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

The synthesis and properties of a novel type of (alpha),(alpha)-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.