Mono- and di-nitroalkyl-(cycloalkyl-)pyrenes in superacid media: dihydroxyiminium-(oxoiminium-)pyrenium dications; cyclisation to long-lived oxazoline-(and 1,2-oxazine-)pyrenium ions, ring opening to form nitrosoalkylpyrenium and nitroso radical cation salts with unprecedented stability

Abstract

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-dinitropyrene 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 possessing buttressed nitro groups, were studied in various superacid media under persistent ion conditions. Nitro group diprotonation was observed in all cases to give N,N-dihydroxyiminium–pyrenium (or oxoiminium–pyrenium) dications. The resulting dications derived from 1, 12, 16 and 23 undergo a facile intramolecular nitro group cyclisation (which is usually complete within minutes at –75 °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 13C and 1H chemical shift analysis) for the iminium–pyrenium dications and their cyclised derivatives illustrate extensive charge delocalisation away from the iminium group at the alternating carbons of the periphery, similar to alkylpyrenium and fluoro(alkyl)pyrenium ions. The remote a positions carry substantial positive charge, which increases with the presence of inductively stabilising alkyl (cycloalkyl) substituents. The assignment of the iminium carbon was proved by independent synthesis and protonation of the 15-N labelled 1. Quenching of 6, 18 and 14(containing 15) does not furnish their N-hydroxyoxazoline derivatives (Like 5) nor the alkylnitropyrene precursors. Instead, the corresponding riitroso-alkylpyrenium and its derived radical cation salts (like 9a and 9RC) are obtained (EPR), which when re-dissolved in CF3SO3H or FSO3H reform the cyclised pyrenium ions. Formation of persistent radical cations was also detected in some cases upon storage of the nitroalkylpyrene samples in CF3SO3H (‘TfOH’) at room temperature.Ring opening of 25 and reduction upon quenching produces a different type of persistent nitrosoalkylpyrene radical (27↔27a) with smaller aN. The mechanistic aspects of nitro group cyclisation/ring opening are discussed.