considered the possibility that the reaction
proceeds in a stepwise manner via biradical 11.
7
4
(1) (a) Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press: New
York, 1971. (b) Baron, W. J.; DeCamp, M. R.; Hendrick, M. E.; Jones, M.,
Jr.; Levin, R. H.; Sohn, M. B. Carbenes; Jones, M., Jr., Moss, R. A., Eds.;
Wiley & Sons: New York, 1973; Vol. 1, p 1. (c) Moss, R. A. In AdVances
in Carbene Chemistry; Brinker, U. H., Ed.; JAI Press: Greenwich, 1994; Vol.
1
, p 59.
2) (a) Friedman, L.: Shechter, H. J. Am. Chem. Soc. 1960, 82, 1002. (b)
(
Shevlin, P. B.; Wolf, A. P. J. Am. Chem. Soc. 1966, 88, 4735. (c) Skell, P.
S.; Plonka, J. H. Tetrahedron 1972, 28, 3571. (d) Shevlin, P. B.; McKee, M.
L. J. Am. Chem. Soc. 1989, 111, 519. (e) Chou, J.-H.; McKee, M. L.; De
Felippis, J.; Squillacote, M.; Shevlin, P. B. J. Org. Chem. 1990, 55, 3291.
(7) Becke, A. D. J. Chem. Phys. 1993, 98, 1372.
(8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.;
Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.;
Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski,
J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J.
V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, (Revision A5);
Gaussian, Inc.: Pittsburgh, PA, 1998.
(
3) (a) Borden, W. T.; Hoo, L. H. J. Am. Chem. Soc. 1978, 100, 6274. (b)
Feller, D.; Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 1981, 103,
2
558. (c) Feller, D.; Borden, W. T.; Davidson, E. R. J. Comput. Chem. 1980,
, 158. (d) Sauers, R. R. Tetrahedron Lett. 1994, 35, 7213 and references
1
therein. (e) See also Lawton, G.; Moody, C. J.; Pearson, C. J. J. Chem. Soc.,
Perkin Trans. 1, 1987, 877.
(
4) Rahman, M.; Shevlin, P. B. Tetrahedron Lett. 1985, 26, 2959.
(
5) The reactor is modeled after that described in Skell, P. S.; Wescott, L.
D., Jr.; Golstein, J. P.; Engel, R. R. J. Am. Chem. Soc. 1965, 87, 2829.
(
6) (a) Lewis, D. K.; Baldwin, J. E.; Cianciosi, S. J. J. Phys. Chem. 1990,
9
4, 7464. (b) Makulski, W.; Collin. G. J. J. Phys. Chem. 1987, 91, 708. (c)
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(10) We calculate a barrier of 8.1 kcal/mol at the B3LYP/6-311+G(d)+ZPC
level.
Shoemaker, J. O.; Carr, R. W., Jr. J. Phys. Chem. 1984, 88, 605. (d) Adam,
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1
0.1021/ja990205b CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/14/1999