High Stabilities of Di(1-azulenyl)(4-hydroxyphenyl)methyl Hexafluorophosphates and Polarized Properties of alpha,alpha-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.PF(6)(-)) bearing 4-hydroxyl groups on the phenyl rings have been converted by treatment with bases to alpha,alpha-di(1-azulenyl)-1,4-benzoquinone methides 6a-f, which revert to 5a-f.PF(6)(-) upon reprotonation with HPF(6). 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.
Ito
Shunji
S
Department of Chemistry, Faculty of Science, Tohoku University, Kawauchi, Aoba-ku, Sendai 980-77, Japan, and Faculty of Education, Iwate University, Ueda, Morioka 020, Japan.
Kikuchi
Shigeru
S
Kobayashi
Hiroki
H
Morita
Noboru
N
Asao
Toyonobu
T