ChemInform Abstract: MECHANISM OF BASE-PROMOTED PHOSPHONIUM SALT HYDROLYSES. KINETICS AND MULTIPLE SUBSTITUENT EFFECTS FOR A NUCLEOPHILIC ATTACK AT PHOSPHORUS

1979 ◽  
Vol 10 (32) ◽  
Author(s):  
B. SIEGEL
Keyword(s):  
Phosphonium Salt ◽  
Substituent Effects ◽  
Nucleophilic Attack

The reactions of mercaptide anions with chlorinated sulfones

1977 ◽  
Vol 55 (12) ◽  
pp. 2316-2322 ◽  
Author(s):  
Richard F. Langler ◽  
James A. Pincock
Keyword(s):  
Substituent Effects ◽  
Nucleophilic Attack ◽  
Leaving Group

Mercaptide anions react with chlorinated sulfones in two modes, i.e. nucleophilic attack on carbon with chlorine as the leaving group and/or nucleophilic attack on chlorine with concomitant carbanion formation. Mercaptide anion pKb, degree of chlorination of the sulfone substrate, and substituent effects are qualitatively assessed in terms of the propensity for nucleophilic attack at carbon or chlorine.

Substituent effects on the reactivity of the silicon–carbon double bond. Arrhenius parameters for the reaction of 1,1 -diarylsilenes with alcohols and acetic acid

1997 ◽  
Vol 75 (10) ◽  
pp. 1393-1402 ◽  
Author(s):  
Christine J. Bradaric ◽  
William J. Leigh
Keyword(s):  
Acetic Acid ◽  
Proton Transfer ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Substituent Effects ◽  
Activation Energies ◽  
Nucleophilic Attack ◽  
Acetonitrile Solution ◽  
Nanosecond Laser ◽  
Arrhenius Parameters

Absolute rate constants for the reaction of a series of ring-substituted 1,1 -diphenylsilene derivatives with methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined using nanosecond laser flash photolysis techniques. The three reactions exhibit small positive Hammett ρ-values at 23 °C, consistent with a mechanism involving initial, reversible nucleophilic attack at silicon to form a σ-bonded complex that collapses to product via rate-limiting proton transfer. Deuterium kinetic isotope effects and Arrhenius parameters have been determined for the reactions of 1,1-di-(4-methylphenyl)silene and 1,1-di-(4-trifluoromethylphenyl)silene with methanol, and are compared to those for the parent compound. Proton transfer within the complex is dominated by entropic factors, resulting in negative activation energies for reaction. The trends in the data can be rationalized in terms of variations in the relative rate constants for reversion to reactants and proton transfer as a function of temperature and substituent. A comparison of the Arrhenius activation energies for reaction of acetic acid with 1,1-diphenylsilene (Ea = +1.9 ± 0.3 kcal/mol) and the more reactive di-trifluoromethyl analogue (Ea = +3.6 ± 0.5 kcal/mol) suggests that carboxylic acids also add by a stepwise mechanism, but with formation of the complex being rate determining. Keywords: silene, substituent effects, kinetics, Arrhenius, flash photolysis.

scholarly journals The kinetics of formation of horseradish peroxidase compound I by reaction with peroxobenzoic acids. pH and peroxo acid substituent effects

1976 ◽  
Vol 157 (1) ◽  
pp. 247-253 ◽  
Author(s):  
D M Davies ◽  
P Jones ◽  
D Mantle
Keyword(s):  
Horseradish Peroxidase ◽  
Active Site ◽  
Rate Constants ◽  
Ph Effect ◽  
Substituent Effects ◽  
Nucleophilic Attack ◽  
Compound I ◽  
Active Intermediate ◽  
Kinetics Of Formation ◽  
Kinetics Of

1. The kinetics of formation of horseradish peroxidase Compound I were studied by using peroxobenzoic acid and ten substituted peroxobenzoic acids as substrates. Kinetic data for the formation of Compound I with H2O2 and for the reaction of deuteroferrihaem with H2O2 and peroxobenzoic acids, to form a peroxidatically active intermediate, are included for comparison. 2. The observed second-order rate constants for the formation of Compound I with peroxobenzoic acids decrease with increasing pH, in the range pH 5-10, in contrast with pH-independence of the reaction with H2O2. The results imply that the formation of Compound I involves a reaction between the enzyme and un-ionized hydroperoxide molecules. 3. The maximal rate constants for Compound I formation with unhindered peroxobenzoic acids exceed that for H2O2. Peroxobenzoic acids with bulky ortho substituents show marked adverse steric effects. The pattern of substituent effects does not agree with expectations for an electrophilic oxidation of the enzyme by peroxoacid molecules in aqueous solution, but is in agreement with that expected for a reaction involving nucleophilic attack by peroxo anions. 4. Possible reaction mechanisms are considered by which the apparent conflict between the pH-effect and substituent-effect data may be resolved. A model in which it is postulated that a negatively charged ‘electrostatic gate’ controls access of substrate to the active site and may also activate substrate within the active site, provides the most satisfactory explanation for both the present results and data from the literature.

Reactions of cyclic oxalyl compounds, 37. Substituent effects on the site of nucleophilic attack at 1H-pyrrole-2,3-diones

1995 ◽  
Vol 1995 (3) ◽  
pp. 537-543 ◽  
Author(s):  
C. Oliver Kappe ◽  
Ewald Terpetschnig ◽  
Gerhard Penn ◽  
Gert Kollenz ◽  
Karl Peters ◽  
...  
Keyword(s):  
Substituent Effects ◽  
Nucleophilic Attack

scholarly journals Regioselective substituent effects upon the synthesis of dipyrrins from 2-formyl pyrroles

2018 ◽  
Vol 96 (7) ◽  
pp. 779-784
Author(s):  
Michael H.R. Beh ◽  
Carlotta Figliola ◽  
Kate-Lyn A.R. Lund ◽  
Aleksandra K. Kajetanowicz ◽  
Ann E. Johnsen ◽  
...  
Keyword(s):  
Systematic Study ◽  
Substituent Effects ◽  
The Self ◽  
Nucleophilic Attack

The synthesis of symmetric α-free meso-H-dipyrrin hydrobromides from 5-H-2-formyl pyrroles was investigated. The self-condensation produces regioisomeric dipyrrins through adoption of two mechanistic pathways. The key difference between the two pathways lies in which position of the pyrrole directs nucleophilic attack. Through a systematic study involving various substituted and (or) isotopically labelled 5-H-2-formyl pyrroles, we herein provide evidence to suggest that not only do two mechanistic pathways exist, but the steric bulk of the substituent adjacent to the 5-unsubstituted position influences which pathway dominates.

ChemInform Abstract: Reactions of Cyclic Oxalyl Compounds. Part 37. Substituent Effects on the Site of Nucleophilic Attack at 1H-Pyrrole-2,3-diones.

ChemInform ◽  
2010 ◽  
Vol 26 (30) ◽  
pp. no-no
Author(s):  
C. O. KAPPE ◽  
E. TERPETSCHNIG ◽  
G. PENN ◽  
G. KOLLENZ ◽  
K. PETERS ◽  
...  
Keyword(s):  
Substituent Effects ◽  
Nucleophilic Attack

Organometallic compounds in synthesis. VIII. Carbon-13 nuclear magnetic resonance spectroscopic study of tricarbonylcyclohexadienyliron salts

1976 ◽  
Vol 29 (8) ◽  
pp. 1671 ◽  
Author(s):  
AJ Birch ◽  
PW Westerman ◽  
AJ Pearson
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Chemical Shift ◽  
Spectroscopic Study ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Organometallic Compounds ◽  
Nucleophilic Attack ◽  
Chemical Shift Data ◽  
Shift Data

The carbon-13 N.M.R. spectra of eleven substituted tricarbonylcyclohexadienyliron salts (1) have been determined and individual resonances assigned. Substituent effects have been deduced and compared with corresponding effects in cyclohexadienyl cations. The structures of the tricarbonylcyclohexadienyliron salts are discussed with reference to chemical shift data. The proportions of irreversible nucleophilic attack at the terminal positions in the unsymmetrical 2-substituted salts are compared with the observed chemical shifts at these carbon atoms.

Theoretical kinetics and thermodynamics study: Peripheral substituent effects on the hydrolysis of verdoheme

2020 ◽  
Vol 24 (10) ◽  
pp. 1233-1241
Author(s):  
Shahriyar Sarabi ◽  
Parisa Rajabali Jamaat ◽  
Hoorieh Djahaniani
Keyword(s):  
Heme Oxygenase ◽  
Substituent Effects ◽  
Dft Method ◽  
The Body ◽  
Nucleophilic Attack ◽  
Prosthetic Group ◽  
Kinetics And Thermodynamics ◽  
Degradation Reactions ◽  
Free Heme ◽  
Hydrolysis Of

The heme oxygenase (HO) enzyme is a free heme protein that binds to heme in the body. Heme acts as both a cofactor and a substrate in this enzyme. The catabolism of heme into biliverdin, monoxide carbon, and free-iron, catalyzed by heme oxygenase via three consecutive oxygenation steps, in which the heme group functions as the prosthetic group as well as the substrate. Investigations of the reactions of the peripheral substituent on the heme ring with 5-oxaporphyrin iron complexes (verdohemes) have been assumed to provide models and largely unknown for the primary step in the hydrolysis of verdohemes. In this work, a theoretical kinetics and thermodynamics study of the degradation reactions of verdohemes was performed, and calculations show that the [Formula: see text] in the hydrolysis of verdohemes with non-peripheral substituents is more negative than hydrolysis of verdohemes with peripheral substituents. In other words, the hydrolysis of verdohemes with non-peripheral substituents is more energy-efficient than verdohemes with a peripheral substituents. Equilibrium constant calculations show that hydrolysis of verdohemes with non-peripheral substituents is much faster than that of verdohemes with peripheral substituents, which is due to a more convenient nucleophilic attack on the cationic ring than the anionic ring. To acquire a good molecular understanding, peripheral substituent effects on the hydrolysis of verdoheme’s inhibitory role was studied using the DFT method.

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