Further insight via 15N NMR spectroscopy into the reactive intermediates formed by superacid protonation of crowded nitro-PAHs: persistent dihydroxyiminiumpyrenium and hydroxyiminiumpyrenium dications

Abstract

Low temperature protonation of 1-nitropyrene and its 15N-labelled isotopomer with FSO3H·SbF5 (1∶1) (‘magic acid’)–SO2ClF (or SO2) or with FSO3H·SbF5 (4∶1)–SO2ClF generates either the dihydroxyiminiumpyrenium dication (NO2 diprotonation) or the hydroxyiminiumpyrenium dication as the principle NMR observable persistent species (depending on the sample concentration, reaction time and the superacid). The latter is independently generated by diprotonation of authentic 1-nitrosopyrene. Variable formation of dihydroxyiminiumpyrenium and hydroxyiminiumpyrenium dications is also observed in the protonation of sterically crowded 1-nitro-2,7-di-tert-butylpyrene, which gives the corresponding dihydroxyiminiumpyrenium dication in FSO3H·SbF5 (1∶1)–SO2ClF (or SO2) and the hydroxyiminiumpyrenium dication by low temperature reaction with FSO3H·SO2ClF or CF3SO3H·SO2. Protonation of the buttressed 1-nitro-2,4,6,8,10-pentaisopropylpyrene and its 15N-labelled isotopomer produces the dihydroxyiminiumpyrenium dication (no NMR evidence for the formation of the hydroxyiminiumpyrenium dication) which, as shown before (J. Chem. Soc., Perkin Trans. 2, 1995, 537), undergoes a facile cyclization to the oxazoline-fused pyrenium cation for which 15N NMR data are now presented. Diprotonation and subsequent cyclization of the singly 15N-labelled 1,3-dinitro-2,4,6,8,10-pentaisopropylpyrene are also studied. Whereas our work reaffirms the generality of NO2 diprotonation in nitropyrenes, it focuses attention on an additional pathway leading to NH(OH) dication formation.