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 (1(1a-m))ZnN(SiMe3)(2) (2a-m) with C-1-symmetric amido-oxazolinate ligands (HL1a-m = 2-(2'-R1NH)phenyl-4-R-2-oxazoline) have been described. Single-crystal X-ray crystallographic studies confirm that 2a (R-1 = 2,6-dimethylphenyl, R-2 = (S)-Pr-i) and 2d (R-1 = 2,6-dimethylphenyl, R-2 = (R)-Bu-i) are three-coordinate, mononuclear complexes, and 2k (R-1 = PhCO, R-2 = (S)-Pr-i) 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 (R-1 = 2,6-diisopropylphenyl, R-2 = (R) Bu-i), and a rare case of syndiotactic PCHC (57% r-centered tetrads) is obtained with 2j (R-1 = (R)-1-(1-naphthyl)ethyl), R-2 = (R) Bu-i) The asymmetric induction is generally low, with up to 71% SS unit in the main chain of the produced PCHCs.