Stereoselective Synthesis of Biphenolate/Binaphtolate Titanate and Zirconate Alkoxide Species: Structural Characterization and Use in the Controlled ROP of Lactide
Stereoselective Synthesis of Biphenolate/Binaphtolate Titanate and Zirconate Alkoxide Species: Structural Characterization and Use in the Controlled ROP of Lactide
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DOI：
10.1021/ic301352t
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发表时间：
2012-10-15
影响因子：
4.6
通讯作者：
Dagorne, Samuel
中科院分区：
文献类型：
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作者：
Azor, Laurine;Bailly, Corinne;Dagorne, Samuel
定义明确的双酚/联萘二甲酸4组醇盐物种[（Ph-Biphen-O）（2）M（（OPr）-Pr-i）]Li（THF）（2a，M = Ti; 4a，M = Zr）和[（Ph-binapht-O）（2）M（（OPr）-Pr-i）]Li（THF）（2b，M = Ti;如图4 b所示，M = Zr）是容易获得的良好的产率，通过醇消除路线和/或取代反应，从相应的前配体Ph-联苯-OH（1a）和外消旋-Ph-联萘-OH（1b）。如通过X射线晶体学分析所建立的，Ti衍生物2a和2b的分子结构由阴离子Ti O’Pr部分的Li+盐组成，其中Ti中心采用扭曲的tbp几何形状，并且被两个联苯酚盐/联萘酚盐单元有效地螯合。值得注意的是，盐物质2a、B的溶液和固态数据与仅存在一种非对映异构体（具有（Delta，aS，aS）/（λ，aR，aR）构型）一致，因此表明Ti和Zr烷氧化物络合物2a、B/4 a、B的形成是立体选择性进行的。相反，中性联苯酚/联萘酸锆络合物（Ph-biphen-O）（2）Zr（THF）（2）（3a）和（Ph-binapht-O）（2）Zr（THF）（2）（3b）均被分离和X射线表征为异手性构型（Delta，aR，aS）/（λ，aS，aR）的立体异构体。发现Ti和Zr醇盐阴离子螯合物以受控的方式引发外消旋丙交酯的ROP，用于生产窄分散和酯端基PIA，如从SEC、动力学和MALDI-TOF数据推断的。Zr-（OPr）-Pr-1衍生物4a、B表现出上级其Ti对应物的性能（无论是关于活性、聚合控制还是立体选择性），以产生窄分散和异规异构富集的PLA（P-r=0.67，PDI < 1.15）。在阴离子4a、B（相对于中性类似物）中的Zr金属中心的显著降低的刘易斯酸性（由于阴离子电荷和四个Zr-O-ArO氧对Zr金属中心的可能的实质性电子π捐赠）可以合理化适度的聚合活性。对照实验表明，抗衡阳离子的性质对丙交酯ROP活性和立体控制的影响很小。
Well-defined biphenol/binaphtolate group 4 alkoxide salt species [(Ph-Biphen-O)(2)M((OPr)-Pr-i)]Li(THF) (2a, M = Ti; 4a, M = Zr) and [(Ph-binapht-O)(2)M((OPr)-Pr-i)]Li(THF) (2b, M = Ti; 4b, M = Zr) were found to be readily accessible in good yields via alcohol elimination routes and/or substitution reactions from the corresponding pro-ligands Ph-Biphen-OH (1a) and rac-Ph-Binapht-OH (1b). As established via X-ray crystallographic analysis, the molecular structures of the Ti derivatives 2a and 2b consist of Li+ salts of anionic Ti O'Pr moieties in which the Ti center adopts a distorted tbp geometry and is effectively chelated by two biphenolate/binaphtolate units. Remarkably, the solution and solid state data for salt species 2a,b agree with the sole presence of one diastereomer (with a (Delta, aS, aS)/(Lambda, aR, aR) configuration), thus indicating that formation of the Ti and Zr alkoxide complexes 2a,b/4a,b proceeds stereoselectively. In contrast, the neutral biphenolate/binaphtolate Zr complexes (Ph-biphen-O)(2)Zr(THF)(2) (3a) and (Ph-binapht-O)(2)Zr(THF)(2) (3b) were both isolated and X-ray characterized as stereomers in a heterochiral configuration (Delta, aR, aS)/(Lambda, aS, aR). The Ti and Zr alkoxide anionic chelates were found to initiate the ROP of rac-lactide in a controlled manner for production of narrowly disperse and ester-end group PIA, as deduced from SEC, kinetic, and MALDI-TOF data. The Zr-(OPr)-Pr-i derivatives 4a,b exhibit superior performance to their Ti counterparts (whether regarding activity, polymerization control, or stereoselectivity) to produce narrowly disperse and heterotactically enriched PLA (P-r=0.67, PDI < 1.15). The significantly decreased Lewis acidity of the Zr metal center in anions 4a,b (versus neutral analogues) due to the anionic charge and a likely substantial electronic pi donation of the four Zr-O-ArO oxygens to the Zr metal center may rationalize the moderate polymerization activity. Control experiments suggest that the nature of the countercation has little influence on lactide ROP activity and stereocontrol.