Base-Strength Effects in Syn Eliminations from trans-2,3-Dichloro-2,3-dihydrobenzofuran in Dimethyl Sulfoxide

Article abstract of DOI:10.1021/jo01293a013

Syn eliminations from trans-2,3-dichloro-2,3-dihydrobenzofuran (1), to give 3-chlorobenzofuran, promoted by substituted phenoxides (substituents: p-CH3, H, p-Br, p-Cl, m-NO2) and 4-chloro-2-nitroanilide anion have been investigated in 99percent Me2SO.The reaction rate is very sensitive to the base strength, and from the Broenstedt plot a β value of 0.67 can be calculated.Introduction of a chlorine substituent at the 5-position causes a 19-fold rate increase for the reaction with phenoxide ions, and with the same base a k_H/k_D value (in comparison with the 3-deuterated counterpart of 1) of 2.4 is observed.These values clearly suggest that the reaction of 1 occurs by a highly carbanionic transition state.The substituent effect and k_H/k_D value remain, however, substantially unchanged when the base strength is changed, thus indicating that the transition-state structure of the eliminations from 1 is not significantly influenced by the strength of the attacking base.These results are discussed in the light of the current theories concerning the effects of structural changes on the E2 transition state.