Electrophilic Assistance Research Articles

Phenolyses of 1-Adamantyl Chloride, Bromide, and p-Toluenesulfonate in Binary Mixtures of Phenol with Benzene. The Rates and the Product Distribution

Abstract Adamantyl derivatives, the bridgehead substrates, have been chosen as the reference compounds in the assessment of the ionizing power of phenolic solvents in the ,SN1-type solvolysis. The SN1-phenolyses of 1-adamantyl chloride, bromide, and p-toluenesulfonate have been studied at several temperatures in binary mixtures of phenol with benzene; the enthalpies and entropies of activation have been calculated. Phenolysis has also been made on t-butyl chloride in order to estimate the Grunwald-Winstein Y values for the phenol-benzene solvents. The solvolysis rates of 1-adamantyl compounds, including the literature values, have been correlated with the Y values and also with the dielectric constants ((D−1)⁄(2D+1)). A significant deviation is found for the phenolic solvents in both correlations; this deviation is discussed in terms of the electrophilic assistance by hydrogen-bonding of the phenolic solvents to the leaving group. The isolated phenolysis products have been characterized as 1-adamantyl phenyl ether, o-, and p-1-adamantylphenols. The product compositions for the reactions of the ion-pair intermediate with the phenol molecule and with the phenoxide ion have been estimated on the basis of the variation in the overall product compositions, caused by the increase in the concentrations of the added phenoxide or amine.

Phenyl ester hydrolysis catalysed by alcalase. Case for electrophilic participation at ester carbonyl oxygen

Kinetic parameters for the hydrolysis of substituted phenyl acetates and hippurates catalysed by alcalase have been measured at 25° and 0·1M ionic strength. The pH-dependencies of the k0/Km parameter for 4-methoxy- and 4-chloro-phenyl hippurates and 4-nitrophenyl acetate are identical and sigmoid in shape for the three substrates. Acylation is probably the rate-limiting step in methyl hippurate hydrolyses because the value of k0 is less than the essentially constant value for the aryl and benzyl esters. Acylation (k0/Km is used as a measure) is insensitive to the σ value for aryl hippurates (ρ= 0·35, r= 0·788) but more sensitive for the acetates (ρ= 1·14, r= 0·868). Electrophilic assistance by the enzyme at the ester carbonyl oxygen atom is advanced to explain part of the catalytic activity. Binding of the leaving group (aryl or benzyl) to the enzyme by lipophilic forces is also suggested to account for part of the efficiency.

Proteolytic enzymes. Papain-catalysed hydrolysis of specific aryl esters

Aryl hippurates (I) and mesylglycinates (II) are hydrolysed in the presence of papain and have constant k0 parameters of ca. 4 s–1 and 12 s–1 respectively at pH 6·00, 35° and 0·3M ionic strength. The low sensitivity of k0/Km to change in phenyl substituent [ρ= 0·47 and 0·78 for (II) and (I) respectively] is argued to arise from electrophilic assistance on the carbonyl oxygen of the ester during acylation but the involvement of imidazolium ion (of histidine-159) as the electrophile is discounted. Evidence is reported for the involvement of a cationic acid (pKaca. 4) in acylation and deacylation and a neutral acid (pKaca. 8) in acylation. 4-Nitrophenyl acetate (III) is a substrate of papain with a typical bell-shaped profile for k0/Km; the ester is shown to differ from specific substrates only in its electronic reactivity.

Proteolytic enzymes: models for electrophilic assistance by NH in acylation

The effect of increasing dioxan content of the medium has been employed to examine the possibility of intramolecular electrophilic assistance by NH in alkaline hydrolysis of the methyl esters of 2-acetamidobenzoic (I), pyrrole-2-carboxylic (VII), and benzimidazole-4(7)-carboxylic (V) acids. Significant enhancement of rate constant is observed over isomers where no participation is possible. Enhancements increased steeply with increasing dioxan concentration in agreement with a ‘built-in’ solvation by the NH of the incipient alkoxide ion (derived from the carbonyl oxygen) of the transition-state. Heterocyclic types [(VII) and (V)] exhibited maxima in their rate enhancements as the dioxan content increased. Since the NH to alkoxide bond distance in the heterocyclic types is larger than normally found for direct hydrogen-bonding the maximum is argued to arise from a water bridge acting as a relay between electrophile NH and incipient alkoxide ion of the transition-state.

Isomérisations en chimie des sucres. II Fermetures de cycles par attaque nucléophile intramoléculaire sur des carbones sp2 à caractère électrophile: Furannoses hybridés sp2 en C3

AbstractWhen heated with one equivalent of H2O, the 1,2: 5,6‐di‐O‐isopropylidene‐α‐D‐ribo‐ and ‐xylo‐hexofuranos‐3‐uloses loose one molecule of acetone and yield the 3, 6‐anhydro‐1, 2‐O‐isopropylidene‐α‐D‐ribo‐ and ‐xylo‐hexofuranos‐3‐ulose ketohydrols. The carbonyl group of the starting material seems to provide some kind of intramolecular electrophilic assistance to the hydrolysis of the 5, 6‐O‐isopropylidene group. When the oxygen of the carbonyl group is replaced by cyanomethylene, an analogous cyclisation takes place under base catalysis, provided that C6 bears a free hydroxyl group.

Chymotrypsin-catalyzed phenyl ester hydrolysis. Evidence for electrophilic assistance on carbonyl oxygen.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChymotrypsin-catalyzed phenyl ester hydrolysis. Evidence for electrophilic assistance on carbonyl oxygenAndrew WilliamsCite this: Biochemistry 1970, 9, 17, 3383–3390Publication Date (Print):August 1, 1970Publication History Published online1 May 2002Published inissue 1 August 1970https://doi.org/10.1021/bi00819a014RIGHTS & PERMISSIONSArticle Views152Altmetric-Citations39LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (774 KB) Get e-Alerts Get e-Alerts

Etude cinétique de la chloration des composés aromatiques par SbCl 5—mécanisme

The kinetics of chlorination of aromatic compounds by antimony pentachloride gives evidence for a mechanism that implies π-complex formation between the Lewis and the aromatic compound. Under these conditions, the rate low observed is: rate = k 2(ArH)(SbCl 5) + k 3(ArH)(SbCl 5) 2 There are two competitive reactions. The slow step is the decomposition of the intermediate ArH/SbCl 5. This decomposition can operate directly or in the presence of a second molecule of SbCl 5. This electrophilic assistance is seen with AlCl 3.

Specific salt effects on the exchange reaction between 4,4′-dimethyldiphenylmethyl thiocyanate and ionic thiocyanate in acetone

Rates of exchange of 4,4′-dimethyldiphenylmethyl thiocyanate and MeSCN (Me = Li, Na, Bun4N) have been measured in acetone at 25°. With NaSCN the kinetics have also been investigated in the presence of a large amount of NaClO4, in all runs the total concentration of electrolytes, (NaSCN)+(NaClO4) being kept constant at 0·1 or 0·03M. With no salt other than MeSCN is the first-order rate coefficient, kobs, linearly dependent on the stoicheiometric concentration of the salt over a large interval (650 fold). With NaClO4, kobs is correlated with the stoicheiometric concentration of the nucleophile by a more complex kinetic expression. From the order of reactivity of different salts, LiSCN > NaSCN > Bun4NSCN, and the form of the kinetic equation a mechanism for the reaction is suggested which includes salt-assisted as well as solvent-assisted ionization paths. It is suggested that electrophilic assistance by salts leads to ion quadruplets which may return to covalent material without undergoing exchange or react with the nucleophilic salt to undergo exchange.

The solvolysis of 2-trimethylsilylpyridine by alcohols and by water

2-Trimethylsilylpyridine, unlike the 3- and 4-isomers, reacts with water, methanol, or ethanol to give pyridine and trimethylsilanol or the alkoxytrimethylsilane. The relative rates of reaction are 740, 120, and 1 in water, methanol, and ethanol respectively. No detectable reaction occurs with higher alcohols. Acids inhibit the reaction, and bases have very little effect. These solvolyses have small activation enthalpies and high negative activation entropies, and the solvent isotope effect is very small. The most likely reaction mechanism involves rate-determining attack on the silicon by the oxygen of the hydrogen-bonded solvent, i.e. nucleophilic substitution with electrophilic assistance via a 5-centred cyclic activated complex.

377. Aspects of stereochemistry. Part II. Intramolecular electrophilic assistance of displacement reactions

377. Aspects of stereochemistry. Part II. Intramolecular electrophilic assistance of displacement reactions