Cyclopropylcarbinyl ring opening as mechanistic probe for differentiation between the singlet and triplet Spin states of trimethylenemethane diradicals (non-kekulé species)

Article abstract of DOI:10.1021/ja00038a018

The cyclopropylcarbinyl rearrangement of the non-Kekulé 2-alkylidene-1,3-cyclopentanediyl diradical (S-2b and T-2b), generated by thermal and direct and triplet-sensitized photochemical denitrogenation of the corresponding azoalkane 1b, was explored. While in the triplet-sensitized photolysis at moderate temperatures no cyclopropylcarbinyl rearrangement took place, in the singlet manifold (thermolysis and direct photolysis) the rearranged hydrocarbon 3 was observed. The amount of the rearranged hydrocarbon 3 increased with increasing temperature, and from the temperature dependence, an activation energy of ca. 5 kcal/mol was estimated for the ring opening. The intact triplet 1,3-diradical T-2b and its ring-opened triplet 1,6-diradical T-2b' were trapped by ³O₂ in the form of their stable endoperoxides 5 and 6. Under the trapping conditions, dimerization of the triplet species T-2b was completely eliminated through dioxygen trapping. Also, 1,4-cyclohexadiene served as a scavenger for the T-2b diradical through hydrogen atom transfer, but less effectively since much dimer was still observed. The present study constitutes the first example of the cyclopropylcarbinyl rearrangement of a non-Kekulé species, in particular the singlet diradical S-2b. Of mechanistic significance is the observation that the much longer lived triplet diradical T-2b does not undergo ring opening. The reason for this is the special trimethylenemethane stability of the ground-state triplet diradical T-2b, which imposes a higher activation energy for ring opening (E_a ca. 14 kcal/mol) than usual (E_a ca. 5 kcal/mol), and consequently cyclopropylcarbinyl rearrangement cannot compete with dimerization (E_a < 2 kcal/mol).