A Mild Synthesis of Unsymmetrical Bis-Alkoxysilanes through Catalyzed Alcoholysis of Hydridosilanes

Abstract

A mild method for the synthesis of unsymmetrical bisalkoxysilanes has been developed. This method utilizes two readily available catalysts (rhodium acetate dimer and 10% palladium on carbon) that, under neutral aprotic conditions and within reasonable time, give very good yields of the product in two steps without need to isolate intermediates. Equation 1The uses of silicon ethers in organic synthesis include the formation of tethered reactants for stereospecific intramolecular reactions, 1,2 protecting agents for many functional groups, 3 and anchoring reagents and substrates for solid support synthesis. 4 Silyl ethers are employed due to their compatibility with a wide range of reaction conditions and the ease with which they are removed. Silicon tethers through bis-alkoxysilanes (silyl ketals) are commonly prepared from the dichlorosilanes by reaction with an alcohol in the presence of base, and these conditions are not compatible with some base labile compounds. To make unsymmetrical bis-alkoxysilanes requires a method for breaking the symmetry of the dichlorosilane. Without such a method, one must accept a statistically determined mixture of mono-alkoxy and bis-alkoxy products. This may be acceptable for inexpensive readily available alcohols, but precludes the use of bis-alkoxysilane tethers for high-value synthetic intermediates. To overcome these limitations to bis-alkoxysilane synthesis, we investigated catalysts for the controlled alcoholysis of hydridosilanes.Catalytic alcoholysis of silanes by a variety of transition metal based catalysts is a very useful method to form silyl ethers under mild conditions. The process is atom-economical; typically hydrogen gas is the only by-product. To date this mild method has not been fully exploited in organic synthesis for the preparation of unsymmetrical bis-alkoxysilanes. A catalytic synthesis using silicon alcoholysis would circumvent the need of bases (and the attendant formation of protic byproducts), and eliminate the need for excess silicon dichlorides in the first ether formation. We sought catalytic methods that would ultimately allow formation of chiral tethers that are asymmetric at the silicon center. Herein we report an efficient method for the synthesis of unsymmetrical bis-alkoxysilanes from dihydridosilanes in two consecutive catalyzed alcoholysis reactions (Equation 1).There have been several reports of catalytic methods for alcoholysis of trialkylsilanes, 5 however only a few of these papers have discussed any reaction with dialkyl(dihydrido)silanes. 5b,e,h,f Among the more active catalysts for the preparation of trialkylsilyl ethers from silyl hydrides are Crabtree's cationic complex [IrH 2 (THF) 2 (PPh 3 ) 2 ] SbF 6 , 5d Wilkinson's catalyst Rh(PPh 3 ) 3 Cl, 5e Stryker's catalyst [PPh 3 CuH] 6 , 5f rhodium(II) perfluorobutyrate Rh 2 (pfb) 4 , 5b and Mn(CO) 5 Br and its dimer [Mn(CO) 4 Br] 2 . 5c A practical catalyst for silane alcoholysis should be compatible with a variety of functional groups, notably unsaturated functional groups which are k...