Rhodium-Catalyzed Synthesis of Chiral Spiro-9-silabifluorenes by Dehydrogenative Silylation: Mechanistic Insights into the Construction of Tetraorganosilicon Stereocenters

Mechanistic insight into the construction of quaternary silicon chiral centers by rhodium-catalyzed synthesis of spiro-9-silabifluorenes through dehydrogenative silylation is reported. The C₂-symmetric bisphosphine ligand, BINAP, was effective in controlling enantioselectivity, and axially chiral spiro-9-silabifluorenes were obtained in excellent yields with high enantiomeric excess. Monitoring of the reaction revealed the presence of a monohydrosilane intermediate as a mixture of two constitutional isomers. The reaction proceeded through two consecutive dehydrogenative silylations, and the absolute configuration was determined in the first silylative cyclization. Competitive reactions with electron-rich and electron-deficient dihydrosilanes indicated that the rate of silylative cyclization increased with decreasing electron density on the silicon atom of the starting dihydrosilane. Further investigation disclosed a rare interconversion between the two constitutional isomers of the monohydrosilane intermediate with retention of the absolute configuration. Silicon-centered chirality: A combination of [{RhCl(cod)}₂] (cod=1,5-cyclooctadiene) and chiral bisphosphine ligand, (R)-BINAP, catalyzes the asymmetric induction of axial chirality through two consecutive dehydrogenative silylations of unactivated C(sp²)-H bonds (see scheme). A mechanistic investigation revealed that the absolute configuration is determined in the first dehydrogenative silylation, and this reaction may involve a rare example of C-Si bond cleavage with retention of the silicon-centered chirality.