Pyrene is not an appropriate model for miniature graphene in edge-functionalization

We claim, on the basis of experimental and theoretical grounds, that pyrene cannot serve as a model for graphene for the purpose of edge-functionalization, as previously reported on the basis of direct Friedel-Crafts acylation of pyrene in polyphosphoric acid. Pyrene has been shown to undergo, at the most, Friedel-Crafts tetraacylation, at positions 1, 3, 6, and 8. Furthermore, pyrene has not undergone any Friedel-Crafts acylations at positions 2, 4, 5, 7, 9 and 10. We show, using DFT calculations (B3LYP/6-31G(d) and M06/6-31G(d)) that the regioselective Friedel-Crafts acylation of pyrene is kinetically controlled and is not thermodynamically controlled. The relative stabilities (ΔG ₂₉₈, kJ/mol) of the σ-complexes of monobenzoylpyrenes (σ-BzPYH⁺) are 1σ-BzPYH⁺ (0.0) >4σ-BzPYH⁺ (37.5) >2σ-BzPYH⁺ (53.1). The most stable σ-complexes of dibenzoylpyrenes are also protonated at position 1 (1σ,3Z-, 1σ,4Z-, 1σ,6Z-, 1σ,8Z-, 1σ,9Z- and 1σ,10Z-Bz₂PYH⁺). 2-BzPY and 2,7-Bz₂PY, which are the most stable isomers in their respective series, are not formed in the Friedel-Crafts benzoylation of pyrene. The introduced electron-withdrawing acyl groups at positions 1, 3, 6, and 8 deactivate further acylation(s) of the pyrene ring system at ortho and peri positions, preventing the formation of, e.g., Bz _n PY with n > 4. DFT calculations of bisanthene (BA) show that the preferred kinetically controlled products of mono- and dibenzoylations of bisanthene are 7-BzBA and 7,14-Bz₂BA, respectively. The formation of neighboring peri and ortho acyl substituents at pyrene and bisanthene may be prohibited, due to considerable overcrowding.