Alkyllithium Reagents as a Hidden Source of Lithium Hydride

Alkyllithium reagents represent one of the most commonly used organometallic compounds. The wide range of applications results mostly from their ability to react as Brønsted bases and as nucleophiles. In this contribution, we took a closer look at their β-hydride elimination. Through the combination of GC/EI-MS analysis, in situ FTIR spectroscopy, and quantum chemical calculations, we were able to reveal the reaction mechanism of a simultaneous β-hydride elimination of the alkyl anion and lithium hydride addition to hexamethylacetone at −50 °C. In addition, the alkyl anion could be reliably identified as the hydride source and its influence on the course of the reaction could be estimated. Most remarkably, when sec-BuLi was used, the selectivity of the conversion of hexamethylacetone could be reversed from a previously pure addition reaction without ligand to an over 90% occurrence of the lithium hydride transfer reaction by adding PMDTA. This suggests that lithium hydride transfer reactions may play a role in asymmetric alkyllithium syntheses on the basis of steric influence.