Substituent effects and charge delocalization mode in chrysenium, benzo[c]phenanthrenium, and benzo[g]chrysenium cations: A stable ion and electrophilic substitution study

The first series of persistent carbocations derived from mono- and disubsti tuted chrysenes Ch (5-methyl- 3, 2-methoxy- 19, 2-methoxy-11-methyl- 20, 2- methoxy-5-methyl- 21, and 9-methyl-4H-cyclopenta[def] chrysene 22), monosub stituted benzo[c] phenanthrenes BcPh (3-methoxy- 23, 3-hydroxy- 24), and mo nosubstituted benzo[g]chrysenes BgCh (12-methoxy- 25; 12-hydroxy- 26) were generated in FSO3H/SO2CIF or FSO3H-SbF5 (4:1)/SO2CIF and studied by low-tem perature NMR at 500 MHz. The methoxy and methyl substituents direct the pro tonation to their respective ortho positions. Whereas parent Ch 1 is proton ated at C-6/C-12, 3 is protonated at C-6 (3aH(+)) and at C-12 (3bH(+)) with the latter being the thermodynamic cation. The 2-methoxy-Ch 19 is protonat ed at C-1 to give two conformationally distinct carboxonium ions (19aH(+)/1 9bH(+)). In the disubstituted Ch derivatives 20 and 21, the 2-methoxy overr ides the 5-methyl and the predominant carbocations formed are via attack or tho to methoxy. For the methano derivative 22 (Me at C-9), a 3:1 mixture of 22aH(+)/22bH(+) is formed. For parent BcPh 13, nitration and benzoylation are directed to C-5. With 3-methoxy-BcPh 23, the site of attack moves to C- 4 thus producing two conformationally distinct carboxonium ions (23aH(+)/23 bH(+)), whereas conventional nitration gave a 2:1 mixture of 23aNO(2) and 2 3bNO(2). In 3-hydroxy-BcPh 24, the carboxonium ion 24H(+) is exclusively fo rmed. For parent BgCh 16, protonation, nitration, and benzoylation are all directed to C-10 (16H(+), 16NO(2), 16COPh), but presence of OMe or OH subst ituent at C-12 changes the site of attack to C-11. Charge delocalization mo de is probed based on magnitude of Delta delta Cs-13 and conformational asp ects via NOED experiments. Complete NMR data are also reported for several benzoylation/nitration products. Using ab initio/GIAO (and NICS), the NMR c hemical shifts (and aromaticity) in model carbocations A-D were evaluated. This work represents the first direct study of the carbocations derived fro m the methyl-, methoxy-/hydroxy-derivatives of three important classes of b ay-region and fjord-region PAHs whose diol-epoxides extensively bind to DNA . It also extends the available data on electrophilic chemistry of BcPh and BgCh.