Bromide Ion Promoted β-Elimination in α-Bromo Ester Substrates. Evidence for an Intermediate in the E2C Reaction

Article abstract of DOI:10.1021/jo00172a012

The TDKIE criteria of transition state geometry in H-transfer reactions have been applied in the title reactions; a bent transition state consistent with the geometry of the E2C reaction has been verified by the results of intramolecular and intermolecular competition modes of determining k_H/k_D as a function of temperature.The extraordinary magnitude measured for the α secondary deuterium isotope effect in the E2C mechanism is reconciled with a very loose transition state and an acute angle of H-abstraction in the course of rearward approach to C_α by the promoter base.The virtual identity of the inter- and intramolecular isotope effects can be correlated by the assumption of a reaction intermediate of trigonal-bipyramid structure surrounding C_α and in which the abstractable H and D atoms are equally available to the action of the promoter base.The properties of this intermediate (4), by way of contrast with the transition state of an S_N2 process, are discussed in detail.