Tautomerism in the solid state and in solution of a series of 6-aminofulvene-1-aldimines

To study systems able to sustain intramolecular proton-transfer, we have prepared a series of six aminofulvene aldimines including several labeled with (15)N and (2)H. These compounds show coupling constants through the hydrogen bond, (1h)J((15)N- (1)H) and (2h)J((15)N-(15)N). The position of the tautomeric equilibria, i.e., on what nitrogen atom is the proton, was determined in the solid state and in solution. The crystal structure of N[[5-[(phenylamino)methylene]-1,3-cyclopentadien-1-yl]methylene]pyrrole-1-amine (3) has been determined by X-ray analysis. In solution, both N-H and C-H tautomers were observed and their structures assigned by NMR spectroscopy. Particularly useful is the value of the (1)J((15)N-(1)H) coupling constant.     

The structure of β-diketones related to curcumin determined by X-ray crystallography,NMR (solution and solid state) and theoretical calculations

Structural Chemistry - Structural data are reported on sixteen ketoenols of β-diketones: solution NMR, solid-state NMR (CPMAS and MAS) and X-ray crystallography (four compounds, where three...     

The use of chemical shifts <Emphasis Type="Italic">vs.</Emphasis> coupling constants for studying tautomerism: a combined experimental and theoretical approach

When observing average NMR signals originated from a rapid equilibrium, the procedure to estimate the composition of the mixture is to use interpolation. To illustrate the difficulties of this approach, the much-studied case of the NH and OH tautomers of pyrazolinones will be reexamined. Calculated absolute shieldings and coupling constants were compared with experimental data. Although the large predominance of the OH tautomer in DMSO was confirmed, the result is a little disappointing because no consistency in the percentages was achieved using chemical shifts and coupling constants.     

Multinuclear NMR spectra and GIAO/DFT calculations of N-benzylazoles and N-benzylbenzazoles

The ¹H, ¹³C, and ¹⁵N chemical shifts of almost the whole series of N-benzyl azoles and benzazoles, with the exception of the unknown 1-benzyl-1H-pentazole (10) and the very unstable 2-benzyl-2H-isoindole (12), have been measured. In addition, the X-ray crystal structure of 1-benzyl-1H-indazole (14) was solved (monoclinic, space group P2₁/n), its geometry being very close to that used for the calculations. The absolute chemical shieldings were calculated at the gauge-independent atomic orbital (GIAO)/Becke, 3-parameter, Lee-Yang-Parr (B3LYP)/6-311++G(d,p) level and then transformed with very robust empirical equations into chemical shifts of the three nuclei showing an excellent agreement with the 313 experimental values.

     

Acid assisted proton transfer in 4‐[(4‐R‐phenylimino)methyl]pyridin‐3‐ols: NMR spectroscopy in solution and solid state,X‐ray and UV studies and DFT calculations

The behaviour of Schiff bases of 3‐hydroxy‐4‐pyridincarboxaldehyde and 4‐R‐anilines (R?H, CH₃, OCH₃, Br, Cl, NO₂) in acid media has been described. ¹H, ¹³C, ¹⁵N‐NMR chemical shifts allow to establish the protonation site and its influence on the hydroxyimino/oxoenamino tautomerism. DFT calculations, electronic spectra and X‐ray diffraction are in agreement with the NMR conclusions. Copyright © 2007 John Wiley & Sons, Ltd.     

Structure of a 4-nitroso-5-aminopyrazole and its salts: tautomerism, protonation, and E/Z isomerism

The structures of 1-benzyl-4-nitroso-5-aminopyrazole (1) and its hydrochloride (1H+) have been determined in the solid state and in solution in DMSO, methanol, and ethanol. The free base exists in solution as a mixture of amino/nitroso tautomers 2a and 2b rather than in the imino/oxime tautomers 3. The conjugated cation 1H+ results from the protonation of the nitroso group. X-ray crystallography showed that both amino hydrogen atoms of 2a form NH...O=N hydrogen bonds: one is intramolecular, the other links adjacent molecules in an infinite chain.     

Visible-light mediated regioselective approach towards synthesis of 7-aroyl-6-methyl-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines

The first visible-light mediated, simple, efficient and ecofriendly protocol for the regioselective synthesis of novel [1,2,4]triazolo[3,4-b][1,3,4]thiadiazines has been developed by the reaction of α-bromodiketones, generated in situ by the bromination of a diverse array of β-diketones with N-bromosuccinimide, with 4-amino-[1,2,4]triazole-3-thiols. The methodology does not require the presence of any additives and this reaction proceeded in the presence of EDG (OMe), EWG (Cl) and heteroarenes (thiophenyl) giving the expected products in good to excellent yields. A solvent free protocol was also established to accomplish the synthesis of target compounds but it required PTSA as a catalyst and yields are comparatively poor. The structure of the regioisomer has been confirmed unambiguously by heteronuclear 2D-NMR [(¹H-¹³C) HMBC, (¹H-¹⁵N) HMBC, (¹H-¹³C) HMQC] spectroscopic and X-ray crystallographic studies.     

Syntheses and structural studies of Schiff bases involving hydrogen bonds

New Schiff bases have been prepared by reacting 3-hydroxy-4-pyridine- carboxaldehyde with various amines. NMR spectroscopic methods provided clear evidence that the Schiff bases exist in the solid state and in solution as hydroxyimino tautomers with the E-configuration. A study of the stabilities of the tautomeric forms and the different conformers has been carried out using density functional calculations at the B3LYP/6-31G** level.