Tautomerism nitroalkanes

Nitronate salts and the tautomeric act-form of nitroalkanes, known as nitronic acids, are converted to gem-dinitro compounds on treatment with dinitrogen tetroxide. Novikov and co-workers synthesized phenyldinitromethane by treating phenylnitromethane with dinitrogen tetroxide in ether and later reported the synthesis of some substituted phenyltrinitromethanes from the direct nitration of the nitronate salts of phenylnitromethanes. 

The tautomeric nitronic acids of secondary nitroalkanes or their nitronate salts react with nitrous acid or alkali metal nitrites to yield pseudonitroles.These pseudonitroles are often isolated as their colourless dimers (78b) but are deep blue in monomeric form (78a). Primary nitroalkanes also form pseudonitroles (80b) but these rapidly isomerise to the nitrolic acid (80a).Reactions are commonly conducted by slowly acidifying a mixture containing the nitronate salt and the metal nitrite, during which, the nitronic acid reacts with the nitrite anion. These reactions, first discovered by Meyer, have been used to prepare 2-nitroso-2-nitropropane (78a) and acetonitrolic acid (80a) from 2-nitropropane (76) and nitroethane (22) respectively. ... 

Aliphatic nitro compounds with the nitro group on a tertiary carbon were reduced to amines with aluminum amalgam [146 or iron [559]. 2-Nitro-2-methylpropane afforded ferf-butylamine in 65-75% yield [146. Even some secondary nitroalkanes were hydrogenated to amines. fra s-l,4-Dinitrocy-clohexane was converted to frans-l,4-diaminocyclohexane with retention of configuration. This may be considered as an evidence that the intermediate nitroso compound is reduced directly and not after tautomerization to the isonitroso compound [560] (see Scheme 54). 

Nitroalkanes are acidic compounds the dissociation of a proton from a nitroalkane produces the nitroalkane anion, or nitronate, whose chemical and physical properties differ from those of the parent nitroalkane. The nitronate form of 2-nitropropane is more mutagenic in S. typhimurium TAIOO and TA 102 than is the neutral parent compound (Fiala et al., 1987b Dayal et al., 1989 Kohl et al., 1994), suggesting that propane 2-nitronate may act as an intennediate in the mechanism by w hich 2-nitropropane exerts its genotoxic and carcinogenic effects. This hypothesis is supported by studies indicating that both bacterial mutagenicity and induction of unscheduled DNA synthesis in rat hepatocytes are decreased by conditions (low pH or deuteration of the secondary carbon atom) that limit formation of the nitronate tautomer, and that the tautomerization of 2-nitropropane can be influenced by hepatic enzymes (Kohl et al., 1994). 

Primary (R CH2 N02), and also secondary (R2CH-N02), nitroalkanes exist in tautomeric equilibrium with the corresponding nitronic acids, the two species being interconvertible via the mesomerically stabilised a-carbanion. These nitroalkanes are therefore soluble in alkali. 

Nitro = ac/-nitro tautomerism occupies a special role as it has been speculated to occur under conditions where high energy explosives detonate. It has been proposed as one of the important initial steps responsible for the cascade of reactions leading to the high energy release during detonation [1], Nitro = aci-nitro tautomerism in nitro compounds RjCH-NO leads to nitronic acids, R2C=N(0)OH. Kinetic studies show similar pKa values for nitronic acids and their carbon counterparts, the carboxylic acids [1]. Nitroalkane = nitronic acid tautomerism has been assumed not to be an acid-catalyzed process because of the poor basicity of N02 group [4]. However, recent work by Erden et al. showed that this tautomerism can, in fact, be achieved under acid catalyzed... 

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. 

A number of methods to effect Nef-type reactions employ reducing agents in place of the acid/base combination. The most commonly used variant of this type was developed by McMurry in 1973 and involves treatment of nitroalkanes with aqueous TiCb. In a representative example, these conditions were used for the conversion of 20 to 21 in 85% yield. These reactions likely proceed through generation of a Ti-0 bond to afford 22, followed by N-0 bond cleavage to yield 23. Tautomerization of the nitroso species (23) would provide oxime 24, which is then hydrolyzed to afford 21. Although these conditions are still somewhat acidic, several functional groups are tolerated... 

The compound given is known as a nitronic acid its more stable tautomeric form is a nitroalkane. 

Nitroalkanes have acidic isomers and tautomerism, in which primary and secondary nitroalkane are acidic because their a-hydrogen atoms are influenced by the nitro group, and can be configured tautomerically into acidic structure. 

The primary and secondary nitroalkanes exist in tautomeric equilibrium with an aci or nitronic acid form. 

It is generally accepted that the condensation of nitroalkanes 1 with aldehydes proceeds with the nitronates 2 as the intermediates. The role of the base is to shift the tautomeric equilibrium towards the formation of nitronic acids, or ac/-nitroalkanes 3. Since 3 are much stronger acids (with a pAa range of 2 6) than 1 (with a pAa range of 9 10), they are more readily deprotonated with the base. After the deprotonation, the formed nucleophilic... 

Properties and Preparation of Nitronates Nitroalkanes exist in equilibrium with a tautomeric form known as a nitronic acid (Scheme 16.19) [103]. The aci-form is usually present in minor concentration with an equilibrium constant of 10 [104]. Salts of nitronic acids, metal nitronates, are formed upon deprotonation of nitroalkanes and are potent nucleophiles [105]. Alkylation of the nitronate salts leads to both a-substituted nitro compounds and the isomeric nitronate esters. 

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