Non-aromatic Claisen ester rearrangements

Allyl esters rearrange to isomeric allyl esters. Reactions are concerted and undoubtedly involve 6-center polar transition states, viz. 

Results for a series of substituted allyl esters are shown in Table 7. Rate coefficient data appear to be reliable however, the Arrhenius parameters in a number of the studies are quite suspect. Transition state estimates give AS = -9.0 2.2 cal.deg. mole , or sec The reported yl-factors 

Equilibrium constants are close to unity, as one might expect on the basis of group additivities and from the very similar structures of the reactant and product allyl esters. However, the large reaction enthalpies and entropies suggested by the kinetic data are not reasonable, even when the rather large error limits indicated are considered. We, therefore, prefer the calculated parameters. These should be accurate to about 1.5 kcal.mole for E . 

Substituent effects in the allyl ester rearrangements are very similar to those observed in the ester reverse ene-type eliminations. This is apparent from the relative rate comparisons of Table 8. At the a- and y-carbons, reaction rates are observed to increase in the order CF3 H CH3. The rate accelerations by methyl substitution for hydrogen at the a-carbons are factors of 40 and 23, and at the y-carbon are factors of 55 and 23. These effects should be compared with the rate accelerations by methyl for hydrogen substitution at the a-carbon in the ester ene reactions, i.e., from Table 2, i-PrOAc/EtOAc = 18.7 and t-BuOAc/i-PrOAc = 53. One may conclude that the positive formal charge densities at the a- and 

CH3COOCH2CHCH3-CHCH3 trani-methyls X, CH3COOCHCH3CH-CHCH3 trans XI, CH3COOCHCH3CH-CHCH3 cm XII, CF3COOCHCH3CH-CH2  

---