Protonation of benzo[a]pyrene dibenzo[a,e]pyrene and benzo[e]pyrene in superacids: NMR studies of charge distribution in persistent arenium ions and AM1 calculations

Abstract

Benzo[a]pyrene 1 and dibenzo[a,e]pyrene 2 are monoprotonated with FSO₃H–SO₂ClF to give persistent arenium ions of protonation at C-6 (1H⁺) and C-8 (2H⁺), respectively. The low temperature protonation of benzo[e]pyrene 3 under a variety of conditions produced a mixture of arenium ions of attack at C-1 (3aH⁺) and C-3 (3bH⁺).

The charge distribution pattern in the resulting arenium ions (as probed by low temperature ¹³C and ¹H NMR spectroscopy) remains very much analogous to those of alkyl(cycloalkyl)pyrenes, showing significant phenalenium ion character (predominant charge alternation at the periphery of the pyrene moiety).

The arenium ion energies and charges were examined by semiempirical theory (AM1). For 1, in agreement with experiment, protonation at C-6 is most favoured and the changes in charge distribution ΔQ[q_c(ion)–q_c(neutral)] indicated a very distinct phenalenium unit. Attack at other sites is less favoured, with that at C-2 being least likely. The extent of positive charge delocalisation becomes more limited as the arenium ion energies increase.

In accord with experiment, AM1 calculations indicate that for 2, the arenium ion of protonation at C-8 is the most stable followed by those of C-1 and C-3 which are 4.5 and 5.1 kcal mol^–1 less stable. Charge delocalisation away from the protonation site in these cations is predicted to be more extensive and involve a phenalenium moiety.

For 3, in line with experiment, the arenium ion energies for attack at C-1 and C-3 are almost identical and most favoured.

The results indicate that despite the pronounced effect that benzannellation has on carcinogenicity, the preferred site(s) of electrophilic attack are those which lead to the most highly delocalised carbocations. Among these, formation of a robust phenalenium ion moiety is always favoured. Possible implications of these findings in relation to carcinogenesis models mediated by arenium ions are discussed.

The oxidation dications of 1–2 were also probed by the AM1 method.