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

Abstract

AbstractFirst examples of stable carbocations are reported from 7H‐benzo[c]fluorene (2), 11H‐benzo[b]fluorene (3), 11H‐benzo[a]fluorene (4), 2‐methoxy‐ (5), 7‐methoxy‐ (6), and 9‐methoxy‐11H‐benzo[a]fluorene (7), 7H‐dibenzo[c,g]fluorene (8), 13H‐dibenzo[a,g]fluorene (9), 2‐methoxy‐13H‐dibenzo[a,g]fluorene (10) and 5,6‐dihydro‐13H‐dibenzo[a,g]fluorene (11). Charge‐delocalization modes in the resulting carbocations were derived based on experimental and/or computed (GIAO‐DFT) Δδ13C values and through the NPA‐derived changes in charges (Δq). 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 3NO2, 5NO2, 7NO2, 8NO2, and 10NO2 relative to their precursors. Compounds 10NO2, 7NO2, and 8NO2 were found to be potent direct‐acting mutagens (with 10NO2, 8NO2 also capable of acting as potent indirect mutagens). The X‐ray structures of 5NO2 and 8NO2 were determined. The angle between the plane of the nitro group and the aromatic ring bearing the NO2 group is 89.4° in 5NO2 and 32.4° in 8NO2. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)