Manifestations of Antiaromaticity in Organic Materials: Case Studies of Cyclobutadiene, Borole, and Pentalene

Abstract

This Review surveys the experimental and synthetic history of three classic antiaromatic cores: cyclobutadiene, borole, and pentalene. The coverage extends from the bare molecular cores to fused or substituted variants that impart thermodynamic or kinetic stability to the antiaromatic systems. Recent studies to utilize the unusual properties of these antiaromatic cores in the context of advanced electronic materials are discussed.

Inorganic salt-assisted assembly of anionic π-conjugated rings enabling7Li NMR-based evaluation of antiaromaticity

“…Aromaticity and antiaromaticity have played a pivotal role in many fields of chemistry including organic chemistry and materials science. 1 Although the concept of aromaticity/antiaromaticity was originally developed for carbocyclic compounds with (4n + 2)/(4n)π-electron systems, as represented by benzene and cyclobutadiene, respectively, the concept has been gradually expanded. Nowadays, a number of unconventional aromatic/antiaromatic systems such as metallaaromatics, 2 heavier element-containing aromatics, 3 Möbius aromaticity, 4 and hyperconjugative aromaticity (also known as σ*-aromaticity), 5 have been developed.…”

Concealed antiaromaticity

“…9 Besides antiaromatic molecules with two degenerate SOMOs and a diradical character y0 of 1 (referred to as truly antiaromatic molecules in this article), there are molecules with a formal 4n πelectron system but reduced diradical character (between 0 and 1). 10 These molecules -including pentalene, dibenzopentalene, s-indacene, and biphenylene (Figure 1c) and their derivatives as well as different cyclooctatetraene derivatives -are often also referred to as antiaromatic, [11][12][13][14][15][16][17][18] even when the diradical character is low. However, the classification of such molecules as antiaromatic has been criticized, particularly for polycyclic molecules that feature both formal 4n and 4n+2 πelectron systems (such as dibenzopentalene and biphenylene, Figure 1c right).…”

Revisiting indeno[1,2-b]fluorene by steric promoted synthesis while isolating the second stable 4nπ indeno[2,1-a]fluorene

“…22,8,5 c This popular strategy lacks the generation of unsymmetrical [1,2- b ]IF or [2,1- a ]IF scaffolds, thereby possibly hindering the growth of 4nπ congeners of unsymmetrically disubstituted (4n + 2)π pentacenes 23 with potential for organic optoelectronics applications. 24 Another possible weakness of this strategy, as discussed in the literature, 5 c could be the harsh reagents and conditions which were employed to synthesize the dione precursors 25 for [1,2- b ]IF 26 and [2,1- a ]IF 27 isomers.…”