the absolute configuration at the metalated carbon center of this alkyllithium compound, which is highly diastereomerically enriched in solution, were determined by single-crystal X-ray diffraction methods. The stereochemical course of an integral sequence of transformations, starting from unmetalated [(benzylsilyl)methyl]amine 1, was experimentally clarified. For the selective reaction of [(R,S)-2] with MeI, inversion of the configuration at the metalated carbon center was found.The synthesis of enantiomerically or diastereomerically enriched alkyllithium compounds, in which the metalated carbon center is the stereogenic center, has been intensively studied for the last 20 years. 1 Among these are several enantiomerically enriched benzyllithium compounds. 1-4 The determination of the absolute configuration at the metalated stereogenic center and the clarification of the stereochemical course of further transformations are most important. However, this is often difficult to realize on the basis of experimental evidence. Instead, stereochemical assumptions are made, which are not always generally applicable. Furthermore, many aspects of the reaction mechanisms have not been understood so far.In 1989, Chan and co-workers reported the lithiation of 1-[(benzyldimethylsilyl)methyl]-(S)-2-(methoxymethyl)pyrrolidine (1) and further substitution reactions with alkyl halides, which showed very high diastereomeric ratios. 3 A significant solvent effect on the diastereomeric ratios (d.r.) of the transformations with methyl iodide was observed when the reaction was carried out in THF (d.r. ) 75:25) and diethyl ether (d.r. g 98:2). Due to the absence of knowledge of the solid state structure for 1-, the authors had to deduce the absolute configuration at the metalated stereogenic center from the absolute configurations of the trapping product (S,S)-3, which is formed "presumably with retention of stereochemistry", 3a and by comparison of product 5 of the Si-C cleavage reaction with a purchased authentic sample. 3a,cBy dynamic 13 C NMR spectroscopy, Fraenkel and coworkers were able to show that the benzyllithium compound (R,S)-2 exists as a monomeric single diastereomer in THF solution between -93 and +27°C. 4 This is in agreement with the results of the trapping reactions at variable temperatures performed by Chan and our research group (see Experimental Section) which prove a stability of configuration on at least the time scale of the reaction.The amazingly high selectivity of reactions of the title compound and the apparent stability of configuration make a closer look at the stereochemical course of its transformations necessary. Thus, it was of great interest to determine the structure (and the absolute configuration) of lithium alkyl (R,S)-2 in the crystal. Furthermore, beginning with a known absolute configuration at the metalated carbon center, we have examined experimentally and explained the stereochemical course of an integral sequence of transformations, starting from the unmetalated silane 1.The lithiation of si...