Nitronic Acids Research Articles

Understanding of the stability of acyclic nitronic acids in the light of molecular electron density theory

In the present work, the electronic structure and stability of acyclic nitronic acids were studied. Depending on the different substituents, the analyzed compounds can be classified as pseudo(mono)radical or zwitterionic nitronic acids. ELF topological analysis of the electron density of nitronic acids containing in their structure EWG substituent (-NO2 (1); –COOCH3 (2); –NO2, –CH3 (5); –COOCH3, –OCH3 (6); –NO2, –H (7); –COOH3, –H (8)) permits establishing a pseudo(mono)radical electronic structure with a pseudoradical centers at the C1 carbon atom. In turn, ELF analysis of the compounds 3, 4, 9 and 10 containing substituent belonging to the EGD group (-CH3 (3); –OCH3 (4); –CH3, –H (9); –COH3, –H (10)) and based on the presence of C1–N2 double bond and absence of pseudoradical centre allows for the classification of these compounds as a zwitterionic nitronic acids. Nitronic acids containing EDG substituents in their structure are the most stable among the analyzed nitronic acids. In turn, nitronic acids containing EWG groups are characterized by higher reactivity in chemical reactions compared to other analyzed nitronic acids.

Electrophilically Activated Nitroalkanes in Reactions With Carbon Based Nucleophiles

Unusual reactivity of nitroalkanes, involving formation of aci-forms (nitronic acids or nitronates) and their subsequent interaction with carbon-based nucleophiles, is surveyed in this review.

The Salts of Nitronic acid as CO2-Corrosion Inhibitors

The salts of nitronic acids were obtained by nitration of higher olefins, both normal and isomeric structure in the presence of aqueous alkali. The resulting salts (Na+, K+, NH4+), were tested in CO2 -media as a corrosion inhibitor. The results showed that, the protective effect of the linear nitronated olefins increases by increasing the number of carbon atoms. This is due to the fact that because of the branched structure of the compounds, it is much more difficult to form a dense protective layer on the metal surface. 20% solutions of nitronated ethanolamine in isopropyl alcohol was synthesized and tested as corrosion inhibitor in carbon dioxide environments. The results showed that diethanolamine nitron based on a mixture of normal olefins C16-C18 at a concentration of 50 ppm has a high protective effect of 99.69%.

The primary kinetic hydrogen isotope effect in the deprotonation of a nitroalkane by an intramolecular carboxylate group

AbstractThe rates of racemization of optically active nitropentanoic acid, and 4‐deuteronitropentanoic have been compared. The rate ratio (kie) is kH/kD = 5.68(±0.17) at 31°C, in good agreement with that determined by Lewis et al. for base‐catalysed deprotonations using iodine‐trapping methods. In a more detailed study, optically active 4‐nitro‐4‐phenylbutanoic acid (NPBA) has also been prepared and rates of racemization measured in dimethoxyethane:water. With less than a full equivalent of triethylamine, rates are proportional to [Et3N:]/[NPBA]. For 1 < [Et3N:]/[NPBA] < 2, rates are independent of the ratio, consistent with racemization being dominated by deprotonation of the nitroalkane by the intramolecular carboxylate group. The solvent isotope effect is = 0.73(±0.04) and rates of exchange with D2O are equal to rates of racemization. Comparison with rates of racemization by acetate of the methyl ester yielded an effective molarity (EM = 13.7) for the intramolecular carboxylate. The kie for racemizations of NPBA and 4‐deutero‐NPBA is kH/kD = 5.78 at 25°C, and for 20 < T < 50°C, E − E = 5.5(±0.1) and AH/AD = 0.63(±1.03). For the acetate catalysed racemizations of the methyl ester, 25°C, kH/kD = 7.43 with E − E = 5.2 kJ mol−1 and AH/AD = 1.08. In neither case is there any indication of a major tunnelling contribution on the isotopic rate ratio. A hitherto unrecognised mode of decomposition of nitronic acids, involving direct reaction with dissolved oxygen, has been identified. Copyright © 2008 John Wiley & Sons, Ltd.

Kinetic study of bleaching of photoinduced forms of o-methylnitrobenzothiazoles

Photochromic properties of benzothiazole derivatives containing nitro and methyl groups in the ortho positions with respect to each other were studied by flash photolysis. The rate constant of bleaching of the corresponding photoinduced nitronic acids and their anions increases as the CH3C-CNO2 bond becomes longer. The application of the thermodynamic approach to predict the kinetic stability of nitronic acids is limited owing to specific intramolecular interactions. The lifetime of photoinduced nitronic acid anions tends to increase with rise in the chemical shift of the methyl protons.

Nitronic Acids and Their Derivatives

AbstractFor Abstract see ChemInform Abstract in Full Text.

Unprecedented in situ Oxidative Ring Cleavage of Isoxazolidines: Diastereoselective Transformation of Nitronic Acids and Derivatives into 3‐Hydroxymethyl 4‐Nitro Tetrahydrofurans and Pyrrolidines.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Unprecedented in situ oxidative ring cleavage of isoxazolidines: diastereoselective transformation of nitronic acids and derivatives into 3-hydroxymethyl 4-nitro tetrahydrofurans and pyrrolidines.

[reaction: see text] Nitronic acids undergo an intramolecular 1,3-dipolar cycloaddition to unactivated double bonds, and the resulting isoxazolidines spontaneously evolve by an unprecedented in situ oxidative ring cleavage. The extension of this transformation to silyl nitronates results in a general diastereoselective construction of hydroxymethyl nitro functionalized tetrahydro-furans and -pyrrolidine having up to four consecutive stereogenic centers.

3,2]‐Sigmatropic Rearrangement of Acyloxy Nitronic Acids into Geminal Acyloxy Nitroso Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Kinetic and Thermodynamic Acidities of Substituted 1-Benzyl-1-methoxy-2-nitroethylenes. Strong Reduction of the Transition State Imbalance Compared to Other Nitroalkanes

Acidity constants of six substituted 1-benzyl-1-methoxy-2-nitroethylenes (2-Z with Z = m-NO(2), m-CF(3), m-Cl, H, p-Me, p-MeO) and their respective nitronic acids were determined in 50% DMSO-50% water (v/v) at 20 degrees C. Kinetic data were obtained on the reversible deprotonation of all six 2-Z by OH(-) and piperidine and on the reversible deprotonation of 2-NO(2)() by piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine in the same solvent. These data allowed a determination of the Brønsted coefficients alpha (dependence on acidity of 2-Z) and beta (dependence on amine basicity). The fact that alpha > beta indicates the presence of a transition state imbalance which, however, is much smaller than that for the deprotonation of arylnitromethanes. The reasons for this reduction in the imbalance and their relevance to a recent study of the deprotonation of Fischer carbene complexes are discussed.

3,2]-Sigmatropic Rearrangement of Acyloxy Nitronic Acids into Geminal Acyloxy Nitroso Compounds

Acylation of secondary nitroalkanes is accompanied by [3,2]-sigmatropic rearrangement of acyloxy nitronic acids into geminal acyloxy nitroso compounds which are formed in preparative yields.

Ketoenol, imineenamine, and nitroaci-nitro tautomerism and their interrelationship in substituted nitroethylenes. Keto, imine, nitro, and vinyl substituent effects and the importance of H-bonding

Tautomeric isomers and conformers of 2-nitrovinyl alcohol (1), 2-nitrovinylamine (2), and 1-nitro-propene (3) are reported at the MP2 and B3LYP levels of theory, using the 6-31G* basis set, with energy evaluation at B3LYP/6-311+G** and G2MP2. The nitroalkenes are the global minima on their respective potential energy surfaces. The barriers for the concerted 1,5-H transfer to the corresponding nitronic acids amount to only 5.0 kcal/mol for 1, 13.2 kcal/mol for 2, and a sizable 37.8 kcal/mol for 3. Whereas the aci-nitro tautomer of 2-nitrovinyl alcohol is easily accessible, beta-iminonitronic acid has little kinetic stability. H-bonding is a strong stabilizing factor in these nitroalkenes, estimated at 7.0 and 3.7 kcal/mol for the OH and NH2 derivatives, respectively, while its stabilization in their nitronic acids amounts to as much as 13 kcal/mol. The H-bonds are evident from the very short O...H and N...H distances and are characterized by bond critical points. The NO2 substituent effect of about 11.4 kcal/mol at G2MP2 on both the classical keto <==> enol and imine <==> enamine tautomeric processes stabilizes the nitroethylene derivatives. The keto, imine, and vinyl substituent effects at G2MP2 on the nitro <==> aci-nitro tautomeric process are also determined as are their pi-resonance components. The substituents have a large influence on the ionization energies of the nitroethylene derivatives.

Correlation between structure and stability of nitronic acids and nitronic esters

Correlation between structure and stability of nitronic acids and nitronic esters

Correlation between structure and stability of nitronic acids and nitronic esters

On the basis of investigation on structural characteristics and stability of more than one hundred nitronic acids and nitronic esters, an equilibrium theory of electronic effects in conjugated systems is put forward and the type of structures is classified. On that basis, an empirical rule for the correlation between structure and stability of nitronic acids and nitronic esters is proposed, which predicts that stable nitronic acids and nitronic esters should have the C type of structure that meets the matching principle of electronic effects.

The reduction mechanism of the >CO group —III. The electrochemical reduction of isonicotinic acid in an aqueous medium

The electrochemical reduction of isonicotinic acid NRCOOH and its protonated from H +NRCOOH has been examined in an aqueous medium between H 0 = 0 and pH 6.7. As shown by cyclic voltammetry and polarography, a global 2e −, 2H + reversible transfer is followed by two successive first-order chemical reactions and a 2e −, 2H + reduction (EC 1C 2E process). A mechanism is proposed, beginning with the 2e −, 2H + reduction of YRCOOH (where Y is N or H +N) to the formal diionized forms YRC −(OH)(OH 2 +), where the first chemical reaction C 1 is an internal proton transfer which yields the hydrate YRCH(OH) 2 this reaction, which is analogous to the internal proton transfer taking place in the case of nitronic acids in acidic medium has been shown previously to occur in the case of methyl isonicotinate. The rate constant of this process is 2 × 10 2 s −1 in neutral medium and 2 × 10 4 s −1 in acidic medium. The second chemical reaction C 2 (dehydration), which is much slower (rate constant k d < 1.6 × 10 −3 s −1), involves the loss of a H 2O molecule to give 4-formylpyridine, which is itself easier to reduce than isonicotinic acid. The variations of the total current and the dehydration constant with pH are analyzed in detail, and compared to those observed for 4-formylpyridine.

Transition State Imbalances in the Deprotonation of Picrylacetophenones by Carboxylate and Phenoxide Bases

The kinetics of the reversible deprotonation of 4-X-substituted picrylacetophenones 3a−c (X = NO2, H, MeO) by a variety of bases have been measured in 50% H2O−50%Me2SO (v/v) at 25 °C. Comparison of Bronsted βB values for the ionization of each carbon acid by phenoxide and carboxylate bases and αCH values for deprotonation of 3a−c by individual buffers bases indicates that the reaction proceeds through strongly imbalanced transition states. The intrinsic reactivities of 3a−c, as determined from the Bronsted plots for phenoxide ion reactions, are typical for the formation of resonance-stabilized polynitrobenzyl-type carbanions, but the intrinsic rate constant k0 decreases regularly on going from the less acidic p-methoxyacetophenone derivative 3c to the more acidic p-nitroacetophenone derivative 3a. This trend is attributed to the fact that the contribution of the benzoyl moiety to the resonance stabilization of the resulting carbanion C-3 is completely negligible for the p-methoxy compound but not for the p-nitro compound. An extensive 1H and 13C NMR study of the ionization of 3a−c confirms this proposal, with steric hindrance to rotation of the picryl ring around the Cα−Cipso linkage being observed at the probe temperature in the p-methoxy-substituted carbanion C-3c, at −40 °C for the unsubstituted carbanion C-3b, but not at all for the p-nitro carbanion C-3a. A major finding, however, is that the three carbanions undergo protonation at the p-nitro group of the picryl ring to form nitronic acids in acidic media. This behavior clearly shows that charge delocalization through the 2,4,6-trinitrophenyl moiety is predominant in the three systems, including C-3a.

Formation of Nitronic Acids by Ring Opening of Nitrocyclopropanes by a Concerted, Acid‐Catalyzed Mechanism

Formation of Nitronic Acids by Ring Opening of Nitrocyclopropanes by a Concerted, Acid‐Catalyzed Mechanism

Kinetic Studies Concerning the Decay of Nitronic Acids in Polymer Matrixes Using a Data Evaluation Method Based on Dispersive Kinetics

Evaluation methods based on dispersive kinetics were applied to derive kinetic parameters for the rearrangement of differently α-substituted nitronic acids generated by flash photolysis (λ inc = 347 nm) of o-nitrobenzyl esters. The rearrangements were performed in polymer matrices of different chemical nature: poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), poly(styrene) (PSt), and poly(dimethylsiloxane) (PDMS). A nonexponential decay behavior was observed in rigid and highly viscous matrices which can be characterized by the average rate constant v and the dispersion factor α

The reduction mechanism of the >CO group: Part 1. The electrochemical reduction of methyl isonicotinate in an aqueous medium

The electrochemical reduction of methyl isonicotinate NRCO(OMe) and its protonated form H +NRCO(OMe) has been examined in an aqueous medium between H 0 = 0 and pH 6.7. As shown by cyclic voltammetry and polarography, a global 2e −, 2H + reversible transfer is followed by two successive first-order chemical reactions and a 2e −, 2H + reduction (EC 1C 2E process). A mechanism is proposed, beginning with the 2e −, 2H + reduction of YRCO(OMe) (where Y is N or H +N) to the formal diionized forms YRC θ(OH 2) +(OMe) or YRC θ(OH)(OHMe) +, where the first chemical reaction C 1 is an internal proton transfer which yields the hemiacetal YRCH(OH)(OMe); this is analogous to the internal proton transfer occurring in the case of nitronic acids in acidic media. The variations of the rate constant of this process with pH (5.2 × 10 2 s −1 < k < 8.7 × 10 3 s −1) are analyzed. The second chemical reaction C 2, which is much slower (rate constant k d < 1.6 × 10 −3 s −1), involves the loss of a MeOH molecule (analogous to a dehydration) to give 4-pyridine aldehyde, which is itself easier to reduce than 4-methyl isonicotinate.

A Herbicidal Inhibitor of Isopropylmalate Isomerase

Nitronate compounds resembling the aci-carboxylate reaction intermediate in the reaction catalyzed by isopropylmalate isomerase were investigated as enzyme inhibitors and potential herbicides. The nitronic acids of 1-hydroxy-2-nitrocyclopentane-1-carboxylic acid, nitroisopropylmalate and of 1-hydroxy -2-nitrocyclohexane-1-carboxylic acid were all potent inhibitors of isopropylmalate isomerase. The nitroalkane/nitronate pK a values of these were 7.3, 9.5 and 11.1, respectively. The perturbed pK a values of the first and last compound were und erstandable on the basis of the opposing effects of 5- and 6-membered ring strain on the ionization of the nitro group. As the only compound significantly in the nitronate form at physiological pH , it was not surprising that 1-hydroxy-2-nitrocyclopentane-1-carboxylic acid was the best herbicide. Its specific mode of action was confirmed by the fact that 1-hydroxy-2-nitrocyclopentane- 1-carboxylic acid inhibited the growth of carrot cells in liquid culture and was specifically reversed by leucine.