Reaction of Xenon Difluoride with Polycyclic Aromatic Hydrocarbons. Fluorination of Pyrene

Fluorine-substituted condensed polycyclic aromatic hydrocarbons and heterocyclics are of interest in experimental carcinogenesis.^3-5 Heretofore, their syntheses were based mostly on the following two general methods, (a) A “tailor-made” sequence analogous to the one applied in a well-established synthesis of the corresponding polycyclic hydrocarbon, but with a fluorine-substituted starting material (e.g., 3-fluorophthalic anhydride,⁶ 4-fluoro-l-bromonaphthalene⁷). (b) A direct electrophilic substitution of the polycyclic aromatic hydrocarbon followed by appropriate transformations of the substituent to fluorine⁸ (e.g., ArH → ArNO₂ → ArNH₂ ArN₂ ⁺BF₄ ^- → ArF and ArH → ArSO₃H → ArOH → ArOCOF → ArF). In the latter method, the fluorine is usually introduced at the most reactive sites of the hydrocarbon. Neither method is very satisfactory. Direct methods for the introduction of a fluorine atom into polycyclic aromatics are still in their infancy. Recently, xenon difluoride has been shown to act as an efficient and selective fluorinating agent for simple aromatic compounds, both in solution and in the vapor phase.^9-14 The reaction is catalyzed by HF and does not proceed without it.^11,15 We report the application of this direct fluorination route in the aromatic polycyclic series. Pyrene (1) was selected [Formula Omitted] as a model substrate for examining the mode of the reaction of xenon difluoride with polycyclic aromatic hydrocarbons. The convenience of pyrene stemmed from its high symmetry (point group D^2h), its tetracyclic structure, and the presence of three characteristic substitution sites (1, 2, and 4) which lend themselves readily to identification by ¹⁹F NMR spectra (vide infra).