The Nature of the Intermediate in Carbonium Ion-Type Interconversion Reactions of Cyclobutyl, Cyclopropylcarbinyl and Allylcarbinyl Derivatives1

The striking ease of interconversion of cyclobutyl and cyclopropylcarbinyl derivatives in carbonium ion-type reactions and the abnormally large solvolytic reactivities of cyclobutyl and cyclopropylcarbinyl halides and sulfonate esters have led to a number of suggestions regarding the nature of the cationic intermediate (or intermediates) which might be involved. Investigation of the course of the reactions using C^(14)-labeled cyclopropylcarbinyl derivatives reveals that the three methylene groups of the starting material achieve a degree of equivalence at some point between reactants and products which is well (although not uniquely) accounted for by a "non-classical" cationic intermediate of structure I.