Reactivity of mono-meso-substituted iron(II) octaethylporphyrin complexes with hydrogen peroxide in the absence of dioxygen. Evidence for nucleophilic attack on the heme

Treatment of the mono- meso-substituted iron(II) octaethylporphyrin complex es, (py)(2)Fe-II(meso-NO2-OEP). (py)(2)Fe-II(meso-CN-OEP), (py)(2)Fe-II(mes o-HC(O)-OEP) (py)(2)Fe-II(meso-Cl-OEP), (py)(2)Fe-II(meso-OMe-OEP), (py)(2) Fe-II(meso-Ph-OEP), and (py)(2)Fe-II(meso-n-Bu-OEP), with hydrogen peroxide in pyridine-d(5) at -30 degreesC in the strict absence of dioxygen has bee n monitored by H-1 NMR spectroscopy. The product oxophlorin complexes are s table as long as the samples are protected from exposure to dioxygen. Hydro gen peroxide reacts cleanly with mono-meso-substituted iron(II) porphyrins in pyridine solution under an inert atmosphere to form mixtures of three po ssible oxygenation products, (py)(2)Fe(cis-meso-R-OEPO), (py)(2)Fe(trans-me so-R-OEPO), and (py)(2)Fe(OEPC). The yields of (py)(2)Fe(OEPO), which resul ts from replacemement of the unique meso substituent, as a function of the identity of the meso substituent decrease in the order NO2 > HC(O) similar to CN similar to Cl > OMe > Ph, Bu, which suggests that the species respons ible for attack on the porphyrin periphery is nucleophilic in nature. A mec hanism involving isoporphyrin formation through attack of hydroxide ion on a cationic iron porphyrin with an oxidized porphyrin ring is suggested. The identity of the unique meso functionality also affects the regiospecificit y of substitution when the unique meso group is retained. Although random a ttack at the two different meso sites is expected to yield a cis/trans prod uct ratio of 2, the observed ratios vary in the following order: cyano, 5.0 ; n-butyl, 4.9; chloro, 3.2; formyl, 2.6: methoxy, 1.9; phenyl 1.4.