Professor Tetsuo Nozoe and My Tropylium Ion Chemistry

Abstract

Needless to say, the discovery of hinokitiol with its "unconventional" aromaticity by Professor Tetsuo Nozoe is one of the most important achievements in organic chemistry in the last century. The essence of this "non-benzenoid" aromaticity in hinokitiol is of course that of tropolone, and it is further related to the aromaticity of tropone and the tropylium ion, i.e., the cycloheptatrienyl cation. In this account, details of the study conducted by the author's group, particularly on the synthesis and properties of tropylium ion derivatives with various unique structures pursuing the ultimately high carbocation stability, are described.

Synthesis and Properties of Azahomocorannulenyl Cations and Radicals

“…According to the Hückel rule, this species is classified as aromatic and displays notable stability, as demonstrated by the isolation of stable tropylium compounds. [3][4][5][6][7] Despite their aromaticity, tropylium cations exhibit electrophilic reactivity as Lewis acids and oxidants, which has led to their utilization as catalysts and stoichiometric reagents in a diverse array of organic transformations [8,9] and as functional organic dyes/chromophores in material sciences. [10][11][12][13][14][15][16][17] In contrast, the neutral tropyl radical (C 7 H 7 * ; B), first experimentally observed in 1960, [18] represents a more elusive species due to its structural and electronic complex-ity primarily attributed to the presence of 7π electrons.…”

Synthesis and Properties of Azahomocorannulenyl Cations and Radicals

“…According to the Hückel rule, this species is classified as aromatic and displays notable stability, as demonstrated by the isolation of stable tropylium compounds. [3][4][5][6][7] Despite their aromaticity, tropylium cations exhibit electrophilic reactivity as Lewis acids and oxidants, which has led to their utilization as catalysts and stoichiometric reagents in a diverse array of organic transformations [8,9] and as functional organic dyes/chromophores in material sciences. [10][11][12][13][14][15][16][17] In contrast, the neutral tropyl radical (C 7 H 7 * ; B), first experimentally observed in 1960, [18] represents a more elusive species due to its structural and electronic complex-ity primarily attributed to the presence of 7π electrons.…”

Stable Red‐Emissive Cationic Dithienotropylium Dyes

“…[5,6] Since the first report by Knox et al in 1954, [5] the structure-stability relationship of tropylium ions has been studied extensively.F or example, Ko-matsua nd co-workers accomplished the thermodynamic stabilizationo ft ropylium ions by the introduction of electron-donatingalkyl groups, [7] intramolecular p-electron donors, [8] or by imposing ab icyclo[2.2.2]octene-fused structure. [5,6] Since the first report by Knox et al in 1954, [5] the structure-stability relationship of tropylium ions has been studied extensively.F or example, Ko-matsua nd co-workers accomplished the thermodynamic stabilizationo ft ropylium ions by the introduction of electron-donatingalkyl groups, [7] intramolecular p-electron donors, [8] or by imposing ab icyclo[2.2.2]octene-fused structure.…”

“…[4] The aforementioned dyes share ac ommon molecular design feature that allowst hem to attainh igh stability toward nucleophiles in combination with attractive photophysical properties:t he presence of electron-donating amino groups on ah ighly extended p-conjugated carbocation scaffold, which allows the delocalization of ap ositive chargeo ver the entire skeleton.In this context, the tropylium ion should be ap articularly useful building block, as it is exceptionally stable on account of its pronouncedH ückel aromaticity. [5,6] Since the first report by Knox et al in 1954, [5] the structure-stability relationship of tropylium ions has been studied extensively.F or example, Ko-matsua nd co-workers accomplished the thermodynamic stabilizationo ft ropylium ions by the introduction of electron-donatingalkyl groups, [7] intramolecular p-electron donors, [8] or by imposing ab icyclo[2.2.2]octene-fused structure. [9] The effect of annulating heterocyclic rings onto the tropylium ion was studied by Gronowitz and co-workers.…”

“…In this context, the tropylium ion should be ap articularly useful building block, as it is exceptionally stable on account of its pronouncedH ückel aromaticity. [5,6] Since the first report by Knox et al in 1954, [5] the structure-stability relationship of tropylium ions has been studied extensively.F or example, Ko-matsua nd co-workers accomplished the thermodynamic stabilizationo ft ropylium ions by the introduction of electron-donatingalkyl groups, [7] intramolecular p-electron donors, [8] or by imposing ab icyclo[2.2.2]octene-fused structure. [9] The effect of annulating heterocyclic rings onto the tropylium ion was studied by Gronowitz and co-workers.…”