Stereochemistry of the thermal isomerization of 1-ethenyl-7-exo-phenylbicyclo[4.1.0]heptane to 7-phenylbicyclo[4.3.0]non-1(9)-ene:: The conformational course of an antarafacial,retention vinylcyclopropane-cyclopentene rearrangement

The thermal vinylcyclopropane-cyclopentene rearrangement of 1-ethenyl-7-exo-phenylbicyclo[4.1.0]-heptane at 220 degrees C gives mostly migration with retention to form 7-exo-phenylbicyclo[4.3.0]non-1(9)-ene. 1-(E-d-Ethenyl)- and 1-(Z-d-ethenyl)-7-exo-phenylbicyclo[4.1.0]heptanes give both 8-exo and 8-endo deuterium-labeled products. The suprafacial, retention and antarafacial, retention paths for this rearrangement are used in 85:15 proportions. The antarafacial process may involve passage through a "semicircular" diradical conformation; it cannot occur through rotation in the opposite sense, passing through an "extended" diradical geometry.