Benzoates, allylic isomerization

A detailed kinetic study10 of a palladium-catalysed reversible cis-trans isomerization of cyclic allylic benzoates has shown how a change to a better leaving group... 

Meisenheimer et were the first to study the kinetics of allylic ester isomerizations. They followed the isomerization of a-phenylallyl p-nitroben-zoate and a-phenylallyl 3,4,5-tribromobenzoate to the corresponding cinnamyl benzoates by a tedious analytical procedure involving use of melting point-composition diagrams. They observed that isomerization of the molten esters at 137°C is autocatalytic due to partial slow decomposition of the esters to substituted benzoic acids and unidentified products, and demonstrated that the reaction is catalyzed by added benzoic acids. The rate law for these reactions is... 

In aqueous 90% acetone at 99.6°C, At for /r Ai.s-a,y-dimethylallyl p-nitro-benzoate is 2.6x10-6 ec" [(II), R = R = CH3 Y = p-O-.NQH.CO., SOH = HoO]. The rate is not affected by addition of small amounts of lithium p-nitrobenzoate or p-nitrobenzoic acid. In aqueous 60% acetone at 60 C, At = 1.5 X lO secBy using carbonyl- o-labeled ester, it was shown that 60 scrambling is about twice as fast as allylic rearrangement, which implies that the carboxyl group of the p-nitrobenzoate ion in the tight ion pair rotates relative to the allylic carbonium ion in some instances before the ions recombine to form enantiomeric esters . This and similar observations on other allylic esters (see below) rules out the concerted Sf i mechanism as the only mechanism for allylic ester isomerization under solvolytic conditions. 

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. 

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