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

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.