Electron spin resonance studies of radical addition to methylenecycloalkanes

Abstract

E.s.r. spectroscopy has been used to investigate homolytic addition to methylenecyclo-propane, -butane, and -pentane and to the acyclic 2-methylpropene. Regiospecific addition of Me₃SiO˙, Bu^tOH⁺, and Me₃Si˙ to the unsubstituted (tail) end of the double bond takes place with 2-methylpropene and methylenecyclo-butane and -pentane. However, although Me₃Si˙ adds to the tail carbon of methylenecyclopropane, substituted but-3-enyl radicals result from the reactions of this alkene with Me₃SiO˙, Bu^tO˙, or Bu^tOH⁺. Ring opening thus accompanies or follows initial addition of the electrophilic radicals to the head carbon atom of methylenecyclopropane. This unusual regioselectivity is attributed to the dominance of polar effects in determining the activation energies for addition.