Structure/reactivity studies on an alpha-lithiated benzylsilane: chemical interpretation of experimental charge...

Modern organic synthesis (e.g., of natural products) is virtually impossible without employment of enantiomerically enriched compounds. In many cases, alkyllithium compounds are key intermediates for the generation of these stereogenic substances. In recent years, the lithiated carbon atom in silicon-substituted benzyllithium compounds has become a focus of interest because it is possible to maintain its stereogenic information. Starting from a highly enantiomerically enriched benzylsilane, (R,S)-2 x quinuclidine could be obtained, and the absolute configuration at the metalated carbon atom was determined by X-ray diffraction analysis. In solution, a quartet was found in the (13)C NMR spectrum for the metalated carbon atom because of coupling between carbon and lithium, indicating a fixed lithium carbon contact at room temperature. After reaction of (R,S)-2 x quinuclidine with trimethylchlorostannane, the trapped product (S,S)-4 was obtained with a dr > or = 98:2 with inversion of the configuration at the metalated carbon. Multipole refinement against high-resolution diffraction data and subsequent topological analysis of the benchmark system (R,S)-2 x quinuclidine provide insight in the electronic situation and thus the observed stereochemical course of the transformations. Surprisingly, the negative charge generated at the carbanion hardly couples into the phenyl ring. The neighboring silicon atom counterbalances this charge by a pronounced positive charge. Therefore, the alpha-effect of the silicon atom is caused not just by a polarization of the electron density but also by an electrostatic bond reinforcement. Furthermore, the experimentally determined electrostatic potential unequivocally explains the observed back side attack of an electrophile under inversion of the stereogenic center with high diastereomeric ratios.