Gas-Phase Diels-Alder Reaction of the o-Quinodimethane Radical Cation and Neutral Styrene

The collision complex formed in the ion-molecule reaction of o-quinodimethane radical cation and neutral styrene was investigated by using mass spectrometry/mass spectrometry (MS/MS) techniques. The reaction was conducted in a high-presure chemical-ionization source, where the reagents were ionized by low-energy charge exchange and the reaction products were collisionally stabilized. The collision complex was shown to have the structure of 2-phenyltetralin through the use of deuterium labeling and direct comparison with the properties of reference radical cations. These results establish the structural integrity of low-energy o-quinodimethane radical ions and are evidence for a Diels-Alder reaction mechanism.