Iron porphyrin moiety

Iron porphyrins display pronounced substrate preferences for alkene cyclopro-panation with EDA. In general, electron-rich terminal alkenes in conjunction with aromatic moiety or heteroatoms can efficiently undergo cyclopropanation with high catalyst turnover and selectivity. In contrast, 1,2-disubstituted alkenes cannot undergo cyclopropanation with diazoesters. Alkyl alkenes are poor substrates, giving cyclopropanated products in low yields. In both cases, the dimerization product diethyl maleate was obtained in high yield [53]. 

Collman et al.99 reported the asymmetric epoxidation of terminal olefins catalyzed by iron porphyrin complex 129. The catalyst was synthesized by connecting binaphthyl moieties to a readily available aa/ / -tetrakis(aminophenyl)-porphyrin (TAPP). Epoxidation of unfunctinalized olefins was carried out using iodosylbenzene as the oxidant. As shown in Scheme 4-46, excellent results were... 

The interaction within the active site can be either in the form of covalent binding or in the form of quasi-irreversible (tight but slowly reversible) binding, and it can involve the protein residues, the porphyrin moiety or the catalytic center (heme iron) [8]. CYP inactivation follows a stoichiometry of one substrate molecule per enzyme molecule inactivated. To measure the stoichiometry of the inactivation, it is necessary to trap all molecules that are not specifically bound to the active site, by using an appropriate scavenger, normally GSH. 

The peroxidase reaction is coupled with the formation of free radicals, either directly at the protein backbone or at the porphyrin moiety or both. This topic is borderline to the scope of this review and we will restrict ourselves to those studies which have at least a strong connection with the heme iron situation. We mention however, some relevant articles which have appeared in the period reviewed. Specific interest was given to a bi-functional enzyme from Mycobacterium tuberculosis which has both catalase and peroxidase activity.286-287,288-289 We also mention that a critical role of cations like Ca2+ and K+ has been described.290-291... 

The first reports on iron-catalyzed aziridinations date back to 1984, when Mansuy et al. reported that iron and manganese porphyrin catalysts were able to transfer a nitrene moiety on to alkenes [90]. They used iminoiodinanes PhIN=R (R = tosyl) as the nitrene source. However, yields remained low (up to 55% for styrene aziridination). It was suggested that the active intermediate formed during the reaction was an Fev=NTs complex and that this complex would transfer the NTs moiety to the alkene [91-93]. However, the catalytic performance was hampered by the rapid iron-catalyzed decomposition of PhI=NTs into iodobenzene and sulfonamide. Other reports on aziridination reactions with iron porphyrins or corroles and nitrene sources such as bromamine-T or chloramine-T have been published [94], An asymmetric variant was presented by Marchon and coworkers [95]. Biomimetic systems such as those mentioned above will be dealt with elsewhere. 

Recently synthesized iron porphyrin-carbene complexes (136) may be considered as carbon analogues of the oxo-iron(IV) porphyrin (HRP-II and CAT-II) species.476 The one-electron oxidation of [Fe(TPP)(C=CR2)] (R = aryl) by CuCl2 affords the stable complex [Fe(TPP)(C=CR2)] Cl (TPP = wieso-tetra-p-tolyl porphyrin and R = p-ClC6H4) which has a visible spectrum very similar to that of CAT-I. X-Ray analysis of this complex has revealed a structure (137) in which the vinylidene moiety has inserted between the iron atom and one pyrrole ring of the porphyrin and it is suggested that this may constitute a model, i.e. structure (138), for the active site of CAT-I as an alternative to the Fe117 cation radical model.477... 

Thianthrene radical cation is also an excellent one-electron oxidant of iron porphyrin complexes. Such oxidation of Fem(0Cl03)(TPP), where TPP is meso-tetraphenylporphyrin, provides the corresponding porphyrin 7r-cation radical analytically pure [32]. Similar oxidation of the AT-methyl porphyrin complex (N-MeTPP)FenCl, where AT-MeTPP is AT-methyl-meso-tetraphenylporphyrin, afforded [N-MeTPPFemCl]+ which was not further oxidized [33]. Thus thianthrene radical cation selectively oxidized the aromatic porphyrin ligand in one case and the metal center in the other. Ligand oxidation at a phenolic moiety has also been reported [34] on treatment of a 1,4,7-triazacyclononane appended with one or two phenol moieties ligated to Cu(II) complex with thianthrene radical cation. 

Any adequate theoretical treatment must also explain how iron-porphyrin systems can bind not only O2, but also CO, NO, alkyl isocyanides, and alkyl-nitroso moieties. A simple qualitative model presented by Wayland and coworkersconveniently summarizes ligand-binding geometries of cobalt and iron porphyrins. Although a reasonable quantitative theoretical consensus exists for 1 1 cobalt-dioxygen species, the same cannot be said yet for iron-dioxygen systems. 

Iron-porphyrin model reactions have been reported for several steps of metabolic oxidation of sydnones by cytochrome P450 [52,96]. Thus, diazoketones react with iron(II) porphyrins to give iron carbene complexes and then AT-alkylporphyrins after a one electron oxidation. In this case, a migration of the carbene moiety to the pyrrole nitrogen atom is observed (Scheme 10) [97]. Similar results were obtained with PhCH2CHN2 [53]. 

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