Chiral N,N,O-Scorpionate Zinc Alkyls as Effective and Stereoselective Initiators for the Living ROP of Lactides

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• 作者：Antonio Otero ; Juan Fern谩ndez-Baeza ; Luis F. S谩nchez-Barba ; Juan Tejeda ; Manuel Honrado ; Andr茅s Garc茅s ; Agust铆n Lara-S谩nchez ; Ana M. Rodr铆guez
• 刊名：Organometallics
• 出版年：2012
• 出版时间：June 11, 2012
• 年：2012
• 卷：31
• 期：11
• 页码：4191-4202
• 全文大小：502K
• 年卷期：v.31,no.11(June 11, 2012)
• ISSN：1520-6041

文摘

The chiral and the enantiopure bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the alcohol compounds (bpzbeH) [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide], (bpzteH) [bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-bpzmmH {(R,R)-bpzmm = (1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]-2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide} have been utilized to obtain new NNO-scorpionate zinc alkyl complexes. The reaction of bpzbeH, bpzteH (racemic mixture), or (R,R)-bpzmmH (enantiopure) with [ZnMe₂] in a 1:1 molar ratio in toluene afforded the mononuclear and monoalkyl zinc complexes [Zn(Me)(魏³-NNO)] (1鈥?b>3), respectively. However, when the same reaction was carried out with [ZnEt₂], [Zn^tBu₂], or [Zn(CH₂SiMe₃)₂], new dinuclear complexes of the type [Zn(R)(魏-NN渭-O)]₂ (魏-NN渭-O = bpzbe, R = Et 4, ^tBu 5, CH₂SiMe₃6; bpzte, R = Et 7, ^tBu 8, CH₂SiMe₃9; (R,R)-bpzmm, R = Et 10, ^tBu 11, CH₂SiMe₃12) were obtained. The single-crystal X-ray structure of derivative 1 confirms a monomeric 4-coordinative structure in which the metal center is in a distorted tetrahedral geometry with the heteroscorpionate ligands in a 魏³ coordination mode, whereas 4路2C₇H₈, 6, and 7 reveal an asymmetric 魏-NN渭-O arrangement in a dimeric molecular disposition. Interestingly, alkyl-containing zinc complexes 1, 4, 5, 6, 9, and 12 can act as single-component initiators for the ring-opening polymerization of lactides at 50 掳C. Thus, lactides were polymerized to afford PLA materials with low molecular weights in a few hours. The polymerizations are living, as evidenced by the narrow polydispersities (M_w/M_n = 1.03) of the isolated polymers in conjunction with the linear nature of the number-average molecular weight versus conversion plot. Inspection of the kinetic parameters for l-LA showed that propagations present the usual pseudo-first-order dependence on monomer and catalyst concentration. ¹H NMR and MALDI-TOF mass spectra confirmed that the initiation occurs through nucleophilic attack of alkyl on the lactide monomer. Microstructural analysis of poly(rac-lactide) by ¹H NMR spectroscopy revealed that the myrtenal substituent on the alkoxide fragment has a moderate influence on the degree of stereoselectivity, producing at mild temperatures enriched-heterotactic PLAs with a P_r value of up to 0.77.