Dinuclear Allylpalladium Complexes of C2-Symmetric Pyrazolate-Bridged Bis(oxazoline) Ligands (pyrbox's): Structures, Dynamic Behavior, and Application in Asymmetric Allyllic Alkylation
Dinuclear Allylpalladium Complexes of C2-Symmetric Pyrazolate-Bridged Bis(oxazoline) Ligands (pyrbox's): Structures, Dynamic Behavior, and Application in Asymmetric Allyllic Alkylation
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DOI：
10.1021/om900835f
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发表时间：
2010-03-08
期刊：
影响因子：
2.8
通讯作者：
Meyer, Franc
中科院分区：
文献类型：
--
作者：
Ficks, Arne;Sibbald, Connor;Meyer, Franc
合成了一系列具有恶唑啉侧臂的新型手性吡唑酸双核n给体（3a-d）。得到了这些配体的双金属甲基钯配合物（4a-d），并用x射线衍射对配合物4a、c的固态结构进行了表征。核磁共振波谱显示，在溶液4a-d中存在三种不同的异构体，两种甲基配体的取向不同。在二维noesi NMR实验中观察到烯丙基氢原子之间的交换（即烯丙基旋转）为同步/同步和反/反相互转化；确定了流体行为的动力学参数。对配合物在钯催化的rac-(E)-1,3-二苯丙烯酯乙酸酯烯丙基烷基化反应中的催化活性进行了测试。通过对具有不同配体支架的一组配合物4a-d的比较，发现空间和电子因素对对映体控制都很重要，并提出了一个模型来合理化观察到的对映体选择性。
A series of new chiral binucleating pyrazolate-based N-donor ligands (3a-d) with oxazoline side arms (coined pyrbox's) have been synthesized. Bimetallic methallylpalladium complexes (4a-d) of these ligands were obtained, and the solid-state structures of complexes 4a,c were characterized by X-ray diffraction. NMR spectroscopy revealed that in solution 4a-d exist as three different isomers that differ in the orientation of the two methallyl ligands. Exchange between the isomers (i.e., allyl rotation) was observed in two-dimensional NOESY NMR experiments as syn/syn and anti/anti interconversion of the allylic hydrogen atoms; kinetic parameters for the fluxional behavior have been determined. The catalytic activity of the complexes was tested in palladium-catalyzed allylic alkylation of rac-(E)-1,3-diphenylallyl acetate. By comparison of the set of complexes 4a-d that feature different ligand scaffolds, both steric and electronic factors were found to be important for enantiocontrol, and a model has been proposed for rationalizing the observed enantioselectivities.