Evidence for a [2+4] cycloaddition in the ion‐molecule reaction of ionized 1,3‐butadiene with vinyl ethyl (methyl) ether in the gas phase

By use of deuterium labelling it is shown that the radical cation of 1,3‐butadiene reacts with vinyl methyl (or ethyl) ether in the gas phase through a [2+4] cycloaddition reaction. This leads to an excited collision complex of ionized 4‐methoxy‐ (or ethoxy) cyclohexene, which expels mthanol (or ethanol) through a 1,3 elimination, as does 4‐methoxycyclohexene itself upon electron impact. The spectrum of daughter ions produced by collision induced decomposition of the resulting ion with m/e 80 has been compared with spectra produced by the collision induced decompositions of the m/e 80 ions given by 1,3‐cyclohexadiene, 1,4‐cyclohexadiene, 1,3,5‐hexatriene and the [M–MeOH]{article}{empty}{document}+_·{document} ion from 4‐methoxycyclohexene.