Abstract
2-Pentanone and 2-pentanone-1,1,1-d3 were treated with three-fold excesses of lithium diisopropylamide (LDA) or lithium hexamethyldisilazide (LHMDS) in tetrahydrofuran (THF) with and without hexamethylphosphoric triamide (HMPA, 3 mol per mol of base) at temperatures ranging from 24 to - 70 °C. The deuterium kinetic isotope effects calculated from the product ratios (measured by GLC as trimethylsilyl enol ethers) showed a range of temperature dependences: none (LDA in THF), attenuated with AH/AD = 2.53 (LHMDS in THF), and normal with AH/AD~0.6 indicating moderate tunneling (LDA and LHMDS in THF-HMPA). The variation in temperature dependence is attributed to reaction via multiple base species in which HMPA affects the equilibria between the base species.
Published Version
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