ADMET Polymerization of Dimeric Cinchona Squaramides for the Preparation of a Highly Enantioselective Polymeric Organocatalyst

Abstract

Under the acyclic diene metathesis (ADMET) reaction condition, the C3-vinyl groups of cinchona alkaloids readily react with each other to form a C-C bond. A novel type of cinchona alkaloid polymers was synthesized from dimeric cinchona squaramides using the Hoveyda-Grubbs’ second-generation catalysts (HG2) by means of ADMET reaction. The chiral polymers, containing cinchona squaramide moieties in their main chains, were subsequently employed as catalysts for the enantioselective Michael reaction to give the corresponding chiral adducts in high yields with excellent enantioselectivity and diastereoselectivity. Both enantiomers from the asymmetric Michael reaction were distinctively prepared while using the polymeric catalysts, possessing pseudoenantiomeric structures. The catalysts were readily recovered from the reaction mixture and recycled several times due to the insolubility of the cinchona-based squaramide polymers.