Unimolecular reactions of ionized methyl allyl ether

The fragmentation of CH₂?CHCH₂OCH₃^+· cation-radicals has been investigated by means of ²H- and ¹³C-labelling experiments and by analysis of collision-induced dissociation spectra. Metastable C₄H₈O^+· species decompose via one of three main channels which involve loss of (a) a hydrogen atom, (b) a methyl radical or (c) a formaldehyde molecule. Extensive, but not complete, exchange of the hydrogen and deuterium atoms in specifically labelled C₄H₈-_nD_nO^+· analogues precedes each of the three fragmentation pathways. The role of distonic ions in the rearrangement steps which bring about hydrogen exchange is discussed. The influence of isotope effects on the relative rates of the major reactions and the associated kinetic energy releases is examined. Only loss of a hydrogen atom is subject to a substantial isotope effect. Elimination of a methyl radical releases a large amount of kinetic energy, as is shown by the broad and dish-topped appearance of the corresponding metastable peak (T_1/2 ≈ 42 kJ mol^?1). The carbon atom of the original methoxy group is specifically expelled in this process. Both the large T_1/2 value and the unusual site selectivity are atypical of methyl and other alkyl radical losses from ionized alkenyl methyl ethers. The carbon atom of the methoxy group also participates specifically in formaldehyde elimination, but the two hydrogen atoms are not always selected from the three contained in the initial methoxy group. The implications of these labelling results for the synchronicity of concert of formaldehyde loss, which can be formu lated as a pericyclic process, is analysed.