Structure and Aromatic Character of Non‐benzenoid Cyclically Conjugated Systems

Abstract

The chemistry of aromatic compounds was originally confined to benzenoid hydrocarbons and their derivatives but has been enriched in the last decade by the discovery of numerous non‐benzenoid, cyclically conjugated systems that are more or less like benzene in their properties. This has necessitated a new conception of “aromatic character”. Theoretical and experimental studies have been applied to this problem. In this connection, several selected representatives of this class of compound are described, mostly from studies by the author's own group, and relationships between structure and “aromatic character” are indicated.

Construction of Polycyclic π‐Conjugated Systems Incorporating an Azulene Unit Following the Oxidation of 1,8‐Diphenyl‐9,10‐bis(phenylethynyl)phenanthrene

“…Moreover, recent advances in transition metal‐mediated functionalizations such as borylation, arylation, and silylation have provided methods to develop fascinating azulene‐based materials . Unlike alternant π‐conjugated systems, the non‐alternant structure of azulene contributes to the construction of peculiar π‐conjugated systems . In addition to the electronic perturbations of the alternant π‐conjugated systems, many three‐dimensional nanocarbon structures with curvature have been proposed by embedding pentagons or heptagons in the hexagonal honeycomb lattice .…”

2H-Dibenzo[b,f]azepin-2-one

“…The crystal structure of 2H-dibenzo[b,f]azepin-2-one, (la), was obtained with the express interest of determining the contribution that the novel nitrenium ion, (lb), makes to the overall structure. The latter contains a central azepine ring that can be looked upon as the nitrogen analogue of the well known tropylium cation (Kolomnikova & Parnes, 1967;Von Doering & Knox, 1954;Pietra, 1973;Hafner, 1964). All attempts to prepare or detect the parent heterotropylium cation, (2), have to date failed, although intermediacy of a benzoazatropylium ion and azatropylium ions has been suggested for the fragmentation of 1-methylisoquinoline or of various substituted phenylazides (Marx & Djerassi, 1968;Abramovitch, 1961).…”

Promotion of Organic Reactions by Non‐Benzenoid Carbocyclic Aromatic Ions

“…[1] Their lesser known analogues,t he non-benzenoid carbocyclic aromatic structures,u sed to be considered of much less synthetic value. [3] Thediscovery of the bromide salt of the cycloheptatrienyl cation, now commonly known as the tropylium ion (3,F igure 1), by Merling in 1891 marked the beginning of the chemical history of non-benzenoid carbocyclic aromatic ions. [3] Thediscovery of the bromide salt of the cycloheptatrienyl cation, now commonly known as the tropylium ion (3,F igure 1), by Merling in 1891 marked the beginning of the chemical history of non-benzenoid carbocyclic aromatic ions.…”

“…[2] These fundamental chemical species normally possess anet charge and are likely to be less stable than the benzenoid systems. [3] Thediscovery of the bromide salt of the cycloheptatrienyl cation, now commonly known as the tropylium ion (3,F igure 1), by Merling in 1891 marked the beginning of the chemical history of non-benzenoid carbocyclic aromatic ions. [4] Theother two primary members of this aromatic ion family,n amely the cyclopentadienyl anion (cyclopentadienide 2,F igure 1) and the cyclopropenyl cation (cyclopropenium 1), were synthesized in 1901 by Thiele [5] and 1957 by Breslow, [6] respectively.T hese charged chemical entities have planar cyclic structures with (4n + 2)p electrons in conjugation, thus satisfying Hückelsr ule for aromaticity [7] and salts of these ions are known to be relatively stable in the solid state or in solution.…”