Alkene loss from metastable methyleneimmonium ions: Unusual inverse secondary isotope effect in ion-neutral complex intermediate fragmentations

The mechanism of propene elimination from metastable methyleneimmonium ions is discussed. The first field‐free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H2C = \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^{\rm +} $\end{document}+R1R2: R1 = R2 = n‐C3H7; R1 = R2 = i‐C3H7; R1 = n ‐C3H7; R2 = C2H5; R1 = n‐C3H7; R2 = CH3; R1 = n‐C3H7; R2 = H) were used to support the mechanism presented. The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly. Propene loss from the ions examined proceeds via ion‐neutral complex intermediates. For the di‐n‐propyl species rate‐determining and H/D distribution‐determining steps are clearly distinct Whereas the former corresponds to a 1,2‐hydride shift in a 1‐propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2‐propyl cation generated by the previous step. For the diisopropyl‐substituted ions which directly form the 2‐propyl cation‐containing complex, the rate‐determining hydride shift vanishes. The 2‐propyl cation‐containing complex can decompose directly or via an intermediate proton‐bridged complex. Competition of these routes is not excluded by the experimental results. Assuming a 2:1:3 distribution, a preference for the α‐ and β‐methylene of the initial n‐propyl chain as the source of the hydrogen transferred is detected for n‐propylimmonium ions containing a second alkyl chain R2. This preference shows a clear dependence on the steric influence of R2. During the transfer step isotopic substitution is found to affect the H/D distribution strongly. For the alternative route of McLafferty rearrangement leading to C2H4 loss, specific γ‐H transfer is observed.