(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.
All Science Journal Classification (ASJC) codes
- Organic Chemistry