The retro-ene reaction of gaseous immonium ions revisited†

The retro-ene reaction is a common reaction of metastable immonium ions, leading to the expulsion of an alkene molecule. The kinetic energy release that accompanies this reaction decreases with alkyl substitution on the γ-carbon of the immonium ion. A number of authors have taken this result to indicate that the retro-ene reaction proceeds in a non-concerted manner via a carbocation intermediate. Using ab initio calculations to test this hypothesis we find that the retro-ene reaction is a concerted process, in which the reactant immonium ion is connected to the products via a single six-membered cyclic transition state. The calculated energy barriers and the measured kinetic energy releases are highly sensitive to alkyl substitution at the γ-position, which indicates that a substantial fraction of the positive charge of the transition state resides at the γ-carbon, not that the retro-ene reaction is a stepwise process with a carbocation intermediate.