Protonation of azuleno [1,2-a]acenaphthylene and 7-bromoazuleno[1,2-a]acenaphthylene in superacids: azulenium, acenaphthenium or naphthalenium cations?

Abstract

Persistent monocation 1aH⁺ has been formed as the major product of low temperature protonation of azuleno [1,2-a] acenaphthylene 1 with FSO₃H·SO₂ClF or FSO₃H·SbF₅(4:1)–SO₂ClF superacid media. Protonation at C-7(→1bH⁺) was observed only as a minor competing process (10–15%). Protonation of the 7-bromoazuleno[1,2-a]acenaphthylene 2 similarly gave the monocation 2aH⁺; the ipso protonated cation 2bH⁺ was not observed.

The NMR characteristics of the resulting ions and their charge distribution patterns are discussed and compared with those of parent acenaphthemum 3H⁺ and azulenium 4H⁺ cations as models. Monocations la and 2a are best viewed as azulenium ions having strong tropylium ion character with limited charge delocalization into the naphthalene moiety. For comparison, the relative energies and changes in charge distribution Δq[q_c(ion)–q_c(neutral)] were calculated for cations 1a, 1b, 1c and 2a, 2b, 2c by the AM1 and PM3 methods.