Acid-catalyzed isomerizations of allylic alcohols

The most thoroughly investigated group of oxotropic isomerizations are those of allylic alcohols. Braude, who conducted extensive studies of allylic alcohol isomerizations. reviewed them in 1950. 

The activating effects of unsaturated a-substituents are probably due to tt-orbital overlap with the allylic system in the transition state for carbonium ion formation. This hypothesis is supported by the fact that a- and /3-methyl substituents which sterically hinder attainment of coplanarity of the allylic system and the unsaturated substituent produce abnormally small rate accelerations, and may even decrease isomerization rate. The activating effects of alkyl substituents probably also arise because of their stabilizing effect on the transition state for carbonium ion formation. Selected data (mostly from Braude s publications) which illustrate substituent effects on reactivity are collected in Table 1. 

Aryl substituents which can release electrons to the allylic system increase the ease of isomerization of a-arylallyl alcohols. Aryl substituent effects support the hypothesis that the rate-limiting transition state has considerable car-bonium-ion character. The specific rate coefficients for acid catalyzed isomerization of a-aryl-y-methylallyl alcohols in aqueous 60% dioxane correlate poorly with Hammett s cr-substituent constants (see ref. 182, p. 184) whereas a plot of log versus Brown s cr -substituent constants is linear, with a slope of 2.7. 

The rate of isomerization of two different allylic alcohols has been found 

SUBSTITUENT EFFECTS ON RATES OF ISOMERIZATION OF ALLYLIC ALCOHOLS IN AQUEOUS 60% DIOXaNE AT30°C 

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