Superacid-catalyzed dimerization/cyclization of isopropenyl-PAHs - Novel pathways to PAH dimers, phenalenes and their stable carbocations

Abstract

The isopropenyl derivatives of representative classes of polycyclic aromatic hydrocarbons (PAHs) having four and five fused-ring systems, namely pyrene, chrysene, benzo[c]phenanthrene (BcPh), dibenzo[a,c]anthracene (benzo[f]tetraphene) and perylene, were synthesized by Wittig olefination from the corresponding acetyl-PAHs. Under the influence of triflic acid (TfOH), the isopropenyl derivatives were converted to novel PAH dimers and/or phenalenes in a simple one-pot procedure. A plausible mechanism for this process has been outlined, and the synthetic scope of this chemistry has been explored. Structural features in the PAH dimers were examined by DFT. As representative initial and final carbocation intermediates in the reaction sequence, stable carbocations derived from 3-isopropenylperylene and from 4,6,6-trimethyl-6H- dibenzo[a,kl]anthracene were generated and studied directly by NMR spectroscopy. The NMR characteristics and charge delocalization modes in the resulting benzylic carbocations are discussed. © Wiley-VCH Verlag GmbH & Co. KGaA, 2008.