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The synthesis of 5-isopropylidenebicyclo[2.1 .O]pentane (4) is accomplished in quantitative yield when 7-isopropylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (15) is photolyzed in CDzClz solution at -78 OC. Hydrocarbon 4 undergoes thermal dimerization to the trimethylenemethane dimers 7-10 in about 3 h at -53 OC. The kinetics of the dimerization are first order, a finding which is consistent with unimolecular rate-determining conversion of 4 to a reactive intermediate. Dimer then can be formed either by combination of two molecules of the intermediate or by attack of a molecule of the intermediate on a molecule of the bicyclopentane 4. The Arrhenius preexponential term for the thermolysis is -lo9, corresponding to AS* --16 eu. The low A factor probably arises from a rate-determining step in which a forbidden transition from the singlet to the triplet energy surface occurs. The dissociation energy of the bridge bond of 4 probably is negative, that is, the triplet biradical 11-T is more stable than the bicyclic hydrocarbon Cleavage of a single carbon-carbon bond hypothetically converts a member of the 5-alkylidenebicyclo[2.l.0]pentane (1) or bicyclo[3.1 .O]hex-1-ene (3) series to a 2-alkylidenecyclopentane-1,3diyl (Z).l Although the latter derivatives of the trimethylenemethane biradical* have triplet ground state^,^ the chemistry of a transient singlet species can be separated from that of the It has been difficult to decide whether the reactive form of the "singlet" actually is a true non-Kekul2 compound,* that is, a biradical (2 with paired electrons), or instead is better formulated as either of the bicyclic compounds 1 or 3. Examination of the 1 2 3 chemistry of authentic examples of these hitherto unknown hydrocarbons obviously is of crucial importance to the resolution of this mechanistic question. Moreover, the hydrocarbons 1 and 3 generate special interest in their own right because their large ring strain and their relationship to a stable biradical may conspire to produce carbon-carbon bonds (C-1-C-4 of 1, C-5-C-6 of 3) with negative dissociation energies. (1) Berson, J. A. Acc. Chem. Res. 1978, 11, 446. (2) For a review of acyclic TMM derivatives, see: Dowd, P. Acc. Chem. Res. 1972, 5, 242. (3) Platz, M. S.; McBride, J. M.; Little, R. D.; Harrison, J. J.; Shaw, A,; Potter, S. E.; Berson, J. A. J. Am. Chem. SOC. 1976, 98, 5725. (4) (a) Berson, J. A.; Convin, L. R.; Davis, J. H. J. Am. Chem. Soc. 1974, 96,6177; (b) Berson, J. A,; Duncan, C. D.; Convin, L. R. Zbid. 1974,96,6175. ( 5 ) Berson, J. A.; Duncan, C. D.; OConnell, G. C.; Platz, M. S. J. Am. Chem. SOC. 1976,98, 2358. (6) (a) Corwin, L. R.; McDaniel, D. M.; Bushby, R. J.; Berson, J. A. J. Am. Chem. SOC. 1980, 102,276. (b) Duncan, C. D.; Corwin, L. R.; Davis, J. H.; Berson, J. A. Zbid. 1980, 102, 2350. (7) Preliminary communication: Rule, M.; Lazzara, M. G.; Berson, J. A. J. Am. Chem. SOC. 1979, 101, 7091. (8) As far as we know, the actual term "non-Kekult compound" was used first by Dewar ((a) Dewar, M. J. S. "The Molecular Orbital Theory of Organic C...