Corrole oxidation

Since meso-substituted corroles with alkyl substituents at a (l-posihon often exhibit reduced stability due to oxidative ring opening (01IC4845), attempt was made to obtain stable analogues by substituting aryl substituents at a -position, which prevented corrole oxidation. Thus, condensation of dipyrromethane 19 (Scheme 11) and appropriately substituted aldehyde 20 under the influence of TFA furnished biladiene 21, and subsequently cyclized to stable corrole 22 derivatives. 

The cobalt(II) corrole anion prepared as above was characterized primarily by electron spin resonance (esr) and absorption spectroscopy. When prepared via sodium film reduction, the cobalt(II) corrole oxidizes rapidly to the corresponding Co(III) corrole on exposure to air. When prepared by the other methods, it is moderately stable in air in the presence of a reducing agent. Attempts to prepare the neutral form of the initial Co(II) corrole anion, by protonation with perchloric acid, resulted in formal oxidation to the Co(III) derivative. Interestingly, further protonation of the Co(III) corrole with perchloric acid led to what appeared to be a protonated Co(III) corrole. Certainly, the absorption spectrum of this species is similar to that of the corresponding neutral nickel(II) corrole complex. However, the exact nature of this protonated material has not been fully elucidated. 

Copper Corrole Oxidation States Experiment and Theory... 

Similarly, the corresponding nickel complex, based on ESR spectroscopy, should be formulated as a nickel(II) corrole-71-radical. The iron corroles exist in the oxidation state + 111 or + IV depending on the nature of additional axial ligands. 

Radish KM, Shao J, Ou Z, Zhan R, Burdet E, Barhe J-M, Gros CP, Guilard R. 2005. Electrochemistry and spectroelectrochemistry of heterobimetalUc porphyrin-corrole dyads. Influence of the spacer, metal ion, and oxidation state on the pyridine binding ability. Inorg Chem 44, 9023-9038. 

The triprotic corroles 64 form monoorganotin complexes of the type RSnlv(Cor). PhSn(Cor) is formed when octaethylcorrole reacts with diphenyltin oxide, or phenylmagnesium bromide reacts with ClSn(Cor). The tin is five-coordinate, and is placed 72.2(3) pm above the plane of the four nitrogen atoms, with the corrole assuming a domed structure. The compound is reversibly reduced by transfer of one electron into the 7r-system.511... 

One example of a tin porphycene has been reported, but as yet no organometallic derivatives have been reported." A small number of tin corrole complexes are known including one organotin example, Sn(OEC)Ph, prepared from the reaction of Sn(OEC)Cl with PhMgBr. A crystal structure of Sn(OEC)Ph shows it to have both shorter Sn—N and Sn—C bonds than Sn(TPP)Ph2, with the tin atom displaced 0.722 A above the N4 plane of the domed macrocycle (Fig. 6). The complex undergoes reversible one-electron electrochemical oxidation and reduction at the corrole ring, and also two further ring oxidations which have no counterpart in tin porphyrin complexes. " " ... 

Corroles are porphyrins which lack the 20-methine group. (7,13-dimethyl-2,3,8,12,17,18-hexa-ethylcorrolato)iron chloride reacts with cyanide to form a low-spin dicyano complex, which is subsequently reduced by excess of cyanide to give (152). The iron in the dicyano complex is still in the 3+ oxidation state, with the reduced correlate as a dinegative radical ligand. ... 

In this section, we present material dealing with the direct oxidation and reduction of a variety of organocobalt species, including complexes with more than one cobalt center, electrodes functionalized with cobalt complexes, cobalt-containing SchifF-base complexes, cobalt porphyrins and corroles, and macrocyclic tetraamines. 

Subsequent investigations were undertaken of cobalt corroles with modified structures [77] and with cobalt in higher oxidation states [78]. In addition, cobalt-containing corrole dimers were studied [79], cobalt biscorroles were investigated ]80, 81], the influence of various alkyl and aryl substituents on the electrochemical behavior of cobalt corroles was probed ]82], and the effects of solvents on the electrochemistry of these compounds was examined [83]. 

Redox equilibrium of Ag(I I [-porphyrin /Ag(III) is characterized with = 0.59 V versus SCE [412]. Evidently, corroles and carbaporphyrins are able to stabilize the Ag(III) oxidation state, presumably due to the presence of 7r-electron donors, which reduce the formal oxidation state of the metal in such complex [396]. It is expected that such complexes have potential practical applications, for example, as the catalysts in the electron-transfer reactions. 

One oxochromium(V) complex, CrO(TETMC), containing the trinegative anion of a corrole (279), has been characterized as the solid.1266 It is prepared (Table 102) simply by exposure to air of a solution presumably containing a Cr11 complex. Aerial oxidation of Cr (TPP) produces the oxochromium(IV) complex CrO(TPP) so the corrole ligand apparently facilitates autoxida-tion. The redox behaviour of CrO(TETMC) has been examined by cyclic voltametry.1267... 

Coproporphyrin I synthesis, 816 Coronands classification, 919 metal ion complexes, 928,938 Corphins, 855 Coninoids, 983 Corrins, 871-888 demetallation, 882 deuteration, 879 electrophilic reactions, 879 metallation, 882 NMR, 878 nucleophilicity, 886 nucleophilic reactions, 879 oxidation, 879 oxidative lactamization, 880 oxidative lactonization, 880 photochemistry, 887 reactions, 879 at metal, 885 rearrangements, 879 redox chemistry, 888 spectra, 877 synthesis, 878 Corroles, 871-888 demetallation, 874 deuteration, 872 hydrogenation, 872 metallation, 874 reactions, 872 at metal, 875 redox chemistry, 876 synthesis, 871 Corticotropin zinc complexes medical use, 966 Cotton effect anils, 717... 

Because of its structure, corrole could be expected to stabilize the + 3 oxidation state for metal ions leading to the formation of neutral complexes. 

The stabilizing effect of an axial ligand has been previously observed in the synthesis of cobalt corrolates. Such an effect has been used to synthesize the complex where no peripheral p substituents are present on the macrocycle, which decomposes if attempts are made to isolate it in the absence of triphenyl-phosphine [10]. The behavior of rhodium closely resembled that of cobalt and it seems to be even more sensitive to the presence of axial ligands. [Rh(CO)2Cl]2 has also used as a metal carrier with such a starting material a hexacoordinated derivative has been isolated. The reaction follows a pathway similar to that observed for rhodium porphyrinates the first product is a Rh+ complex which is then oxidized to a Rh3+ derivative [29]. 

It is well known that the oxidation state of the cobalt atom changes in the enzymatic B12 dependent reactions. The redox chemistry of cobalt corrinoids received a great deal of attention in the seventies. The Co3 +/Co2 + reaction of different corroles was investigated in the past although no mechanistic details have been reported. The resulting chemistry has already been reviewed [11]. 

The first example reported in the literature is the cyclization of dihydrobilin to octadehydrocorrin [51-54]. The reaction is catalyzed by the presence of nickel or cobalt salts. As in the case of corrole and its metal complexes such ring closure reaction has been carried out in alcoholic solution, it is oxidative and base catalyzed. It has been demonstrated that the formation of the corrin ring is part of an equilibrium where the oxidative ring closure is coupled with a reductive ring opening reaction [55]. 

Scheme 6 Corrole synthesis through oxidation of bilanes.

A simple and versatile one-pot synthesis of meso-substituted trans-A2B-corroles (Scheme 7) was reported by Gryko and Jadach (01JOC4267). It affords regioisomerically pure frans-A2B-corroles 11 through the TFA-catalyzed condensation of a dipyrromethane 10 and an aldehyde followed by oxidation with DDQ. The synthesis is compatible with diverse functionalities ester, nitrile, ether, fluoro, hydroxy, etc. on the aryl group of the... 

Synthesis of regioisomerically pure corroles 15 (Scheme 9), possessing up to three different substituents (ABC-type corroles) at the meso-positions, has been achieved (02OL4491) through acid-catalyzed condensation of a dipyr-romethane-dicarbinol 14 with pyrrole followed by oxidation with DDQ. 

Corrphycenes and their metal complexes undergo four distinct one-electron redox steps, two are reduction steps and two are oxidation steps. A comparison with porphyrins and porphycenes indicates that the first reduction potentials of the free base and of metallo-corrphycenes are between those of porphycene 38, the easiest to reduce molecules, and those of porphyrins 2. The oxidation potentials of corrphycenes and porphyrins, however, are quite similar (00IC2850). The synthesis of hemi-porphycene 70 (Scheme 33) has been achieved (05ACI3047) from corrole 8 (R=C6H5). 

The reaction of tripyrranes with pyrrolecarboxaldehyde in the presence of TFA catalyst followed by oxidation with chloranil results in a simultaneous oxidative coupling and condensation to generate a meso-free corrole <02OL4233>. A fluorescence receptor based on triaza-18-crown-6 ether combined with two /V-guanidinium groups (and one A-anthracen-9-ylmethyl moiety) could bind several biologically important amino acids in aqueous methanol... 

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