THE BARRIER TO ROTATION ABOUT THE DOUBLE-BOND IN METHYLENECYCLOPROPANE

Activation parameters have been determined in the gas phase for the ge ometrical isomerization of the double bonds in two phenyl-substituted methylenecyclopropanes (2 and 8). Comparison with the analogous values for three sterically non-restricted styrene derivatives (15, 16 and 1 7) shows that the enthalpies of activation for the methylenecyclopropa nes are lower by 3.7 kcal mol(-1). This value, which is an upper Limit to the amount of strain energy released by pyramidalization of the ri ng carbon atom in the orthogonal diradical transition state when under going geometrical isomerization, is considerably smaller than the valu e of 12-14 kcal mol(-1) by which the strain energy of methylenecyclopr opane is larger relative to that of cyclopropane. Our kinetics experim ents thus show that the angle strain, associated with incorporating a trigonal carbon atom into the three-membered ring, is not responsible for the majority of the additional strain energy of methylenecycloprop ane, relative to that of cyclopropane.