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1,3-dipolar cycloadditions with porphyrins Flemming, Jeffrey

Abstract

The objective of this work is to use 1,3-dipolar cycloadditions to synthesize novel aromatic compounds based on meso-phenyl substituted porphyrins. These compounds are potential photosensitizers for use in photodynamic therapy. Tetraphenylporphyrins with a variety of substituents were reacted with selected 1,3-dipoles. Tetraphenylporphyrin (TPP), 5,10-diphenylporphyrin (DPP), and tetrakis(pentafluorophenyl)porphyrin (pFTPP) were used in reactions with azomethine ylides, carbonyl ylides, diazoalkanes, nitrile oxides, thiocarbonyl ylides, nitrile ylides, and ozone, with varying results. Centrally metallated porphyrins were also used. Azomethine ylides reacted with pFTPP efficiently giving cycloadducts of pFTPP. Three n-substituted azomethine ylides (R = benzyl, 3,5-dimethoxybenzyl, and propyl) were used. The different substituents on the pyrrolidine ring did not affect the ultra-violet / visible (UV-Vis) spectra of the chlorin. Carbonyl ylides form cycloadducts with both TPP and pFTPP. The electron deficient pFTPP reacts with non-stabilized carbonyl ylides, while TPP reacts with the electron deficient tetracyano-substituted carbonyl ylide. These results highlight the differences in reactivity of these two porphyrins. The cyano groups of 42 were hydrolyzed with concentrated hydrochloric acid, and then methylated giving 41. Changes were observed in the UV-Vis spectra as the substituents were modified. Diazomethane gave the cycloadduct pyrazoline with pFTPP (45). Compound 45 was then subjected to heat or light, both of which caused the loss of N₂ giving the cyclopropane 46. The UV-Vis a spectrum of the cyclopropane derivative was red-shifted as compared to the pyrazoline, possibly by distortion in the porphyrin ring system as a result of the cyclopropane ring.

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