3‑(Dialkoxyphosphoryl)-N-confused Phlorin and Porphyrin. Synthesis, Stereochemistry, and Coordination Properties

A 3-phosphonated N-confused phlorin 3 was
synthesized by the reaction of N-confused porphyrin 1 and trimethyl or triethyl phosphite 2 in the presence
of acetic acid in good yield. The presence of hydrogen and aryl substituents
in one of the meso positions (C5) generates a stereogenic
center, resulting in configurationally stable enantiomers. The enantiomers
were separated by HPLC and characterized by the circular dichroism
method for the first time in the case of phlorin. Further oxidation
of 3 by DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)
afforded the achiral 3-phosphonated N-confused porphyrin 4. Chiral chlorozinc 4–Zn and chlorocadmium 4–Cd, as well as achiral nickel­(II) complexes 4–Ni were also characterized. For 4–Cd
in the solid state, formation of a dimer consisting of heterochiral
subunits joined by two H-bonds was established by a single crystal
X-ray analysis. For 4–Cd, separation of enantiomers
was achieved. Slow racemization of 4–Cd in solution
prevented the absolute configuration determination by the X-ray method
indicating the labile character of the complex. The relationship between
circular dichroism and absolute configuration of 3a and 4–Cd was established on the basis of TD-DFT calculations.

以N-混淆卟啉（N-confused porphyrin）1与亚磷酸三甲酯或亚磷酸三乙酯2为原料，在乙酸存在下经反应以良好收率合成了3-膦酰化N-混淆菲啉3。其中一个中位碳原子（C5）上连有氢原子与芳基取代基，由此形成手性中心，得到构型稳定的对映异构体。本研究首次针对菲啉类化合物，通过高效液相色谱（High Performance Liquid Chromatography, HPLC）分离得到该对映异构体，并采用圆二色谱（circular dichroism）法对其进行表征。以DDQ（2,3-二氯-5,6-二氰基-1,4-苯醌）为氧化剂对3进行进一步氧化，得到非手性3-膦酰化N-混淆卟啉4。研究还对手性氯化锌配合物4-Zn、氯化镉配合物4-Cd以及非手性镍(II)配合物4-Ni进行了表征。通过单晶X射线衍射分析证实，固态下4-Cd会形成由两个异手性单元通过两条氢键连接而成的二聚体。本研究成功分离得到了4-Cd的对映异构体，但4-Cd在溶液中会发生缓慢的外消旋化，这使得无法通过X射线衍射法确定其绝对构型，体现了该配合物的不稳定性。基于含时密度泛函理论（Time-Dependent Density Functional Theory, TD-DFT）计算，本研究明确了3a与4-Cd的圆二色谱信号与绝对构型之间的关联。