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

Abstract

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_slow = 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.