Journal of Organic Chemistry p. 4509 - 4513 (1983)
Update date:2022-08-11
Topics:
Kwart, Harold
Gaffney, Anne
Wilk, Kazimiera A.
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 SN2 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 <ΔEa>DH is found to be temperature independent and AH/AD >> 1.2.The α-secondary deuterium isotope effect, (kH/kD)α = 1.27, is the largest value ever found (thus far) for either SN2-like or β-elimination processes.On these and other grounds it is concluded that the E2C mechanism is not SN2 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.
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