Chiral Amido-Oxazolinate Zinc Complexes for Asymmetric Alternating Copolymerization of CO2 and Cyclohexene Oxide

The synthesis and characterization of a series of chiral zinc complexes (L1a–m)­ZnN­(SiMe3)2 (2a–m) with C1-symmetric amido-oxazolinate ligands (HL1a–m = 2-(2′-R1NH)­phenyl-4-R2-oxazoline) have been described. Single-crystal X-ray crystallographic studies confirm that 2a (R1 = 2,6-dimethylphenyl, R2 = (S)-iPr) and 2d (R1 = 2,6-dimethylphenyl, R2 = (R)-iBu) are three-coordinate, mononuclear complexes, and 2k (R1 = PhCO, R2 = (S)-iPr) exists as an amide oxygen-bridged dimer in the solid state with zinc in a distorted tetrahedral geometry. These complexes are viable initiators for alternating copolymerization of carbon dioxide (CO2) and cyclohexene oxide (CHO), yielding poly­(cyclohexene carbonate) (PCHC) with good to high carbonate linkage and moderate molecular weights and PDI values, depending on the substituents. The PCHCs produced are typically isotactic, containing up to 72% m-centered tetrads by 2h (R1 = 2,6-diisopropylphenyl, R2 = (R)-iBu), and a rare case of syndiotactic PCHC (57% r-centered tetrads) is obtained with 2j (R1 = (R)-1-(1-naphthyl)­ethyl), R2 = (R)-iBu). The asymmetric induction is generally low, with up to 71% SS unit in the main chain of the produced PCHCs.