Solvent effects on the tautomeric equilibrium of 2,4-pentanedione

1982 ◽  
Vol 60 (10) ◽  
pp. 1178-1182 ◽  
Author(s):  
J. N. Spencer ◽  
Eric S. Holmboe ◽  
Mindy R. Kirshenbaum ◽  
Daniel W. Firth ◽  
Patricia B. Pinto
Keyword(s):  
Hydrogen Bond ◽  
Solvent Effects ◽  
Tautomeric Equilibrium ◽  
Spectroscopic Techniques ◽  
Hydrogen Bond Formation ◽  
Entropy Changes ◽  
Tautomeric Forms ◽  
Self Association ◽  
Enol Tautomer ◽  
Intramolecular Hydrogen

The influence of solvent on the equilibrium position of the tautomeric forms of 2,4-pentanedione was studied by calorimetric and nmr spectroscopic techniques. For solvents such as CCl4 and cyclohexane the intramolecular bond of the enol form persists and bulk solvent effects account for the equilibrium enol–keto content. In solvents such as DMSO, disruption of the intramolecular bond occurs and the percentage of enol falls due to unfavorable entropy changes. The enol intramolecular bond is disrupted by the solvents water and methanol. Enol hydrogen bond formation through self-association and with the solvent accounts for the entropy changes upon enolization in these solvents. The thermodynamic parameters for enolization in neat 2,4-pentanedione are rationalized by the disruption of the enol intramolecular hydrogen bond through consequent polymerization of the enol tautomer.

scholarly journals Synthesis of a Conformationally Stable Atropisomeric Pair of Biphenyl Scaffold Containing Additional Stereogenic Centers

Molecules ◽  
2019 ◽  
Vol 24 (3) ◽  
pp. 643 ◽  
Author(s):  
Chi-Tung Yeung ◽  
Wesley Chan ◽  
Wai-Sum Lo ◽  
Ga-Lai Law ◽  
Wing-Tak Wong
Keyword(s):  
Hydrogen Bond ◽  
Intramolecular Hydrogen Bond ◽  
Bond Formation ◽  
Aldol Reaction ◽  
Spatial Arrangement ◽  
Hydroxyl Groups ◽  
Hydrogen Bond Formation ◽  
X Ray ◽  
Stereogenic Centers ◽  
Intramolecular Hydrogen

The synthesis of a new CF3-containing stereogenic atropisomeric pair of ortho-disubstituted biphenyl scaffold is presented. The atropisomers are surprisingly conformationally stable for isolation. X-ray structures show that their stability comes from an intramolecular hydrogen bond formation from their two hydroxyl groups and renders the spatial arrangement of their peripheral CF3 and CH3 groups very different. The synthesized stereogenic scaffold proved to be effective in catalyzing the asymmetric N-nitroso aldol reaction of enamine and nitrosobenzene. Compared to similar scaffolds without CF3 groups, one of our atropisomer exhibits an increase in enantioselectivity in this reaction.

An infrared study of the interaction between ethyl N-(diphenylmethylene)glycinate with proton donors: comparison with N-benzylidenemethylamine

1986 ◽  
Vol 64 (12) ◽  
pp. 2305-2309 ◽  
Author(s):  
Marleen Ruysen ◽  
Thérèse Zeegers-Huyskens
Keyword(s):  
Hydrogen Bond ◽  
Ir Spectra ◽  
Equilibrium Constants ◽  
Lone Pair ◽  
Imino Group ◽  
Infrared Study ◽  
Hydrogen Bond Formation ◽  
Hydroxy Proton ◽  
Proton Donors ◽  
Intramolecular Hydrogen

The interaction between ethyl N-(diphenylmethylene)glycinate (DPG) and hydroxy proton donors or pyrrole has been investigated by ir spectrometry. The equilibrium constants, enthalpies, and entropies of complex formation have been determined in carbon tetrachloride solution and compared with the data obtained for the complexes involving N-benzylidene-methylamine and the same proton donors. The ir spectra studied mainly in the νOH, νC=N, νC=O, and νC—O regions suggest that hydrogen bond formation occurs at the N atom of the imino group and at the O atom of the carbonyl group. The results are discussed in terms of the basicity at the two acceptor sites and of the accessibility of the lone pair of electrons. The ir spectra of the solid adduct of DPG with HCl show that protonation takes place on the N atom. The protonated structure is possibly stabilized by an intramolecular hydrogen bond.

scholarly journals New theoretical insights on tautomerism of hyperforin—a prenylated phloroglucinol derivative which may be responsible for St. John’s wort ( Hypericum perforatum ) antidepressant activity

2019 ◽  
Vol 31 (2) ◽  
pp. 657-666
Author(s):  
Wojciech P. Oziminski ◽  
Agata Wójtowicz
Keyword(s):  
Density Functional ◽  
Parent Compound ◽  
Antidepressant Activity ◽  
Functional Theory ◽  
Tautomeric Forms ◽  
Stability Order ◽  
Enol Tautomer ◽  
Intramolecular Hydrogen ◽  
Gradual Transformation ◽  
Tautomeric Mixture

AbstractThe thermodynamic aspects of keto-enol tautomerism of hyperforin were investigated theoretically using density functional theory methods. At the B3LYP/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ level of theory the enol tautomer dominates the tautomeric mixture and the second enol tautomer 1OH-HB has Gibbs free energy higher by 1.2 kcal/mol, despite possessing an intramolecular hydrogen bond. The purely keto tautomer is less stable by 3.3 kcal/mol compared with the 1OH tautomer, which means that the percentage of the keto tautomer in the tautomeric mixture is only about 0.4%. This is a different picture than in the parent compound of hyperforin—the phloroglucinol, where the keto tautomer is more stable than corresponding enol 1OH tautomer by 0.6 kcal/mol. To explain this difference, several in-between model molecules reflecting gradual transformation from phloroglucinol to hyperforin were build, and all the tautomeric forms were optimized for each molecule. It turned out that the addition of an aliphatic three-carbon bridge to phloroglucinol ring is crucial for the reversal of the tautomer stability order to that for hyperforin. The probable reason is the unfavorable strain in the keto tautomer introduced by the carbon bridge, which forces a specific geometric configuration which destabilizes in consequence the keto tautomer. This picture of hyperforin tautomerism underlines the dominance of enol tautomers, which can be important when studying the antidepressant activity of hyperforin—its interactions with neurotransmitters receptors.

Intramolecular hydrogen bonds: common motifs, probabilities of formation and implications for supramolecular organization

2000 ◽  
Vol 56 (5) ◽  
pp. 849-856 ◽  
Author(s):  
Clair Bilton ◽  
Frank H. Allen ◽  
Gregory P. Shields ◽  
Judith A. K. Howard
Keyword(s):  
Hydrogen Bond ◽  
Bond Formation ◽  
Intermolecular Hydrogen Bond ◽  
The Other ◽  
Supramolecular Organization ◽  
Intermolecular Bond ◽  
Hydrogen Bond Formation ◽  
Ability To Act ◽  
Structural Database ◽  
Intramolecular Hydrogen

A systematic survey of the Cambridge Structural Database (CSD) has identified all intramolecular hydrogen-bonded ring motifs comprising less than 20 atoms with N and O donors and acceptors. The probabilities of formation Pm of the 50 most common motifs, which chiefly comprise five- and six-membered rings, have been derived by considering the number of intramolecular motifs which could possibly form. The most probable motifs (Pm > 85%) are planar conjugated six-membered rings with a propensity for resonance-assisted hydrogen bonding and these form the shortest contacts, whilst saturated six-membered rings typically have Pm < 10%. The influence of intramolecular-motif formation on intermolecular hydrogen-bond formation has been assessed for a planar conjugated model substructure, showing that a donor-H is considerably less likely to form an intermolecular bond if it forms an intramolecular one. On the other hand, the involvement of a carbonyl acceptor in an intramolecular bond does not significantly affect its ability to act as an intermolecular acceptor and thus carbonyl acceptors display a substantially higher inclination for bifurcation if one hydrogen bond is intramolecular.

Tautomerism in 3{5}-(dimethoxyphenyl)pyrazoles

1996 ◽  
Vol 52 (4) ◽  
pp. 746-752 ◽  
Author(s):  
M. A. Halcrow ◽  
H. R. Powell ◽  
M. J. Duer
Keyword(s):  
Hydrogen Bond ◽  
Nmr Spectroscopy ◽  
Solvate Water Molecule ◽  
X Ray ◽  
Nmr Data ◽  
Tautomeric Forms ◽  
Oxygen Donor ◽  
Intramolecular Hydrogen ◽  
Pyridinic N ◽  
C Nmr

The single-crystal X-ray structures of 3{5}-(2′,5′-dimethoxyphenyl)pyrazole (HI) and the hemihydrate of 3 {5}-(3/,4′-dimethoxyphenyl)pyrazole (IV) have been determined. Compound (HI) exists purely as the 5-substituted prototropomer in the crystal; the pyrazole pyrollic N—H proton is involved in a three-way hydrogen bond, involving an intramolecular contact with a methoxy oxygen donor and an intermolecular interaction to the pyridinic N atom of a neighbouring molecule, forming discrete hydrogen-bonded dimers. There is no evidence of degenerate proton transfer within the dimeric units from CPMAS 13C NMR spectroscopy, in contrast to other known pyrazoles that associate in this manner. In (IV).1/2H2O, however, the pyrrolic proton is disordered over both N(1) and N(2) via hydrogen bonding to the solvate water molecule. CPMAS 13C NMR spectroscopy shows that the prototropic disorder in (IV).1/2H2O is static at temperatures up to 370 K. Solution 1H and 13C NMR data in DMSO-d 6 show that for both (HI) and (IV) the 3- and 5-substituted tautomeric forms are similarly populated in this solvent, suggesting both that the intramolecular hydrogen bond in (HI) has been disrupted and that the two tautomers of (HI) and (IV) are close in energy.

scholarly journals Salicylaldehyde Hydrazones: Buttressing of Outer-Sphere Hydrogen-Bonding and Copper Extraction Properties

2017 ◽  
Vol 70 (5) ◽  
pp. 556 ◽  
Author(s):  
Benjamin D. Roach ◽  
Tai Lin ◽  
Heiko Bauer ◽  
Ross S. Forgan ◽  
Simon Parsons ◽  
...  
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonding ◽  
Density Functional ◽  
Density Functional Theory Calculations ◽  
Outer Sphere ◽  
Copper Extraction ◽  
Hydrogen Bond Acceptor ◽  
Hydrogen Bond Formation ◽  
Oxime Derivatives ◽  
Intramolecular Hydrogen

Salicylaldehyde hydrazones are weaker copper extractants than their oxime derivatives, which are used in hydrometallurgical processes to recover ~20 % of the world’s copper. Their strength, based on the extraction equilibrium constant Ke, can be increased by nearly three orders of magnitude by incorporating electron-withdrawing or hydrogen-bond acceptor groups (X) ortho to the phenolic OH group of the salicylaldehyde unit. Density functional theory calculations suggest that the effects of the 3-X substituents arise from a combination of their influence on the acidity of the phenol in the pH-dependent equilibrium, Cu2+ + 2Lorg ⇌ [Cu(L–H)2]org + 2H+, and on their ability to ‘buttress’ interligand hydrogen bonding by interacting with the hydrazone N–H donor group. X-ray crystal structure determination and computed structures indicate that in both the solid state and the gas phase, coordinated hydrazone groups are less planar than coordinated oximes and this has an adverse effect on intramolecular hydrogen-bond formation to the neighbouring phenolate oxygen atoms.

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