Preparation of enantiopure biimidazoline ligands and their use in asymmetric catalysis
作者:Nicola A. Boland、Mike Casey、Stephen J. Hynes、Jonathan W. Matthews、Helge Müller-Bunz、Philippa Wilkes
DOI:10.1039/b407743c
日期:——
A convenient new method for the preparation of 2,2′-biimidazolines is reported. Amino alcohols were reacted with dimethyl oxalate, and the product hydroxy amides converted into chloroamides by reaction with thionyl chloride. Treatment with PCl5, followed by diamines (ethanediamine, propane-1,3-diamine, 2,2-dimethylpropane-1,3-diamine) furnished a series of enantiopure tricyclic biimidazolines. Complexes of two of the ligands with PdCl2 were prepared and their X-ray crystal structures were determined. The biimidazolines were tested as ligands for asymmetric Pd-catalysed allylations. Moderate enantioselectivity (up to 80% ee) was found for the reaction of dimethyl malonate with diphenylallyl acetate, with the 5,7,5 fused tricyclic systems outperforming the 5,6,5 analogues. The corresponding reaction of pentenyl acetate gave lower enantioselectivity (44–57% ee), and proved very sensitive to the donor strength of the ligands, the stronger donors giving lower yields. The results provide a further demonstration of the value of the ‘tunability’ of imidazoline ligands.
报道了一种制备2,2′-双咪唑啉的便捷新方法。该方法通过氨基酸醇与草酸二甲酯反应,得到的羟基酰胺再与亚硫酰氯反应转化为氯酰胺。随后用五氯化磷处理,接着与二胺(乙二胺、丙烷-1,3-二胺、2,2-二甲基丙烷-1,3-二胺)反应,得到一系列手性纯的三环双咪唑啉。制备了其中两种配体与PdCl2的配合物,并测定了其X射线晶体结构。测试了这些双咪唑啉作为不对称Pd催化烯丙基化反应的配体。在二甲基丙二酸酯与二苯基烯丙基乙酸酯的反应中,发现了中等程度的手性选择性(最高达80% ee),其中5,7,5融合三环体系优于5,6,5类似物。相应的戊烯基乙酸酯反应则得到了较低的手性选择性(44-57% ee),并且对配体的给电子能力非常敏感,较强的给电子配体得到的产率较低。这些结果进一步展示了咪唑啉配体的“可调节性”价值。