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STRUCTURE AND TAUTOMERISM OF 3(5)-AMINO-5(3)-ARYLPYRAZOLES IN THE SOLID STATE AND IN SOLUTION : AN X-RAY AND NMR STUDY

Abstract The crystal and molecular structures of five 3(5)-amino-5(3)-arylpyrazoles differing in the nature of the substituent at the para position of the phenyl ring (1: X = H; 3.H2O: X = OCH3; 4: X = Cl; 5: X = Br and 6: X = NO2) have been determined by X-ray analysis. Three situations were detected in the crystal structures: the 3-tautomer is present in 1, 3 and 4; the 5-tautomer is only found in 6 and both tautomers (1:1) are observed in 5. The crystal packings are governed by NH… N O hydrogen bonds and also by OH…N interactions in the monohydrate of 3. It is worth noting that in 1, 3, 4 and 5 there are N-H…π(arene) contacts that might play a role in stabilizing the packing. Solid state 13C NMR results are consistent with the above crystallographic conclusions, thus allowing to determine that the only compound for which no good crystals have been obtained, the p-methyl derivative 2 should be a 3-amino tautomer. NMR solution studies (1H and 13C) allow to determine the 3-amino/5-amino tautomeric equilibrium constant, KT, which obeys a Hammett relationship with σp. Geometry optimizations of the 3 and 5-tautomers at semi-empirical level (AM1) were performed. In all compounds, the 3-tautomer has been found to possess a relatively lower energy by approximately 2 kcal mol−1. The potential energy surface as a function of the hybridization of the amino group and its conformation have also been analyzed.

The annular tautomerism of the curcuminoid NH-pyrazoles

The structures of four NH-pyrazoles, (E)-3,5-bis[β-(4-hydroxy-3-methoxyphenyl)-ethenyl]-1H-pyrazole (3), (E)-3(5)-[β-(4-hydroxy-3-methoxyphenyl)-ethenyl]-5(3)-methyl-1H-pyrazole (4), (E)-3(5)-[β-(4-hydroxy-3-methoxyphenyl)-ethenyl]-4,5(3)-dimethyl-1H-pyrazole (5) and (E)-3(5)-[β-(3,4-dimethoxyphenyl)-ethenyl]-4-methyl-5(3)-phenyl-1H-pyrazole (8), have been determined by X-ray crystallography. Compounds that have a phenol residue crystallize forming sheets that are stabilized by a complex pattern of hydrogen bonds between a unique tautomer (4), or by a 2 : 1 mixture of both tautomers (5) (these tautomers being identical in the case of 3). Pyrazole 8, which lacks OH groups, crystallizes in cyclic dimers that are stabilized by N–H⋯N hydrogen bonds. The tautomerism in solution and in the solid state was determined by 13C and 15N CPMAS NMR spectroscopy. For compounds 4, 5 and 8, the solid state results agree with those observed by crystallography; the most abundant tautomer in solution coincides with the tautomer present in the solid state (4 and 8) or with the most abundant tautomer in the crystal (5).

Structure of NH-benzazoles (1H-benzimidazoles, 1H- and 2H-indazoles, 1H- and 2H-benzotriazoles)

The structure and properties (crystallography, NMR, theoretical calculations) of the three N-unsubstituted benzazoles (1H-benzimidazoles, 1H- and 2H-indazoles, 1H- and 2H-benzotriazoles) have been reviewed for the period 2000–2012 with some results from previous years. The study of these compounds will greatly increase in the coming years and it is expected that the present review will contribute to it.

Computational Thermochemistry of Six Ureas, Imidazolidin-2-one, N,N '-Trimethyleneurea, Benzimidazolinone, Parabanic Acid, Barbital (5,5 '-Diethylbarbituric Acid), and 3,4,4 '-Trichlorocarbanilide, with an Extension to Related Compounds

A computational study of the structural and thermochemical properties of N-phenyl (open) and N-alkyl (cyclic) ureas, through the use of M05-2X and B3LYP density functional theory calculations has been carried out. The consistency of the literature experimental results has been confirmed, and using mainly isodesmic reactions, the unknown Delta(f)H(0)(g) of the other urea derivatives were estimated. The experimental results together with the theoretical information have permitted the study of the effect of phenyl, p- and m-chlorophenyl, alkyl, and carbonyl substitutions on the thermodynamical stability of urea and its cyclic derivatives. The peculiar behavior of the N-tert-butyl substituent in cyclic ureas has been related to geometric deformations.

Synthesis and structural studies of some [14]paracyclo-bis-(1,2)pyrazolium- and (1,3)imidazolium-phanes

Abstract The crystal and molecular structure of [14]paracyclo-bis-(1,2)pyrazoliumphane dibromide (1a) has been determined. The compound exists in the solid state in the chair (C) conformation while both chair (C) and boat (B) conformations are present in solution in comparable amounts. The barrier to the C ⇌ B interconversion has been determined by 1H NMR spectroscopy ( ΔG ‡ ≈ 17 kcal mol −1 ). AM1 semiempirical calculations conveniently reproduce the difference in stability between the chair (C) and the boat (B) conformations. [14]Paracyclo-bis-(1,2)pyrazolium-phane dibromide exists in the solid state in the chair conformation (X-ray crystallography) while both chair and boat conformations are present in solution in comparable amounts. The barrier to the chair boat interconversion has been determined by 1H NMR spectroscopy ( ΔG‡ ≈ 17 kcal mol −1 ).

The molecular structure of 2-phenylbenzimidazole: A new example of incommensurate modulated intramolecular torsion

Abstract Molecular crystals of 2-phenylbenzimidazole, C13H10N2, exhibit a one-dimensional incommensurate structure. The structure has been solved by charge flipping and refined using the superspace formalism in the (3 + 1) D superspace group C2/c(0b0) s0 with modulation wave-vector q = 0.368b*. The unit cell contains 8 molecules, each one disorderly occupying two configurations related by inversion center. The refinement based on a molecular model with two rigid-body parts per molecule includes up to second order Fourier amplitudes for the modulation. The displacive modulation involves a significant intra-molecular twist between the phenyl and the benzimidazole parts.

Adamantylation of N-unsubstituted pyrazole derivatives: mechanistic and structural studies

Reaction of NH-pyrazoles with 1 -bromoadamantane in a high pressure stainless steel autoclave gives regioselectively 1-(1 -adamantyl)or 4-(1-adamantyl)pyrazoles depending on the temperature. A series of cross-experiments allows to establish the mechanism of this reaction. The molecular structure of the most simple derivative, 4-(1-adamantyl) pyrazole (3a), has been determined. Intermolecular N−H...N hydrogen bonds hold the four independent molecules together in a helix system parallel to the c axis. The crystal is built up of two centrosymmetrically related helices

Substituent and ring effects on enthalpies of formation: 2-methyl- and 2-ethylbenzimidazoles versus benzene- and imidazole-derivatives

The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-methylbenzimidazole (2MeBIM) and 2-ethylbenzimidazole (2EtBIM) are reported and the results compared with those of benzimidazole itself (BIM). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311+G(d,p) on B3LYP/6-31G* optimized geometries. The comparison of experimental and calculated values of benzenes, imidazoles and benzimidazoles bearing H (unsubstituted), methyl and ethyl groups shows remarkable homogeneity. The energetic group contribution transferability is not followed, but either using it or adding an empirical interaction term, it is possible to generate an enormous collection of reasonably accurate data for different substituted heterocycles (pyrazole-derivatives, pyridine-derivatives, etc.) from the large amount of values available for substituted benzenes and those of the parent (pyrazole, pyridine) heterocycles.

Studies of intramolecular hydrogen bonds (IMHB): crystal and molecular structure of 2-(2′-hydroxy-phenyl)imidazoles

Abstract The molecular and crystal structure of 2-(2′-hydroxyphenyl)imidazole (2) and 1-methyl-2-(2′-hydroxyphenyl)imidazole (5) have been determined by X-ray analysis. Compound (2) presents a strong intramolecular hydrogen bond (IMHB) responsible for the planarity of the molecule. In both compounds the molecules form chains through NH…O (compound 2) and OH…N hydrogen bonds (compound 5) but giving rise to the same packing mode. Ab initio calculations (6–31G∗∗) have been carried out on both compounds in order to study the effect of the IMHB on the structure.

4-(1H-1,2,4-Triazol-1-yl)phenol

The secondary structure of the title compound, C 8 H 7 N 3 O, consists of O-H…N hydrogen-bonding catemers, similar to those of the parent 1,2,4-triazole, where the molecules are linked by N-H…N bonds. Chains related by translation are linked by C-H(triazole)…O/N interactions to give a tertiary structure of corrugated sheets parallel to the ac plane. The quaternary structure is due to C-H(Ph)…O contacts and to the stacking of triazole rings from centrosymmetrically related sheets.