Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA.
Inorg Chem. 2010 Mar 1;49(5):2360-71. doi: 10.1021/ic902271x.
A series of chiral NNO-tridentate Schiff base ligands derived from natural amino acids were reacted with zinc(bis-trimethylsilylamide)(2) to provide metal complexes which have been fully characterized. One of these derivatives was further reacted with p-fluorophenol to yield a phenoxide complex. X-ray crystallographic studies reveal the zinc Schiff base amide complexes to be monomeric, whereas, the p-fluorophenolate complex was shown to be dimeric with bridging phenoxide ligands. All zinc complexes were shown to be very effective catalysts for the ring-opening polymerization (ROP) of lactides at ambient temperature, producing polymers with controlled and narrow molecular weight distributions. These enantiomerically pure zinc complexes did not show selectivity toward either L- or D-lactide, that is, k(D(obsd))/k(L(obsd)) approximately = 1. However, steric substituents on the Schiff base ligands exhibited moderate to excellent stereocontrol for the ROP of rac-lactide. Heterotactic polylactides were produced from rac-lactide with P(r) values ranging from 0.68 to 0.89, depending on the catalyst employed and the reaction temperature. The reactivities of the various catalysts were greatly affected by substituents on the Schiff base ligands, with sterically bulky substituents being rate enhancing.
一系列源自天然氨基酸的手性 NNO-三齿席夫碱配体与锌（双三甲基甲硅烷基酰胺）（2）反应，得到了已充分表征的金属配合物。其中一种衍生物进一步与对氟苯酚反应，得到了酚氧络合物。X 射线晶体学研究表明，锌席夫碱酰胺配合物为单体，而对氟苯氧络合物则表现为桥连酚氧配体的二聚体。所有锌配合物都被证明是在环境温度下开环聚合（ROP）丙交酯的非常有效的催化剂，产生具有受控和窄分子量分布的聚合物。这些手性纯锌配合物对 L-或 D-丙交酯没有选择性，即 k(D(obsd))/k(L(obsd))≈1。然而，席夫碱配体上的空间位阻取代基对 rac-丙交酯的 ROP 表现出中等至优异的立体控制。异质的丙交酯聚合物是由 rac-丙交酯通过 ROP 得到的，P(r)值范围为 0.68 到 0.89，这取决于所使用的催化剂和反应温度。各种催化剂的反应性受席夫碱配体上取代基的影响很大，空间位阻大的取代基具有促进反应的作用。
