Acid-catalyzed isomerization of allylic esters

Allylic esters undergo both uncatalyzed and acid-catalyzed heterolysis. There is an important difference between the two processes uncatalyzed alkyl carbon-oxygen bond fission produces a carbonium carboxylate ion pair, while acid-catalyzed alkyl carbon-oxygen fission results in formation of an allylic carbonium ion and an uncharged carboxylic acid molecule. 

In aprotic solvents, the allylic carbonium ion and carboxylic acid molecule produced by acid-catalyzed heterolysis of an allylic ester may recombine to form the starting material and its allylic isomer. Kinetic studies show that a-phenylallyl p-nitrobenzoate undergoes concurrent uncatalyzed and p-nitro-benzoic acid-catalyzed isomerization to cinnamyl benzoate in chlorobenzene solutions containing p-nitrobenzoic acid . Similarly, benzoic acid catalyzes the isomerization of a-phenylallyl benzoate in chlorobenzene. When this reaction was carried out using 0-labeled benzoic acid, appeared in both the starting material and its allylic isomer under the conditions used, only about a quarter of the isomerization reaction is intramolecular. 

In protic solvents, the allylic carbonium ion formed by acid-catalyzed alkyl carbon-oxygen bond fission can recombine either with the carboxylic acid molecule or with a solvent molecule. The electrostatic attraction between the carbonium and carboxylate ions, which is a major factor in isomerization of allylic esters by ion-pair internal return during solvolysis, is absent in the acid-catalyzed reaction. The more numerous, usually more nucleophilic, solvent molecules in the solvation shell of the carbonium ion should compete effectively with the departed carboxylic acid molecule and solvolysis rather than isomerization should be the predominant reaction. For example, in the presence of 0.05 M perchloric acid, solvolyses of cis- and //- //7.s-5-methyl-2-cyclohexenyl p-nitrobenzoates are not only very much faster than in the absence of the acid, but polarimetric and titrimetric rates of solvolysis of optically-active esters were identical within experimental error. For these esters, the acid-catalyzed solvolysis was not accompanied by a detectable amount of isomerization. Braude reported, on the basis of indirect evidence, that isomerization accompanies acid-catalyzed hydrolysis of a-ethynyl-y-methylallyl acetate in aqueous dioxane. It was shown that, under some experimental conditions, the spectrophotometrically determined rate of appearance of the rearranged 1 -yne-3-ene chromophore exceeds the titrimetrically determined rate of hydrolysis, 

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