3-(Dialkoxyphosphoryl)-N-confused Phlorin and Porphyrin and Its Zinc, Cadmium, and Nickel(II) Complexes. Synthesis and Stereochemistry

Bin, L.; Li, X.; Xu, X.; Stępień, M.; Chmielewski, P. J.
J. Org. Chem. 2013, 78, 1354-1364 | view at publisher's site

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.