Exploring Steric Effects in Diastereoselective Synthesis of Chiral Aminophenolate Zinc Complexes and Stereoselective Ring-Opening Polymerization of rac-Lactide.

A series of tridentate chiral aminophenol proligands and corresponding zinc complexes, LZnX (L = (S)-2-{[(1-R(4)-2-pyrrolidinyl)CH2N(R(3))-]CH2}-6-R(1)-4-R(2)-C6H2O, X = N(SiMe3)2, R(3) = (n)Bu, R(4) = Bn: R(1) = R(2) = Cl (1), R(1) = R(2) = Me (2), R(1) = R(2) = (t)Bu (3); X = N(SiMe3)2, R(1) = trityl, R(2) = Me: R(3) = n-octyl, R(4) = Bn (4), R(3) = Bn, R(4) = Bn (5), R(3) = (n)Bu, R(4) = naphthalen-1-ylmethyl (6), R(3) = (n)Bu, R(4) = (i)Pr (7); R(1) = R(2) = cumyl, R(3) = Et, R(4) = Bn: X = N(SiMe3)2 (8), X = O(t)Bu (9), X = Et (10), X = Cl (11)), have been synthesized. Complexes 4, 6, and 11 were obtained as enantiopure products (4 and 6 as enantiopure a; 11 as enantiopure b), while complexes 1-3, 5, and 7-10 as a pair of diastereomers, but in different ratios, which have been proved by X-ray diffraction and NMR spectroscopic studies. When exposed to the ring-opening polymerization of rac-lactide, most of these complexes can effectively produce PLAs with narrow polydispersities, desirable molecular weights, and moderate to high isotacticities. The structure-selectivity relationships, including the relationships of structure-synthesis diastereoselectivity and structure-polymerization stereoselectivity, have been further investigated. Consistent trends of diastereoselectivity and stereoselectivity are observed with the variations of the R(1) group at the ortho-position of the phenolate ring and the R(3) group in the pyrrolidinyl moiety. The decrease of the steric bulkiness of the R(4) group on the central amine has less influence on the diastereoselectivity, but leads to considerable loss of the stereoselectivity, whereas the decrease of the steric bulkiness of the X group results in a reverse of the diastereoselectivity, but shows no influence on the stereoselectivity. There is probably no direct relationship between the diastereoselectivity in complex synthesis and the stereoselectivity of the complex toward the ROP of rac-LA. The stereocontrol of these complexes might largely rely on the substituents in the ligand framework rather than their diastereomer ratios.