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

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Abstract
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]\documentclass{article}\pagestyle{empty}$\mathop +\limits_\cdot$ ion from 4‐methoxycyclohexene.