New preparative methods of electrophilic nitration and transfer nitration are reviewed, including reactions relating to the ambident reactivity of the nitronium ion. Recent aspects of the mechanism of electrophilic aromatic substitution are discussed.Nitration is one of the most studied and best understood of organic reactions (for reviews, see refs. [1][2][3][4] Typical results of preparative nitrations of arenes, nitroarenes, ary1carboxylic acid esters and halides, and arylnitriles are summarized in Tables 1-4. In mono-or dinitration of aromatics, nitronium salts generally react under quite mild conditions, and yields are 80-100%.Trinitration of aromatics, particularly of benzene, represented a challenging problem. Although the preparation of 1,3,5-trinitrobenzene from rn-dinitrobenzene has been reported in low yield (11), 1,3,5-trinitrobenzene is usually prepared by indirect methods (12). The nitration of m-dinitrobenzene to 1,3,5-trinitrobenzene with nitronium tetrafluoroborate in fluorosulfuric acid solution in contrast could be carried out with ease (62% yield).Nitronium salts in strongly acidic solvents (such as FS03H or HF) are thus capable of effecting an efficient preparative nitration of benzene to 1,3,5-trinitrobenzene which, due to its increased stability relative to TNT, can be of significance.Nitronium salts also readily nitrate aliphatics. Alkanes and cycloalkanes, such as cyclohexane or adamantane, give high yields. The difficulty is not in lack of reactivity but due to the fact that tertiary (and reactive secondary) nitroalkanes themselves can react further under acidic conditions via cleavage of the C-,N bond. Thus, reaction conditions must be found to minimize further reaction; in contrast, this is not the case in aromatic nitrations. Primary nitroalkanes are less susceptible but nitration of their less reactive C-H bonds is also more difficult. The nitration of methane, for example, is feasible only in strongly acidic solution (such as FSO3H).Nitronium salts also add readily to oilefins in the presence of 4487
