Relationships between E2 and E1cB mechanisms of β-elimination

It is shown that for base-catalysed β-elimination of 9-fluorenylmethyl derivatives, the stability of the 9-fluorenylcarbanion is too great to be consistent with a continuity of transition states between E2 and E1cB mechanisms. A model is formulated in which the reactants, products, and transition states for concerted elimination are represented on a common potential-energy surface with the carbonium ion and carbanion intermediates of stepwise reaction paths. The effect of energy changes in each stable species upon the energy and structure of the E2 transition state is considered within the framework of the Hammond postulate. The model provides an interpretation of the effects of substituents and reaction conditions upon β-eliminations. It is concluded that a more stable carbanion causes the E2 transition state to approach the carbanion in structure, leading to an apparent relation between E2 and E1cB mechanisms. At the borderline between the mechanisms a continuity of transition states, or operational indistinguishability between them, can be expected only when the reaction of the carbanion approaches diffusion control.