Persistent oxidation dications of carcinogenic PAHs: charge delocalization mapping in 7,12-dimethylbenzo[a]anthracenium, 3-methylcholanthrenium, 1-methylbenzo[a]anthracenium and in parent benzo[a]anthracenium dications†

Abstract

Persistent oxidation dications of 7,12-dimethylbenzo[a]anthracene 7,12-DMA (1²⁺), 3-methylcholanthrene 3-MC (2²⁺), 1-methylbenzo[a]anthracene 1-MBA (3²⁺) and parent benzo[a]anthracene BA (4²⁺) were generated by low temperature reaction with SbF₅–SO₂ClF and studied directly by NMR. Whereas the positive charge is highly delocalized throughout their periphery, the meso-positions of the anthracene moiety (the “L-region”) experience the largest Δδ_¹³C values with C-7 being the most positive. Methyl introduction at C-7 results in a dramatic increase in the Δδ_¹³C value at this position (80 ppm in 1²⁺ as compared to 56 ppm in the case of 4²⁺), whereas a methyl group at C-12 is less effective in enhancing carbon deshielding. In 3-methylcholanthrenium dication 2²⁺, the ring carbons attached to the ethano-bridged C-12b/C-2a sustain the largest positive charge, followed by C-6. The ΣΔδ_¹³C values for the dications are between 412.6–432.7 ppm and their AM1 calculated ΔΔ_fH ^o values are between 431.2–443.7 kcal mol^–1. The resulting 16π-dications exhibit strong proton shielding and are paratropic. Dications 1²⁺ and 2²⁺ derived from potent carcinogens have lower ΔΔ_fH ^o values and are less paratropic as compared to 3²⁺ and 4²⁺. Charge delocalization mapping allows the most likely site(s) for nucleophilic attack to be identified for comparison with the available data on chemical generation and nucleophile trapping of PAH radical cations. Quenching of 1²⁺ with hexane and adamantane to selectively produce the arenium ion of C-7 by hydride abstraction gave inconclusive results.