FORBIDDEN REACTIONS .2. THE DISROTATORY C YCLOBUTENE RINGOPENING

The energy profiles for the con- and disrotatory opening of benzocyclo butene, methylenecyclobutene, and cyclobutene derivatives were establi shed by NO and oxygen trapping. The enthalpy for the transition states for the ''forbidden'' reactions proofed to be identical with the heat of formation of the orthogonal diradicals derived by geometrical isom erization of the dienes formed in these reactions. These diradicals de scribe very well the activation barriers observed for the disrotatory opening of bicycle cyclobutenes ([3.2.0] and [2.2.0] systems), but not for bicyclo[2.1.0]pent-2-ene, indicating here a truly forbidden react ion.