The retro-een reaction is a common reaction of metastable immonium ions. leading to the expulsion of alkene molecule. The kinetic energy release that accompanies this reaction decreases with alkyl substittution of the #gamma#-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 prodcts via a single six-membered cyclic transition state . The calculated energy barriers and the measured kinetic energy releases are higherly sensitive to alkyl substitution at the #gamma#-position , which indicates that a substantial fraction of the positiove charge of the transition state resides at the #gamma#-carbon , not that the retro-ene reaction is a stepwise process with a carbocation intermediate
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