Nitronium salts alkanes, nitration

Mixture of nitric and sulfuric acid (mixed acid) used extensively in nitration of aromatic hydrocarbons, is generally unsuitable for nitration of alkanes, since primary nitroalkanes are rapidly hydrolyzed by hot sulfuric acid and secondary and tertiary nitroalkanes form tars (in all probability via rapid alkene-forming elimination and subsequent polycondensation, polymerization, etc.). It is, however, significant to point out that it is not necessarily the lack of reactivity of paraffins with mixed acid that makes the nitration of saturated hydrocarbons unsuitable, but that fast secondary reactions of any nitro products formed (as well as oxidative side reactions) can take place. This difficulty can be, at least in part, overcome by using preformed nitronium salts as nitrating agents. 

The selection of solvents for quantitative work is not easy. Nitro-alkanes are sufficiently inert, but nitronium tetrafluoroborate is poorly soluble in them c. 0-3 %). Nitronium salts react rapidly with acetic anhydride, and less rapidly with acetic acid, A, A -dimethylformamide and acetonitrile, although the latter solvent can be used for nitration at low temperatures. Sulpholan was selected as the most suitable solvent ... 

Aliphatic Nitration. Alkanes undergo electrophilic nitration with nitronium salts such as (N02) piotic solvent such as CH2CI2... 

Electrophilic nitration of olefins can also be carried out with nitronium salts in pyridinium poly (hydrogen fluoride) (PPHF) solution491 (which also acts as solvent) to give high yields of nitrofluorinated alkanes. In the presence of added halide ions (iodide, bromide, chloride) the related haloalkanes are formed, and these can be dehydrohalogenated to nitroalkenes492 [Eq. (5.183)]. 

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). 

Electrophilic nitration of alkanes has been performed with nitronium salts, for example, NO2 PFe and with HNO3-H2SO4 mixtures, but mixtures of nitration and cleavage products are obtained and yields are generally low. ° The reaction of alkanes with nitric acid and A-hydroxysuccinimide (NHS), however, gave moderate-to-good yields of the corresponding nitroalkane. Similar nitration was accomplished with NO2, NHS and air. "... 

Perhaps the most fascinating aspect of nitrations with nitronium salts, was the realization that alkanes also undergo with relative ease electrophilic nitrations. 

Whereas our studies centered so far primarily on the feasibility of the nitration of alkanes (cycloalkanes) with nitronium salte in aprotic solvent systems, once reactions are demonstrated they can, of course, also be carried out under more conventional conditions. Mixed acid (HNO -H-SO.) nitration of isobutane at 50° gave about 10% nitroproducts, with the relative ratio of CH3NO2, 28% ... 

It is significant to note, however, that with highly deactivated substrates, such as for example m-dinitrobenzene, nitration with nitronium salts is not talking place with any appreciable rate in aprotic solvent media (6 ). On the other hand, when using protic solvents, such as fluorosulfuric acid, the nitronium ion in the strongly acidic media can with ease trinitrate benzene (12,6 ). Similar observations were made in nitration of alkanes (22). 

A solution of a stable nitronium salt (generally the hexafluorophosphate NOa PF but also the hexafluoroantimonateNO SbFft ortetrafiuoroborate NO2BF4) in methylene chloride-tctramethylene sulfone solution was allowed to react with the alkane (cycloalkane), with usual precautions taken to avoid moisture and other impurities. Reactions were carried out at room temperature (25°C) in order to avoid or minimize the possibility of radical side reactions and/or protolytic cleavage reactions (tertiary nitroalkanes particularly readily undergo protolytic cleavage, even if the system initially is acid free but nitration forms acid). Data obtained are summarized in Table XXI. 

HF and HSO3F (or other superacid) solutions (see subsequent discussion). Higher alkanes and isoalkanes gave yields of 5-10% and adamantane was nitrated in 30% yield. Data indicate that nitratioa (nitrolysis) of alkanes with nitronium salts proceeds in accordance with the generalized concept of electrophilic reactions of single bonds [77] involving two-electron, three-center bond carbocationic intermediates (transition states) as illustrated with case of the nitration of methane. 

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