Nitration by nitronium salts

Hydrocarbons are efficiently nitrated by nitronium salts under anhydrous conditions as shown by Ingold el al. and subsequently developed by Olah et al. (54] as a general preparative nitration method... 

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). 

Olah, G. A., Laali, K. K., and Sandford, G. (1992a). Comparison of the nitration of poly-fluoronitrobenzenes by nitronium salts in superacidic and aptotic media Activation of the nitronium ion by protosolvation. Proc. Natl. Acad. Sci. U.S.A. 89, 6670-6672. 

Reaction of a primary aliphatic amine, n-butylamine, with nitronium tetrafluoroborate in methylene chloride or acetonitrile produced not n-butylnitramine, but n-butyl nitrate in about 20% yield. However, treating an electronegatively substituted primary aromatic amine, picramide, with nitronium tetrafluoroborate did give the primary nitramine, N,2,4,6-tetra-nitroaniline, in 85% yield. Olah (16) had reported previously that aniline was oxidized vigorously by nitronium salts. 

Results of preparative nitration arenes, haloarenes, and nitroarenes are summarized in Tables XV XVII. Since HF and BF3 are the only by-products of the reaction, nitration with nitronium salts can be carried out under anhydrous conditions. This is advantageous in nitration of aromatics containing functional groups sensitive to hydrolysis. Thus aromatic nitriles, acid halides, and esters can be nitrated in high yield without difficulty (Tables XVIII-XIX). 

Nitronium salts axe the most effective electrophilic nitrating agents for nitration of aromatic compounds under very mild conditions. They are also widely applied in the nitration of heterocyclic aromatic compounds. The nitration of heterocyclic compounds by nitronium salts was first studied in the case of pyridine [16,19,70). N-nitration giving N-nitropyridinium ion is followed by ring opening, if excess pyridine is present, yielding glutaconic... 

If the nitration of olefins by nitronium salts is carried out in acetonitrile, the nitrocarbenium ion intermediates undergo Ritter reaction with the solvent to form nitroacetamides [87]. 

In the conversion of alcohols to alkyl nitrates with nitronium salts, the reaction proceeds via the formation of an iV-nitrooxonium ion followed by proton loss. 

The first report of the nitration of amines by nitronium salts was by Olah and Kuhn [106]. 

As discussed, one of the known methods of synthesis of dialkylnitra-mines involves the reaction of dialkylamides with nitric acid, which leads to the substitution of the acyl group by the nitro group (nitrolysis). However, when nitric acid or its mixtures with acetic anhydride are used, the yield of nitramines is as a rule low and only the use of the HNOj-fFjCCOjjO mixture makes it possible to raise the yield to 90% [118], For preparative purposes, it is more convenient to nitrate the dialkylamides by nitronium salts [119]. The reaction takes place at 20 C in acetonitrile solution. The dialkylnitramines are formed in yields up to 90% and the acyl group is converted into acylium tetrafluoroborate. 

The nitration of olehns, particularly those containing electron donating substituents, by nitronium salts is complicated by side reactions and does not always lead to the expected result. In many instances nitration with nitronium tetrafluoroborate takes place with higher yields in the presence of a-picoline [91,132]. In this case the nitrating agent is apparently not the nitronium salt but 2-methyl-W-nitropyridinium tetrafluoroborate, which is formed rapidly when NO2 BFJ is mixed with a-picoline [73b],... 

This scheme explains the loss of substrate selectivity in nitration with nitronium salts and the high position selectivity which is determined by the spin density distribution in the radical cation (see above) and by the relative stability of the formed a-complexes. In favour of this scheme naphthalene is electrochemically oxidized in CH3CN with NO2 at a lower oxidation potential than that of NOj but sufficient to fonn the radical cation of naphthalene 1- and 2-nitronaphthalenes are formed in the same ratio (9 1) as from nitration by HNO3 and H SO in CH3CN. Later, however, the formation of nitronaphthalenes was shown to be mainly, if not completely, due to the nitration of naphthalene by catalyzed by the acid... 

The Raman spectrum of nitric acid shows two weak bands at 1050 and 1400 cm. By comparison with the spectra of isolated nitronium salts ( 2.3.1), these bonds were attributed to the nitrate and nitronium ion respectively. Solutions of dinitrogen pentoxide in nitric acid show these bands , but not those characteristic of the covalent anhydride , indicating that the self-dehydration of nitric acid does not lead to molecular dinitrogen pentoxide. Later work on the Raman spectrum indicates that at —15 °C the concentrations of nitrate and nitronium ion are 0-37 mol 1 and 0 34 mol 1 , respectively. The infra-red spectrum of nitric acid shows absorption bands characteristic of the nitronium ion. The equivalence of the concentrations of nitronium and nitrate ions argues against the importance of the following equilibrium ... 

Olah and his co-workers compared the behaviour of nitronium salts in competitive nitrations with the behaviour of other nitrating systems. The results are given in table 4.1, columns (a)-(j), and also in table 4.2. The results obtained from competitive nitrations using solutions of nitric acid in organic solvents (table 4.1, columns (6)-(e)) are in line with those obtained by earlier workers. The evidence that in nitromethane,... 

Kinetic studies on the nitration of nitrobenzene by nitronium borofhioride in the polar solvents sulphuric acid, methane-sulphuric acid, and acetonitrile show the reaction to be first-order in both nitronium salt and aromatic110. With the first two solvents, the rate coefficients are similar for nitration by nitric acid and by the nitronium salts, indicating a common nitrating entity. With acetonitrile the rate coefficients are very much lower, consistent with a much lower concentration of free nitronium ions in this medium and thus with the nitronium salts existing as ion pairs in organic solvents (see Table 25). 

The fact that nitronium salts in which nitronium ion is known to be present (by X-ray studies) nitrate aromatic compounds shows that this ion does attack the ring. 

Oxidation of either alkyl or aryl sulphoxides to sulphones in 65-90% yields may be accomplished by treatment with a nitronium salt . In the case of aryl sulphoxides no nitration is observed (which is in contrast to the results of nitric acid oxidation). The reaction was shown to proceed through intermediate nitratosulphonium and nitritosulph-oxonium ions, as depicted in equation (7), which were studied by nmr spectroscopy. 

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