Nitronium salts alcohols

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. 

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)]. 

When necessary the selectivity of nitronium salts, such as nitronium tetrafluoroborate in sulfolane, or nitronium hexafluo-rophosphate in nitromethene can be modified with added complexing agents such as dimethyl ether, dimethyl sulfide tetrahydrofuran, or methyl alcohol (13,23). The results are shown in Table VIII. 

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],... 

Nitronium salts as discussed react extremely readily with ir-donor aromatic compounds as well as with alkenes and alkynes (vide infra). They also show high reactivity towards -donors, such as alcohols, ethers, amines, amides, imides, carbodiimides, oximes, hydrazones, sulfides, sulfoxides, halides, and phosphines. However, their reactivity toivards cf-donors is understandably much lower. 

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. 

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. 

No oxidation of alcohols is observed under the reaction conditions. The N-nitrocollidinium salt is generally less reactive than nitronium tetra-fluoroborate itself, but gives better control of conditions and superior yields as shown, for example, in the preparation of 1-adamantyl nitrate. Adamantanol gives less than 2% yield of nitrate ester upon treatment with nitronium tetrafluoroborate while the N-nitro salt forms the ester in 82 yield. 

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