  Diels-Alder cycloadditions are efficient routes for the synthesis of cyclic
organic compounds. There has been a long-standing discussion whether these
reactions proceed via stepwise or concerted mechanisms. Here, we adopt a new
experimental approach to explore the mechanistic details of the model polar
cycloaddition of 2,3-dibromo-1,3-butadiene with propene ions by probing its
conformational specificities in the entrance channel under single-collision
conditions in the gas phase. Combining a conformationally controlled molecular
beam with trapped ions, we find that both conformers of the diene, gauche and
s-trans, are reactive with capture-limited reaction rates. Aided by
quantum-chemical and quantum-capture calculations, this finding is rationalised
by a simultaneous competition of concerted and stepwise reaction pathways,
revealing an interesting mechanistic borderline case.
