Deuterium Isotope Effects and the Mechanism of Kinetic Enolate Formation

Article abstract of DOI:10.1021/jo00268a037

3-Methyl-3-pentanone, 2-methyl-3-pentanone-2-d, 2-methyl-3-pentanone-4,4-d2, 3-methyl-2-butanone, and 3-methyl-2-butanone-1,1,1-d3 have been treated with lithium dialkylamide (LDA, LDEA, and LTMP) in the dipolar aprotic solvents THF and DME, with and without added HMPA.Ratios of the product enolates were determined by quenching with trimethylsilyl chloride followed by GC analysis.Isotope effects calculated from these product ratios are all appreciable, k_H/k_D = 2.3-5.9 at 0 deg C, but below the "maximum" of ca. 8.4 expected at this temperature.A reactant-like transition state is proposed.The temperature dependences of the isotope effects are abnormal (A_aH/A_aD = 1.6-3.2) without, but normal with, HMPA (A_aH/A_aD = 0.35-1.0).It is suggested that two or more different base species are active in proton removal and that the concentrations of these base species depend on temperature and solvent.Variations in the E/Z and regioisomer ratios are discussed in terms of models previously advanced by Ireland, Narula, and Dauben.It is concluded that different factors govern the E/Z and regioisomer ratios and that none of the models accounts satisfactorily for all of the observed variations.