Polyols, transfer nitration

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. 

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)]. 

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],... 

The method was further improved using N-nitrocoIlidinium tetra-fluoroborate as transfer nitrating agent for alcohols (polyols) [73]. 

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