Synthesis and Properties of α,α-Bis(heteroazulen-3-yl)-1,4-benzoquinonemethides

The synthesis and properties of a novel type of α,α-bis(heteroazulen-3-yl)-1,4-benzoquinonemethides 11a–f are studied. The synthetic method was based on a TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes with 4-hydroxybenzaldehyde to afford the corresponding methane derivatives, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HBF4 or aq. HPF6 and neutralization. The polarization of 11a–f was evaluated by their 13C NMR and UV–vis spectral data. The thermodynamic stability of the conjugated acid of 11a–f was evaluated to be in the order 11a < 11b < 11c and in the order 11d < 11e < 11f based on their pKa values (<0–5.4) obtained spectrophotometrically. The substituent effect of t-Bu was discussed based on the UV–vis spectra and pKa values. In CV measurements, the reduction waves of quinonemethides 11a–f were reversible, suggesting a stabilizing effect of heteroazulenes toward the radical and anion species. Moreover, quinonemethides 11a–f showed two oxidation waves, and that the first oxidation potentials (E1ox) of 11a–c, which have two t-Bu groups, are reversible.