Highly active rare-earth metal catalysts for heteroselective ring-opening polymerization of racemic lactide

Abstract

Rare-earth metal complexes usually exhibit high activities in the ring-opening polymerization (ROP) of lactide, yet only a few scandium complexes have shown satisfactory activity. Herein, we report the synthesis of a series of chiral anilido-oxazoline-supported scandium and yttrium complexes that exhibit high activity in the ROP of racemic lactide (rac-LA). Complexes L^a–f-Ln(CH₂SiMe₃)₂THF (L^a–f = 2-(2,6-R₂PhN)-phenyl-4-(S)-R′-oxazoline; for 1a–f: L = L^a–f, Ln = Sc; for 2a–d: L = L^a–d, Ln = Y) were synthesized via the convenient one-pot reaction of Ln(CH₂SiMe₃)₃(THF)₂ (Ln = Sc, Y) with the corresponding proligands. The crystal structures of 1a, 1d, 1e, and 1f were isostructural, adopting a distorted trigonal bipyramidal configuration. Sc complexes 1 showed outstanding activity in the ROP of rac-LA with heteroselectivity. TOFs of up to 720 h⁻¹ and 2910 h⁻¹ were obtained in THF at room temperature and toluene at 60 °C, respectively. To our knowledge, these are the highest activities reported for Sc systems. Y complexes 2 showed higher activity and heteroselectivity than the Sc complexes, with TOFs of up to 1176 h⁻¹ in THF at room temperature. Compared with the ortho-substituent on the anilido moiety, the bulky substituent at the chiral center of the oxazoline ring had a greater effect on controlling the heteroselectivity.