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 (<i>L</i>- and <i>D</i><b>-1</b>) bearing <i>tert</i>-butyloxycarboryl-protected <i>L</i>-prolinol ester using chiral Pd(II)/L^{<i>R</i>,or<i>,S</i>} catalysts coordinated with <i>R</i>- or <i>S</i>-bidentate phosphine ligand showed high enantioselectivity (<i>k</i>_fast/<i>k</i>_<i>s</i>low = 142). Interestingly, the living polymerization of racemic <i>D</i>/<i>L</i><b>-1</b> shows high helix-sense selectivity and affords left- and right-handed helices by using Pd(II)/L^{<i>R</i>,and,<i>S</i>} catalysts, respectively. Removing the <i>tert</i>-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 (<i>ee</i>) 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.