Rosa M. Claramunt

Substituent effects on the 15N NMR Parameters of Azoles

The 15N chemical shifts and a large collection of coupling constants pertaining to azoles have been gathered from the literature. To complete this collection and to check some anomalies, the spectra of 14 compounds in several solvents were recorded again and 31 compounds were studied for the first time; in all, data for 420 compounds (pyrroles, imidazoles, pyrazoles, triazoles, tetrazoles, indoles, benzimidazoles, indazoles, benzotriazoles and carbazoles) are reported. Additive models are used to discuss the substituent chemical shifts.

Determining hydrogen-bond strengths in the solid state by NMR: the quantitative measurement of homonuclear J couplings

Hydrogen-bonding strengths in the solid state are quantitatively determined by the accurate measurement of 15N-15N J couplings using a straightforward 2D MAS NMR spinecho approach.

Polymorphism vs. Desmotropy: The Cases of 3-Phenyl- and 5-Phenyl-1H-pyrazoles and 3-Phenyl-1H-indazole

Two desmotropes, 3-phenyl-1H-pyrazole (1a) and 5-phenyl-1H-pyrazole (1b) have been isolated and the conditions for their interconversion established. The X-ray structure of 1b has been determined (a=10.862(1), b=5.7620(5), c=12.927(2) A, β=111.435(2)°, space group P21/c), and both tautomers 1a and 1b were characterized by NMR in the solid state (13C- and 15N-CPMAS). In the case of 3-phenyl-1H-indazole (2a), two concomitant polymorphs have been analyzed by X-ray crystallography, and their NMR spectral properties were determined. The low-melting-point polymorph, at 106.7°, contains three molecules in the asymmetric unit (a=41.086(1), b=7.3860(2), c=23.391(1) A, β=117.697(1)°, space group C2/c) and the high-melting-point one, 115.3°, six molecules (a=13.7818(4), b=13.7976(5), c=18.9445(5) A, α=94.300(3), β=95.131(3), γ=119.428(3)°, space group P-1). Here, too, it has been experimentally determined how to transform one form into the other. Density-functional-theory calculations at the B3LYP/6-31G** level have been performed in both examples to rationalize the stability of the different tautomers.

Substituent and solvent effects on the proton transfer equilibrium in anils and azo derivatives of naphthol. Multinuclear NMR study and theoretical calculations

Abstract The tautomeric equilibrium due to proton transfer is compared in a series of anils of 2-hydroxynaphthalene-1-carbaldehyde and azo derivatives of 2-naphthol. Although structurally similar, these systems suffer different substituent effects on the solution-state proton transfer properties. The comparative study was performed using 1 H, 13 C and 15 N NMR spectroscopy in a variety of solvents. Hartree–Fock ab initio calculations involving relative stability of tautomers and full geometry optimization for the ground state are in agreement with the experimental observations.

Photophysics of the 2-(2′-hydroxyphenyl)perimidine: On the fluorescence of the enol form

Abstract The photophysical behaviour of 2-(2′-hydroxyphenyl)perimidine (6), which possesses a strong intramolecular hydrogen bond is evidenced as due to the non-transferred enol form. This compound is highly stable to light, with a photodegradative quantum yield in degassed cyclohexane of 7 × 10 −6 , suggesting that the involvement of a lightinduced excited state intramolecular proton transfer (ESIPT) process in which the phenolic proton is transferred from the enol form to the neighbouring nitrogen atom is less important for conferring photostability than has hitherto been supposed for such type of systems.

Tris(pyrazol-1-yl)methane-rhodium(I) and -iridium(I) complexes; cyrstal structure of [Rh(COD)(tpzm)][RhCl2(COD)]·3CHCl3

Abstract Fourteen new rhodium and iridium complexes of the tris(pyrazol-1-yl)methane (tpzm) ligand have been prepared. They are of the three types [MCl(diolefin)(tpzm)], [M(diolefin)(tpzm)]Clo 4 , and [M(diolefin)(tpzm)] [MCl 2 (diolefin)], where M is Rh I of Ir I and (diolefin) is a cyclic diolefin (1,5-cyclooctadiene, bicyclo-2,2,1-heptadiene, 5,6,7,8-tetrafluoro-1,4-dihydro-1,4-ethenoaphtalene, or 1,3-dimethyl-5,6,7-,8-tetrafluoro-1,4-dihydro-1,4-ethenonapthalene, or 1,3-dimethyl-5,6,7,8-tetrafluoro-1,4-dihydro-1,4-[9-methyletheno]-napthalene). Addition of [IrCl(COD)] 2 to [RhCl(COD)(tpzm)] gives the complex [Ir(COD)(tpzm)] [RhCl 2 (COD)] owing to the greater tendency of iridium to form five-coordinated species. The crystal structure of [Rh(COD)(tpzm)] [RhCl 2 (COD)] has been determined by X-ray diffraction. The space group is P 1 with a 12.4256(21), b 15.4113(25), c 12.0152(16) A, α 101.48(1), β 105.03(1) and λ 67.21(1)°. The complex exhibits an ionic dinuclear structure and crystallizes with six CHCl 3 molecules per unit cell. In the anion, the Rh(2) atom is in a square-planar arrangement and in the cation the coordination around Rh(1) is that of a distorted trigonal bipyramid. A careful 13 C and 1 H NMR spectra study has been carried out, with particular emphasis on the assignment of the pyrazole signals. The shifts induced by complexation (larger in the 1 H NMR spectra for iridium than for rhodium), the dynamics aspects, and the COD signals are discussed.

Structure of bis-, tris- and tetrakispyrazolylborates in the solid state (sodium and potassium salts of tetrakispyrazolylborate by X-ray crystallography) and in solution (1H, 11B, 13C and 15N NMR)

Abstract The knowledge of the properties of pyrazolylborates has been considerably improved both in the solid state and in solution. The hydrates of the sodium and the potassium salts of tetrakis(l-pyrazolyl)borate crystallize in the space group C2/c, both salts being isomorphous. Regular pyrazole rings conform with a pseudosymmetry plane around a distorted tetrahedral boron atom. Chains along the b axis of two different octahedra of coordination, sharing water oxygens, are linked by H bonds. In solution, a careful 1H, 13C, 11B and 15N NMR study was carried out. The applicability of the rules used to assign H(3) versus H(5) and C(3) vcrsus C(5) in the case of pyrazolylborates was reevaluated. Only for tetrakis(1- pyrazolyl)borate were couplings observed with 1B in 1H, 3C and 5N resonances. The regularity of the properties of bis-, tris- and tetrakispyrazolylborates, allows the calculation of the spectral parameters of the still unknown tris-hydrido(1-pyrazolyl)borate, pzBH3−. Finally, the electronic properties, as measured by σp, of the substituents BH3−, pzBH2−, pz2BH− and pz3B− were estimated.

Synthesis and biological evaluation of curcuminoid pyrazoles as new therapeutic agents in inflammatory bowel disease: effect on matrix metalloproteinases.

Seven N-unsubstituted curcuminoid pyrazoles have been synthesized from the corresponding beta-diketones (including curcumin). We evaluated the possibility of curcuminoid pyrazoles regulating the activity of matrix metalloproteinases (MMPs) by human intestinal epithelial cells in vitro. Zymographic analysis revealed that three compounds significantly down-regulated MMP-9 activity on inflammation-induced intestinal epithelial cells, making them original candidates for the treatment of inflammatory bowel disease (IBD).

Symmetrization of Cationic Hydrogen Bridges of Protonated Sponges Induced by Solvent and Counteranion Interactions as Revealed by NMR Spectroscopy

The properties of the intramolecular hydrogen bonds of doubly (15)N-labeled protonated sponges of the 1,8-bis(dimethylamino)naphthalene (DMANH(+)) type have been studied as a function of the solvent, counteranion, and temperature using low-temperature NMR spectroscopy. Information about the hydrogen-bond symmetries was obtained by the analysis of the chemical shifts delta(H) and delta(N) and the scalar coupling constants J(N,N), J(N,H), J(H,N) of the (15)NH(15)N hydrogen bonds. Whereas the individual couplings J(N,H) and J(H,N) were averaged by a fast intramolecular proton tautomerism between two forms, it is shown that the sum |J(N,H)+J(H,N)| generally represents a measure of the hydrogen-bond strength in a similar way to delta(H) and J(N,N). The NMR spectroscopic parameters of DMANH(+) and of 4-nitro-DMANH(+) are independent of the anion in the case of CD(3)CN, which indicates ion-pair dissociation in this solvent. By contrast, studies using CD(2)Cl(2), [D(8)]toluene as well as the freon mixture CDF(3)/CDF(2)Cl, which is liquid down to 100 K, revealed an influence of temperature and of the counteranions. Whereas a small counteranion such as trifluoroacetate perturbed the hydrogen bond, the large noncoordinating anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate B[{C(6)H(3)(CF(3))(2)}(4)](-) (BARF(-)), which exhibits a delocalized charge, made the hydrogen bond more symmetric. Lowering the temperature led to a similar symmetrization, an effect that is discussed in terms of solvent ordering at low temperature and differential solvent order/disorder at high temperatures. By contrast, toluene molecules that are ordered around the cation led to typical high-field shifts of the hydrogen-bonded proton as well as of those bound to carbon, an effect that is absent in the case of neutral NHN chelates.

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.