THEORY AND MECHANISM OF THE ALLYLIDENECYCLOPROPANE TO METHYLENECYCLOPENTENE THERMAL-ISOMERIZATION

The thermally induced rearrangement of 7-(trans-butenylidene)bicyclo[4 .1.0]heptane gives 8-methylbicyclo[5.3.0]undeca-1(2),9-diene with ster eospecificity that is opposite that predicted by conservation of orbit al symmetry. Further, when optically active starting material is therm olyzed, the product is racemic, but recovered starting material is unc hanged. Identical results were obtained upon thermolysis of 6-(trans-b utenylidene)bicyclo[3.1.0]hexane and of 2-oxo-7-(trans-butenylidene)bi cyclo[4.1.0]heptane. These results require that the reaction proceeds through one or more achiral species, that at least one of them is plan ar, that they never return to starting material, and that there is a s tereospecific pathway from the intermediate(s) to product, 6-Electron- 6-orbital CASSCF calculations with a 6-31G basis set reveal that the lowest energy singlet intermediates on the parent C6H8 energy surface are the achiral cisoid and transoid orthogonal 2-(1'-allyl)allyl birad icals. The cisoid form undergoes conrotatory closure to the methylenec yclopentene product via a transition state that is 2.8 kcal/mol lower in energy than the disrotatory transition state. The conrotatory trans ition state is consistent with the observed stereospecificity. The hig hest energy transition state is that from starting allylidenecycloprop ane to an orthogonal 2-methylene-transoid-1,3-pentadienyl singlet spec ies which then forms the transoid orthogonal bisallyl singlet biradica l which undergoes bond rotation via a planar transoid vinyltrimethylen emethane transition state as well as slower closure to 2-vinylmethylen ecyclopropane. Reopening of the latter material to a cisoid orthogonal bisallyl singlet biradical then provides the pathway for the stereosp ecific conrotatory closure to 3-methylenecyclopentene. Besides the cyc lopropanes and methylenecyclopentene, the lowest energy species on the entire energy surface is the planar transoid vinyltrimethylenemethane triplet. The orthogonal bisallyl singlets are 8 kcal/mol higher in en ergy than the planar triplet at this level of theory.