Dibenzoheterole-Fused <i>s</i>-Indacenes
Hemonta Kumar Saha, Dibyendu Mallick, Soumyajit Das
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 1 H NMR chemical shifts of s -indacene protons. Such observation was explained by the electron-withdrawing effect of the sulfone group and loss of 4 n + 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.