SOME ASPECTS OF THE REACTIONS OF ORTHO-QUINODIMETHANE RADICAL CATIONSAND THEIR ISOMERS IN GAS-PHASE CYCLOADDITION REACTIONS

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 prod ucts of the ion-molecule reactions and the reaction efficiencies were determined. The isomeric C8H9+. radical cations can be generated in th e gas phase under carefully controlled conditions as stable, non-inter converting species. Styrene radical cations were produced directly fro m styrene by electron impact ionization or by charge exchange. ortho-Q uinodimethane radical cations were generated by loss of H2O from ioniz ed 2-methylbenzyl alcohol or by electron impact ionization of benzocyc lobutene. However, charge exchange of benzocyclobutene with toluene mo lecular ions produced mixtures of ortho-quinodimethane and benzocyclob utene radical cations. The latter ions are unreactive toward the alken es studied (other than charge exchange). The radical cations of styren e and ortho-quinodimethane are distinguished unambiguously by the rate constants of their reactions with alkenes and by the reaction product s. The reaction of the styrene radical cations with propene are best e xplained by the formation of an intermediate open-chain adduct, which undergoes fragmentation in competition with cyclization to a [4+2] cyc loadduct. All fragmentation products from the reactions of ortho-quino dimethane radical cations are derived from the [4+2] cycloadduct, sugg esting either a concerted process or a fast two-step process. The diff erence between the reactivity of the radical cations of styrene and or tho-quinodimethane is particularly clear for the reaction with norborn adiene. Ionized styrene reacts predominantly by charge exchange while ionized ortho-quinodimethane forms mainly product ions by a [4+2] cycl oaddition - [4+2] cycloreversion reaction sequence.