Institute of Chemical Research of Catalonia (ICIQ) , Avda. Països Catalans 16, 43007 Tarragona, Spain.
J Am Chem Soc. 2012 Aug 8;134(31):13121-32. doi: 10.1021/ja305684m. Epub 2012 Jul 30.
We describe the design, synthesis and conformational assignment of three diasteromeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole tetrol scaffold. The diastereoisomers differ in the relative spatial orientation of the P═O groups installed at their upper rims. We demonstrate that these compounds act as heteroditopic receptors for ion pairs forming ion-paired 1:1 complexes with alkylammonium (quaternary and primary) chloride salts in dichloromethane (DCM) solution and in the solid-state. (1)H NMR titrations indicate that the complexes are highly stable thermodynamically and kinetically. In the case of tetraalkyl-phosphonium/ammonium chloride guests, the host featuring the two P═O groups directed outwardly with respect to the aromatic cavity, 4oo, produces the most thermodynamically stable complexes. Conversely, for the primary alkyl ammonium chloride, the most effective receptor is the diastereoisomer 4ii with the two P═O groups converging on top of the aromatic cavity. In the nonpolar DCM solvent, the size of the quaternary cation has a strong impact in the thermodynamic stability of the complexes and their binding geometry. We use 2D-ROESY experiments to map out the binding geometries of the 1:1 complexes formed in solution. The 1:1 complexes of the 4oo host with the chloride salts have a separated arrangement of the bound ion-pair. In contrast, those of the 4ii host display a close-contact arrangement. We also investigate the same complexation processes in acetonitrile (ACN) solution. Both the salt and the initially formed anionic complex are fully dissociated in this more polar solvent. The receptors show an analogous trend in their binding affinities for quaternary phosphonium/ammonium chloride salts to the one seen in DCM solution. However, in ACN solution, the magnitudes of the binding affinities are reduced significantly and the size of the cation does not play a role. In addition, the inversion in the trend of relative binding affinities of the complexes, which was revealed in DCM solution, is eradicated in ACN when changing the cation substitution from quaternary to primary.
我们描述了三种基于芳基扩展杯[4]吡咯四醇支架的手性双膦酸酯 Cavitands 的设计、合成和构象分配。非对映异构体在安装在上边缘的 P═O 基团的相对空间取向上有所不同。我们证明这些化合物作为异双位受体，可与烷基铵（季铵和伯铵）氯化物盐在二氯甲烷（DCM）溶液中和固态中形成离子对 1:1 配合物。（1）H NMR 滴定表明，这些配合物在热力学和动力学上都非常稳定。在四烷基膦/铵氯化物客体的情况下，具有两个 P═O 基团相对于芳香空腔向外指向的主体 4oo 产生最热力学稳定的配合物。相反，对于伯烷基铵氯化物，最有效的受体是两个 P═O 基团在芳香空腔顶部收敛的非对映异构体 4ii。在非极性 DCM 溶剂中，季阳离子的大小对配合物的热力学稳定性及其结合几何形状有很强的影响。我们使用 2D-ROESY 实验来绘制在溶液中形成的 1:1 配合物的结合几何形状。4oo 主体与氯化物盐形成的 1:1 配合物具有分离的结合离子对排列。相比之下，4ii 主体的那些显示出紧密接触的排列。我们还在乙腈（ACN）溶液中研究了相同的络合过程。在这种极性更大的溶剂中，盐和最初形成的阴离子配合物都完全解离。受体对季膦/铵氯化物盐的结合亲和力表现出与在 DCM 溶液中相同的趋势。然而，在 ACN 溶液中，结合亲和力的大小显著降低，阳离子的大小不再起作用。此外，当将阳离子取代从季铵变为伯铵时，在 DCM 溶液中揭示的配合物相对结合亲和力趋势的反转在 ACN 中被消除。
