A Route to Stereochemically Pure Benzyllithium Reagents by Stereocontrolled Epimerisation

Alkyllithium compounds are not generally stable at room temperature, therefore strategies were established to overcome this limitation. We present a systematic approach to obtain a stereochemically enriched benzyllithium compound, starting with the simple addition of a chiral auxiliary and ending by incorporation of the auxiliary in the substrate. Thereby, a very unusual dimer of a lithiated benzylsilane was obtained, which could be split into stereochemically enriched monomers by the addition of Lewis bases. Furthermore, we were able to understand the factors responsible for this stereochemical enrichment by using quantum chemical calculations and clarify the configuration of the lithiated compound and the corresponding trapping product by crystallisation. This enabled us to determine the stereochemical course of the deprotonation and the subsequent metathesis reaction. A systematic approach to obtain a highly stereochemically enriched benzyllithium compound is presented (see figure). The stereochemical enrichment was explained by a combination of X‐ray analysis and quantum chemical calculations. Furthermore, the absolute configuration of the lithiated compound, as well as the stereochemical course of the trapping reaction, was clarified.