94849-68-0Relevant academic research and scientific papers
Charge delocalization in persistent benz[a]anthracenium cations bah+ and related α-carbocations/carboxonium ions: modeling epoxide ring opening in potent carcinogens
Laali, Kenneth K.,Tanaka, Mutsuo
, p. 7280 - 7285 (2007/10/03)
Parent BA 1 protonates at C-7/C-12 to give 1H+ and laH+ in 3:1 ratio which remains unaffected by variation in temperature and the superacid system. Increasing steric crowding at the bayregion by introduction of a methyl at C-l (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 FSO3H/ SO2C1F. Ketone 6 is O-protonated in TFAH and in TFAH/H2SO4 to give the bay-region carboxonium ion 6H+; its diprotonation in FSO3H·SbFs (4:1)/SO2C1F gave the first example of the oxoniumarenium dication 6H22+. α-BA-substituted secondary carbocation 7+ and the carboxonium ion 8H+ were generated to probe charge delocalization into the α-BA moiety via C-7. To gauge the importance of the benz[α] 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 Δδ13C values. AMI 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 electrondeficient 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.
Solvolysis of K-region arene oxides: Substituent effects on reactions of benz[a]anthracene 5,6-oxide
Nashed, Nashaat T.,Balani, Suresh K.,Loncharich, Richard J.,Sayer, Jane M.,Shipley, David Y.,Mohan, Ram S.,Whalen, Dale L.,Jerina, Donald M.
, p. 3910 - 3919 (2007/10/02)
The solvolytic reactivity and products formed from benz[a]anthracene 5,6-oxide (BA-O) on substitution of a methyl group at positions 1 (1-MBA-O), 4 (4-MBA-O), 7 (7-MBA-O), 11 (11-MBA-O), and 12 (12-MBA-O), on 7,12-dimethyl substitution (7,12-DMBA-O), and on 7-bromo substitution in 1:9 dioxane-water and in methanol at 25°C are reported. These substitutions result in > 150-fold differences in their rates of acid-catalyzed solvolysis and cause marked changes in the distribution of solvent adducts and phenols resulting from isomerization. Optically pure BA-O, 7-MBA-O, 12-MBA-O, and 7,12-DMBA-O as well as their optically pure trans dihydrodiols were utilized to determine the point of attack by water in the hydrolysis reactions. In general, the reactions in aqueous dioxane (0.1 M NaClO4) obeyed the rate equation kobsd = kH[H+] + k0, where kH is the second-order rate constant for acid-catalyzed reaction and k0 is the first-order rate constant for spontaneous reaction, to provide biphasic pH-rate profiles. When ionic strength was maintained with 0.5 M KCl, however, more complex pH-rate profiles were observed for some of the arene oxides due to attack of chloride on the neutral epoxide to produce steady-state concentrations of chlorohydrins. Rate enhancement on methyl substitution is largest (kH, ca. 5-fold) when the methyl group is present in the hindered bay region (C1 or C12) or adjacent to the epoxide at C7. The combined effect of two methyl groups (7,12-DMBA-O) is additive (ca. 25-fold). Theoretical calculations (molecular mechanics by PCMODEL-PI and ab initio by GAUSSIAN 86 and 88 programs) of carbocation stability indicate the importance of steric factors in determining relative reactivity and types of products formed from substituted benz[a]anthracene 5,6-oxides.
