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Enolization of 3-Pentanone-2,2-d2: Deuterium Kinetic Isotope Effects and Stereoselectivity

✍ Scribed by Gary Held; Linfeng Xie

Publisher: Elsevier Science
Year: 1997
Tongue: English
Weight: 86 KB
Volume: 55
Category: Article
ISSN: 0026-265X
DOI: 10.1006/mchj.1996.1420

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✦ Synopsis

3-Pentanone-2,2-d 2 was subjected to enolization by lithium diisopropylamide (LDA), lithium N-isopropyl(trimethylsilyl)amide (LITA), and lithium hexamethyldisilazide (LHMDS). Internal deuterium kinetic isotope effects were measured by 1 H-NMR analysis of the corresponding trimethylsilyl enol ethers after quenching the enolates with chlorotrimethylsilane. The k H /k D values range from 2.3 (LDA) to 5.2 (LITA) to 6.6 (LHMDS), suggesting that the proton transfer is involved in the rate-determining step of kinetic enolization with the transition state changing from reactant-like for LDA to more symmetric for LHMDS. An improved E/Z ratio of 85%:15% at 0ЊC was observed for the deprotonation of 3-pentanone by LITA, compared to 71%:29% by LDA and 44%:66% by LHMDS.

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