Copper-induced fluoroalkylation of porphyrins: solvent-dependent synthesis of fluoroalkyl chlorins and porphyrins from fluoroalkyl iodides

Abstract

Treatment of copper(II) or nickel(II) porphyrins with fluoroalkyl iodides ( R f I ) in the presence of copper powder in dioxane, diglyme(DG) or tetrahydrofuran ( THF ) at 70-90°C gave β-fluoroalkyl chlorins in moderate yields. Unexpectedly, when dimethylsulphoxide ( DMSO ), hexamethylphosphoric triamide ( HMPA ) or N , N -dimethylformamide ( DMF ) was used as the solvent, with chlorobenzene as a co-solvent, the β-fluoroalkylporphyrins were obtained. In the first three solvents the reaction can be completely suppressed by p-dinitrobenzene (p- DNB ) or hydroquione ( HQ ), but not at all in the last three solvents. Apparently, a single electron-transfer mechanism may be involved in the formation of β-fluoroalkyl chlorins while fluoroalkylcopper intermediates may play a role in the production of β-fluoroalkylporphyrins. The metallofluoroalkylchlorins can be converted quantitatively to 2-hydoxyl-3-fluoroalkylchlorins by air/ SiO 2 or air/ Al 2 O 3. Additionally, the metallofluoroalkylchlorins and metallofluoroalkylporphyrins can be converted into the corresponding free-base chlorins and porphyrins, respectively, with H 2 SO 4. On the other hand, the metallohydoxyfluoroalkylchlorins under the same conditions afforded β-fluoroalkylporphyrins. Metallohydroxyfluoroalkylchlorins were readily also etherified to the corresponding metallomethoxyfluoroalkylchlorins with dimethyl sulfate ( Me 2 SO 4).