Chiral Control in Pentacoordinate Systems: The Case of Organosilicates.

Chirality at the central element of pentacoordinate systems can be controlled with two identical bidentate ligands. In such cases the topological Levi-Desargues graph for all the Berry pseudorotations (BPR, max. 20) reduces to interconnected inner and outer "circles" that represent the dynamic enantiomer pair. High enough barriers of the BPR crossovers between the two circles is all what is needed to ascertain chiral integrity. This is illustrated computationally and experimentally for the organosilicates 7 and 10 that carry besides a Me (a), Et (b), Ph (c), or F (d) group two bidentate 2-(phenyl)benzo[ b]-thiophene or 2-(phenyl)naphthyl ligands, respectively. The enantiomers of tetraorganosilane precursor 9 could be separated by column chromatography. Their chiral integrity persisted on forming the silicates. CD spectra are reported for 10c. Fluoro derivative 10d is shown to have its electronegative F substituent in an equatorial position, is stable toward hydrolysis, and its enantiomers do not racemize at ambient temperatures, while those of 10c racemize slowly.

Toward Asymmetric Synthesis of Pentaorganosilicates.

Introducing chiral silicon centers was explored for the asymmetric Rh-catalyzed cyclization of dihydrosilanes to enantiomerically enriched spirosilanes as targets to enable access to enantiostable pentacoordinate silicates. The steric rigidity required in such systems demands the presence of two naphthyl or benzo[b]thiophene groups. The synthetic approach to the expanded spirosilanes extends Takai's method (Kuninobu et al. in Angew Chem Int Ed 52(5):1520-1522, 2013) for the synthesis of spirosilabifluorenes in which both a Si-H and a C-H bond of a dihydrosilane are activated by a rhodium catalyst. The expanded dihydrosilanes were obtained from halogenated aromatic precursors. Their asymmetric cyclization to the spirosilanes were conducted with [Rh(cod)Cl]2 in the presence of the chiral bidentate phosphane ligands (R)-BINAP, (R)-MeO-BIPHEP, and (R)-SEGPHOS, including derivatives with P-(3,5-t-Bu-4-MeO)-phenyl (DTBM) groups. The highest enantiomeric excess of 84% was obtained for 11,11'-spirobi[benzo[b]-naphtho[2,1-d]silole] with the DTBM-SEGPHOS ligand.