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

Abstract

AbstractSeveral 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/SO2ClF or in FSO3H/SbF5 (1:1)/SO2ClF 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 α‐pyrenyl carbocation. With all other isolated nitropyrenyl carbinol isomers, low‐temperature protonation (with FSO3H/SO2ClF) led to polymerization within 5 min standing at dry‐ice–acetone temperature. For these cases, nitro substituent effects on the α‐pyrenyl carbocations were gauged by DFT and GIAO‐DFT studies. An interesting relationship between the computed nitro tilt angles and the GIAO‐derived charge delocalization modes was observed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)