3-Methylenecyclobutyl, cyclopent-3-enyl, and 3-methylenecyclobutylmethyl radicals; absence of homoallylic conjugation

Abstract

The 3-methylenecyclobutyl radical and the cyclopent-3-enyl radicals show such small e.s.r. hyperfine splittings from the δ- and γ-hydrogens respectively that homoallylic conjugation can be ruled out. Semi-empirical SCF-MO calculations indicated that the through-space interaction of the p-orbital at Ċ_α with the π-orbitals is negligible because they are >2 Å apart. The 3-methylenecyclobutylmethyl radical rearranges by β-scission to give the 2-allylallyl radicals. The Arrhenius parameters of the rearrangement were determined by kinetic e.s.r. spectroscopy and by study of the reduction of 3-methylenecyclobutyl-methyl bromide with tri-n-butyltin hydride. The resonance stabilisation of the rearranged radical causes no significant lowering of the activation energy for β-scission.