Dication species stabilized by heteroazulenes: synthesis and properties of 1,3- and 1,4-bis[bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyliumyl]-, bis[bis(1,2-dihydro-N-methyl-2-oxocyclohepta[b]pyrrol-3-yl)methyliumyl]benzene, and their related dications

Abstract

A general synthetic route to a novel type of heteroazulene analogue of exceptionally stable dimethyliumbenzenes (16a–c·2PF₆66⁻−− and 17a–c·2PF₆66⁻−−) bearing 1,3-di- and 1,4-dimethylium groups substituted with four 2H-cyclohepta[b]furan-2-one 9a, four 1,2-dihydro-N-methylcyclohepta[b]pyrrol-2-one 9b, and two 9a and two 9b, is reported. The synthetic method is based on a single and stepwise TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes 9a and 9b with isophthalaldehyde and terephthalaldehyde to afford the corresponding 1,3- and 1,4-dimethylbenzene derivatives, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HPF₆ solution. In spite of the dicationic nature of 16a–c and 17a–c, they exhibited high stability with large pK_R⁺ values due to the stabilizing effect of the heteroazulene units; however, we could not determine pK_R⁺ and pK_R⁺⁺ values separately. Thus, the pK_R⁺ values correspond to the average of the pH used to form neutralized dications and half-neutralized monocations. The electrochemical reduction of the cations exhibits irreversible waves and low reduction peak potentials upon cyclic voltammetry (CV); the values are discussed on the basis of the comparison with those of the related monocation species.