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, 69451 Weinheim, Germany, 2008)

Application of Electron Delocalization Indicators in the Study of Electrophilic Aromatic Substitution Reactions

“…Brule et al [59] reported an example of the interpretation of the charge distribution from NMR chemical shifts in several benzylic carbocations. We take 3-isopropenylperylene as an example.…”

“…(8). 13 C NMR-derived charge delocalization modes in the carbocation resulting from the protonation of 3-isopropenylperylene [59]. The circles are related to the magnitude of 13 C. 9 The Gauge invariant atomic orbital (GIAO) method [57; 58] allows the obtention of gauge invariant wavefunctions from which NMR chemical shifts can be calculated and interpreted in terms of charge delocalization modes as in references [52][53][54][55][56].…”

Reaction mechanisms : Part (iii) Polar reactions

“…The isopropenyl derivatives of a range of polycyclic aromatic hydrocarbons (PAHs) having four and five-membered fused ring systems were prepared by Wittig olefination of the corresponding acetyl PAHs. 25 In the presence of triflic acid these derivatives converted to PAH dimers 37 and/or phenalenes 38 (shown in Scheme 10 for the isoprenyl derivative of phenanthrene). The mechanism of formation of dimers and subsequently phenalenes was suggested to proceed via several carbocation intermediates.…”