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Deprotonation Reactions (deprotonation + reaction)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Novel Enantiomerically Pure Heteroleptic Magnesium Complexes for Use in Enantioselective Deprotonation Reactions.

CHEMINFORM, Issue 41 2003
Emma L. Carswell
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

Solid-Phase Supported Chiral Lithium Amides Used in Deprotonation Reactions.

CHEMINFORM, Issue 37 2003
Anna Johansson
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

Gas-phase basicities for ions from bradykinin and its des-arginine analogues

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 8 2001
Nigel P. Ewing
Abstract Apparent gas-phase basicities (GBapps) for [M + H]+ of bradykinin, des-Arg1 -bradykinin and des-Arg9 -bradykinin have been assigned by deprotonation reactions of [M + 2H]2+ in a Fourier transform ion cyclotron resonance mass spectrometer. With a GBapp of 225.8 ± 4.2 kcal mol,1, bradykinin [M + H]+ is the most basic of the ions studied. Ions from des-Arg1 -bradykinin and des-Arg9 -bradykinin have GBapp values of 222.8 ± 4.3 kcal mol,1 and 214.9 ± 2.3 kcal mol,1, respectively. One purpose of this work was to determine a suitable reaction efficiency ,break point' for assigning GBapp values to peptide ions using the bracketing method. An efficiency value of 0.1 (i.e. approximately 10% of all collisions resulting in a deprotonation reaction) was used to assign GBapps. Support for this criterion is provided by the fact that our GBapp values for des-Arg1 -bradykinin and des-Arg9 -bradykinin are identical, within experimental error, to literature values obtained using a modified kinetic method. However, the GBapps for bradykinin ions from the two studies differ by 10.3 kcal mol,1. The reason for this is not clear, but may involve conformation differences produced by experimental conditions. The results may be influenced by salt-bridge conformers and/or by conformational changes caused by the use of a proton-bound heterodimer in the kinetic method. Factors affecting the basicities of these peptide ions are also discussed, and molecular modeling is used to provide information on protonation sites and conformations. The presence of two highly basic arginine residues on bradykinin results in its high GBapp, while the basicity of des-Arg1 -bradykinin ions is increased by the presence of two proline residues at the N-terminus. The proline residue in the second position folds the peptide chain in a manner that increases intramolecular hydrogen bonding to the protonated N-terminal amino group of the proline at the first position. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Lithiation of 2-Aryl-2-(chloroaryl)-1,3-dioxolanes and Its Application in the Synthesis of New ortho -Functionalized Benzophenone Derivatives

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2004
Gyula Lukács
Abstract 2-Aryl-2-(chloroaryl)-1,3-dioxolanes 4 were lithiated ortho to the ketal group of the chloroaryl ring by treatment with butyllithium in THF between ,78 and 0 °C. The site selectivity of some of the deprotonation reactions was rationalized by the long-range effect of the 4-chloro substituent. The lithio species thus generated were treated with various electrophiles to give ortho -functionalized benzophenone derivatives. Intramolecular competition between the aryl rings was observed in the lithiation of 2-(4-chlorophenyl)-2-(4-fluorophenyl)-1,3-dioxolane (4s). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Gas-phase basicities for ions from bradykinin and its des-arginine analogues

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 8 2001
Nigel P. Ewing
Abstract Apparent gas-phase basicities (GBapps) for [M + H]+ of bradykinin, des-Arg1 -bradykinin and des-Arg9 -bradykinin have been assigned by deprotonation reactions of [M + 2H]2+ in a Fourier transform ion cyclotron resonance mass spectrometer. With a GBapp of 225.8 ± 4.2 kcal mol,1, bradykinin [M + H]+ is the most basic of the ions studied. Ions from des-Arg1 -bradykinin and des-Arg9 -bradykinin have GBapp values of 222.8 ± 4.3 kcal mol,1 and 214.9 ± 2.3 kcal mol,1, respectively. One purpose of this work was to determine a suitable reaction efficiency ,break point' for assigning GBapp values to peptide ions using the bracketing method. An efficiency value of 0.1 (i.e. approximately 10% of all collisions resulting in a deprotonation reaction) was used to assign GBapps. Support for this criterion is provided by the fact that our GBapp values for des-Arg1 -bradykinin and des-Arg9 -bradykinin are identical, within experimental error, to literature values obtained using a modified kinetic method. However, the GBapps for bradykinin ions from the two studies differ by 10.3 kcal mol,1. The reason for this is not clear, but may involve conformation differences produced by experimental conditions. The results may be influenced by salt-bridge conformers and/or by conformational changes caused by the use of a proton-bound heterodimer in the kinetic method. Factors affecting the basicities of these peptide ions are also discussed, and molecular modeling is used to provide information on protonation sites and conformations. The presence of two highly basic arginine residues on bradykinin results in its high GBapp, while the basicity of des-Arg1 -bradykinin ions is increased by the presence of two proline residues at the N-terminus. The proline residue in the second position folds the peptide chain in a manner that increases intramolecular hydrogen bonding to the protonated N-terminal amino group of the proline at the first position. Copyright © 2001 John Wiley & Sons, Ltd. [source]

4-Alkylbenzopyrylium perchlorates as C,H Acidic compounds

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2002
U.-W. Grummt
Abstract 7-Alkyl-5,6-dihydro-3-methoxy(naphtho[1,2b]-1-benzopyrylium) compounds are acidochromic due to CH acidity of the 7-alkyl substituent. Their acidity exponents are 4 < pKa < 5 and 1 < pKa < 3 regardless of whether the carbocation generated by deprotonation is resonance stabilized by a phenyl substituent or not. The thermodynamic equilibrium data were obtained via UV,vis spectroscopy. The activation parameters for protonation and deprotonation reactions were derived from rapid mixing experiments. Ab initio calculations with model compounds are presented in order to rationalize the experimental findings. The title compounds are useful objects to study the kinetics of proton-transfer kinetics to and from carbon. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Reaction Mechanism of Porphyrin Metallation Studied by Theoretical Methods

CHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2005
Yong Shen Dr.
Abstract We have studied the reaction mechanism for the insertion of Mg2+ and Fe2+ into a porphyrin ring with density functional calculations with large basis set and including solvation, zero-point and thermal effects. We have followed the reaction from the outer-sphere complex, in which the metal is coordinated with six water molecules and the porphyrin is doubly protonated, until the metal ion is inserted into the deprotonated porphyrin ring with only one water ligand remaining. This reaction involves the stepwise displacement of five water molecules and the removal of two protons from the porphyrin ring. In addition, a step seems to be necessary in which a porphyrin pyrrolenine nitrogen atom changes its interaction from a hydrogen bond to a metal-bound solvent molecule to a direct coordination to the metal ion. If the protons are taken up by a neutral imidazole molecule, the deprotonation reactions are exothermic with minimal barriers. However, with a water molecule as an acceptor, they are endothermic. The ligand exchange reactions were approximately thermoneutral (±20 kJ,mol,1, with one exception) with barriers of up to 72 kJ,mol,1 for Mg and 51 kJ,mol,1 for Fe. For Mg, the highest barrier was found for the formation of the first bond to the porphyrin ring. For Fe, a higher barrier was found for the formation of the second bond to the porphyrin ring, but this barrier is probably lower in solution. No evidence was found for an initial pre-equilibrium between a planar and a distorted porphyrin ring. Instead, the porphyrin becomes more and more distorted as the number of metal,porphyrin bonds increase (by up to 191 kJ,mol,1). This strain is released when the porphyrin becomes deprotonated and the metal moves into the ring plane. Implications of these findings for the chelatase enzymes are discussed. [source]