Synthesis and reactions of phosphaporphyrins: Reconstruction of π-skeleton triggered by oxygenation of a core phosphorus atom

(Chemical Equation Presented) The synthesis, structures, optical and redox properties, and reactivity of phosphaporphyrins are reported. The 21-phosphaporphyrin (P,N₃-porphyrin) and 23-phospha-21-thiaporphyrin (P,S, N₂-porphyrin) were prepared via acid-promoted dehydrative condensation between a phosphatripyrrane and the corresponding 2,5-bis[hydroxy(phenyl)methyl]heteroles followed by 2,3-dichloro-5,6- dicyanobenzoquinone oxidation. Experimental (NMR, UV-vis, and X-ray analyses) and theoretical (DFT calculations) results suggest that the 18π aromaticity inherent in regular N4-porphyrins was maintained in these phosphaporphyrins. X-ray crystallography revealed a slightly distorted 18π aromatic ring for the P,N₃-porphyrin with the phosphole and three pyrrole rings tilted from the 24-atoms mean plane by 9.6° and 3.8-15.4°, respectively. DFT calculations on model compounds showed that the P,X,N₂-porphyrins (X = N, S) possess considerably small HOMO-LUMO gaps as compared with N ₄- and S,N₃-porphyrins, which is reflected in the red-shifted absorptions, low oxidation potentials, and high reduction potentials of the phosphaporphyrins. The P-oxygenation of the P,X,N₂-porphyrins with H₂O₂ has been found to lead to the formation of different types of products. The 18π P,N₃-porphyrin was transformed into the 22π aromatic P(O), N₃-porphyrin accompanied by the π extension at the peripheral C₃ bridge, whereas the 18π P,S,N₂-porphyrin was converted to the isophlorin-type 20π antiaromatic P(O),S,N₂-porphyrin. In both of the reactions, simple P-oxygenated 18π P(O),X,N₂-porphyrins were formed as the initial products, which were subsequently transformed into the 22π or 20π porphyrins. The two reaction courses from 18π to 20π/22π are apparently determined by the combination of the core heteroatoms (i.e., P,N₃ or P,S,N₂) and the structure of the peripherally fused carbocycles. The present results demonstrate that the incorporation of a phosphorus atom into the core is not only a highly promising way to modify the fundamental properties of the porphyrin 18π system but also a reliable tool to stabilize uncommon 22π and 20π systems through the chemical modifications at the core phosphorus atom.