Efficient intramolecular general acid catalysis of enol ether hydrolysis. Hydrogen-bonding stabilisation of the transition state for proton transfer to carbon

1994 ◽  
pp. 643 ◽  
Author(s):  
Anthony J. Kirby ◽  
Nicholas H. Williams
Keyword(s):  
Hydrogen Bonding ◽  
Proton Transfer ◽  
Transition State ◽  
Acid Catalysis ◽  
Enol Ether ◽  
General Acid

A mechanism for efficient proton-transfer catalysis. Intramolecular general acid catalysis of the hydrolysis of 1-arylethyl ethers of salicylic acid

1999 ◽  
Vol 77 (5-6) ◽  
pp. 792-801 ◽  
Author(s):  
Sarah E Barber ◽  
Kathryn ES Dean ◽  
Anthony J Kirby
Keyword(s):  
Hydrogen Bond ◽  
Salicylic Acid ◽  
Proton Transfer ◽  
Acid Catalysis ◽  
Enzyme Mechanism ◽  
Base Catalysis ◽  
Strong Hydrogen Bond ◽  
Cooh Group ◽  
Tert Butyl ◽  
General Acid

The tert-butyl (1) and 1-arylethyl ethers (2) of salicylic acid are hydrolyzed with efficient general acid catalysis by the ortho-COOH group. The half-life of the neutral COOH form of the tert-butyl ether is 15.2 min at 39°, and the estimated acceleration by the COOH group of 2, X = Me, Y = H is 2.13 × 105. The salicylate leaving group from 2 (X = Me, Y = H) has an effective pKa of 2.9, compared with a nominal pKa of 8.52. Analysis of substituent effects in both arylethyl and leaving groups provides the most detailed available mechanistic insight into a reaction involving efficient intramolecular proton-transfer catalysis. The mechanism is very different from classical general acid-base catalysis. Proton transfer takes place very rapidly within a developing strong hydrogen bond, and though an integral part of the C—O cleavage process is practically uncoupled from it. "Strategic delay" of the proton-transfer step, relative to C—O cleavage, makes a significant contribution to efficiency by setting up the conditions for the formation of the strong, intramolecular hydrogen bond.Key words: catalysis, carboxyl, hydrogen bond, proton transfer, enzyme mechanism.

Reactivity of proton and general acid with 1,3-bis-(4-methylphenyl)triazene in aqueous methanol

1990 ◽  
Vol 55 (11) ◽  
pp. 2701-2706 ◽  
Author(s):  
Oldřich Pytela ◽  
Taťjana Nevěčná ◽  
Jaromír Kaválek
Keyword(s):  
Benzoic Acid ◽  
Transition State ◽  
Rate Constant ◽  
Acid Catalysis ◽  
Methanol Concentration ◽  
Binary Solvent ◽  
Aqueous Methanol ◽  
Solvent Water ◽  
General Acid ◽  
Activated Complex

The effect of concentration of benzoic acid and composition of the binary solvent water-methanol on the rate of decomposition of 1,3-bis(4-methylphenyl)triazene has been studied. It has been found that both general acid catalysis by undissociated benzoic acid and catalysis by the proton are significant. The rate constant kHA of general acid catalysis decreases monotonously with decreasing amount of water in the mixture due to preferred solvation of the activated complex as compared with the educts. The rate constant kH of the catalysis by proton in its dependence on methanol concentration exhibits a minimum for 80% (by wt.) of methanol in the mixture. This phenomenon is caused by formation of the conjugated acid from more basic methanol and proton with simultaneous solvation by water and methanol; the particle thus formed is a weaker acid as compared with the complexes existing in water or in methanol. The kH value is higher in methanol than in water due to preferred solvation of the educts as compared with that of the transition state.

Reactions of aryldiazonium salts and alkyl arylazo ethers. VIII. General acid catalysis of the ionization of alkyl (E)-arylazo ethers in alcoholic solvents

1982 ◽  
Vol 35 (5) ◽  
pp. 961 ◽  
Author(s):  
TJ Broxton ◽  
AC Stray
Keyword(s):  
Ionic Strength ◽  
Proton Transfer ◽  
Aromatic Ring ◽  
Acid Catalysis ◽  
Bond Breaking ◽  
General Acid

Further studies of the general acid-catalysed ionization of alkyl (E)-arylazo ethers in alcoholic solvents have been carried out. The magnitude of catalysis and the degree of proton transfer as a function of the substituent on the aromatic ring, the alkoxide ion nucleofuge, the solvent and the ionic strength have been studied. The results are consistent with a mechanism for the acid-catalysed ionization of alkyl (E)-arylazo ethers in which proton transfer and N-O bond breaking are concerted. 1H and 13C n.m.r. data for a number of alkyl p-nitrophenylazo ethers are reported.

Intramolecular general acid catalysis of sulfate transfer — Nucleophilic attack by oxyanions on the SO3– group

2005 ◽  
Vol 83 (9) ◽  
pp. 1629-1636 ◽  
Author(s):  
Anthony J Kirby ◽  
José Carlos Gesser ◽  
Florian Hollfelder ◽  
Jacks P Priebe ◽  
Faruk Nome
Keyword(s):  
Hydrogen Bond ◽  
Transition State ◽  
Acid Catalysis ◽  
Nucleophilic Attack ◽  
General Acid ◽  
State Transfer ◽  
Carboxylate Anions ◽  
Mechanism Of Hydrolysis ◽  
Intramolecular Hydrogen ◽  
Hydrolysis Of

The mechanism of hydrolysis of 8-N,N-dimethylaminonaphthyl sulfate closely resembles that of the corresponding phosphate monoester. Nucleophilic attack by water on the sulfate group of the zwitterion is catalyzed by the neighbouring dimethylammonium group, acting as a particularly efficient general acid through the intramolecular hydrogen bond. This hydrogen bond is present in both reactant and product, but is strongest in the transition state. Transfer of the sulfuryl group to oxygen nucleophiles, including water and carboxylate anions, shows steric and electrostatic effects, and a sensitivity to basicity which is low, but significantly higher than expected for uncatalyzed transfer of the SO3– group.Key words: sulfate, sulfatase, intramolecular, general acid catalysis, promiscuity.

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