Preparation and thermolysis of exo-and endo-Tricyclo[6,2,1,0^2,7] undeca-3,5,9-triene

Abstract

Syntheses of exo- and endo-tricyclo[6,2,1,0^2,7]undeca-3,5,9-triene, (3) and (4) respectively, are described. Both trienes undergo ready _π4_s+ _π2₈ cycloreversion to yield cyclopentadiene and benzene at 40-60ºC, the exo-triene (3) being slightly more reactive. At 60ºC, for (3) k = 2.29x 10^-3 s^-1, ΔH? = 104.1 ± 4.2kJ mol^-1, ΔS? =14.6 ± 1.6 mol^-1 K^-1, while for (4) k = 5.25 x 10^-3s^-1, ΔH? =ll5.0 ± 3.3 kJ mol^-1, ΔS? = 34.3 ± 13.0 J mol^-1 K^-1 .  The cycloreversion of (3) and (4) are believed to be concerted reactions. The related dienes exo- and endo-tricyclo[6,2,1,0^2,7]undeca-3,5-diene, (25) and (28) respectively, have also been prepared, and are stable up to 200ºC. At 300ºC, interconversion between (25) and (28) occurs, apparently via bicyclo[6,2,1]undeca-2,4,6-triene. The relationship of the trienes (3) and (4) to the recently described bicyclo[6,2,l]undeca-2,4,6,9-tetraene is discussed.