Aci-Nitro compounds

The intermediacy of an aci-nitro compound has been proposed for the sulfuric acid cyclization of o-nitrophenylacetic acid to yield a mixture of 2,1-benzisoxazole and 2,1-benzisoxazole-3-carboxylic acid. The acid does not decarboxylate under the reaction conditions. The proposed aci-nitro intermediate cyclized to an A/ -hydroxy compound which decomposed to the products (Scheme 179) (70JCS(C)2660). 

Reaction of nitromalon-bis-A -methylanilide (105) with sulfuric acid gives A -methylisatin- -oxime (107) and not 4-methylquinoxalin-3-one 1-oxide (108) as originally suggested. This transformation may involve a Beckmann-type rearrangement of the protonated aci-nitro compound (106) prior to dehydrative ring closure. ... 

However, if protons at the a-carbon atom of the O-alkyl fragment are absent, another process can occur resulting in elimination of olefin and generation of the respective aci-nitro compound (Eq.2). In particular, this anomalous decomposition was found by Nenitzescu and Isacescu (234) for nitronate 74a (Eq. 3). 

The UV spectra of nitronates, which are not functionalized at the a-C atom, have an intense absorption at 230 to 240 nm, which is very similar in characteristics to UV absorption of salts of nitro compounds and solutions of aci-nitro compounds in protic solvents. Since standard alkyl- or silyl nitronates cannot have ionic structures, the presence of the above mentioned absorption in the UV spectra of nitronates, unambiguously confirms, that these compounds have the structures of O-esters. 

Figure 13.2 Biosynthetic pathways of (A) cyanogenic glucosides and (B) glucosinolates. The CYP79s are assumed to catalyze the same reaction in both pathways. It is not known whether the oxime is oxidized to an aci-nitro compound or a nitrile oxide in the glucosinolate pathway.

Experiment.—Dissolve 1 c.c. of nitromethane in water and test the solution with litmus paper. Then add some phenolphthalein and, drop by drop from a burette, OliV-sodium hydroxide solution. Before a permanent pink colour develops about 2 c.c. of the alkali will be added—a sign that an acid, aci-nitromethane, H2C NOOH, has been formed from the neutral nitromethane. A small sample of this solution gives with ferric chloride a blood-red colour, characteristic of aci-nitro-compounds. The salts of the oci-compound undergo extensive hydrolysis. This is shown by further addition of 0-1 N-alkali which produces a deep red colour. If 10 c.c. of alkali were added and 5 c.c. of 0-1 JV-hydrochloric acid are now run in the solution is decolorised because the liberated oci-compound restricts the hydrolysis of its salt. But the conversion of H2C N02H into H3C.N02 proceeds so rapidly that the red colour reappears in a few moments. 

The remainder of the aci-nitro-compound, dissolved in alcohol, is allowed to stand over night. The solution then neither takes up bromine nor gives the colour reaction with ferric chloride. If a few particles of the substance are left exposed on a watch-glass over night they are transformed into an oil. 

The mechanism of the coupling reaction has been very exhaustively studied. Summarising first what has already been mentioned, it must be noted that the reaction is not confined to the aromatic series, for diazo-compounds condense also with enols and with the very closely related aliphatic aci-nitro-compounds. The final products of these reactions are not azo-compounds, but the isomeric hydrazones formed from them by rearrangement. 

In the presence of sodium or potassium ethoxide, nitric esters react with substances containing an active methylene group to form salts of aci-nitro compounds (Wislicenus [64]) as quoted above in eqn. (59). Esters are hydrolysed on this occasion. 

The A -oxidation of oximes leads to the formation of aci-nitro compounds. In a polar organic solvent such as acetonitrile these compounds are isomerized to nitro compounds (Scheme 26) and are thus protected from further oxidation. The reagent of choice is trifluoroperacetic acid. The oxidation of oximes derived from a-epoxy ketones results in the formation of a nitroalkene with opening of the epoxide, as illustrated in Scheme 27. ... 

The sodium aci-nitro compound is a greenish-yellow substance, formed when the anhydride is dissolved in sodium h droxide and the solution evaporated with exclusion of atmospheric carbon dioxide. 

Protonation of the anion of the aci form, the nitronate anion, takes place at the highest rate on the oxygen the aci form is unstable and either tautomerizes to the nitroalkane or is hydrolyzed to a carbonyl compound and N2O. The reduction of an aci nitro compound, prepared from the anion, is thus generally difficult to study, but its polaro-graphic behavior can be investigated during the reduction of a-halonitroalkanes. 

Electron attracting substituents in the nearest vicinity to the nitro group tend to increase the screening constant for the latter. Nitrogen chemical shifts differentiate clearly nitroalkanes> nitroaromatics, Af-nitro and 0-nitro groups. They are also helpful in distinguishing between the true nitro structure and that of an aci-nitro compound ... 

At — 50 the aci-nitro compound separates as a gray, pasty solid. When the solution becomes acid the colloidal precipitate tends to undergo coagulation. 

Reduction of heterocyclic or aromatic ketones containing reactive halogen atoms are accompanied by exchange of these halogens nitro compounds, which are stabilized as aci-nitro compounds in alcoholic solution, are not reduced and in certain circumstances ether groups may be cleaved. 

These selective protonations include the soft Nef reaction (Nef reaction in buffers at high pH), in which a nitronate ion is selectively protonated (proper buffer) to form the aci-nitro compound aci-11 that ultimatively gives the corresponding carbonyl compound Due... 

Potassium hydrogen sulfate/sodium sulfate Aliphatic nitro compounds from aci-nitro compounds... 

---