The first examples of persistent dimethyldihydropyrenium cations: reversal of ring current effects

Abstract

Diatropic[14]annulene trans-15,16-dimethyldihydropyrene 1 and its 2-bromo (2) and 2-nitro (3) derivatives are transformed into stable paratropic [12]annulenium ions (dimethyldihydropyrenium cations) by low temperature protonation with FSO₃H–SO₂ClF. Strong preference for ipso protonation (attack at C-2) is observed with 2 and 3.

Stable ion studies provide the first charge delocalization mapping at the periphery of these [12]annulenium ions and a comparison with pyrenium cations.

The predicted (AM1 and PM3) annulenium ion energies and charges are compared with experiment. A theoretical study of 1 and 1H⁺ at the RHF/6-31G* level is included.