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Helix-Sense-Specific and Enantiomer-Specific Living Polymerizations of Phenyl Isocyanides Using Chiral Palladium(II) Catalysts
- Xu, Lei, Yang, Li, Guo, Zongxia, Liu, Na, Zhu, Yuan-Yuan, Li, Zhibo, Wu, Zong-Quan
- Macromolecules 2019 v.52 no.15 pp. 5698-5706
- atomic force microscopy, catalysts, circular dichroism spectroscopy, enantiomers, ligands, molecular weight, palladium, polymerization, polymers
- Controlled synthesis of a single-handed helical polymer from an achiral monomer is still a great challenge. In this work, we report two chiral Pd(II) catalysts bearing commercially available R- or S-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (R- or S-BINAP) ligands. Polymerization of achiral phenyl isocyanide (A-1) by S-BINAP/Pd(II) afforded a single right-handed helical poly(phenyl isocyanide)s in high yields with controlled molar mass (Mₙs) and narrow molar mass distributions (Mw/Mₙs). While the R-BINAP/Pd(II) catalyst leads to the formation of a single left-handed helix under the same conditions. The single-handed helices were determined by circular dichroism, UV–vis analyses, and direct atomic force microscopy (AFM) observations as well. Moreover, the chiral Pd(II) catalysts showed enantiomer specificity on polymerizations of isocyanide enantiomers (l- and d-1), although the chiral center was remote from the polymerization site. S-BINAP/Pd(II) can effectively promote the living polymerization of l-1, afforded a right-handed helical poly-l-1ₘ(S), while it failed on the polymerization of the d-1 enantiomer and the d/l-1 racemate under the same conditions. Accordingly, R-BINAP/Pd(II) can only catalyze the polymerization of the d-1 enantiomer.