Destabilized carbenium ions: Secondary and tertiary α‐carbomethoxybenzyl cations

Abstract

AbstractThe secondary α‐carbomethoxybenzyl cations a and the tertiary α‐carbomethoxybenzyl cations d have been generated by electron impact‐induced fragmentation from appropriately α‐substituted methyl phenylacetate and 2‐phenylpropionates 1–4. The ions a and d are further examples of destabilized carbenium ions with a push–pull substitution at the carbenium ion centre. The characteristic reaction of these ions is a rearrangement by a 1,2‐shift of the methoxy group concomitant to the elimination of CO. This rearrangement reaction is associated with a very large and non‐statistical kinetic energy release (a : T 50 = 570 meV; d : T 50 = 760 meV), which is attributed to tight transition states along the reaction coordinates corresponding to the three‐membered cyclic oxonium ions b and h, respectively. The tertiary ion d can be distinguished from its more stable isomers f and g by the mass‐analysed ion kinetic energy and collisional activation spectra. The investigation of specifically deuterated analogues of ions d and g reveals an isomerization of d to g via a species protonated at the phenyl group but no equilibration between d and g. This isomerization exhibits a large isotope effect for the hydrogen transfer, indicating similar energy barriers for the isomerization and for the CO elimination of d.