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 = (S)-{[(1-R(4), 2-pyrrolidinyl)CH2N(R(3))-]}CH2-(6-R(1)-4-R(2))-C6H2O(-)] in a 1 : 1 molar ratio in toluene gave the chiral aminophenolate magnesium silylamido complexes L(1-10)MgN(SiMe3)2 as enantiopure compounds or as pairs of diastereomers. As determined by X-ray analysis, structures and of diastereomer mixtures adopt the configuration of SCSNRNRMg- and SCSNSNSMg-, 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 (Pm = 0.67) to heteroselective-enriched (Pr = 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 can efficiently initiate the ring-opening polymerization of rac-BBL in a controlled manner, giving moderate syndiotactic (Ps up to 0.73) PHBs.