Amine Base Promoted β-Elimination in α-Bromo Ester Substrates. Evidence for Permutational Isomerism in the TBP Carbon Intermediate

Article abstract of DOI:10.1021/jo00172a013

Amine bases show reaction rates and activation parameters that suggest close similarity to the E2C mechanism previously identified for the Br(1-)-promoted reaction with α-bromo ester.Proton Sponge (PS), an amine base, which cannot participate in an S_N2 process, nonetheless, is an effective promoter of the E2C.This is confirmed by applying the temperature-dependent isotope effect (TDKIE) criteria of transition-state geometry to PS and other amine reactions with α-bromo esters with the general result that <ΔE_a>_D^H is found to be temperature independent and A_H/A_D >> 1.2.The α-secondary deuterium isotope effect, (k_H/k_D)_α = 1.27, is the largest value ever found (thus far) for either S_N2-like or β-elimination processes.On these and other grounds it is concluded that the E2C mechanism is not S_N2 like but is regarded as a precedented nucleophilic trigger mechanism.The virtual identity of intra- and intermolecular isotope effects among amine base promoters cannot be explained by the formation of a symmetrical trigonal-bipyramid (TBP) intermediate, as was the case for the Br(1-)-promoted reaction with α-bromo ester substrates.But the obviously equal availability of abstractable H and D in the transition states of the intramolecular competition isotope effect is possibly to be explained by invoking permutational isomerism in the precursor trigonal-bipyramid (TBP) intermediate.The reasoning which seems to justify this proposal is considered in detail.