| Optically active 6-methylenebicyclo(3.2.0)heptane was pyrolyzed at 320(DEGREES) in the gas phase. Racemization occurred with a first-order approach-to-equilibrium rate constant of 1.93 x 10('-5) sec('-1). This implies that the rate of C-1/C-5 bond cleavage to produce a 1,4-biradical (which inverts and rebonds to effect enantiomerization) is 9.6 x 10('-6) sec('-1). From a similar study of the automerization of 6-dideuteriomethylenebicyclo(3.2.0)heptane, the first order approach-to-equilibrium rate constant was 7.9 (+OR-) 0.6 x 10('-6) sec('-1). Since K(,eq) was measured as 1.06, the rate constant for C-1/C-7 fission is 4.07 x 10('-6) sec('-1). Hence, at 320(DEGREES), the C-1/C-5 bond cleaves 2.4 times more rapidly than the C-1/C-7 bond. Comparison of the rate of automerization in this encumbered system with that of parent methylenecyclobutane (W. von E. Doering, J. C. Gilbert, Tet. Suppl. 7, 397 (1966)) reveals the bicyclic analog rearranges nine times more slowly, in spite of its ability to form a more stable diradical. This rate retardation is ascribed to the required configurational retention of the migrating carbon, which limits full utilization of the con-rotatory-bevel ring-opening formulated by Gajewski (J. Am. Chem. Soc. 98, 5254 (1976)).;The interconversions of exo- and endo-7-methyl-6-methylenebicyclo(3.2.0)heptanes (1 and 2) and their isomers E- and Z-6-ethylidenebicyclo(3.2.0)heptanes (3 and 4) were also studied in the gas phase at 320(DEGREES). A Runge-Kutta analysis afforded the following first order rate constants (all x 10('6) sec('-1)): k(,12) = 8.61, k(,13) = 9.44, k(,14) = 0.28, k(,21) = 18.6, k(,23) = 9.44, k(,31) = 4.64, k(,32) = 1.75, k(,41) = 0.14. All other values were 0. The equal rates for 1 and 2 forming 3 were attributed to operation of the con-bevel mechanism in opposite rotational senses to avoid methyl-cyclopentyl steric interaction. The fast 2 to 1 reaction probably indicates substantial C-1/C-5 cleavage and ring inversion is operating in this system, as was shown rigorously in the normethyl parent. Z-ethylidene 4 is protected by a substantial kinetic barrier, as has been observed for the analogous dimethylmethylenecyclobutane (J. E. Baldwin, R. H. Fleming, J. Am. Chem. Soc., 95, 5249 (1973)) network. A novel product, 6,7-dimethylbicyclo(3.2.0)hept-6-ene, was formed in low conversion from the pyrolysis of the endo-methyl isomer 2. |
