Use of solvent isotope effect to identify an intermediate carbanion in the β-elimination reactions from N-[2-(4-pyridyl)ethyl]quinuclidinium and N-[2-(2-pyridyl)ethyl]quinuclidinium induced by acetohydroxamate/acetohydroxamic acid buffers

Abstract

Solvent isotope effect is a useful technique for identifing and characterizing an intermediate carbanion in the base-induced β-elimination reaction from N-[2-(4-pyridyl)ethyl]quinuclidinium, 1, and N-[2-(2-pyridyl)ethyl]quinuclidinium, 2. While at high [buffer]kobs(D2O) > kobs(H2O) due to the presence of a primary kinetic solvent isotope effect on the reprotonation of the intermediate carbanion by BD, at low [buffer] no solvent isotope effect is observed, and kobs(D2O) kobs(H2O). The data are in agreement with a reversible E1cb mechanism in which carbon deprotonation occurs from NH+, the substrate protonated at the nitrogen atom of the pyridine ring. In the absence of solvent isotope effect at low [buffer], and with the similarity of the results obtained with the two isomers, 1 and 2, the significance of an intramolecular proton transfer in the intermediate carbanion can be excluded in these processes.