Diastereoselective synthesis of chiral aminophenolate magnesium complexes and their application in the stereoselective polymerization of rac-lactide and rac-β-butyrolactone

Abstract

A series of magnesium silylamido complexes supported by chiral aminophenolate ligands were synthesized, and their catalytic behaviors toward the ring-opening polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of Mg[N(SiMe₃)₂]₂ with chiral tridentate aminophenols L^1–10H [L = (S)-{[(1-R⁴, 2-pyrrolidinyl)CH₂N(R³)-]}CH₂-(6-R¹-4-R²)-C₆H₂O⁻] in a 1 : 1 molar ratio in toluene gave the chiral aminophenolate magnesium silylamido complexes L^1–10MgN(SiMe₃)₂ as enantiopure compounds or as pairs of diastereomers. As determined by X-ray analysis, structures a and b of diastereomer mixtures adopt the configuration of S_CS_NR_NR_Mg-a and S_CS_NS_NS_Mg-b, respectively. It was found that most of these magnesium complexes served as highly active initiators for the ring-opening polymerization of rac-LA. Besides, the stereoselectivity of chiral magnesium complexes gradually changed from isoselective-biased (P_m = 0.67) to heteroselective-enriched (P_r = 0.81) via a stepwise adjustment of the substituents of the ligand framework. It was noted that, as the rare reported active magnesium initiator for the ROP of rac-BBL, complexes 3–9 can efficiently initiate the ring-opening polymerization of rac-BBL in a controlled manner, giving moderate syndiotactic (P_s up to 0.73) PHBs.