Despite their signi ficance in organic synthesis, the mechanistic details of Diels-Alder cycloadditions, in which a diene and a dienophile react to form a cyclic product, still remain an unsolved and extensively discussed question. It has proven difficult to determine whether only the s-cis conformer (concerted mechanism) or both conformers (stepwise mechanism) of the involved diene react to form the cyclic product. Due to their high stereo- and regioselectivity, a concerted mechanism is generally assumed [1]. For polar Diels-Alder reactions, however, where one of the reactants contains a charge, the reaction may instead favor a stepwise mechanism [2].
In order to shed light on these questions we investigate the reactivities of individual conformers by employing molecular beam technology [3]. Owing to their diff erent dipole moments, conformers can be spatially separated in an inhomogeneous electric fi eld. Directing a molecular beam of spatially separated conformers into a cloud of sympathetically cooled molecular ions in an ion trap allows us to study highly conformationally selective reactions such as Diels-Alder cycloadditions. Previously, this method has been successfully applied to study reactive collisions of 3-aminophenol with a Coulomb crystal of Ca+ ions [4][5]. As a representative example, we want to study the conformationally resolved ionic cycloaddition reaction of 2,3-dibromo-1,3-butadiene which exhibits two di fferent conformers, s-cis and s-trans, with ionic maleic anhydride. The products of the reactive collisions are analyzed by an integrated high-resolution time-of-flight mass-spectrometer. The performance of the setup was recently confi rmed by measuring the rate constant of the reaction between laser cooled Ca+ ions with N2O [6]. This advancement will enable us to trace the complex mechanistic pathways of Diels-Alder reactions.
Besides conformers, our setup also enables us to study state selective reactions. In the presence of a strong inhomogeneous electric fi eld, water molecules in a molecular beam can be separated into their two nuclear spin isomers ortho and para which correlate with di fferent rotational states [7]. Currently we are investigating the reaction of rotational state selected H2O molecules with N2+ and N2H+ ions. The flexibility of our setup allows us to access the individual spin isomers and study their reactivities.
[1] M. Eberlin, Int. Journal of Mass Spectrometry, 2004, 235, 263.
[2] U. Rivero et. al., Chem. Phys. Lett., 2017, dx.doi.org/10.1016/j.cplett.2017.03.063.
[3] F. Filsinger et al., Phys. Rev. Lett., 2008, 100, 133003.
[4] Y.-P. Chang et al., Science, 2013, 342, 98.
[5] D. Rösch et al., JCP, 2014, 140, 124202).
[6] D. Rösch et. al., EPJ techniques and instrumentation, 2016, 3:5.
[7] D. A. Horke et al., Angew. Chem. Int. Ed., 2014, 53, 11965- 11968.