Synthesis of Crosslinked Chiral Polysiloxanes of Cinchona Alkaloid Derivatives and their Applications in Asymmetric Catalysis

Abstract

AbstractCrosslinked chiral polysiloxanes containing cinchona alkaloid derivatives were synthesized from poly(methylhydrosiloxane) by hydrosilylation reaction. The C3‐vinyl group of cinchona alkaloid derivatives were easily hydrosilylated with Si−H functional groups of poly(methylhydrosiloxane) by using Pt catalyst. The reaction between cinchona alkaloid dimers and poly(methylhydrosiloxane) and afforded the crosslinked chiral polysiloxanes. Partial hydrolysis of Si−H bonds with Pt catalyst during reaction resulted in a formation of extra crosslinkages of chiral polysiloxanes. Mixed solvent (toluene and DMF) system was employed for a successful immobilization of C6′−OH free cinchona alkaloid derivatives into poly(methylhydrosiloxane). Both cinchona alkaloid C9‐ester and C9‐urethane derivatives were incorporated into poly(methylhydrosiloxane) through Si−C bond. The resulted crosslinked chiral polysiloxanes were insoluble in common organic solvents. Their catalytic performance was evaluated in asymmetric Michael addition reactions. The chiral polysiloxanes of cinchona alkaloid having C6′−OH showed higher enantioselectivities (up to 99% ee) with sufficient catalytic activities in the Michael addition reactions. The chiral polysiloxanes were easily recovered by filtration or centrifugation method and could be reused for several times without losing their catalytic performance.