Search results for "tautomer"
showing 10 items of 141 documents
Specificity of 15N NMR chemical shifts to the nature of substituents and tautomerism in substituted pyridine N-oxides
2012
1H, 13C, and 15N NMR chemical shifts have been measured for 2-aminopyridine N-oxide (1), its eleven derivatives (2–10, 13, 14), and 3-Cl and 3-Br substituted 4-nitropyridine N-oxides (11, 12). δ(15N) of pyridine ring nitrogen in 2-acetylaminopyridine N-oxides are 5.9–11.5 ppm deshielded from those in 2-aminopyridine N-oxides. When amino and acetylamino substituents are in 4-position, δ(15N) of ring nitrogen is 21.3 ppm deshielded in the acetylated derivative. The strong resonance interaction between 2-amino and 5-nitro groups reflects in the decrease of amino nitrogen shielding about 5.3–17.9 ppm. Also, 1H and 13C NMR spectral data are in agreement with 15N NMR results reflected as deshield…
Electrochemical and DFT studies of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H3 (M = Mo, W) in acetonitrile
2006
International audience; A detailed electrochemical study of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H3 (M = Mo, W) in acetonitrile is presented. For the Mo complex, the decomposition occurs by four different pathways involving classical and non-classical tautomers, whereas only the classical form is accessible for the W derivative. Each of the decomposition pathways has been quantitatively assessed by analyses of the linear sweep voltammograms. In addition to the previously established (B. Pleune, D. Morales, R. Meunier-Prest, P. Richard, E. Collange, J. C. Fettinger and R. Poli, J. Am. Chem. Soc., 1999, 121, 2209–2225) deprotonation, disproportionation, and H2 red…
Structure and properties of hydroxyl radical modified nucleic acid components II. 8-Oxo-adenine and 8-oxo-2′-deoxy-adenosine
1997
Abstract The tautomerism of the 8-oxo-adenine (8-oxo-A) and 8-oxo-2′-deoxy-adenosine (8-oxo-dA) was analysed on the basis of semiempirical, SCF ab initio and DFT density functional quantum chemistry calculations. The results of full gradient geometry optimisation of all possible 8-oxo-A and 8-oxo-dA structures lead to the conclusion that the most stable form is 8-keto-6-amino-tautomer. The second stable tautomer corresponds to 8-hydroxy-isomer. Such an order was unchanged after solvating process. In all studied solvents: water, methanol, acetone, cyclohexane the keto-tautomer proceeds the enol one. The preferred N-glycoside torsion angle corresponds to syn rotamer of 8-oxo-dA in all studied…
Long-range substituent and temperature effect on prototropic tautomerism in 2-(acylmethyl)quinolines
2001
Tautomeric equilibria between 2-(cinnamoylmethyl)quinoline, (Z)-1,2-dihydro-2-(cinnamoylmethylene)quinoline and (Z)-4-phenyl-1-(2-quinolyl)-1,3-butadien-2-ol were studied by 1H, 13C and 15N NMR methods. The —CHCH— fragment conjugated with phenyl and a strong electron donor p-(1-pyrrolidine) substituent were found to favour the enolimine tautomer. This undergoes fast exchange (on the NMR time-scale) with the enaminone form. The amount of the latter tautomer was found to increase at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd.
Lewis Acid Mediated Domino Reaction between 2-Cyclohexenone and Methyl Azide - A DFT Study
2005
We have studied the Lewis acid mediated domino reaction between 2-cyclohexenone and methyl azide at the B3LYP/6-31G* and B3LYP/6-31+G**//B3LYP/6-31G* levels of theory. A complete survey of the potential energy surface is presented, and the structures of the stationary points and the corresponding molecular mechanism are discussed. The calculations suggest that the product, a cyclopentanone derivative, is formed by means of a domino process that comprises three consecutive reactions. The first and rate-limiting step is a concerted 1,3-dipolar cycloaddition between the Lewis acid coordinated cyclohexenone and methyl azide that yields the corresponding bicyclic 1,2,3-triazoline. The second rea…
(Z)- und (E)-Enole von diarylsubstituierten β-Ketocarbonsäurederivativen
2006
(Z) and (E) Enols of Diaryl-Substituted β-Ketocarboxylic Acid Derivatives Thermolysis of the 1,3-diaryl-2-diazo-1,3-propanediones 4 in the presence of alcohols or amines yielded the compounds 7–11 which exist as chelated (Z) enols or as tautomeric ketones. Contrary to the steric interaction of two mesityl groups the steric interaction between two naphthyl or two anthryl groups or between naphthyl and mesityl is not sufficient to stabilize the corresponding (E) enols. The latter isomers were generated in irradiation processes of some (Z) enols. Detailed NMR studies were performed in order to characterize the enols and their molecular dynamics.
Evidence that steric factors modulate reactivity of tautomeric iron-oxo species in stereospecific alkane C-H hydroxylation
2014
A new iron complex mediates stereospecific hydroxylation of alkyl C-H bonds with hydrogen peroxide, exhibiting excellent efficiency. Isotope labelling studies provide evidence that the relative reactivity of tautomerically related oxo-iron species responsible for the C-H hydroxylation reaction is dominated by steric factors This work has been supported by the European Union (the Erasmus Mundus program), the International Research Training Group Metal Sites in Biomolecules: Structures, Regulation and Mechanisms (www.biometals.eu), and COST Action CM1003. M.C. acknowledges ERC-29910, MINECO of Spain for CTQ2012- 37420-C02-01/BQU and CSD2010-00065, catalan DIUE (2009SGR637) and an ICREA academ…
Chlorodicyclohexylborane-mediated aldol additions of alpha,alpha'-dioxygenated ketones.
2001
Boron aldol additions of variously O-protected alpha,alpha'-dioxygenated ketones using dicyclohexylboron chloride and a tertiary amine have been investigated. The stereoselectivity of the process was dependent on the protecting group on the alpha-oxygen atoms. Notably, ketones with bulky silyloxy groups gave syn aldols, most likely via Z enolates. This rules out the participation of chelates during the enolization process, at least in the presence of such sterically crowded protecting groups. An alternative explanation is offered.
2-Acylamino-6-pyridones: breaking of an intramolecular hydrogen bond by self-association and complexation with double and triple hydrogenbonding coun…
2012
2-Acylamino-6-pyridones (acyl = RCO, where R = Me, Et, i-Pr, t-Bu, and 1-adamantyl) were previously characterized by X-ray diffractometry and solid-state NMR techniques by us. One of these compounds was used recently in organocatalysis. The series is now studied in solution and by computational methods recommended for noncovalent interactions (DFT/M05). These compounds showed interesting behavior during dilution and titration experiments monitored by (1)H NMR. 2-Acylamino-6-pyridones change their conformation at higher concentrations, forming double hydrogen-bonded dimers and trimers in which an uncommon steric effect is observed. To the best of our knowledge, this is the first example of s…
Durch sterische effekte stabilisierte ß-ketocarbonsäuren
1989
Abstract Increasing steric hindrance in β-keto carboxylic acids leads to an increasing kinetic stability towards decarboxylation, till systems are reached which are completely stable at room temperature. Simultaneously the tautomeric equilibrium is changed in favour of the (Z)-enol, and finally in favour of the (E)-configurated enol.