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

Abstract

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.

Construction of Chiral Tetraorganosilicons by Tandem Desymmetrization of Silacyclobutanes/Intermolecular Dehydrogenative Silylation

“…Carbene insertion [10] and alcoholysis [11] are also very efficient and highly enantioselective.T he more challenging cross-coupling reaction with aryl iodide has been described. [13] Other desymmetrization reactions that did not involve silicon atom in the bond formation, such as the [2+ +2+ +2] cycloaddition, [14] C À H bond activation, [15] and b-elimination of silacyclopentene oxides, [16] were also developed. [13] Other desymmetrization reactions that did not involve silicon atom in the bond formation, such as the [2+ +2+ +2] cycloaddition, [14] C À H bond activation, [15] and b-elimination of silacyclopentene oxides, [16] were also developed.…”

“…[1] However,t oo ur knowledge,o nly three examples have been reported for the synthesis of dibenzosiloles with silicon stereogenic centers. [13][14][15] We commenced our study by using SCB [23,24] 1a and 2chlorothiophene 2a as the model substrates under the catalysis of Rh(cod)Cl (10 mol %) ( Table 1). No reaction took place in the absence of al igand (entry 1).…”

Mechanistic Studies on Rhodium-Catalyzed Enantioselective Silylation of Aryl C–H Bonds

“…4–9 However, little information on the mechanism of enantioselective variants of these reactions is available. 10–18 Preliminary mechanistic data on enantioselective silylation of C–H bonds have been reported, 11,17,18 but systematic mechanistic studies involving the isolation of reaction intermediates and kinetic analysis of the process have not been conducted. Most relevant to the work reported here, little information on the origins of enantioselectivities has been gained.…”

Enantioselective Synthesis of Chiral‐at‐Sulfur 1,2‐Benzothiazines by CpxRhIII‐Catalyzed C−H Functionalization of Sulfoximines

“…[15,16] Examples of using this concept to create stereogenic phosphorus [17] or silicon atoms [18] are more scarce.R elated methods to access chiral sulfur atoms are,tothe best of our knowledge,elusive, except for as ingle example for chiral sulfoxides reported by Wang and co-workers. [15,16] Examples of using this concept to create stereogenic phosphorus [17] or silicon atoms [18] are more scarce.R elated methods to access chiral sulfur atoms are,tothe best of our knowledge,elusive, except for as ingle example for chiral sulfoxides reported by Wang and co-workers.…”