Nitronium salts preparation

Another aspect of my early research in Budapest was in nitration chemistry, specifically the preparation of nitronium tetrafluoroborate, a stable nitronium salt. 1 was able to prepare the salt in a simple and efficient way from nitric acid, hydrogen fluoride, and boron trifluoride. 

Nitronium salts in solution in inert organie solvents have been used in reeent years to nitrate a wide range of aromatic compounds. Yields are generally good, but in preparative work the method is advantageous only in speeial cases, notably where the aromatie contains a hydrolysable substituent ( 4.4). 

Nitronium salts of many acids have been prepared, " and some are commercially available. They have been used as nitrating agents ( 4-4-2). 

Preparation and properties of nitronium salts The first preparation of a nitronium salt by Hantzsch, who isolated the perchlorate mixed with hydroxonium perchlorate, and some of the subsequent history of these salts has already been recounted ( 2.3.1). 

Nitronium tetrafluoroborate was first prepared by adding a mixture of anhydrous hydrofluoric acid and boron trifluoride to a solution of dinitrogen pentoxide in nitromethane. Nitric acid can be used in place of dinitrogen pentoxide, and by replacing boron trifluoride by other Lewis-acid fluorides Olah and his co-workers prepared an extensive series of stable nitronium salts. ... 

Olah s original preparative nitrations were carried out with mixtures of the aromatic compound and nitronium salt alone or in ether, and later with sulpholan as the solvent. High yields of nitro-compounds were obtained from a wide range of aromatic compounds, and the anhydrous conditions have obvious advantages when functional groups such as cyano, alkoxycarbonyl, or halogenocarbonyl are present. The presence of basic fimctions raises difficulties with pyridine no C-nitration occurs, i-nitropyridinium being formed. ... 

A convenient method for the preparation of nitronium tetrafluoroborate involves treating a mixture of absolute nitric acid or an alkyl nitrate " with anhydrous hydrogen fluoride and an excess of boron trifluoride. Pure nitronium triflate can be synthesized by treating triflic anhydride or triflic acid with dinitrogen pentoxide in an inert solvent. Other methods for nitronium salt synthesis are discussed by Olah. ... 

Secondary nitramines are conveniently prepared from the nitrolysis of A, A-dialkylamides with nitronium salts in acetonitrile or ethyl acetate at 20 °C where the acyl group is converted into an acylium tetrafluoroborate (Equation 5.14). Problems can occur if commercial nitronium salts like the tetrafluoroborate are used without purification. The presence of nitrosonium salts can then lead to nitrosamines via nitrosolysis. Yields of secondary nitramine up to 90 % have been reported with solutions of nitronium tetrafluoroborate in acetonitrile di-n-butylnitramine is obtained in 82 % yield from the nitrolysis of corresponding acetamide. ... 

PF6, SbF6, C104-, FSO3 counterions. The X-ray crystal structure of nitronium ion is known with hydrosulfate anion.515 The most widely used tetrafluoroborate nitronium salt (N02+BF4 ) is prepared by treating a mixture of nitric acid and anhydrous hydrogen fluoride wit boron trifluoride516 [Eq. (4.145)]. 

Stable nitronium salts, which are readily prepared from nitric acid (or nitrates) with HF and BF3 (and other Lewis acids such as PF5, SbF5, etc.) [Eqs. (5.171) and (5.172)], will nitrate aromatics in organic solvents generally with close to quantitative yield. Because HF and PF5 (or BF3) can be easily recovered and recycled, the method can be considered as a nitric acid nitration using a superacid catalyst [Eq. (5.173)]. 

Nitronium ions such as IV-nitropyridinium salts, which are readily prepared from the corresponding pyridine and nitronium salts,488 489 act as convenient transfer nitrating agents. Transfer nitrations are applicable to C-nitrations [Eq. (5.181)] as well as to a variety of heteroatom nitrations. For example, they allow safe, acid-free preparation of alkyl nitrates and polynitrates from alcohols (polyols) in nearly quantitative yield490 [Eq. (5.182)]. 

Woolf Mid Emeleus [54] have prepared a number of other crystalline nitronium salts, for example ... 

Dinitrogen pentoxide readily forms nitronium salts. As mentioned, Weber [27] and other authors [28-31] obtained a number of sulphates of NO. Bachman and Dever [45] prepared a complex with BF3 which most likely possesses the structure NOi Bp3 0NO7 - Kuhn and Olah [33] obtained nitronium tetrafluoroboiate by adding anhydrous HF as a solvent to N2OS and ... 

The existence of two nitryl halides, NO2F and NO2CI, is well established the existence of N02Br is less certain. The fluoride can be conveniently prepared by the action of fluorine on slightly warm dry sodium nitrite NaN02 + F2 -> NO2F + NaF, and is a reactive gas which converts many metals to oxide and fluoride or oxyfluoride, and reacts with many non-metals (Br and Te excepted) to form nitronium salts. Examples of nitronium salts are (N02)BF4, (N02)2SiFe, (N02)PFe, and (N02)IFg. 

The related nitronium salt, N02 (F3CS03 )2H30, has also been prepared. Nitration Limited experimental data suggest that nitronium trifluoromethanesulfonate is more effective than nitronium tetrafluoroborate (1,742-743) under heterogenous conditions (1,2-dichloroethane), but less effective under homogenous conditions (sulfolane and 1,2-dichloroethane). 

A nearly quantitive yield of the stable nitronium salt can be obtained. An undergraduate in an afternoon s work, can prepare up to a pound of the salt which is now also commercially available. To my best knowledge in its two decades of use, there was never any difficulty connected with its stability. Nitronium tetrafluoroborate, and subsequently other complex fluoride salts,such as the hexafluorophosphate,have gained widespread application as most reactive nitrating agents. 

Nitronium salts in strongly acidic solvent (such as FSO3H or HF) are thus capable to affect an efficient preparative nitration of benzene to 1,3,5-trinitrobenzene, which due to its increased stability over TNT can be of significance. 

Bachman (3T) used the stable N20 BF2 complex prepared as a crystalline, insoluble salt from the components in aromatic nitrations. He suggested a nitronium salt structure for the complex... 

The nitronium salts of pentafluoroselenate(IV) and pentafluorotellurate(IV) of composition NO2XF5 have been prepared from the reaction of selenium or tellurium dioxide with nitryl fluoride. 

Methods for Preparing Energetic Nitrocompounds Nitration with Superacid Systems, Nitronium Salts, and Related Complexes George A. Olah... 

Varied nitrocompounds ranging from such Onitrocompounds as TNT to N-nitrocompounds such as RDX and HMX to 0-nitrocompounds such as trinitroglycerol play a most significant role in energetic compounds. Their preparation is thus of substantial importance. This chapter reviews the most frequently used electrophilic nitration chemistry with particular emphasis on the use of superacidic systems, nitronium salts, and related complexes on which my research group for 30 years has continued to cairv out intensive research. 

Low meta substitution allows favorable regiocontrol in the subsequent preparation of dinitrotoluenes. In general, the nitric add-trifluoro-methanesulfonic acid system shows less meta substitution than other nitrating systems at comparable temperatures (Table IV). The major factor, however, effecting low meia nitration is the use of extremely low temperatures. Solubility of the formed nitronium salt at low temperature in halomethane solutions is limited and unusual ortho/para ratios ma> be also a consequence of the heterogeneous nature of the reaction mixtures. 

Alkyl nitrates must be prepared and stored with care, as particularly in the presence of even traces of adds they can become explosive [15]. They should not be stored over prolonged periods of times. This is particularly the case for nitrates of polyols. The simplest and effective means of their preparation involves reaction of alcohols with nitronium salts in the presence of an acid-binding agent. Transfer nitrations with. -nitropyridinium salts are particularly suited for their acid-free preparation [16],... 

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... 

---