Studies of the Electron-Promoted Cope Cyclization of 2,5-Phenyl-Substituted 1,5-Hexadiene Radical Anions

Abstract

This work describes studies of the electron-promoted Cope cyclization of 2,5-phenyl-1,5-hexadiene radical anions in a flowing afterglow triple quadrupole mass spectrometer. The electronic properties of the hexadienes have been systematically modified by using aromatic substituents at the 2- and 5-positions of the hexedienes, including those with nitro, trifluoromethyl, fluoro, chloro, and acetyl groups. Ions were formed by the thermal attachment of electrons in the gas phase. Structures of the molecular radical anions were probed to determine whether they undergo cyclization to a cyclohexane-1,4-diyl anion structure by examining chemical reactivity with neutral reagents including carbon dioxide, carbon disulfide, and nitric oxide. First-order rate constants for the reactions of ions were measured, and the reaction efficiencies were determined. Based on the reactivity results, a thermochemical model has been developed, which predicts the reaction thermochemistry by using thermochemical properties of model systems. The observed reactivity from ion-molecule reactions and the study of reaction rates show that the ion of 2,5-dicyanohexadiene and 2,5-di(4,4'-trifluoromethyl phenyl)-1,5-hexadiene undergo Cope cyclization, whereas the radical anions having substituents such as the fluoro, nitro, chloro, and acetyl groups do not.