Destabilized carbenium ions: α‐carbomethoxy‐α,α‐dimethyl‐methyl cations

Tertiary α‐carbomethoxy‐α,α‐dimethyl‐methyl cations a have been generated by electron impact induced fragmentation from the appropriately α‐substituted methyl isobutyrates 1–4. The destabilized carbenium ions a can be distinguished from their more stable isomers protonated methyl methacrylate c and protonated methyl crotonate d by MIKE and CA spectra. The loss of I — and Br˙ from the molecular ions of 1 and 2, respectively, predominantly gives rise to the destabilized ions a , whereas loss of Cl˙ from [3] + ˙ results in a mixture of ions a and c . The loss of CH 3 ˙ from [4] + ˙ favours skeletal rearrangement leading to ions d . The characteristic reactions of the destabilized ions a are the loss of CO and elimination of methanol. The loss of CO is associated by a very large KER and non‐statistical kinetic energy release ( T 50 = 920 meV). Specific deuterium labelling experiments indicate that the α‐carbomethoxy‐α,α‐dimethyl‐methyl cations a rearrange via a 1,4‐H shift into the carbonyl protonated methyl methacrylate c and eventually into the alkyl‐O protonated methyl methacrylate before the loss of methanol. The hydrogen rearrangements exhibit a deuterium isotope effect indicating substantial energy barriers between the [C 5 H 9 O 2 ] + isomers. Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.