Synthesis and Properties of Extremely Stable Tris(6-methoxy-1-azulenyl)methyl Cation and a Series of Di(1-azulenyl)phenylmethyl and (1-Azulenyl)diphenylmethyl Cations Stabilized by Methoxy Substituents

Abstract

Extremely stable carbocations, tris(6-methoxy-1-azulenyl)methyl (8), bis(6-methoxy-1-azulenyl)(4-methoxyphenyl)methyl (9a), and (6-methoxy-1-azulenyl)bis(4-methoxyphenyl)methyl (10a) cations and a series of di(1-azulenyl)phenylmethyl and (1-azulenyl)diphenylmethyl cations having methoxy substituents on each phenyl group, i.e., di(1-azulenyl)(4-methoxyphenyl)methyl (9b) and (1-azulenyl)bis(4-methoxyphenyl)methyl (10b) cations and 3-methyl-1-azulenyl (9cand 10c) and 3,6-di-t-butyl-1-azulenyl (9d and 10d) analogues, were synthesized by hydride abstraction from the corresponding methane derivatives and their properties were fully characterized. The pKR+ values of 8 and 9a were well beyond 14.0. The value of 10a was determined as 13.2, which is higher by 10.2 pK units than that of (1-azulenyl)diphenylmethyl cation. The value also considerably increased by the methoxy substitution on each phenyl group. The values of 9b—d (pKR+ 11.7—13.4) and 10b—d (pKR+ 5.2—7.0) are higher by 1.0—1.4 and 2.2—2.4 pK units than those of the corresponding analogous benzyl and diphenylmethyl cations. The electrochemical reduction of 8, 9a—d, and 10a—d showed a wave at −0.88, −0.71 — −0.83, and −0.56 — −0.71 V (V vs. Ag/Ag+), respectively, upon cyclic voltammetry (CV). The relatively high reduction potentials also exhibited the stabilization of the methyl cations by the methoxy substituents. The oxidation of 8 in acetonitrile exhibited barely separated two-step, one-electron oxidation waves at a potential range of +0.90 — +0.98 V upon CV, although 9a—d and 10a—d did not show two similar waves at a narrow potential range. The wave is ascribed to the oxidation of two azulene rings to generate a trication species.

Carbocations Confined in a Thiatriazuliporphyrin Frame

“…The easily accessible organic tetracation incorporating the 21-thiatriazuliporphyrinf rame provides ar are instanceo fm acrocyclic tetracations precedented by an electron-deficient tetracationic cyclophane [23] (cyclobis-(paraquat-p-phenylene)w idely used in the construction of molecular machines or switches. [24] By considering the apparent importance of acyclic azulene-stabilized cations, dications, and trications, [25][26][27][28][29][30] as well as the azulene cation in the design of electronic materials, these readily available macrocyclic polycation species may be regardeda sacontrollable building block for conductive or semiconductive applicationso rf ormation of stabilized electrochromic and polyelectrochromic materials. [31,32] Experimental Section General Dichloromethane was distilled from calcium hydride before use as the reaction solvent.…”

Tetraazuliporphyrin Tetracation

“…The signals in the UV/Vis spectrum of 6 a‐2 (Figure 4, inset), which bears the characteristic features of free azulene, are replaced by a set of intense bands in the visible region with the most intense signal located at 588 nm. The spectrum of 5 a resembles those of azulene‐containing methylium salts 31–33…”

Azulene Moiety as Electron Reservoir in Positively Charged Systems; A Short Survey

“…It has been suggested that this increase can be attributed to the steric effect and less to hyperconjugation [16]. The contribution of the donor substituents such as OMe [22,23] or NMe 2 [24,25] to phenyl and/or azulene is more important. As shown in Scheme 7, for cation 10 (R ph = R az = Me 2 N) the charge is distributed between the resonance structures A, A' and A", namely methylium, tropylium, and ammonium ions.…”