Nitronium salts superacid reactions

Nitrations can be performed in homogeneous media, using tetramethylene sulfone or nitromethane (nitroethane) as solvent. A large variety of aromatic compounds have been nitrated with nitronium salts in excellent yields in nonaqueous media. Sensitive compounds, otherwise easily hydroly2ed or oxidized by nitric acid, can be nitrated without secondary effects. Nitration of aromatic compounds is considered an irreversible reaction. However, the reversibihty of the reaction has been demonstrated in some cases, eg, 9-nitroanthracene, as well as pentamethylnitrobenzene transnitrate benzene, toluene, and mesitylene in the presence of superacids (158) (see Nitration). 

Superacidic FSO3H (fluorosulfonic acid, Ho — 15) has also been used in some studies involving superelectrophilic activation. However, due to its tendency for sulfonation and oxidation, this acid has found only limited use in synthetic conversions involving superelectrophiles. Fluorosulfonic acid has been shown effective to activate nitronium salts in their reactions with weak nucleophiles, and again it was suggested that the protosolvated species (6) is involved in the reactions.28 Both fluorosulfonic acid and triflic acid have been reported to give the diprotonated species (14) from 3-arylindenones (eq 12) 29... 

Remarkably, nitronium salts have been shown to react even with methane under superacidic conditions.106 Nitronium hexafluorophosphate (NO2PF6.) reacts with methane, albeit in low yields, when the reaction is carried out in the presence of boron tris(triflate) in triflic acid (eq 63). 

HF and HSO3F (or other superacid) solutions (see subsequent discussion). Higher alkanes and isoalkanes gave yields of 5-10% and adamantane was nitrated in 30% yield. Data indicate that nitratioa (nitrolysis) of alkanes with nitronium salts proceeds in accordance with the generalized concept of electrophilic reactions of single bonds [77] involving two-electron, three-center bond carbocationic intermediates (transition states) as illustrated with case of the nitration of methane. 

Nitration of dinitrobenzene to trinitrobenzene and other deactivated aromatics with nitronium salts is greatly facilitated by carrying out the reactions in superacids (FSO3H. CF3SO3H, etc.). 

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