The kinetics of water-assisted tautomeric 1,2-proton transfer in azoles: a computational approach

Solvent- and functional-group-assisted tautomerism of 3-alkyl substituted 5-(2-pyridyl)-1,2,4-triazoles in DMSO-water

The tautomerism of a series of 5-alkyl substituted 3-(2-pyridyl)-1,2,4-triazoles in DMSO-d6-containing water has been investigated by 1H, 13C and 15N NMR spectroscopy. The populations of the three possible regioisomers in the tautomeric equilibrium (A [3-alkyl-5-(2-pyridyl)-1H], B [5-alkyl-3-(2-pyridyl)-1H] and C [5-alkyl-3-(2-pyridyl)-4H]) were determined. Isomers A (17-40%) and B (54-79%) are the major components and their ratio is insensitive to the substitution pattern, except for the unsubstituted and the methoxymethyl substituted derivatives. The isomer C (3-5%) has been fully characterised for the first time by NMR spectroscopy. Activation energies of tautomerisation (14.74-16.78 kcal mol-1) were determined by EXSY experiments, which also supported the involvement of water in the tautomerisation. Substituent effects on the 15N chemical shifts are relatively small. The DFT study of the tautomerism in DMSO-water showed that both A/B and B/C interconversions are assisted by the pyridine substituent and catalysed by solvent molecules. The NH-A/NH-B tautomerisation takes place via a relayed quadruple proton transfer mediated by three water molecules in the hydrogen-bonded cyclic substructure of a triazole·4H2O complex. The equilibrium B ⇄ C involves three steps: NH-B transfer to the pyridyl nitrogen mediated by a water molecule in a 1 : 1 cyclic complex, rotamerisation to bring the pyridinium NH close to N4 of the triazole catalysed by complexation to a DMSO molecule and transfer of the NH from the pyridinium donor to the N4 acceptor via a 1 : 1 complex with a bridging water molecule. This mechanism of 1,3-prototropic shift in triazoles is unprecedented in the literature.

A theoretical study of dynamic processes observed in trimethylsilyl-1H-pyrazoles: prototropy and silylotropy

The 1H, 13C, 15N and 29Si chemical shifts of three trimethylsilyl-1H-pyrazoles were calculated and compared with literature results; the calculations were carried out at the GIAO/B3LYP/6–311 +  + G(d,p) level resulting in a very good agreement that allows to predict with confidence the missing experimental values. The prototropic barrier of 4-trimethylsilyl-1H-pyrazole (1) as well as the silylotropic barriers of 1-trimethylsilyl-1H-pyrazole (2) and 1-trimethylsilyl-4-methyl-1H-pyrazole (3) were also calculated and the mechanism was established, the accordance with the experimental values being satisfactory.

Structure and Reactivity of Alloxan Monohydrate in the Liquid Phase

Alloxan is an important toxic glucose analogue used to induce diabetes in lab test animals. Once regarded as a "problem structure," the condensed-phase structure of anhydrous alloxan has largely been settled, but literature inconsistencies remain for the structure of the typically employed reagent alloxan monohydrate. Due to the criticality of structure-function relationships, we have used ¹H/¹³C{¹H} NMR, IR spectroscopy, as well as quantum mechanical (QM) calculations to probe the liquid-phase structure and reactivity of alloxan monohydrate. In protic solvents (D₂O and acetic acid-d₄), hydration at the C5 carbonyl of alloxan monohydrate occurs quantitatively to form the C5 gem-diol (5,5'-dihydroxybarbituric acid). In the aprotic solvent dimethyl sulfoxide (DMSO)-d₆, there exists a mixture of the C5 gem-diol and planar tetraketo form of alloxan monohydrate. QM calculations explain the solvent-dependent hydration reactivity, where a solvent-assisted H-atom transfer mechanism lowers the activation energy of water addition at the C5 carbonyl by ∼16 or 27 kcal/mol in water or acetic acid, respectively, compared to the unassisted hydration reaction. Prompt recrystallization of alloxan monohydrate from boiling water does not alter the structure of the reagent. These findings probe the exact structure of alloxan monohydrate to guide future research efforts in biological sciences and in organic synthesis.

Large Stabilization Effects by Intramolecular Beryllium Bonds in Ortho-Benzene Derivatives

Intramolecular interactions are shown to be key for favoring a given structure in systems with a variety of conformers. In ortho-substituted benzene derivatives including a beryllium moiety, beryllium bonds provide very large stabilizations with respect to non-bound conformers and enthalpy differences above one hundred kJ·mol⁻¹ are found in the most favorable cases, especially if the newly formed rings are five or six-membered heterocycles. These values are in general significantly larger than hydrogen bonds in 1,2-dihidroxybenzene. Conformers stabilized by a beryllium bond exhibit the typical features of this non-covalent interaction, such as the presence of a bond critical point according to the topology of the electron density, positive Laplacian values, significant geometrical distortions and strong interaction energies between the donor and acceptor quantified by using the Natural Bond Orbital approach. An isodesmic reaction scheme is used as a tool to measure the strength of the beryllium bond in these systems in terms of isodesmic energies (analogous to binding energies), interaction energies and deformation energies. This approach shows that a huge amount of energy is spent on deforming the donor–acceptor pairs to form the new rings.

Cation-Anion and Anion-CO2 Interactions in Triethyl(octyl)phosphonium Ionic Liquids with Aprotic Heterocyclic Anions (AHAs)

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for postcombustion CO₂ capture applications. The anions of AHA ILs play a significant role in tuning anion-CO₂ complexation. In addition, AHAs are able to trigger the abstraction of acidic protons located at the α position of phosphonium cations by forming hydrogen bonds between cations and anions, eventually leading to cation-driven CO₂ complexation. Here we investigate the role of the anion in cation-anion hydrogen bonding and ylide formation. Using CO₂ uptake measurements, ³¹P nuclear magnetic resonance (NMR), attenuated total reflection-Fourier transform infrared (ATR-FTIR) deuterium exchange equilibrium and rates, two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY), and density functional theory calculations, we show that the key is the proximity of the negatively charged nitrogen atoms on the anion to the α protons, which is governed not just by anion basicity but by sterics. Thus, we show that triethyl(octyl)phosphonium 3-methyl-5-trifluoromethylpyrazolide is much more effective in hydrogen-bonding with and deprotonating the cation than the equivalent [P₂₂₂₈] ILs with more basic 2-cyanopyrrolide and 3-trifluoromethylpyrazolide anions.

Revisiting the Structure and Chemistry of 3(5)-Substituted Pyrazoles

Pyrazoles are known as versatile scaffolds in organic synthesis and medicinal chemistry, often used as starting materials for the preparation of more complex heterocyclic systems with relevance in the pharmaceutical field. Pyrazoles are also interesting compounds from a structural viewpoint, mainly because they exhibit tautomerism. This phenomenon may influence their reactivity, with possible impact on the synthetic strategies where pyrazoles take part, as well as on the biological activities of targets bearing a pyrazole moiety, since a change in structure translates into changes in properties. Investigations of the structure of pyrazoles that unravel the tautomeric and conformational preferences are therefore of upmost relevance. 3(5)-Aminopyrazoles are largely explored as precursors in the synthesis of condensed heterocyclic systems, namely pyrazolo[1,5-a]pyrimidines. However, the information available in the literature concerning the structure and chemistry of 3(5)-aminopyrazoles is scarce and disperse. We provide a revision of data on the present subject, based on investigations using theoretical and experimental methods, together with the applications of the compounds in synthesis. It is expected that the combined information will contribute to a deeper understanding of structure/reactivity relationships in this class of heterocycles, with a positive impact in the design of synthetic methods, where they take part.

Irbesartan desmotropes: Solid-state characterization, thermodynamic study and dissolution properties

Irbesartan (IBS) is a tetrazole derivative and antihypertensive drug that has two interconvertible structures, 1H- and 2H-tautomers. The difference between them lies in the protonation of the tetrazole ring. In the solid-state, both tautomers can be isolated as crystal forms A (1H-tautomer) and B (2H-tautomer). Studies have reported that IBS is a polymorphic system and its forms A and B are related monotropically. These reports indicate form B as the most stable and less soluble form. Therefore, the goal of this contribution is to demonstrate through a complete solid-state characterization, thermodynamic study and dissolution properties that the IBS forms are desmotropes that are not related monotropically. However, the intention is also to call attention to the importance of conducting strict chemical and in solid-state quality controls on the IBS raw materials. Hence, powder X-ray diffraction (PXRD) and Raman spectroscopy (RS) at ambient and non-ambient conditions, differential scanning calorimetry (DSC), hot stage microscopy (HSM), Fourier transform infrared (FT-IR) and scanning electron microscopy (SEM) techniques were applied. Furthermore, intrinsic dissolution rate (IDR) and structural stability studies at 98% relative humidity (RH), 25 °C and 40 °C were conducted as well. The results show that in fact, form A is approximately four-fold more soluble than form B. In addition, both IBS forms are stable at ambient conditions. Nevertheless, structural and/or chemical instability was observed in form B at 40 °C and 98% RH. IBS has been confirmed as a desmotropic system rather than a polymorphic one. Consequently, forms A and B are not related monotropically.

Complexes of Zn(II)-Triazoles with CO2 and H2O: Structures, Energetics, and Applications

Using a first-principle methodology, we investigate the stable structures of the nonreactive and reactive clusters formed between Zn²⁺-triazoles ([Zn²⁺-Tz]) clusters and CO₂ and/or H₂O. In sum, we characterized two modes of bonding of [Zn²⁺-Tz] with CO₂/H₂O: the interaction is established through (i) a covalent bond between Zn²⁺ of [Zn²⁺-Tz] and oxygen atoms of CO₂ or H₂O and (ii) hydrogen bonds through N-H or C-H of [Zn²⁺-Tz] and oxygen atoms of H₂O or CO₂, N-H···O. We also identified intramolecular proton transfer processes induced by complexation. Indeed, water drastically changes the shape of the energy profiles of the tautomeric phenomena through strong lowering of the potential barriers to tautomerism. The comparison to [Zn²⁺-Im] subunits formed with Zn²⁺ and imidazole shows that the efficiency of Tz-based compounds for CO₂ capture and uptake is due to the incorporation of more accessible nitrogen donor sites in Tzs compared to imidazoles. Since [Zn²⁺-Tz] clusters are subunits of an organometallic nanoporous materials and Zn-proteins, our data are useful for deriving force fields for macromolecular simulations of these materials. Our work also suggests the consideration of traces of water to better model the CO₂ sequestration and reactivity on macromolecular entities such as pores or active sites.

Solvent and Substituent Effects on the Phosphine + CO2 Reaction

A theoretical study of the substituent and solvent effects on the reaction of phosphines with CO2 has been carried out by means of Møller-Plesset (MP2) computational level calculations and continuum polarizable method (PCM) solvent models. Three stationary points along the reaction coordinate have been characterized, a pre-transition state (TS) assembly in which a pnicogen bond or tetrel bond is established between the phosphine and the CO2 molecule, followed by a transition state, and leading finally to the adduct in which the P–C bond has been formed. The solvent effects on the stability and geometry of the stationary points are different. Thus, the pnicogen bonded complexes are destabilized as the dielectric constant of the solvent increases while the opposite happens within the adducts with the P–C bond and the TSs trend. A combination of the substituents and solvents can be used to control the most stable minimum.

Synthesis of 3-(5-amino-1H-1,2,4-triazol-3-yl)propanamides and their tautomerism

Two complementary pathways for the preparation of N-substituted 3-(5-amino-1H-1,2,4-triazol-3-yl)propanamides (5) were proposed and successfully realized in the synthesis of 20 representative examples. These methods use the same types of starting materials viz. succinic anhydride, aminoguanidine hydrochloride, and a variety of amines. The choice of the pathway and sequence of the introduction of reagents to the reaction depended on the amine nucleophilicity. The first pathway started with the preparation of N-guanidinosuccinimide, which then reacted with amines under microwave irradiation to afford 5. The desired products were successfully obtained in the reaction with aliphatic amines (primary and secondary) via a nucleophilic opening of the succinimide ring and the subsequent recyclization of the 1,2,4-triazole ring. This approach however failed when less nucleophilic aromatic amines were used. Therefore, an alternative pathway, with the initial preparation of N-arylsuccinimides and their subsequent reaction with aminoguanidine hydrochloride under microwave irradiation, was applied. The annular prototropic tautomerism in the prepared 1,2,4-triazoles 5 was studied using NMR spectroscopy and X-ray crystallography.

Two complementary pathways for the preparation of N-substituted 3-(5-amino-1H-1,2,4-triazol-3-yl)propanamides were proposed and successfully realized in the synthesis of 20 representative examples.