Recyclable Phase-Anchored Silylation Reagents
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Chlorosilanes are common protecting groups. However, in use, they introduce an extra step and after use they are discarded. As part of an effort to make these species’ use greener, we have been exploring effective ways to efficiently isolate silyl-protected species and to recycle and regenerate trialkylsilylchlorides after the protecting group is removed from its substrate. Prior work in the Bergbreiter group has shown that attaching a highly nonpolar group to molecules makes them selectively soluble in nonpolar solvents. This selective solubility allows such phase-anchored species to be separated from the reaction mixture with a biphasic extraction. We have extended this work to silyl species showing that a chlorosilane with a long alkyl chain, octadecyldimethylchlorosilane can make silylated species phase-selectively soluble. This was shown by allowing octadecyldimethylchlorosilane to react with several primary alcohols to form dimethyloctadecylsilyl ethers. These ethers were placed in a DMF/cyclohexane solvent system where they showed >96% retention of the silylated alcohols in the nonpolar phase. Octadecyldimethylchlorosilane was also used as a protecting group in a palladium-catalyzed cross-coupling reaction. In this case, the silylated product could be removed from polar phase soluble catalyst residues by a simple extraction, avoiding a waste generating chromatographic purification step. To further improve phase-selectivity, we replaced the octadecyl group with PIB, a soluble polymer support. PIB-chlorosilane was synthesized using hydrosilylation and chlorination reactions and ongoing work involves removing unreacted PIB with a PEG protection and precipitation. Octadecyldimethylsilyl triflate, a Lewis acid catalyst, was also synthesized.
Leibham, Ashley M (2016). Recyclable Phase-Anchored Silylation Reagents. Undergraduate Research Scholars Program. Available electronically from