Destabilized Carbenium Ions: α-Carbomethoxy-α,α-dimethyl-methyl Cations

Article abstract of DOI:10.1002/oms.1210240607

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 CH3^. 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 (T₅₀ = 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 ⁺ isomers.Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.