Some Aspects of the Reactions of ortho-Quinodimethane Radical Cations and their Isomers in Gas-Phase Cycloaddition Reactions.

Abstract

The reactions of the radical cations of ortho-quinodimethane, styrene, and benzocyclobutene with ethene, propene and norbornadiene have been studied by FT-ICR spectrometry using an external ion sourer. The products of the ion-molecule reactions and the reaction efficiencies were determined. The isomeric C8H9+. radical cations can be generated in the gas phase under carefully controlled conditions as stable, non-interconverting species. Styrene radical cations were produced directly from styrene by electron impact ionization or by charge exchange. ortho-Quinodimethane radical cations were generated by loss of H2O from ionized 2-methylbenzyl alcohol or by electron impact ionization of benzocyclobutene. However, charge exchange of benzocyclobutene with toluene molecular ions produced mixtures of ortho-quinodimethane and benzocyclobutene radical cations. The latter ions are unreactive toward the alkenes studied (other than charge exchange). The radical cations of styrene and ortho-quinodimethane are distinguished unambiguously by the rate constants of their reactions with alkenes and by the reaction products. The reaction of the styrene radical cations with propene are best explained by the formation of an intermediate open-chain adduct, which undergoes fragmentation in competition with cyclization to a [4+2] cycloadduct. All fragmentation products from the reactions of ortho-quinodimethane radical cations are derived from the [4+2] cycloadduct, suggesting either a concerted process or a fast two-step process. The difference between the reactivity of the radical cations of styrene and ortho-quinodimethane is particularly clear for the reaction with norbornadiene. Ionized styrene reacts predominantly by charge exchange while ionized ortho-quinodimethane forms mainly product ions by a [4+2] cycloaddition - [4+2] cycloreversion reaction sequence.