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

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posted on 2015-07-13, 00:00 authored by Manuel Honrado, Antonio Otero, Juan Fernández-Baeza, Luis F. Sánchez-Barba, Andrés Garcés, Agustín Lara-Sánchez, Jaime Martínez-Ferrer, Sonia Sobrino, Ana M. Rodríguez
The preparation of the first racemic bis­(pyrazol-1-yl)­methane-based NNCp-donor hybrid scorpionate/cyclopentadienyl magnesium and zinc complexes [Mg­(R)­(κ25-NNCp)] (16) [NNCp = bpztcp, R = Me (1), Et (2), CH2SiMe3 (3); bpzpcp, R = Me (4), Et (5), CH2SiMe3 (6)] and [Zn­(R)­(κ21-NNCp)] (712) [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 (16) have been employed in a protonolysis reaction with 2,6-dimethylphenol to yield the aryloxide magnesium complexes [Mg­(OAr)­(κ25-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′H (16b) {bpmycp′H = 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)­(κ25-R,S-bpmycp)] (1719) (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)­(κ21-R,S-bpmycp)] (2022) (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 13, 59, 1114, 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 Pi value up to 0.77, the racemic mixtures of initiators impart more discrete levels of heteroselectivity in the production of PLAs, with a Ps value of 0.72.

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