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

A general synthesis and properties of a novel type of heteroazulene analogues of fairly stable trimethyliumyl-benzenes (14a-i . 3BF(4)(-)) 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 a ord 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.