Dibenzoheterole-Fused s-Indacenes

Abstract

Heterole (pyrrole, thiophene, furan, thiophene-S,S-dioxide)-fused s-indacenes are known for their enhanced paratropic ring-current strength. However, the outcome of the antiaromatic properties for dibenzoheterole-fused s-indacene antiaromatics remained underexplored. Carbazole-, dibenzothiophene-, dibenzofuran-, and dibenzo[b,d]thiophene-5,5-dioxide-fused s-indacenes 1–4, respectively, were synthesized and characterized by experimental (NMR, single-crystal, UV–vis, CV) and computational (DFT) approaches to study the ground-state antiaromatic properties. Sulfone-containing 4 showed the weakest paratropic ring-current strength for the s-indacene unit, while 1–3 showed a relatively greater paratropicity for the s-indacene unit, as evidenced by the changes in 1H NMR chemical shifts of s-indacene protons. Such observation was explained by the electron-withdrawing effect of the sulfone group and loss of 4n + 2 aromaticity of the heterole unit for 4 reducing its s-indacene paratropicity strength as the nonaromaticity of the heterole unit reduces the π-bond character at the dibenzo[b,d]thiophene-5,5-dioxide/s-indacene fusion site to avoid antiaromatic s-indacene ring formation. The modulation of the paratropic ring-current strength of s-indacene for 1–4 was further supported by the NICS(1)zz and ring-current (ACID) calculations.

Is azulene's local aromaticity and relative stability driven by the Glidewell–Lloyd rule?

“…8 This rule establishes that the population of π-electrons in conjugated polycyclic systems tends to form the smallest 4n+2 groups and to avoid the formation of the smallest 4n groups and has been successfully used to indicate the influence of local aromaticity on the stability of indole and its most known isomers. 9 Additionally, by applying this rule, the aromaticity patterns of more complex systems such as heteroatomic s-indacene and fuzed derivatives, 10–12 among others, 13,14 can be studied, allowing, as mentioned, the design of compounds with photovoltaic properties and even biological activity. 13,15…”

The Glidewell–Lloyd rule explains the local aromaticity and the relative stability of benzoborepin isomers

“…This rule has recently been validated in a series of polycyclic aromatic hydrocarbons and also in heteroaromatic compounds such as indole and its isomers, where the GLR rule was used to relate the local aromaticity of these compounds to their relative stability. 10–14…”