Stereoselectivity Switch between Zinc and Magnesium Initiators in the Polymerization of rac-Lactide: Different Coordination Chemistry, Different Stereocontrol Mechanisms

The preparation and characterization of several zinc and magnesium complexes bearing chiral aminophenolate ligands, [(S)- or (R)-L]ZnN(SiMe3)(2) (1, 2), [(S)- or (R)-L](ZnOBu)-Bu-t (3, 4) and [(S)- or (R)-L]MgN(SiMe3)(2) (5, 6) ((S)- or (R)-L = (S)- or (R)-2-{N-benzyl-N-[(1-(n)butyl-2-pyrrolidinyl)methyl]}aminomethyl-4-methyl-6-tritylphenolate), have been reported. While the X-ray diffraction studies as well as the NMR spectroscopic data revealed that zinc and magnesium silylamido complexes possess similar structures both in the solid state and in solution, their catalytic performance toward rac-LA polymerization exhibited significant difference. The replacement of zinc center by magnesium realized an interesting stereoselectivity switch from isoselective (P-t = 0.80) to heteroselective (P-r = 0.81). The reactions of enantiopure zinc complexes 1 and 2 with (S)-methyl lactate or (R)-tert-butyl lactate resulted in the target lactate complexes 79 as a mixture of two diastereomers, but in different ratios. Two typical zinc lactate isomers 8a and 9-beta/9 beta' were isolated and further characterized by the X-ray diffraction method to have pentacoordinated geometry where the coordination of lactate carbonyl group displays some hemilabile nature. The molecular structures of 8a and 9 beta/9 beta' indicated that the specific configuration of lactate moiety in combination with the aminophenolate ligand of given chirality would favor the formation of one of the two diastereomers in each case. Based on the structures of zinc lactate model complexes and the active propagating species, as well as the preferential enchainment of one specific lactide monomer in the polymerization, a cooperation of enantiomorphic-site control and chain-end control mechanisms involving three types of active species was proposed for the formation of stereoblock PLAs from rac-LA by zinc initiators. Results of apparent rate constants for rac-, D-, and L-LA polymerizations by magnesium initiators are consistent with a chain-end control mechanism. The C-13 NMR spectra of magnesium lactate complexes proved that the bonding of carbonyl group to magnesium center is stronger than that in zinc active species. The coordination number of magnesium species can expand easily to six in the presence of coordinative molecules, suggesting that the heteroselective magnesium active species have a hexacoordinated core structure. The difference of coordination geometry between zinc and magnesium active species should be responsible for the stereoselectivity switch in the polymerization of rac-LA.