Charge Delocalization in Persistent Benz[a]anthracenium Cations BAH(+) and Related alpha-Carbocations/Carboxonium Ions: Modeling Epoxide Ring Opening in Potent Carcinogens.

Abstract

Parent BA 1 protonates at C-7/C-12 to give 1H(+)() and 1aH(+)() in 3:1 ratio which remains unaffected by variation in temperature and the superacid system. Increasing steric crowding at the bay-region by introduction of a methyl at C-1 (1-MBA) changes the ratio of C-7/C-12 protonated arenium ions 2H(+)()/2aH(+)() to 10:1. The highly potent 7,12-dimethylbenz[a]anthracene, 7,12-DMBA, gives a 1:1 mixture of the two ipso-protonated cations 3aH(+)()/3H(+)() whose composition changes to 50:1 overtime in favor of 3aH(+)() (ipso-attack at bay-region), showing it to be the thermodynamic cation. 3-Methylcholanthrene, 3MC, is exclusively protonated at C-6 (-->4aH(+)()). Cation 5(+)() (a simplified model for bay-region epoxide ring opening) is cleanly formed via its carbinol 5-OH with FSO(3)H/SO(2)ClF. Ketone 6 is O-protonated in TFAH and in TFAH/H(2)SO(4) to give the bay-region carboxonium ion 6H(+)(); its diprotonation in FSO(3)H.SbF(5) (4:1)/SO(2)ClF gave the first example of the oxonium-arenium dication 6H(2)()(2+). alpha-BA-substituted secondary carbocation 7(+)() and the carboxonium ion 8H(+)() were generated to probe charge delocalization into the alpha-BA moiety via C-7. To gauge the importance of the benz[a] ring and for comparison, the anthracene-substituted carbocations 9(+)() and carboxonium ion 10H(+)() were generated and studied. Charge delocalization pathways into the PAH periphery are evaluated on the basis of Deltadelta(13)C values. AM1 was used as a complementary tool for qualitative comparison with experiment. The resulting arenium ions and benzylic carbocations exhibit strong anthracenium ion character emphasizing the importance of an electron-deficient anthracene moiety. The present study constitutes the first direct investigation of carbocations in the BA series, whose bay-region and K-region diol epoxides are considered the ultimate carcinogens en route to PAH-DNA adduct formation.