Heme cleavage with remarkable ease: paramagnetic intermediates formed by aerobic oxidation of a meso-amino-substituted iron porphyrin

Metal-Free Synthesis of meso-Aminoporphyrins through Reduction of meso-Azidoporphyrins Generated in Situ by Nucleophilic Substitution Reactions of meso-Bromoporphyrins

A facile and metal-free method for the preparation of free base meso-aminodiarylporphyrins from readily available meso-bromodiarylporphyrins is described. Simple treatment of meso-bromoporphyrins with sodium azide and sodium ascorbate in DMF affords the corresponding meso-aminoporphyrins in very good yields. This method involves the aromatic nucleophilic substitution (S_NAr) of a bromo group with an azido group and the subsequent in situ reduction of the introduced azido group by sodium ascorbate. This amination reaction can be scaled up to gram scale without any decrease of the product yield. The amination reaction of free base meso-dibromoporphyrin affords a monoaminated product selectively, whereas that of the Ni(II) complex furnishes a diaminated product that is oxidized by air under ambient conditions but isolable as a trifluoroacetyl ester. Metal-insertion reactions of the obtained free base aminoporphyrins afford the corresponding metal complexes (Ni(II), Cu(II), Zn(II), and Pd(II)) all in good yields except the Pd(II) complex. Synthetic methods for the preparation of N-mono- or dialkylaminoporphyrins from the free base meso-aminoporphyrins have been also established.

Ring Opening of a meso-Triaryl 25-Oxasmaragdyrin Macrocycle by m-Chloroperoxybenzoic Acid

Smaragdyrin, a class of expanded porphyrin macrocycles, upon treatment with meta-chloroperoxybenzoic acid (mCPBA) underwent oxidative ring opening to form an unprecedented linear pentaheterocyclic compound. The linear pentaheterocyclic compound was freely soluble in common organic solvents and characterized in detail by HRMS, 1D and 2D NMR spectroscopy, and X-ray crystallography. Our preliminary studies indicated that the linear pentaheterocyclic compound can specifically sense anions such as H₂ PO₄^- and CN^- ions, which was corroborated by absorption and fluorescence studies.

Pitfalls in bromination reactions of zinc porphyrins: two-sided ring opening of the porphyrin macrocycle

Reaction of [Zn(II)(TTP)] (1) (TTP = dianion of 5,10,15,20-meso-tetrakis(p-tolyl)porphyrin) with 16 equiv of N-bromosuccinamide (NBS) in methanol at reflux led to the unexpected two-sidedd open-ring brominated product [Zn(II)(C26H20N2O2Br5)2] (2). Similar observations have been made with other meso-substituted zinc porphyrins as well [Zn(II)(por)] {por = dianion of 5,10,15,20-meso-tetrakis(aryl)porphyrin; aryl = phenyl (TPP), p-(t)Bu-phenyl (TBPP), m-Cl-phenyl (TClPP)}. The respective products [Zn(II)(C24H16N2O2Br5)2] (3), [Zn(II)(C32H32N2O2Br5)2] (4), and [Zn(II)(C24H14N2O2Cl2Br5)2] (5) have been isolated in good to moderate yields and characterized by elemental analysis and UV-vis, (1)H NMR, and mass spectrometry. Additional bromination reaction of 1 with 8 equiv of NBS in a chloroform/methanol mixture led (after the two-sided ring opening) to nonmetalated brominated bi(pyrrole) product, C36H34N2O4Br4 (6). The detailed structures of complexes 1, 2, 3, and 6, available in a single crystal form, have been confirmed by X-ray diffraction analysis.

Pseudohalogenido complexes of iron-2,2′-bidipyrrins

The chlorido iron(III) complex of octaethyl-2,2′-bidipyrrin has been transformed to a series of pseudohalide complexes by ligand exchange reactions with azide, cyanate, thiocyanate and selenocyanate anions. All new complexes show the expected N-coordination of the axial ligand to the iron(III) center. In the solid state, all four species display an intermediate spin (S = 3/2) ground state, with a gradual increase of a high spin (S = 5/2) contribution at elevated temperatures for the members with the smallest ligand field strengths, i.e. the cyanato and the azido derivatives. In solution, proton NMR, and in particular IR spectroscopic studies support the interpretation of a high-spin state at ambient temperature throughout the series. The dependency of the spin state on the crystalline or dissolved state thus resembles that found for a similar series of halide derivatives before. In dichloromethane solution, the thiocyanato and selenocyanato complexes are very sensitive to aerial oxidation, forming oxacorrole and thiacorrole complexes as the only isolated products. These complexes show a S = 3/2 spin state in the solid as well as in solution, and their structural analyses prove the expected strong π-binding of the linear pseudohalide ion to the iron(III) central metal.

Synthetic transformations of porphyrins – Advances 2002-2004

Contemporary methods for the modification of porphyrins are presented. In association with the Third International Conference on Porphyrins and Phthalocyanines (ICPP-3) a survey of current method developments and reactivity studies is made. The review focuses on synthetic transformations of porphyrins currently in use for various applications and on functional group transformations. A brief survey of important developments covers selectively the literature from late 2001 to early 2004.

Copper(II) and Copper(III) Complexes of Pyrrole-Appended Oxacarbaporphyrin

The reaction of an O-confused porphyrin with a pendant pyrrole 4 and copper(II) acetate yields an organocopper(III) diamagnetic complex 4-Cu(III) substituted at the C(3) position by the pyrrole and H. The transformation of 4-Cu(III), performed in aerobic conditions, gave a rare copper(II) organometallic compound 6-Cu(II). In the course of this process, the tetrahedral-trigonal rearrangement originated at the C(3) atom but effects the whole structure. The electron paramagnetic resonance spectroscopic features correspond to a copper(II) oxidation state. A crystallographic analysis of 6-Cu(II) confirmed the formation of a direct metal-C bond [Cu(II)-C 1.939(4) A]. It was found that the Cu(II) complex of O-confused oxaporphyrin is sensitive to oxidative conditions. The degradation of 6-Cu(II) to yield copper(II) tripyrrinone complexes has been observed, which was considered as a peculiar case of dioxygen activation in a porphyrin-like environment. This process is accompanied by regioselective oxygenation at the inner C to form the 2-oxa-3-(2'-pyrrolyl)-21-hydroxycarbaporphyrinatocopper(II) complex ((pyrr)OCPO)CuII (8). The reaction of 6-Cu(II) with hydrogen peroxide, performed under heterophasic conditions, resulted in quantitative regioselective hydroxylation centered at the internal C(21) atom, also producing 8. Treatment of 8 with acid results in demetalation to form the nonaromatic 21-hydroxy O-confused porphyrin derivative ((pyrr)OCPOH)H (9).

Unexpected bromination ring-opening of tetraarylporphyrins

An unexpected, mild, efficient bromination ring-opening method has been developed for convenient synthesis of various novel biladienes or brominated porphyrins by controlling the amounts of NBS used.

Metal Complexes of meso-Amino-octaethylporphyrin and the Oxidation of NiII(meso-amino-octaethylporphyrin)

The crystal structures of meso-NH2-OEPH2, NiII(meso-NH2-OEP), and CuII(meso-NH2-OEP) (where OEP is the dianion of meso-amino-octaethylporphyrin) have been determined to examine the effects of the meso-substituent on the geometry of the ligand. CuII(meso-NH2-OEP) has a nearly planar geometry while the free ligand itself and NiII(meso-NH2-OEP) have ruf conformations. NiII(meso-NH2-OEP) is much less reactive toward oxidation than are (py)2FeII(meso-NH2-OEP), ClFeIII(meso-NH2-OEP), or NiII(meso-HO-OEP), which all undergo oxidation in pyridine solution when exposed to dioxygen. Treatment of NiII(meso-NH2-OEP) with iron(III) chloride in chloroform solution does result in oxidation of the ligand in two separate processes. One involves oxygenation at the trans-meso position, while the other results in ring cleavage and removal of the amino function. The open-chain tetrapyrrole complex, NiII(OEB-CO2Et), has been characterized by single-crystal X-ray diffraction and shown to contain a helical ligand with a four-coordinate nickel ion.

Facile Ring Opening of Iron(III) and Iron(II) Complexes of meso-Amino-octaethylporphyrin by Dioxygen

Pyridine solutions of ClFe(III)(meso-NH(2)-OEP) undergo oxidative ring opening when exposed to dioxygen. The high-spin iron(III) complex, ClFe(III)(meso-NH(2)-OEP), has been isolated and characterized by X-ray crystallography. In the solid state, it has a five-coordinate structure typical for high-spin (S = 5/2) iron(III) complex. In chloroform-d solution, ClFe(III)(meso-NH(2)-OEP) displays an (1)H NMR spectrum characteristic of a high-spin, five-coordinate complex and is unreactive toward dioxygen. However, in pyridine-d(5) solution a temperature-dependent equilibrium exists between the high-spin (S = 5/2), six-coordinate complex, [(py)ClFe(III)(meso-NH(2)-OEP)], and the six-coordinate, low spin (S = 1/2 with the less common (d(xz)d(yz))(4)(d(xy))(1) ground state)) complex, [(py)(2)Fe(III)(meso-NH(2)-OEP)](+). Such pyridine solutions are air-sensitive, and the remarkable degradation has been monitored by (1)H NMR spectroscopy. These studies reveal a stepwise conversion of ClFe(III)(meso-NH(2)-OEP) into an open-chain tetrapyrrole complex in which the original amino group and the attached meso carbon atom have been converted into a nitrile group. Additional oxidation at an adjacent meso carbon occurs to produce a ligand that binds iron by three pyrrole nitrogen atoms and the oxygen atom introduced at a meso carbon. This open-chain tetrapyrrole complex itself is sensitive to attack by dioxygen and is converted into a tripyrrole complex that is stable to further oxidation and has been isolated. The process of oxidation of the Fe(III) complex, ClFe(III)(meso-NH(2)-OEP), is compared with that of the iron(II) complex, (py)(2)Fe(II)(meso-NH(2)-OEP); both converge to form identical products.

IsdG and IsdI, heme-degrading enzymes in the cytoplasm of Staphylococcus aureus

Staphylococcus aureus requires iron for growth and utilizes heme as a source of iron during infection. Staphylococcal surface proteins capture hemoglobin, release heme from hemoglobin and transport this compound across the cell wall envelope and plasma membrane into the bacterial cytoplasm. Here we show that Staphylococcus aureus isdG and isdI encode cytoplasmic proteins with heme binding properties. IsdG and IsdI cleave the tetrapyrrol ring structure of heme in the presence of NADPH cytochrome P450 reductase, thereby releasing iron. Further, IsdI complements the heme utilization deficiency of a Corynebacterium ulcerans heme oxygenase mutant, demonstrating in vivo activity of this enzyme. Although Staphylococcus epidermidis, Listeria monocytogenes, and Bacillus anthracis encode homologues of IsdG and IsdI, these proteins are not found in other bacteria or mammals. Thus, it appears that bacterial pathogens evolved different strategies to retrieve iron from scavenged heme molecules and that staphylococcal IsdG and IsdI represent examples of bacterial heme-oxygenases.

Formation and Isolation of an Iron-Tripyrrole Complex from Heme Degradation

Addition of dioxygen to a solution of (py)2Fe(OEPO) (1) (where OEPO is the trianion of octaethyloxophlorin) in pyridine-d5 at 23 degrees C results in a series of oxidative changes that have been followed by 1H NMR spectroscopy. After 4 h, the resonances of 1 have vanished, while several sets of new resonances have developed. The most intense of these are identified as belonging to (py)2FeIII(OEB) (2) by comparison with the previously obtained spectrum of this species, which contains a ring-opened tetrapyrrole. After the sample was left standing for 1 day, further changes occur which produce a sample that is stable, because 1H NMR spectroscopic examination reveals no further changes on continued exposure to dioxygen or air. The product, (py)2FeIII(HETP) (3), where HETP is the dianion of a hexaethyltripyrrole, has been converted into (n-Bu4N)[ClFeIII(HETP)] by treatment with tetra(n-butyl)ammonium chloride. Red (n-Bu4N)[ClFeIII(HETP)] has been isolated and characterized by single-crystal X-ray diffraction.

Metallobiliverdin radicals--DFT studies

Several aspects of the molecular and electronic structure of biliverdin derivatives have been studied using density functional theory (DFT). The calculations have been performed for complexes of trianion (BvO2)3- and dianion [BvO(OH)]2-, derived from two tautomeric forms of biliverdin, BvO2H3 and [BvO(OH)]H2, with redox innocent metal ions: lithium(I), zinc(II), and gallium(III). One-electron-oxidized and reduced forms of each complex (cation and anion radicals) have been also considered. The molecular structures of all species investigated are characterized by a helical arrangement of tetrapyrrolic ligands with the metal ion lying in the plane formed by the two central pyrrole rings. The spin density distribution in four types of metallobiliverdin radicals--[(BvO2.)Mn+]n-2,[[BvO(OH).]Mn+]n-1 (cation radicals),[(BvO2.)Mn+]n-4,[[BvO(OH).]Mn+]n-3 (anion radicals)--has been investigated. In general, the absolute values of spin density on meso carbon atoms were larger than for the beta-carbon atoms. Sign alteration of spin density has been found for meso positions, and also for the beta-carbon atoms of at least two pyrrole rings. The calculated spin density maps accounted for the essential NMR spectroscopic features of iron biliverdin derivatives, including the considerable isotropic shifts detected for the meso resonances and shift alteration at the meso and beta-positions.