Anionotropic isomerizations

Anionotropic rearrangements involve migrations of electronegative groups. They comprise two mechanistically related groups of reactions substitutions and isomerizations. Allylic substitution reactions in which skeletal rearrangement accompanies nucleophilic substitution, viz. 

For purposes of discussion, allylic isomerizations are divided into two groups  

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Allylic alcohols, esters, and ethers undergo acid-catalyzed isomerizations, and allylic alcohols and ethers isomerize at appreciable rates only in the presence of acids. The migrating groups in these reactions are uncharged molecules derived from the oxonium ion conjugate acids of the starting materials. For this reason, and because they are mechanistically related to anionotropic isomerizations, Braude named these reactions oxotropic rearrangements. 

Allyl and -methylallyl thiocyanates isomerize at almost the same rate . In cyclohexane solution, y-methylallyl thiocyanate isomerizes about 10 times faster, and y,y-dimethylallyl thiocyanate about 30 times faster than allyl thiocyanate The effect of y-methyl substitution on isomerization rate is somewhat larger in acetonitrile than in cyclohexane solutions. The substituent effects are several orders of magnitude smaller than would be expected for SnT anionotropic isomerizations. 

A number of interesting and useful organic reactions involve isomerizations of substances having one or more carbon-carbon double bonds. This chapter deals with the kinetics of reactions of alkenes, cycloalkenes and substituted alkenes which involve migration of carbon-carbon double bonds, with or without structural alteration of the carbon skeleton of the starting materials. These reactions include prototropic and anionotropic rearrangements, several concerted unimolecular isomerizations such as the Cope and Claisen rearrangements, and a number of non-concerted thermal isomerization reactions. 

Qualitative studies of the effects of allylic substituents, acyl substituents, and reaction conditions on rates of isomerization of allylic esters played a prominent role in the development of mechanistic theories of anionotropic rearrangements . Burton and Ingold observed that a-phenylallyl p-nitrobenzoate isomerizes more rapidly than a-phenylallyl acetate, that electron-releasing a-substituents facilitate isomerization of a-substituted allyl acetates, and that the rate of isomerization of a-phenylallyl p-nitrobenzoate increases as the dielectric constant of the solvent increases. All of these observations support the mechanism, which involves dissociation of the allylic ester into an allylic carbonium carboxylate ion pair, which reassociates to the starting material and its allylic isomer, viz. 

Most of the anionotropic ester isomerizations whose kinetics have been studied involve simple allylic compounds. A few kinetic investigations of solvolysis and isomerization reactions of doubly unsaturated allylic esters and non-allylic unsaturated esters have been reported. Two examples of such reactions are mentioned here. Spirodienyl p-nitrobenzoate (V) undergoes simultaneous allylic isomerization to VI, viz. 

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