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61.
Yoann Leydet Dr. A. Jorge Parola Dr. Fernando Pina Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(2):545-555
Substitution of the phenyl group in 2‐hydroxychalcones by a 4‐pyridine unit dramatically changes the network of chemical reactions of this compound: trans‐chalcone‐type ( Ct ), cis‐chalcone‐type ( Cc ), and a hemiketal (hydroxy‐4‐pyridinechromene) ( B ) and their protonated forms are formed, but the presence of a flavylium‐type cation could not be detected even at very acidic pH values. Moreover, whereas in 2‐phenyl‐2‐benzopyrylium compounds B and Cc are generally elusive species whose kinetic processes in aqueous solutions occur on the sub‐second timescale, in the present compound these species equilibrate on a timescale four orders of magnitude lower. Complete characterization of the equilibrium and kinetics of the reaction network could thus be achieved by 1H NMR spectroscopy and UV/Vis spectrophotometry. The network of chemical reactions exhibits cis–trans photoisomerization, as well as photochromism between the hemiketal and the chalcone‐type species. The irradiation of Ct in MeOH/H2O (1:1) at 365 nm produces B almost quantitatively through two consecutive photochemical reactions: Ct → Cc photoisomerization followed by Cc → B photo ring closure with a global quantum yield of 0.02. On the other hand, irradiation of B at 254 nm leads to a photostationary state composed by 80 % Ct and 20 % B , with a quantum yield of 0.21. 相似文献
62.
Emi Evangelio Dr. Marie‐Laure Bonnet Dr. Miquel Cabañas Dr. Motohiro Nakano Dr. Jean‐Pascal Sutter Dr. Andrea Dei Prof. Vincent Robert Dr. Daniel Ruiz‐Molina Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(22):6666-6677
The different thermally induced intermolecular electron transfer (IET) processes that can take place in the series of complexes [M(Cat‐N‐BQ)(Cat‐N‐SQ)]/[M(Cat‐N‐BQ)2], for which M=Co ( 2 ), Fe ( 3 ) and Ni( 4 ), and Cat‐N‐BQ and Cat‐N‐SQ denote the mononegative (Cat‐N‐BQ?) or dinegative (Cat‐N‐SQ2?) radical forms of the tridentate Schiff‐base ligand 3,5‐di‐tert‐butyl‐1,2‐quinone‐1‐(2‐hydroxy‐3,5‐di‐tert‐butylphenyl)imine, have been studied by variable‐temperature UV/Vis and NMR spectroscopies. Depending on the metal ion, rather different behaviors are observed. Complex 2 has been found to be one of the few examples so far reported to exhibit the coexistence of two thermally induced electron transfer processes, ligand‐to‐metal (IETLM) and ligand‐to‐ligand (IETLL). IETLL was only found to take place in complex 3 , and no IET was observed for complex 4 . Such experimental studies have been combined with ab initio wavefunction‐based CASSCF/CASPT2 calculations. Such a strategy allows one to solicit selectively the speculated orbitals and to access the ground states and excited‐spin states, as well as charge‐transfer states giving additional information on the different IET processes. 相似文献
63.
64.
Shaheen Faizi Humaira Siddiqi Aneela Naz Samina Bano Lubna 《Helvetica chimica acta》2010,93(3):466-481
An H/D exchange process in patuletin ( 1 ) and its derivatives in D‐donor solvents (e.g., CF3COOD), which occurs regioselectively at C(8) was observed for the first time during NMR studies. The effect of substituents and temperature on the deuteration of various flavonoids (see Fig. 1) which include apigenin, chrysin, galangin, kaempferol, luteolin, morin, myricetin, patuletin, patulitrin, and quercetin, as well as derivatives of patuletin was examined extensively under NMR conditions. The rate constant of deuteration at C(8) of patuletin ( 1 ) and two flavones, luteolin ( 3 ) and apigenin ( 12 ), was also determined in CF3COOD. The D‐atom was introduced into the flavonoids via a keto–enol tautomerism (Scheme 1). During these studies, monodeuterated patuletin was also obtained as a new compound. The examined flavonoids have been reported to possess significant pharmacological activities, and their deuterated derivatives would be of importance for the identification and quantification of these compounds in biological matrices. 相似文献
65.
66.
Atanas G. Chapkanov Bojidarka B. Koleva Michail G. Arnaudov Ivan K. Petkov 《Chemical Papers》2008,62(3):294-301
A detailed experimental and theoretical UV-spectral analysis of the tautomeric forms of 3-methyl-1-phenyl pyrazol-5-one and
1,3-diphenyl pyrazol-5-one as well as of the UV-irradiated photoinduced products of the latter compound were carried out.
The experimental UV-spectra were compared with the theoretical ones obtained by CIS/6-31G**. Selected geometrical parameters
of the different tautomers were estimated by ab initio calculations using the RHF/6-31G** level of theory and basis set, which
have also been discussed with a view to the corresponding UV-results. 相似文献
67.
Stanley Rehn 《Tetrahedron》2005,61(12):3115-3123
The reaction between 3-aminocrotonates and 3-acetonylideneoxindole in refluxing toluene resulted in 2-pyrrolo-3′-yloxindoles in high yields (around 90%). At room temperature the 2-pyrrolo-3′-yloxindoles exists as keto-enol tautomers. Treatment with POCl3 yielded the 2-chloro-3-pyrrolyl indole, which gave the pyrrolo annulated indolopyran-2-one upon basic hydrolysis of 2-chloro-3-pyrrolyl indole methyl ester. 相似文献
68.
S. A. Hovakimyan A. V. Babakhanyan V. S. Voskanyan V. E. Karapetian G. A. Panosyan S. T. Kocharian 《Chemistry of Heterocyclic Compounds》2004,40(8):1047-1051
The reaction of methyl esters of 3-methyl-2-oxo- and 2-oxo-3-phenyl-3-pentenoic acids with hydrazine hydrate and phenylhydrazine was used to synthesize new pyridazinone derivatives. These products are formed through intermediate hydrazides with subsequent cyclization. 4-Hydroxy-3-oxotetrahydropyridazines are mainly formed using equimolar amounts of the starting reagents, while the corresponding hydrazones of 3,4-dioxohexahydropyridazines are formed in the case of a two-fold excess of hydrazine hydrate or phenylhydrazine. Evidence was obtained indicating the existence of a keto–enol tautomerism for 4-hydroxy-3-oxopyridazines. 相似文献
69.
70.
T. Schnherr R. Linder U. Rosellen V. Schmid 《International journal of quantum chemistry》2002,86(1):90-99
The molecular and electronic structure of the three tautomeric forms of dithizone has been calculated by using semiempirical, density functional theory (DFT), and ab initio methods. Comparison of ground‐state energies shows the symmetric form most stabilized, but there is only a small barrier (<3 kcal/mol) for the hydrogen transfer from N? H toward H? S (enol form). For understanding the origin of the optical transitions intermolecular interactions have to be taken into account. By using the supermolecule method, the absorption band pattern can be rationalized already on the level of the PM3 model. The nuclear magnetic resonance (NMR) solution spectrum is interpreted in terms of an equilibrium between the symmetric and the enol forms of dithizone. The appearance of strong EPR signals only for the solid state reflects a considerable lowering of the triplet state (symmetric form). Experimental features are discussed in view of calculated energies (stabilization), chemical shifts (NMR), and SOMO orbitals (EPR). © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 相似文献