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* 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 numerous pathways have been observed. 1 Particularly interesting examples are two-bond  cleavages, such as that of cyclopropyl- methylenes 1 to an alkene and an alkyne 2 and the cleavage of 2,5-dioxacyclopentylidene 2a to CO 2 and an alkene. 3 In the latter reaction, the stability of CO 2 provides a thermodynamic driving force. We now report that the parent cyclopentylidene, 2b, will undergo an analogous cleavage when generated with sufficient excess energy. 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, 2c,4 and the carbon atom deoxygenation of cyclopentanone, 3, is expected to generate highly energetic 2b. Co-condensation of arc-generated carbon 5 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 Since we observe that generation of 2b from diazo compound 7 by pyrolysis of tosylhydrazone lithium salt 8 at 180 °C gives 4 as the only detectable carbene product, it seems likely that the excess energy in the C-atom reaction is responsible for the observed cleavage. 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. 7,8 Table 1 shows energies of relevant species relative to the ground state of 2b. Since deoxygenation of carbonyl compounds by carbon occurs along a singlet energy surface, 9 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 to methylenecyclobutane 9 (∆H q ) 51.0 kcal/mol) and  C-H insertion to give bicyclo[2.1.0]pentane 10 (∆H q ) 27.5 kcal/mol) 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 C 2V 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 in which a calculated barrier of 10 ( 1 kcal/mol has been reported. 3d,10 Since a careful search of the closed-shell surface 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. (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. 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(9) Ahmed, S. N.; Shevlin, P. B. J. Am. Chem. Soc. 1983, 105, 6488. (10) We calculate a barrier of 8.1 kcal/mol at the B3LYP/6-311+G(d)+ZPC level. 7150 J. Am. Chem. Soc. 1999, 121, 7150-7151 10.1021/ja990205b CCC: $18.00 © 1999 American Chemical Society Published on Web 07/14/1999