188866-61-7Relevant articles and documents
High Stabilities of Di(1-azulenyl)(4-hydroxyphenyl)methyl Hexafluorophosphates and Polarized Properties of α,α-Di(1-azulenyl)-1,4-benzoquinone Methides
Ito, Shunji,Kikuchi, Shigeru,Kobayashi, Hiroki,Morita, Noboru,Asao, Toyonobu
, p. 2423 - 2431 (2007/10/03)
Acid-catalyzed condensation of azulenes 8a-c with 4-hydroxy- and 3,5-di-tert-butyl-4-hydroxy-benzaldehyde 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 pKa 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.
