Precise Synthesis of Racemate-Based One-Handed Helical Polymers as Recyclable Homogeneous Chiral Catalysts for Multiple Asymmetric Reactions

Developing chiral catalysts from racemic materials while retaining the merits of both homogeneous and heterogeneous catalysts is a highly desirable yet formidable challenge to date. In this work, we synthesized optically active, one-handed helical polycarbenes from racemic monomers using chiral Pd(II) catalysts. The living polymerization of enantiomeric diazo acetate monomers ( L - and D -1 ) bearing tert -butyloxycarboryl-protected L -prolinol ester using chiral Pd(II)/L R,or,S catalysts coordinated with R - or S -bidentate phosphine ligand showed high enantioselectivity ( k fast / k s low = 142). Interestingly, the living polymerization of racemic D / L -1 shows high helix-sense selectivity and affords left- and right-handed helices by using Pd(II)/L R,and, S catalysts, respectively. Removing the tert -butyloxycarboryl-protected groups on prolinol esters, the racemate-based helical polycarbenes bearing amine pendants showed excellent catalytic activity and enantioselectivity in asymmetric Aldol reaction, Michael addition, and Domino oxa-Michael/Aldol condensation. All of the reactions gave the target products in high yields (>78%) with enantiomeric excess ( ee ) up to 99%. Surprisingly, enantioselectivity is solely determined by the backbone helicity, regardless of the pendant chirality; thus, enantiomeric products are easily obtained using helical polymer-based catalysts in opposite handedness. The polymer catalysts could be recovered from the homogeneous reaction solution via solvent precipitation and were recycled at least 5 times while maintaining excellent activity and enantioselectivity, confirming that these racemate-based helical polymers have the merits of both homogeneous and heterogeneous catalysts.