Synthesis and Reactions of Phosphaporphyrins: Reconstruction of π-Skeleton Triggered by Oxygenation of a Core Phosphorus Atom
作者:Takashi Nakabuchi、Makoto Nakashima、Shinya Fujishige、Haruyuki Nakano、Yoshihiro Matano、Hiroshi Imahori
DOI:10.1021/jo902060b
日期:2010.1.15
The synthesis, structures, optical and redox properties, and reactivity of phosphaporphyrins are reported. The 21-phosphaporphyrin (P,N-3-porphyrin) and 23-phospha-21-thiaporphyrin (P,S, N-2-porphyrin) were prepared via acid-promoted dehydrative condensation between a phosphatri-pyrrane 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 18 pi aromaticity inherent in regular N-4-porphyrins was maintained in these phosphaporphyrins. X-ray crystallography revealed a slightly distorted 18 pi aromatic ring for the P,N-3-porphyrin with the phosphole and three pyrrole rings tilted from the 24-atoms mean plane by 9.6 degrees and 3.8-15.4 degrees, respectively. DFT calculations on model compounds showed that the P,X,N-2-porphyrins (X = N, S) possess considerably small HOMO-LUMO gaps as compared with N-4- and S,N-3-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,N-2-porphyrins with H2O2 has been found to lead to the formation of different types of products. The 18 pi, P,N-3-porphyrin was transformed into the 22 pi aromatic P(O), N-3-porphyrin accompanied by the T extension at the peripheral C-3 bridge, whereas the 18 pi P,S,N-2-porphyrin was converted to the isophlorin-type 20 pi antiaromatic P(O),S,N-2-porphyrin. In both of the reactions, simple P-oxygenated 18 pi P(O),X,N-2-porphyrins were formed as the initial products, which were subsequently transformed into the 22 pi or 20 pi porphyrins. The two reaction courses from 18 pi to 20 pi/22 pi are apparently determined by the combination of the core heteroatoms (i.e., P,N-3 or P,S,N-2) 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 18 pi system but also a reliable tool to stabilize uncommon 22 pi and 20 pi systems through the chemical modifications at the core phosphorus atom.