Positive charge delocalization and anti-aromaticity of cations generated by protonation of benzo[a]fluoranthenes in superacid

Benzo[a]fluoranthene (4a) is one of non-alternate polycyclic aromatic hydrocarbons. A reaction of 4a in CF₃SO₃H yielded a dark yellow solution. Direct NMR observation indicated the exclusive formation of carbocation 4aH⁺ by protonation at the C(8) position. The most deshielded ¹H and ¹³C signals were observed at 8.99 ppm for H(12) and 8.29 ppm for H(1), 182.6 ppm for C(12b), 155.6 ppm for C(8a), and 149.4 ppm for C(7a). The signals for H(4) and H(5) were significantly more shielded than those of 4a. A reaction of 3-tert-butylbenzo[a]fluoranthene (4b) in CF₃SO₃H afforded carbocation 4bH⁺ by the protonation at the C(8) position. 4bH⁺ was gradually converted to 4aH⁺. The changes in ¹³C NMR chemical shifts (Δδ¹³C) suggested that positive charge was delocalized into mainly seven carbons in 4aH⁺ and 4bH⁺. The observed cations were found to be the most stable cations among the possible protonation cations by the DFT method. The NICS(1)_zz values for the five-membered rings were calculated to be 35.6 for 4aH⁺ and 34.4 for 4bH⁺ by GIAO-B3LYP/6-311+G(2d,p). The experimental NMR and the NICS(1)_zz data indicated that the five-membered rings in 4aH⁺ and 4bH⁺ exhibited anti-aromaticity.