Nitration with Nitronium Salts

It can be concluded, as already stated above, that the diminution in intermolecular selectivity observed in these nitrations with nitronium salts in organic solvents does not of itself require any special mechanistic considerations as regards the process of substitution. 

It has already been noted that, as well as alkylbenzenes, a wide range of other aromatic compounds has been nitrated with nitronium salts. In particular the case of nitrobenzene has been examined kinetically. Results are collected in table 4.4. The reaction was kinetically of the first order in the concentration of the aromatic and of the nitronium salt. There is agreement between the results for those cases in which the solvent induces the ionization of nitric acid to nitronium ion, and the corresponding results for solutions of preformed nitronium salts in the same solvent. 

In nitration with nitronium salts in sulpholan, nitrobenzene was substituted in the following proportions 8% ortho, 90% meta and 2% paraf under the same conditions benzylidyne trifluoride yielded 8%, 88% and 4% of 0-, m- and p-nitro compound respectively Both of these aromatic compounds were stated to be io -10 times less reactive than benzene. "... 

Work on the nitration of durene also gives evidence for the importance of the speed of mixing in nitration with nitronium salts. ... 

Even so, the results were claimed to show a greater resemblance to nitrations with nitronium salts than to nitrations in organic solvents. However, reaetion at the eneounter rate ( 3.3) imposes a limit to the rate of reaetion in these media, whieh deereases from 40 times the rate for benzene in 68 % sulphuric acid to 6 times the rate in 80 % sulphurie acid. Therefore it is reasonable to expeet that in stronger solutions even under homogeneous eonditions, the rates of these eompounds would approximate to that of benzene. 

The nitronium ion is the electrophile in nitrations with nitronium salts in organic solvents. 

The possibility mentioned in (4) is of great importance, and centres on the persistenee in nitrations with nitronium salts of positional seleetivities in the nitration of alkylbenzenes. It is discussed in 6.2. 

The work of Olah et al. on nitration with nitronium salts in organic solvents has already been discussed in some of its aspects ( 4.4). It will... 

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). 

Aliphatic Nitration. Alkanes undergo electrophilic nitration with nitronium salts such as (N02) piotic solvent such as CH2CI2... 

Nitronium tetrafluoroborate is a very efficient reagent for the D-nitration of alcohols and glycols containing both primary and secondary hydroxy functionality (Equation 3.7). Yields of nitrate ester are high and frequently quantitative. 0-Nitration with nitronium salts produces a strong acid and so an acid-binding agent should be present for acid sensitive substrates. 

Fig. 2 The energy profile of nitration with nitronium salts W is energy, % stands for the reaction coordinate, and AE denotes the total energy of activation...

Olah et al. have found out that the reaction rates on the nitration with nitronium salts are in good agreement with the stability of ir-complexes [6, 13-16], On this basis the authors have assumed that the nitronium salts serve as the nitrating agent the stage limiting the reaction rate is the formation of ir-complcx (Fig. 2) that is rather uncommon in aromatic substitution. 

Perhaps the most fascinating aspect of nitrations with nitronium salts, was the realization that alkanes also undergo with relative ease electrophilic nitrations. 

It is significant to note, however, that with highly deactivated substrates, such as for example m-dinitrobenzene, nitration with nitronium salts is not talking place with any appreciable rate in aprotic solvent media (6 ). On the other hand, when using protic solvents, such as fluorosulfuric acid, the nitronium ion in the strongly acidic media can with ease trinitrate benzene (12,6 ). Similar observations were made in nitration of alkanes (22). 

Sulfolane (tetramcthylene sulfone) is a relatively good solvent for nitronium salts. NO2 BF has about a 7% solubility [63]. Acetonitrile is also applicable for nitrations with nitronium salts but the nitrile group strongly interacts with NOJ and causes acetonitrile to slowly oligomerize even at room temperature. Thus only freshly prepared solutions should be used at low temperature. Nitromethane, nitroethane, and nitropropane can also... 

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). 

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. 

Acid-sensitive systems represent a particularly useful application for desilylative nitration with nitronium salts. Mononitration of imidazoles and triazoles is difficult because acid formed in the reactions even with NO2 BF4 tends to catalyze denitration. However, the nitration of tri-methylsilyl derivatives with NO2 BF overcomes this difficulty. It has been shown for 2-isopropyl-1-trimethyl-silylimidazole that it is possible to obtain its nitro compounds which could not be obtained previously (c.g., 2-isopropyl-1,4-dinitroimidazole) [149], Desilylative N-nitration is involved in the reaction. 

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... 

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