New Racemic and Single Enantiopure Hybrid Scorpionate/Cyclopentadienyl Magnesium and Zinc Initiators for the Stereoselective ROP of Lactides

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• 作者：Manuel Honrado ; Antonio Otero ; Juan Fern谩ndez-Baeza ; Luis F. S谩nchez-Barba ; Andr茅s Garc茅s ; Agust铆n Lara-S谩nchez ; Jaime Mart铆nez-Ferrer ; Sonia Sobrino ; Ana M. Rodr铆guez
• 刊名：Organometallics
• 出版年：2015
• 出版时间：July 13, 2015
• 年：2015
• 卷：34
• 期：13
• 页码：3196-3208
• 全文大小：528K
• ISSN：1520-6041

文摘

The preparation of the first racemic bis(pyrazol-1-yl)methane-based NNCp-donor hybrid scorpionate/cyclopentadienyl magnesium and zinc complexes [Mg(R)(魏²-畏⁵-NNCp)] (1鈥?b>6) [NNCp = bpztcp, R = Me (1), Et (2), CH₂SiMe₃ (3); bpzpcp, R = Me (4), Et (5), CH₂SiMe₃ (6)] and [Zn(R)(魏²-畏¹-NNCp)] (7鈥?b>12) [NNCp = bpztcp, R = Me (7), Et (8), CH₂SiMe₃ (9); bpzpcp, R = Me (10), Et (11), CH₂SiMe₃ (12)] has been carried out by the reaction of the corresponding racemic NNCp-H-donor scorpionate pro-ligands with Grignard reagents RMgCl, after deprotonation with ⁿBuLi, or directly with ZnR₂. The resulting alkyl magnesium complexes (1鈥?b>6) have been employed in a protonolysis reaction with 2,6-dimethylphenol to yield the aryloxide magnesium complexes [Mg(OAr)(魏²-畏⁵-NNCp)] (13 and 14) [NNCp = bpztcp (13), bpzpcp (14)]. Subsequently, the enantiopure scorpionate pro-ligand (R,R)-bpmycpH (16a) {bpmycpH = 1-{2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2R)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]ethyl}cyclopentadiene} and (R,R)-bpmycp鈥睭 (16b) {bpmycp鈥睭 = 2-{2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2R)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]ethyl}cyclopentadiene} reacted with RMgCl (after addition of ⁿBuLi) or with ZnR₂ to give the single enantiopure complexes [Mg(R)(魏²-畏⁵-R,S-bpmycp)] (17鈥?b>19) (R = Me (17), Et (18), CH₂SiMe₃ (19)) {R,S-bpmycp = 2-{2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2S)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]ethyl}cyclopentadienyl} and [Zn(R)(魏²-畏¹-R,S-bpmycp)] (20鈥?b>22) (R = Me (20), Et (21), CH₂SiMe₃ (22)). The structures of these complexes were elucidated by ¹H and ¹³C{¹H} NMR spectroscopy, and the X-ray crystal structures of 2, 3, 7 and 21 were also established. These racemic and single enantiopure complexes 1鈥?b>3, 5鈥?b>9, 11鈥?b>14, 19, and 21 can act as single-component initiators for the ring-opening polymerization of rac-lactide under mild conditions. Microstructural analysis of poly(rac-lactide)s revealed that, whereas the myrtenal substituent on the single enantiomerically pure initiator 21 exerts an appreciable influence on the degree of stereoselectivity to produce isotactic-enriched poly(lactide)s with a P_i value up to 0.77, the racemic mixtures of initiators impart more discrete levels of heteroselectivity in the production of PLAs, with a P_s value of 0.72.