Stable‐Ion NMR and GIAO‐DFT Study of the Carbocations from Benzofluorenes and Dibenzofluorenes; Synthesis of Nitro Derivatives; Mutagenicity Assay and X‐ray Analysis

First examples of stable carbocations are reported from 7 H ‐benzo[ c ]fluorene ( 2 ), 11 H ‐benzo[ b ]fluorene ( 3 ), 11 H ‐benzo[ a ]fluorene ( 4 ), 2‐methoxy‐ ( 5 ), 7‐methoxy‐ ( 6 ), and 9‐methoxy‐11 H ‐benzo[ a ]fluorene ( 7 ), 7 H ‐dibenzo[ c , g ]fluorene ( 8 ), 13 H ‐dibenzo[ a , g ]fluorene ( 9 ), 2‐methoxy‐13 H ‐dibenzo[ a , g ]fluorene ( 10 ) and 5,6‐dihydro‐13 H ‐dibenzo[ a , g ]fluorene ( 11 ). Charge‐delocalization modes in the resulting carbocations were derived based on experimental and/or computed (GIAO‐DFT) Δ δ 13 C values and through the NPA‐derived changes in charges (Δ q ). Whereas protonation regioselectivity in the parent systems ( 2 , 3 , 4 , 8 , and 9 ) corresponds to the energetically most favored carbocations computed by DFT, selectivity in the OMe‐substituted derivatives ( 5 , 6 , 7 , 10 , and 11 ) is strongly controlled by the methoxy group. Benzofluorenes 3 , 5 , 6 , and 7 and dibenzofluorenes 8 , and 10 were nitrated under very mild conditions. Nitration selectivity in the parent systems 3 and 8 parallels those in stable‐ion protonation, whereas regioselectivity in the MeO derivatives ( 6 , 7 , and 10 ) corresponds more closely to relative arenium ion energies in the parent unsubstituted systems. Comparative mutagenicity assays (Ames tests) were performed on 3NO 2 , 5NO 2 , 7NO 2 , 8NO 2 , and 10NO 2 relative to their precursors. Compounds 10NO 2 , 7NO 2 , and 8NO 2 were found to be potent direct‐acting mutagens (with 10NO 2 , 8NO 2 also capable of acting as potent indirect mutagens). The X‐ray structures of 5NO 2 and 8NO 2 were determined. The angle between the plane of the nitro group and the aromatic ring bearing the NO 2 group is 89.4° in 5NO 2 and 32.4° in 8NO 2 . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)