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.