The E2C mechanism in elimination reactions. IX. Re-examination of the olefin-like transition-state

Abstract

The kinetics and product olefins arising from E2C-like elimination reactions of secondary bromides and tosylates having various alkyl and conjugating substituents at Cα and Cβ are re-examined in the light of the theoretical Pross-Shaik valence-bond model and the apparent lack of a β-phenyl effect. Comparison of the effect of substituents at Cα and Cβ shows that whilst there are differences in reactivity and products of E2C reactions there is also a large degree of double-bond character in the transition state. There is a good correlation between the kinetics of E2C reactions and the double-bond stabilization energy arising from both alkyl and conjugating substituents. Contributing valence-bond structures, giving asymmetry and partial charge at Cα and Cβ appear to vary in importance according to leaving group and substituent type.