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Electron-deficient porphyrins Terazono, Yuichi

Abstract

The search for effective porphyrin catalysts for oxygenations that mimic the functions of cytochrome P-450 has led to the synthesis of electron-deficient porphyrins. Novel β-ttifluoromerhyl-meso-tetraphenylporphyrins were prepared by the copper assisted trifluoromethylation using β -bromo-meso-tetraphenylporphyrins and in-situ generated CF₃Cu via the pyrolysis of CF₃CO₂Na/CuI or the metathesis of trifluoromethylcadmium ((CF₃)₂Cd + CF₃CdBr/CuBr). Although multiple trifluoromethylation was difficult due to the steric bulk of the —CF₃ group, the existence of various meso-tetraphenylporphyrins with perfluoroalkyl moieties was affirmed. Partially trifluoromethylated porphyrins H₂TPP(CF₃)₂ (46a), H₂TPP(CF₃)₃ (47a), H₂TPP(CF₃)₄ (48a), and H₂TPP(CF₃)₃ (CF₂CF₃ ) (52a) were obtained by trifluoromethylation of Zn(TPPBr₄) (45b) followed by demetallation. Zn(TPP(CF₃)₄) (48b), Co(TPP(CF₃)₄) (48e) Fe(TPP(CF₃)₄ Cl (48f), and Fe(TPP(CF₃)₃ (CF₂CF₃))Cl (52c) were also synthesized from the corresponding free-base porphyrins. These metalloporphyrins, as well as free-base porphyrins, were used for the analysis of the electronic and the steric effects of the —CF₃ groups on the [Chemical Diagrams] porphyrin macrocycle by UV-visible and ¹H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography. These analyses not only suggested that novel p-trifluoromethylporphyrins were electron-deficient but also showed that those porphyrins take bacteriochlorin-like distorted electronic structures; a fixed 18π-electronic pathway. An X-ray crystal structure of 48b revealed severe macrocycle distortion into a saddle shape due to the steric interaction between the —CF₃ and the meso-phenyl groups. Catalytic oxidations of cyclohexane and cyclohexene using 48f and 52c as catalysts, iodosylbenzene (PhIO) as an oxidant and the above-mentioned substrates showed that they were not superior to one of the best porphyrin catalysts, Fe(TDCPPCl₈)Cl (10d) and porphyrin 48f and 52c were not very stable in the oxidation runs. Although the prepared novel porphyrins were not satisfactory as P-450 model compounds, interesting electronic and structural properties of the meso-tetraphenylporphyrins partially trifluoromethylated at the pyrrolic β-positions of antipodal pyrroles were revealed. The steric and electronic effects of the —CF₃ groups on the pyrrolic β-positions of meso-tetraphenylporphyrin were also compared to those of β-methyl analogues (57a, 58a, 59a, and 59b) in order to compare these effects distinctly. [Chemical Diagrams].

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