MONO-NITROALKYL-(CYCLOALKYL-)PYRENES AND DI-NITROALKYL-(CYCLOALKYL-)PYRENES IN SUPERACID MEDIA - DIHYDROXYIMINIUM-(OXOIMINIUM-)PYRENIUM DICATIONS - CYCLIZATION TO LONG-LIVED OXAZOLINE-PYRENIUM (AND 1,2-OXAZINE) PYRENIUM IONS - RING OPENING TO FORM NITROSOALKYLPYRENIUM AND NITROSO RADICAL-CATION SALTS WITH UNPRECEDENTED STABILITY

The low temperature protonations of sterically crowded molecules 2,4,6 ,8,10-pentaisopropyl-1-nitropyrene 1, 2,4,6,8,10-pentaisopropyl-1,3-di nitropyrene 12, 3,6,8,10-tetracyclohexyl-1-nitropyrene 16, 2,7-di-tert -butyl-1-nitropyrene 19, 2,7-di-tert-butyl-1,8-dinitropyrene 21, 1,3,6 ,8-tetraisopropyl-4-nitropyrene 23 and parent 1-nitropyrene 28, all po ssessing buttressed nitro groups, were studied in various superacid me dia under persistent ion conditions. Nitro group diprotonation was obs erved in all cases to give N,N-dihydroxyiminium-pyrenium (or oxoiminiu m-pyrenium) dications. The resulting dications derived from 1, 12, 16 and 23 undergo a facile intramolecular nitro group yclisation (which i s usually complete within minutes at -75 degrees C --> room temp.) to give oxazoline-(or oxazine-)pyrenium cations 6 (and 11), 14 (and 15), 18 and 25. The charge distribution patterns (probed by C-13 and H-1 ch emical shift analysis) for the iminium-pyrenium dications and their cy clised derivatives illustrate extensive charge delocalisation away fro m the iminium group at the alternating carbons of the periphery, simil ar to alkylpyrenium and fluoro(alkyl) pyrenium ions. The remote a posi tions carry substantial positive charge, which increases with the pres ence of inductively stabilising alkyl (cycloalkyl) substituents. The a ssignment of the iminium carbon was proved by independent synthesis an d protonation of the 15-N labelled 1. Quenching of 6, 18 and 14 (conta ining 15) does not furnish their N-hydroxyoxazoline derivatives (like 5) nor the alkylnitropyrene precursors. Instead, the corresponding nit roso-alkylpyrenium and its derived radical cation salts (like 9a and 9 RC) are obtained (EPR), which when re-dissolved in (CFSOH)-S-3-H-3 or (FSOH)-H-3 reform the cyclised pyrenium ions. Formation of persistent radical cations was also detected in some cases upon storage of the ni troalkylpyrene samples in (CFSOH)-S-3-H-3 ('TfOH') at room temperature . Ring opening of 25 and reduction upon quenching produces a different type of persistent nitrosoalkylpyrene radical (27 <-> 27a) with small er a(N). The mechanistic aspects of nitro group cyclisation/ring openi ng are discussed.