Bicyclobutonium ion versus cyclopropylcarbinyl cation chemistry: The cycloaddition reaction with ethene in the dilute gas phase

The C₄H⁺₇ cations were formed from cyclopropylcarbinyl, cyclobutyl and homoallyl chlorides via chloride atom loss from the radical cations in a chemical ionization source. Their structures were probed on a short time-scale (of the order of 10^-6 s) as a function of internal energy by utilizing collisionally activated dissociation with tandem mass spectrometric methods. The results show that the radical cations of cyclopropylcarbinyl and homoallyl chlorides generate primarily the cyclopropylcarbinyl cation, 3, whereas the radical cation of cyclobutyl chloride generates a substantial amount of bicyclobutonium ion, 4. Ion 4 reacts with nucleophiles via multiple competing reaction pathways, unlike 3. Ion 3 undergoes a cycloaddition reaction with ethene to generate the cyclopentylcarbinyl cation. Ion 4, although reactive with ethene, does not generate, to any appreciable degree, the cyclopentylcarbinyl cation.