Synthesis and Stable-Ion Studies of Regioisomeric Acetylnitropyrenes and Nitropyrenyl Carbinols and GIAO-DFT Study of Nitro Substituent Effects on α-Pyrenyl Carbocations

Several regioisomeric acetylnitropyrenes were synthesized from isomeric acetylpyrenes by mild protic nitration. Nitration of 1-acetylpyrene gave the 3-, 6-, and 8-nitro derivatives (with 8-nitro as the major isomer), from which the corresponding carbinols [NO2-Py-CH(OH)CH3; Py = pyrene] were synthesized. Isomeric 4-acetylnitropyrenes and their corresponding carbinols were synthesized by starting from hexahydropyrene through nitration/aromatization/reduction or aromatization/nitration/reduction sequences. The molecular structures of 4-acetyl-3-nitropyrene and 1-(6-nitropyren-1-yl) ethanol were established by X-ray analysis. Tetrahydropyrene was the starting point for the synthesis of isomeric nitro-2-acetylpyrenes. Low-temperature protonation of 1-acetyl-8-nitropyrene, 4-acetyl-3-nitropyrene, and 2-acetyl-6-nitropyrene in FSO3H/SO2CIF or in FSO3H/SbF5 (1:1)/SO2CIF resulted in the formation of onium dications (by C=O and NO2 protonation). Charge delocalization (pyrenium ion character) in the carboxonium ions is strongly influenced by the position of the carboxonium group, with the 4-acetyl-3-nitropyrene dication being the most delocalized. Superacid protonation of 1-(3-nitropyren-4-yl)ethanol gave a persistent onium dication rather than an alpha-pyrenyl carbocation. With all other isolated nitropyrenyl carbinol isomers, low-temperature protonation (with FSO3H/SO2CIF) led to polymerization within 5 min standing at dry-ice-acetone temperature. For these cases, vitro substituent effects on the alpha-pyrenyl carbocations were gauged by OFT and GIAO-DFT studies. An interesting relationship between the computed nitro tilt angles and the GIAO-derived charge delocalization modes was observed. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)