Diastereomeric hexacoordinate silicon complexes: Preparation, structure and epimerization

The synthesis of a neutral hexacoordinate silicon complex (6) with a chiral silicon and an adjacent chiral carbon center, via an intramolecular nucleophilic displacement, is described. 6 exists in a mixture of two diastereomers. One of the diastereomers has been characterized by an X-ray crystallographic analysis, and is found to have a nearly octahedral geometry about the silicon. 6 undergoes two intramolecular ligand-site exchhange processes, observable by NMR spectroscopy: reversible dissociation of the N-Si dative bond, leading to interchange of the N-methyl groups (ΔG*=16. 7±0.2kcalmol^-1 at 365 K), and inversion of configuration at the silicon center, measured by selective inversion recovery NMR spectroscopy (21.0kcalmol^-1 at 350 K).