Isolation of a Metastable Geometrical Isomer of a Hexacoordinated Dihydrophosphate: Elucidation of Its Enhanced Reactivity in Umpolung of a Hydrogen Atom of Water

Two of five conceivable geometrical isomers of a hexacoordinated dihydrophosphate bearing two sets of a bidentate ligand were investigated. X-ray crystallographic analysis of both of isomers, 1a-TPP and 1b-TEA, revealed their octahedral geometries of C2 and C1 symmetry, respectively, which were consistent with the NMR spectra. The isomer 1b-TEA underwent both hydride reduction of an aldehyde and proton exchange with water at room temperature in DMSO without any additive. A one-pot reaction of both of the reactions of 1b-TEA with D2O and an aldehyde or a ketone under the above conditions proceeded successfully to give the deuterated alcohol. Thus, umpolung of a hydrogen atom of water with 1b-TEA was achieved under much milder conditions than those used in the reaction with another isomer, 1a-TEA. Quantitative isomerization of 1b-TEA to 1a-TEA occurred in methanol at room temperature. Calculations on the five conceivable geometrical isomers of the anionic part of the dihydrophosphate revealed their relative stability, which reasonably explained the isomerization, and the larger negative charge at the atoms located at the trans positions of the oxygen atoms. The smaller coupling constants of the P–H and P–C bonds located at the rear of an oxygen atom in the NMR spectra resulted in the smaller s character of these bonds. The differences in both hydride-donation and proton-exchange reactivities between 1a-TEA and 1b-TEA could be explained by the differences in the atomic charge of the hydrogen atom and the stability difference of the initially formed phosphorane intermediates, respectively.

本研究针对带有两组双齿配体的六配位二氢磷酸根（hexacoordinated dihydrophosphate）的五种潜在几何异构体中的两种展开了系统探究。对两种异构体1a-TPP与1b-TEA开展的X射线晶体衍射分析（X-ray crystallographic analysis）结果显示，二者分别具有C2和C1对称性的八面体几何构型，该结果与核磁共振谱（NMR spectra）的表征结果相吻合。在无任何添加剂的二甲基亚砜（DMSO）溶剂中，室温条件下异构体1b-TEA可同时发生醛基的氢化物还原反应以及与水的质子交换反应。在上述条件下，将1b-TEA与重水（D2O）、醛或酮分别进行一锅法反应（one-pot reaction），可顺利得到氘代醇（deuterated alcohol）。由此可见，相较于另一异构体1a-TEA参与的反应，利用1b-TEA实现水的氢原子极性反转（umpolung）的反应条件更为温和。在室温甲醇溶剂中，1b-TEA可发生定量异构化（quantitative isomerization）转化为1a-TEA。针对该二氢磷酸根阴离子部分的五种潜在几何异构体的计算结果揭示了它们的相对稳定性，该结果合理解释了上述异构化过程，同时印证了氧原子反位处的原子电荷更大这一规律。核磁共振谱中，位于氧原子后方的磷-氢（P–H）与磷-碳（P–C）键的耦合常数更小，这意味着这些化学键的s成分（s character）更低。1a-TEA与1b-TEA在氢化物给体能力及质子交换反应活性上的差异，可分别通过二者氢原子的原子电荷差异以及初始生成的磷烷中间体（phosphorane intermediates）的稳定性差异加以解释。