The halogenation, crystallization induced asymmetric transformation, and epimerization of silicon oligomers

This study involved the development of methods for the preparation of silicon oligomers of the type H(ArMeSi)_nH the conversion of terminal Si-H bonds in the oligomers to Si-X (X = halogen), and characterization of the resulting stereoisomers. Preparation of oligomers of this type required reductive coupling of silane monomer units, ArMeSiH ₂, by dehydrocoupling. Subsequently, additional oligomers were prepared by replacement of Si-Ph in H(PhMeSi)_nH with Si-Ar or Si-R groups. Methods for the halogenation of terminal Si-H bonds in the oligomers were adapted from methods utilized for monosilanes and required modifications to prevent the cleavage of Si-Si bonds. Disilanes, H(ArMeSi) ₂H, were successfully fluorinated using CuF₂·2H ₂O/CCl₄, CuCl₂/CuI/KF, or chlorination followed by treatment with ethanolic HF. The combination CuCl₂/CuI/KF was found to yield the best fluorination results. The disilane, H(PhMeSi) ₂H, was successfully chlorinated using NCS, SOCl₂, CuCl ₂/CuI with the last system yielding the best results. Bromination with CuBr₂/CuI was only partially successful.; Higher oligomers, H(PhMeSi)_nH (n = 3, 4) were also successfully fluorinated employing CuCl₂ /CuI/KF and in all cases, F(ArMeSi)_nF ( n = 2, 3, 4), the number of diastereomers could be identified by 19F{lcub}1H{rcub} NMR spectroscopy. The diastereomers were shown to correspond to a statistical (n = 2, 3) or near statistical (n = 4) distribution.; The disilanes, F(ArMeSi)₂F, exhibited the phenomenon of crystallization induced Asymmetric Transformation (AT) whereby one diastereomer was converted to the other in solution allowing a single diastereomer, the meso -form as verified by x-ray crystallography, to be isolated in a yield greater than 50%. The disilane, F(BuMeSi)₂F, did not crystallize from solution and did not exhibit the AT phenomenon, however, the diastereomer distribution changed in solution. The observation of AT required an equilibrium between the meso- and racemic-forms of the disilane.; Catalytic sources of anionic fluoride and chloride were found to induce epimerization of the meso-form in solution resulting in a return to the statistical distribution of both diastereomers (∼1:1). Kinetic data were collected for this isomerization by 1H NMR spectroscopy and demonstrated a bimolecular process that was promoted by an increase in positive charge at the silicon center. Both nucleophilic substitution and a pseudorotation process are invoked to explain the observations.