Nitronium salts reaction

When mixed with Lewis acids, dinitrogen pentoxide yields crystalline white solids, which were identified as the corresponding nitronium salts by their infra-red spectra. The reaction with boron trifluoride can be formulated in the following way... 

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. 

With the more concentrated solution the results, as regards loss of intermolecular selectivity, were similar to those obtained with nitronium salts (table 4.1, column a), whilst with the more dilute solution a more usual situation was revealed. The significance of the former observations is again open to doubt because of the likelihood that mixing was relatively slow, and also because reaction upon encounter is here a serious probability. 

It has been necessary to comment upon these various studies because Olah and his co-workers have suggested that whilst nitrations, like those with nitronium salts, which give a relative rate of reaction of toluene with respect to benzene not much greater than unity involve the nitronium ion as the electrophile, this is not so in other cases. It is important to consider these opinions closely. In the earlier of the two relevant papers it is agreed that since nitrations of toluene with nitronium tetrafluoroborate in sulpholan show no abnormal o -ratio there... 

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). 

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). 

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. 

Because nitration of alkenes with nitronium salts proceeds via carbocation intermediates, nitration of electron-deficient alkenes with nitronium salts is rare. Only a few cases are reported. The reaction of a,(3-unsaturated esters with nitronium salts affords products via highly reactive ot-carbonyl cations.75... 

The reaction of alkenes with nitronium salts proceeds through a nitrocarbocation. The product(s) obtained depends on both the nature of the starting alkene and the conditions used. ... 

The reaction of primary aliphatic amines and nitramines with nitronium salts also leads to deamination and the formation of alkyl nitrates. 

Nitronium salts in sulfuric, triflic or fluorosulfuric acids are extremely reactive and well suited for the polynitration of deactivated substrates. Olah and Lin" studied the nitration of m-dinitrobenzene to 1,3,5-trinitrobenzene with a solution of nitronium tetrafluoroborate in fluorosulfuric acid at 150 °C. An optimum yield of 66 % was obtained after a reaction time of 3 hours. However, the crude reaction mixture was found to contain 17 % unreacted m-dinitrobenzene. After a reaction time of 3.8 hours the yield of 1,3,5-trinitrobenzene dropped to 50 % but the product was free from m-dinitrobenzene and was essentially pure. 

Unlike the direct nitration of amines under acidic conditions, nucleophilic nitration is an excellent route to both primary and secondary nitramines. In these reactions the amine or the conjugate base of the amine is used to attack a source of NO2. This source may be a nitrogen oxide, nitronium salt, cyanohydrin nitrate, alkyl nitrate ester or any other similar source of nitronium ion. 

While nitramines are formed from the reaction of secondary amines with nitronium salts the success of the reaction depends on the basicity of the amine (Equation 5.11). Thus, amines of low to moderate basicity are A-nitrated in good yields. The nitration of more basic amines is slow and the nitrosamine is often observed as a significant by-product, a consequence of the partial reduction of the nitronium salt to the nitrosonium salt during the reaction. Increased reaction temperature is also found to increase the amount of nitrosamine formed. The amine substrate is usually used in excess to compensate for the release of the strong mineral acid formed during the reactions. Both nitronium tetrafluoroborate and the more soluble hexafluorophosphate are commonly used for A-nitrations. Solvents like acetonitrile, methylene chloride, nitromethane, dioxane, sulfolane, ethyl acetate and esters of phosphoric acid are commonly used. 

The reaction of nitronium salts with primary amines is not usually a feasible route to primary nitramines, except in the case of some electron-deficient arylamines . Picramide... 

The nitration of nitramine (153) with nitronium tetrafluoroborate, followed by neutralization of the resulting dinitraminic acid with ammonia, also generates ammonium dinitramide (152). Neutralization of this reaction with alkylamines, instead of ammonia, yields the corresponding alkylammonium salts of dinitramide. The nitration of ammonia with dinitrogen pentoxide (15 %) or nitronium salts like the tetrafluoroborate (25 %) yield ammonium dinitramide (152) through the initial formation of nitramine. 

The nitronium salts of pentafluoroselenate(IV) and pentafluorotellurate(IV) of composition N02XF5 have been prepared23 from the reaction of selenium or tellurium dioxide with nitryl fluoride. 

Nitronium salts are also available from the reaction of N02F with SbCl5 followed by anion exchange517 [Eq. (4.146)], and methods have also been developed for the synthesis of the triflate salt.517... 

The applications of nitronium salts as a synthetic reagent530 are discussed in Chapter 5. Until recently, the nitronium ion was recognized only as a nitrating agent. However, it has been found that it possesses significant ambident reactivity. This has been recently shown in the oxidation of sulfides, selenides, and phosphines. In fact, the sulfide reaction has been monitored by 15N NMR spectroscopy wherein both nitrosulfonium and nitritosulfonium ions 88 and 89 were detected as distinct intermediates [Eq. (4.147)]. 

On the other hand, some of these species are stable enough to be isolated as salts. Nitronium cation N02 exists in equilibrium with nitric acid at ambient temperature however, more than 15 crystalline nitronium salts with a variety of counter ions have been isolated and characterized [ 1 ]. The most widely used salt N02+BF4- is made by the reaction of HN03 and BF3 in anhydrous hydrogen fluoride [41] ... 

About the same time, a similar type of activation was observed in the reactions of nitronium salts.20 Nitronium salts (such as NC>2+BF4 or NC>2+ PF6-) show little or no tendency to react with deactivated arenes or alkanes in aprotic media. However, in fluorosulfuric acid or HF-BF3 solution, nitration takes place giving nitration products even nitromethane (eq 15). 

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). 

The comparison of the observed values of k and EA with those calculated from (32) and (33) therefore provides evidence on whether or not the reaction occurs on encounter. At lower acidities, allowance has to be made for the extent of conversion of nitric acid to 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, reaction at the encounter rate ( 3.3) imposes a limit to the rate of reaction in these media, which decreases from 40 times the rate for benzene in 68 % sulphuric acid to 6 times the rate in 80 % sulphuric acid. Therefore it is reasonable to expect that in stronger solutions even under homogeneous conditions, the rates of these compounds would approximate to that of benzene. 

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