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

Abstract

Crosslinked chiral polysiloxanes containing cinchona alkaloid derivatives were synthesized from poly (methylhydrosiloxane) by hydrosilylation reaction. The C3vinyl group of cinchona alkaloid derivatives were easily hydrosilylated with SiÀ 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 SiÀ 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'À OH free cinchona alkaloid derivatives into poly(methylhydrosiloxane). Both cinchona alkaloid C9-ester and C9-urethane derivatives were incorporated into poly(methylhydrosiloxane) through SiÀ 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'À 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.[a] B. T. Kumpuga, Prof. S. Itsuno

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring

“…These cinchona-based organocatalysts are usually used with saturated or unsaturated substituent (ethyl, vinyl, ethynyl) on the quinuclidine ring, although the effect of that saturation has not been examined before. Vinyl or ethynyl group is often the target for further transformations (Figure 2) such as polymerization or immobilization [17,18,19,20,21,22]; however, several methods for modification are described in the literature in other positions of the cinchonas as well (C9 position, or modification after demethylation of quinoline) [23,24].…”

Preparations of spherical nanoparticles of chiral Cinchona alkaloid-based bridged silsesquioxanes and their use in heterogeneous catalysis of enantioselective reactions

“…40,41 Soon after the pioneering work of the Sharpless' team, research began to prepare analogous solid catalysts that could be easily isolated from the reaction mixture and recycled. Since then, many heterogeneous catalysts have been prepared in which Cinchona alkaloid derivatives are bound to insoluble organic polymers, [42][43][44][45][46][47][48] silicones [49][50][51] or grafted onto prefabricated silica materials. [52][53][54][55][56] However, these materials often contain, as linkers between the alkaloid derivative and the support, functional groups that are hydrolytically labile under conditions of Sharpless dihydroxylation, causing leaching of the ligand from the solid material.…”