There has been a long-standing discussion whether Diels–Alder cycloadditions proceed via stepwise or concerted mechanisms. By combining a conformationally controlled molecular beam with trapped ions, we explored the mechanism of the model polar cycloaddition of conformer-specific 2,3-dibromo-1,3-butadiene with propene ions under single-collision conditions in the gas phase. Both conformers of the diene are reactive with capture-limited reaction rates and a simultaneous competition of concerted and stepwise reaction pathways was found.
Read more