High Stabilities of Di(1-azulenyl)(4-hydroxyphenyl)methyl Hexafluorophosphates and Polarized Properties of α,α-Di(1-azulenyl)-1,4-benzoquinone Methides

Acid-catalyzed condensation of azulenes 8a−c with 4-hydroxy- and 3,5-di-tert-butyl-4-hydroxybenzaldehyde leads to substituted di(1-azulenyl)(4-hydroxy- and 3,5-di-tert-butyl-4-hydroxyphenyl)methanes 7a− f, which are easily converted into substituted di(1-azulenyl)(4-hydroxy- and 3,5-di-tert-butyl-4-hydroxyphenyl)methyl cations 5a−f by oxidation with DDQ. The spectroscopic data are consistent with the protonated cationic structures of 5a−f. The electrochemical reduction of 5a−f showed a reversible wave at −0.74 to −0.86 V (V vs Ag/Ag+) upon cyclic voltammetry (CV), although 5d and 5e showed an irreversible wave at −0.79 V. The relatively high reduction potentials of 5a−f, compared with those of di(1-azulenyl)phenylmethyl cations 2a−c, exhibit the stabilization by 4-hydroxy substituents on the phenyl groups. These salts (5a−f·PF6 -) bearing 4-hydroxyl groups on the phenyl rings have been converted by treatment with bases to α,α-di(1-azulenyl)-1,4-benzoquinone methides 6a−f, which revert to 5a−f·PF6 - upon reprotonation with HPF6. These quinone methides (6a−f) are highly polarized by the extreme-electrodonating properties of 1-azulenyl groups. The highly polarized properties of 6a−f reflected to the high pK a values of their conjugate acids (5a−c, 6.5−7.1, and 5d−f, 3.4−3.8). The strong solvatochromic effects also provide strong evidence of a large contribution of dipolar forms (6‘) in the ground state. The relatively low oxidation potentials of 6a−f (+0.35 to +0.47 V vs Ag/Ag+) reflected facile formation of phenoxy radical cations, stabilized by two 1-azulenyl groups.