Nitronium salts, electrophilic nitration with

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

G. A. Olah in Chapter 7 reviews some of the most useful methods in preparing nitro compounds (i.e., electrophilic nitrations with superacid systems, nitronium salts, and related Friedel-Crafts type complexes). Polynitro compounds were traditionally and still are the most widely used explosives [e.g., nitroglycerol, trinitrotoluene (TNT), and Af-nitramines (RDX and HMX)]. Methods of preparing nitro compounds thus remain a key part of the synthesis of energetic materials. 

It has been necessary to comment upon these various studies because Olah and his co-workers have suggested that whilst nitrations, like those with nitronium salts, which give a relative rate of reaction of toluene with respect to benzene not much greater than unity involve the nitronium ion as the electrophile, this is not so in other cases. It is important to consider these opinions closely. In the earlier of the two relevant papers it is agreed that since nitrations of toluene with nitronium tetrafluoroborate in sulpholan show no abnormal o -ratio there... 

The nitronium ion is the electrophile in nitrations with nitronium salts in organic solvents. 

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)]. 

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. "... 

Pyridine itself can be converted into 3-nitropyridine only inefficiently by direct nitration, even with extremely vigorons conditions, however a couple of ring methyl groups facihtate electrophilic substitution snfficiently to allow nitration both collidine (2,4,6-trimethylpyridine) and its M-methyl quaternary salt are nitrated at similar rates under the same conditions, showing that the former reacts via its A -protonic salt. Steric or/and inductive inhibition of M-nitration allows C-3-substitution using nitronium tetrafluo-roborate an example is the nitration of 2,6-dichloropyridine or of 2,6-difluoropyridine using tetramethyl-ammoninm nitrate with trifluoromethansulfonic anhydride. ... 

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

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. 

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. 

The regioselectivity of nitration of toluene with nitronium salts has been successfully altered by their prior complexation with crown ethers. Complexation of NO2 BFJ by 18-C-6 crown ether substantially altered the selectivity in nitration of toluene and benzene as reported by Elsenbaumet and Wasserraan [128]. Similar effect was observed with polyethylene oxides. Savoie al. reported isolation of the IS-C-b-NOa BFJ complex and its characterization [129], Masci carried out the yet most detailed study on the effect of crown ethers on the selectivity of electrophilic aromatic nitration [130]. 

Mixed acid or anhydrous nitric acid has been used for the nitration of em-nitronitronate salts to the corresponding 1,1,1-trinitromethyl compounds. A convenient route to hexan-itroethane (75) involves treating the dipotassium salt of 1,1,2,2-tetranitroethane (74) with mixed acid the nitration proceeding via electrophilic addition of the nitronium cation to the bis-nitronitronate ion. ... 

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