Synthesis and properties of bis(heteroazulen-3-yl)methyl cations and bis(heteroazulen-3-yl)ketones

The synthesis and properties of a novel type of bis(heteroazulen-3-yl)methyl cations, bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyl cation salt and nitrogen analogues, () and (), as well as bis(heteroazulen-3-yl)ketones () are studied. The synthetic method was based on a TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes () with paraformaldehyde to afford the corresponding disubstituted methane derivatives , followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HPF6 or aq. HBF4. In addition, the reaction of with 2.2 equiv. amounts of DDQ afforded carbonyl compounds . The delocalization of the positive charge of was evaluated by the 1H and 13C NMR spectral data. The thermodynamic stability of cations was evaluated to be in the order << on the basis of their reduction potentials measured by cyclic voltammetry (CV) and pKR+ values (2.6–10.3) obtained spectrophotometrically. The reduction waves of cations were irreversible, suggesting the dimerization of the radical species generated by one-electron reduction. This was demonstrated by the reduction of with Zn powder to give dimerized product . In addition, the quenching of with MeOH/NaHCO3 gives ether derivative , which is proposed for the precursor for synthesizing tris(heteroazulene)-substituted methyl cations bearing two different heteroazulene-units.