Charging Pyracylene: Steering Aromaticity and Reactivity of [12]Annulene by Substitution

Two tetrasubstituted pyracylene derivatives were synthesized and chemically reduced using alkali metals. X-ray crystallographic evidence of the neutral, monoanionic, and dianionic states was obtained for both the n-propyl and 4-tert-butylphenyl-substituted derivatives, which were further characterized by NMR, EPR, and UV-vis spectroscopy studies. The aromaticity of each state was comprehensively examined using both experimental and computational methods. Both compounds feature an antiaromatic [12]annulene carbocycle in the neutral state, which turns aromatic upon 2-fold reduction to a 14 π-electron system. n-Propyl substitution enhances the localized negative charge density and aromaticity within the five-membered rings in the dianionic state. In contrast, aryl substitution stabilizes the aromatic annulene character of the dianion, triggers photoluminescence, and enables further reduction coupled with enhanced reactivity. The use of potassium metal gives a highly reactive third reduction state of 4-tert-butylphenyl derivative, which transforms the carbon scaffold into a phenanthrene-pyracylene hybrid via 2-fold reductive cyclodehydrogenation.