Nitropyridinium salts

We also found N-nitropyridinium salts such as C5HjN N02BF4 as convenient transfer nitrating reagents in selective, clean reaetions. Transfer nitrations are equally applicable to C- as well as to 0-nitra-tions, allowing, for example, safe, acid-free preparation of alkyl nitrates and polynitrates from alcohols (including nitroglycerine). 

The same type of rearrangement could be observed with 3-cyano-5-nitropyridinium salt (123) when it reacts with methylamine, affording 124... 

Olah and co-workers ° conducted a comprehensive study into the use of N-nitropyridinium salts for nitration. Such salts are easily prepared from the slow addition of the appropriate heterocyclic base to an equimolar suspension of nitronium tetrafluoroborate in acetonitrile. Olah studied the effect on nitration of changing both the structure of the heterocyclic base and the counter ion. Three of these salts (20, 21, 22) illustrated above have been synthesized and used for the 0-nitration of alcohols with success. Transfer nitrations with Al-nitropyridinium salts are particularly useful for the preparation of nitrate esters from acid-sensitive alcohols and polyols because conditions are essentially neutral. 

The transformations of 3-nitropyridinium salts into indole derivatives by the action of ketonic carbanions seem to be a very promising and useful method for the preparation of indoles (Scheme 65) (85KGS509 85KGS522). The mechanism of this reaction is not quite clear, but bicyclic meta-bridged adducts 79 may be intermediates (Scheme 65). 

An unusual indole synthesis consists of reaction between a 1-methyl-3-nitropyridinium salt (60) and the -me thy limine of a dialkyl ketone (61). 

M. A. Yurovskaya, A. V. Karchava, A. Z. Afanas ev, and Yu. G. BundeT, Indoles from 3-nitropyridinium salts. 9. Methyl ethyl ketone A-methylimine in the indolization of l-methyl-3-nitropyridinium salts, Chem. Heterocycl. Cpds., 1992, 409-413. 

Fig. 21.10 Structures (X-ray diffraction) of (a) [Cr207] in the 2-amino-5-nitropyridinium salt [J. Pecaut et al. (1993) Acta Crystallogr., Sect. B, vol. 49, p. 277], and (b) [Cr30i()] in the guanidinium salt [A. Stepien et al. (1977) Acta Crystallogr., Sect. B, vol. 33, p. 2924]. Colour code Cr, green O, red.

Recently, for the photochemical oxidation of indan, tetralin, 2-ethyl-naphthalene and cyclohexane by nitropyridinium salts, the mechanism has been proposed [39d], which also involves the stages of electron and hydrogen atom transfer (Scheme 11.3). 

N02 BF,. NOJPF and other salts N-Nitropyridinium salts N-Niiropyraxole 9-Nitrounthraeenc C lCHri No ... 

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],... 

Reverse addition of pyridine to excess nitronium salt gives stable fV-nitropyridinium salts [16. No N-C migration of the nitro-group is, however, observed even on heating. On the other hand, alkylnitropyridiiuum salts are good transfer nitration agents [16,71,72,73]. 

The IV-nitropyridinium and A -nitroquinolinium salts are stable (but moisture sensitive) crystalline reagents, well characterized by spectroscopic methods (NMR, IR, Raman). They are prepared in essentially quantitative yield by the slow addition of the corresponding pyridine to an equivalent amount of the nitronium salt in acetonitrile, nitromethane. or sulfolane solution. It is important to add the pyridine to the solution of the nitronium ion, because excess pyridine present during the reaction can lead to opening of the pyridinium ring. The iV-nitropyridinium salts can be used as isolated compounds or they can be generated in situ. 

V-Nitrito-4-nitropyridinium salts are isomeric with the previously discussed W-nitropyridinium ions. Similarly, dimethylnitritosulfonium salts are isomeric with IV-nitrosulfonium ions, formed from nitronium salts and dimethyl sulfide. The nitro-onium salts are prepared from nitrosonium hexafluorophosphate with 4-nitropyridine-/V-oxide and dimethyl sulfoxide, respectively [131]. 

Upon aqueous workup of the reaction mixture, sulphur dioxide should form sulphurous acid (H SOj) and sulphurous acid anion (HSOj-)" which make a nucleophilic attack on N-nitropyridinium salt to form adducts, eventually leading to the final products of (Equations 5.84 and 5.85). The 3-nitropyridine is most likely to be derived from the unstable adduct via the allylic rearrangement of the nitro group. 

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