Electrophilic and oxidative chemistry of 4-methyl[6]helicene, [6]helicene and coronene; persistent ion generation in superacid media, gas phase mass spectrometric studies and AM1 calculations

Abstract

4-Methyl[6]helicene 3 is protonated in FSO₃H–SO₂ at C-3 to give a persistent methylhexahelicenium cation; there is minor competing oxidation. The monoarenium ion of parent [6]helicene 2 was observed in the less oxidizing CF₃SO₃H (TfOH)–SO₂ClF. Competing oxidation is extensive and the arenium ion slowly disappears from the NMR spectrum. A monoprotonated coronene cation could not be observed. The coronene radical cation formed in attempted protonation was characterized by EPR.

The protonation, acetylation and trimethylsilylation of coronene and the [6]helicenes were studied in the gas phase by Cl–MS and the decomposition of the resulting adducts were examined by tandem mass spectrometry. The oxidations of 1–3 were also probed by El–MS and MS–MS experiments.

The energies of arenium ions of protonation of 2–3 were calculated by the AM1 method. The monocations observed under stable ion conditions are not the theoretically predicted lowest energy arenium ions. AM1 calculations were also performed on coronene radical cation, and its singlet and triplet dications. The previously observed singlet dication is predicted to be less stable than the triplet dication by 6.54 kcal mol^–1.