Are perpendicular alkene triplets just 1,2-biradicals? Studies with the cyclopropylcarbinyl clock

Article abstract of DOI:10.1021/ja00085a005

Thermochemical studies of perpendicular alkene triplets demonstrate that their energies are almost exactly those expected from Benson estimations using group equivalents for appropriate free radicals as models for the termini (the two essential carbons of the alkene triplet). If these species are to be described as 1,2-biradicals, their reactivities in reactions for which the termini act independently should be similar to corresponding reactivities for appropriate free-radical models. We have used the "cyclopropylcarbinyl clock", the rearrangement of a cyclopropylcarbinyl radical to the corresponding homoallyl radical, to test such a model. We have synthesized cis- and trans-β-cyclopropylstyrene (cis-1 and trans-1), α-cyclopropylstyrene (2), α-cyclopropylindene (3), and β-cyclopropylindene (4). We have studied their triplet chemistry and have examined the transients produced upon triplet sensitization by nanosecond laser flash photolysis. The results are consistent with two basic hypotheses: (1) that, when able to access the perpendicular configuration, cyclopropylcarbinyl ring opening occurs at the rate expected from an appropriate monoradical model and (2) that, when constrained in a higher energy (near-planar) geometry, the rate of ring opening is accelerated.