FURTHER INSIGHT VIA N-15 NMR-SPECTROSCOPY INTO THE REACTIVE INTERMEDIATES FORMED BY SUPERACID PROTONATION OF CROWDED NITRO-PAHS - PERSISTENT DIHYDROXYIMINIUMPYRENIUM AND HYDROXYIMINIUMPYRENIUM DICATIONS

Low temperature protonation of 1-nitropyrene and its N-15--labeled iso topomer with FSO3H . SbF5 (1:1) ('magic acid))-SO2ClF (or SO2) or with FSO3H . SbF5 (4:1)SO2ClF generates either the dihydroxyiminiumpyreniu m dication (NO, diprotonation) or the hydroxyiminiumpyrenium dication as the principle NMR observable persistent species (depending on the s ample concentration, reaction time and the superacid). The latter is i ndependently generated by diprotonation of authentic l-nitrosopyrene. Variable formation of dihydroxyiminiumpyrenium and hydroxyiminiumpyren ium dications is also observed in the protonation of sterically crowde d 1-nitro-2,7-di-tert-butylpyrene, which gives the corresponding dihyd roxyiminiumpyrenium 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 N-15-labelled isotopomer prod uces the dihydroxyiminiumpyrenium dication (no NMR evidence for the fo rmation of the hydroxyiminiumpyrenium dication) which, as shown before (J; Chem. Sec., Perkin Trans. 2, 1995, 537), undergoes a facile cycli zation to the oxazoline-fused pyrenium cation for which N-15 NMR data are now presented. Diprotonation and subsequent cyclization of the sin gly N-15-labelled 1,3-dinitro-2,4,6,8,10-pentaisopropylpyrene are also studied. Whereas our work reaffirms the generality of NO2 diprotonati on in nitropyrenes, it focuses attention on an additional pathway lead ing to =NH(OH) dication formation. PM3 calculations are used as a comp lementary tool to examine the geometries and energies of the resulting dications.