Some interesting aspects of N-oxides

N-oxidation Regioselectivity and Risk Prediction Using DFT-ALIE Calculations

INTRODUCTION

The formation of N-oxide degradants is a major concern in development of new drugs due to potential effects on a compound's pharmacological activity. Such effects include but are not limited to solubility, stability, toxicity, and efficacy. In addition, these chemical transformations can impact physicochemical properties that affect drug manufacturability. Hence identification and control of N-oxide transformations is of critical importance in the development of new therapeutics.

OBJECTIVE

This study describes the development of an in-silico approach to identify N-oxide formation in APIs with respect to autoxidation.

METHODS

Average Local Ionization Energy (ALIE) calculations were carried out using molecular modeling techniques and application of Density Functional Theory (DFT) at the B3LYP/6-31G(d,p) level of theory. A total of 257 nitrogen atoms and 15 different oxidizable nitrogen types were used in developing this method.

RESULTS

The results show that ALIE could be reliably used to predict the most susceptible nitrogen for N-oxide formation. A risk scale was developed that rapidly categorizes nitrogen's oxidative vulnerabilities as small, medium, or high.

CONCLUSIONS

The developed process presents a powerful tool to identify structural susceptibilities for N-oxidation as well as enabling rapid structure elucidation in resolving potential experimental ambiguities.

Thermogravimetric analysis, kinetic study, and pyrolysis-GC/MS analysis of 1,1'-azobis-1,2,3-triazole and 4,4'-azobis-1,2,4-triazole

BACKGROUND

In general, the greater the number of directly linked nitrogen atoms in a molecule, the better its energetic performance, while the stability will be accordingly lower. But 1,1'-azobis-1,2,3-triazole (1) and 4,4'-azobis-1,2,4-triazole (2) show remarkable properties, such as high enthalpies of formation, high melting points, and relatively high stabilities. In order to rationalize this unexpected behavior of the two compounds, it is necessary to study their thermal decompositions and pyrolyses. Although a great deal of research has been focused on the synthesis and characterization of energetic materials with 1 and 2 as the backbone, a complete report on their fundamental thermodynamic parameters and thermal decomposition properties has not been published.

METHODS

Thermogravimetric-differential scanning calorimetry were used to obtain the thermal decomposition data of the title compounds. Kissinger and Ozawa-Doyle methods, the two selected non-isothermal methods, are presented for analysis of the solid-state kinetic data. Pyrolysis-gas chromatography/mass spectrometry was used to study the pyrolysis process of the title compounds.

RESULTS

The DSC curves show that the thermal decompositions of 1 and 2 are at different heating rates involved a single exothermic process. The TG curves provide insight into the total weight losses from the compounds associated with this process. At different pyrolysis temperatures, the compositions and types of the pyrolysis products differ greatly and the pyrolysis reaction at 500 °C is more thorough than 400 °C.

CONCLUSIONS

Apparent activation energies (E) and pre-exponential factors (lnA/s^-1) are 291.4 kJ mol^-1 and 75.53 for 1; 396.2 kJ mol^-1 and 80.98 for 2 (Kissinger). The values of E are 284.5 kJ mol^-1 for 1 and 386.1 kJ mol^-1 for 2 (Ozawa-Doyle). The critical temperature of thermal explosion (T _b ) is evaluated as 187.01 °C for 1 and 282.78 °C for 2. The title compounds were broken into small fragment ions under the pyrolysis conditions, which then might undergo a multitude of collisions and numerous other reactions, resulting in the formation of C₂N₂ (m/z 52), etc., before being analyzed by the GC/MS system.

Computational assessment of several hydrogen-free high energy compounds

Tetrazino-tetrazine-tetraoxide (TTTO) is an attractive high energy compound, but unfortunately, it is not yet experimentally synthesized so far. Isomerization of TTTO leads to its five isomers, bond-separation energies were empolyed to compare the global stability of six compounds, it is found that isomer 1 has the highest bond-separation energy (1204.6kJ/mol), compared with TTTO (1151.2kJ/mol); thermodynamic properties of six compounds were theoretically calculated, including standard formation enthalpies (solid and gaseous), standard fusion enthalpies, standard vaporation enthalpies, standard sublimation enthalpies, lattice energies and normal melting points, normal boiling points; their detonation performances were also computed, including detonation heat (Q, cal/g), detonation velocity (D, km/s), detonation pressure (P, GPa) and impact sensitivity (h50, cm), compared with TTTO (Q=1311.01J/g, D=9.228km/s, P=40.556GPa, h50=12.7cm), isomer 5 exhibites better detonation performances (Q=1523.74J/g, D=9.389km/s, P=41.329GPa, h50= 28.4cm).

Investigations of the Vicarious C-Aminations of 5,7-Dinitrobenzotriazole and 4,6-Dinitrobenzotriazol-3-ium-1-oxide and Their Energetic Properties

This combined experimental and theoretical study details the vicarious nucleophilic substitution by amination of 5,7-dinitrobenzotriazol (1) and 4,6-dinitrobenzotriazole-3-ium-1-oxide (4) with trimethylhydrazinium iodide to afford the new corresponding one- and two-time aminated compounds and investigations of its mechanism by EPR spectroscopy. The preferred position for the first amination is computed by spin density population and verified by X-ray crystallography. The zwitterionic structure of 4 is investigated in solution by (1) H NMR spectroscopy and in solid state by X-ray diffraction. Furthermore, the crystal structure of 1 is presented. The energetic behavior of the aminated products as well as the starting materials 1 and 4 was investigated, regarding sensitivities and performance.

The stability and decomposition mechanism of the catenated nitrogen compounds

Through theoretical calculation and experimental data, the decomposition mechanism of the N,N'-azo azole poly-nitrogen compounds were obtained. The compounds with the decomposition caused by the rupture of the -N-N=N-N- bridge are more stable than the ones with the ring-opening decomposition mechanisms. The catenated -N-N=N-N- bridge can strengthen the stability of the nitrogen chain. Preventing the hybridization change of the N atom where the ring-opening reaction occurs is essential for the nitrogen chain elongation on the ring system. Moreover, the introduction of energetic substituents like nitro group can further improve the performances of poly-nitrogen compounds; and such a modification should be based on a stable poly-nitrogen backbone.

Improving an insensitive low-energy compound, 1,3,4,6,7,9-hexaazacycl[3.3.3]azine, to be an insensitive high explosive by way of two-step structural modifications

We improved an insensitive low-energy compound, 1,3,4,6,7,9-hexaazacycl[3.3.3]azine (HAA), to be an insensitive high explosive by a useful approach of two-step structural modifications. First, the three carbon atoms in HAA are substituted by three new nitrogen atoms symmetrically to form a new energetic compound, 1,2,3,4,5,6,7,8,9-nonaazacycl[3.3.3]azine (NAA). Then, introducing three N-oxides into NAA symmetrically generates another new energetic compound, 1,2,3,4,5,6,7,8,9-nonaazacycl[3.3.3]azine-2,5,8-trioxides (NAATO). The energetic properties and sensitivity of NAATO were studied by using density functional theory. The results indicate that NAATO has higher detonation performance than RDX and comparative sensitivity to TNT, indicating that it has outstanding overall performance and may be considered as a potential candidate of insensitive high explosives. Thus, the insensitive low-energy compound HAA was successfully improved to be an insensitive high explosive by the two-step structural modifications.

Theoretical study on the structure and stability of [1,2,5] oxadiazolo [3,4-e] [1,2,3,4]-tetrazine-4,6-Di-N-dioxide (FTDO)

Although many 1,2,3,4-tetrazine-1,3-dioxide derivates have been synthesized, [1,2,5] oxadiazolo [3,4-e] [1,2,3,4]-tetrazine-4,6-di-N-dioxide (FTDO) is the only one with high enthalpy of formation and high detonation velocity. Whereas, its stability has not been studied. In the present work, the structure of FTDO was investigated using density functional theory (DFT) method, and its stability was calculated by potential energy surface scanning and structure interconvert thermodynamics under different temperatures. The spontaneous isomerization of FTDO and its effect on the stability of FTDO were investigated. The dissociation of FTDO to N2, N2O and furoxan fragments was studied, and the possibility of synthetic route from FTDO to TTTO was discussed.