Trication species stabilized by heteroazulenes: synthesis and properties of 1,3,5-tris[bis(heteroazulen-3-yl)methyliumyl]benzenes

Abstract

A general synthesis and properties of a novel type of heteroazulene analogues of fairly stable trimethyliumylbenzenes (14a–i·3BF₄⁻) bearing 1,3,5-trimethyliumyl groups substituted with six 2H-cyclohepta[b]furan-2-one 8a, six 1,2-dihydro-N-phenylcyclohepta[b]pyrrol-2-one 8b, six 1,2-dihydro-N-methylcyclohepta[b]pyrrol-2-one 8c, and their related compounds are reported. The synthetic method is based on a single and stepwise TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes 8a, 8b, and 8c with 1,3,5-triformylbenzene 9, mono- and diformylbenzene having di-and monoheteroazulene-substituted methyl groups to afford the corresponding 1,3,5-trimethylbenzene derivatives, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HBF₄ solution in Ac₂O. In spite of their tricationic nature, 14a–i exhibited high stability with large [pK_R+] values due to the stabilizing effect of the heteroazulene units. In the case of trications 14b, three methyliumyl-units were neutralized stepwise at the pH of 10.4, 11.5, and 13.0. However, we could not determine pK_R+, pK_R++, and pK_R+++ values separately in the cases of other trications 14a and 14c–i. Thus, some [pK_R+] values were obtained as the average values of pK_R+++ and pK_R++ values as well as of pK_R++ and pK_R+ values. The electrochemical reduction of most of the trications exhibits irreversible waves and low reduction peak potentials upon cyclic voltammetry (CV); the values are discussed on the basis of a comparison with those of the related monocation and dication species to clarify the reduction process of trications 14a–i. The reduction waves of 14a–e,h,i were irreversible, while those of 14f,g seem to be reversible; this feature would be ascribed to their large steric constraints.