Corrole catalysts

The synthesis of a chiral meso-ABC (oxo)Cr(V) corrole complex (Fig. 7) was recently described [84] the free-base starting material was obtained using the method of Gryko [85]. A meso-ABC corrole possesses different substituents at each of the meso-positions on the ring and is prochiral. This complex is not as efficacious an oxygen atom transfer catalyst as Cr(tpfc)(0) [86]. However, it could be a starting point for a new generation of chiral corrole catalysts. 

It was observed in a 2005 article that Co(II) porphyrin-Co(III) corrole dimers are more effective dioxygen reduction electrocatalysts than analogous Co(III)-Co(III) corrole dimers or monomeric Co(III) corroles [145], The heterodimers operated effectively at lower overpotentials and promote complete reduction to water (the average number of electrons transferred per 02 molecule approaches 4 in the best porphyrin-corrole catalyst). It was suggested that the inferior catalytic performance of the corrole homodimers could be due to a reduction in the basicity of the activated intermediate when two Co(III) moieties are involved, leading to a less favorable 4-electron reduction. Heterobimetallic catalysts containing formally Co (IV) corroles were also examined as potential dioxygen reduction catalysts [146]. 

While the Co corrole-Fe/Mn porphyrin dyads were active electrocatalysts, the homobimetallic Co porphyrin-corrole dyads operate at lower overpotentials and favor 4-electron reduction to a greater extent. This was attributed to the locus of reduction in each complex apparently, the porphyrin is reduced first in the homometallic dimer, whereas the corrole is the site of the first reduction in the heterometallic dimers. A later article presented EPR and spectroelectrochemical evidence supporting the assignment of a Co(III) Ji-cation electronic configuration for the oxidized derivatives of monomeric Co triarylcorrolates, suggesting that the dimeric five-coordinate Co corrole catalysts are also Co(III) 7i-cation radicals [147]. Further study is needed to elucidate this point. 

Carbenoid N-H insertion of amines with diazoacetates provides a useful means for the synthesis of ot-amino esters. Fe(III) porphyrins [64] and Fe(III/IV) corroles [65] are efficient catalysts for N-H carbenoid insertion of various aromatic and aliphatic amines using EDA as a carbene source (Scheme 16). The insertion reactions occur at room temperature and can be completed in short reaction times and with high product yields. It is performed in a one-pot fashion without the need for slow... 

A more practical, atom-economic and environmentally benign aziridination protocol is the use of chloramine-T or bromamine-T as nitrene source, which leads to NaCl or NaBr as the sole reaction by-product. In 2001, Gross reported an iron corrole catalyzed aziridination of styrenes with chloramine-T [83]. With iron corrole as catalyst, the aziridination can be performed rmder air atmosphere conditions, affording aziridines in moderate product yields (48-60%). In 2004, Zhang described an aziridination with bromamine-T as nitrene source and [Fe(TTP)Cl] as catalyst [84]. This catalytic system is effective for a variety of alkenes, including aromatic, aliphatic, cyclic, and acyclic alkenes, as well as cx,p-unsaturated esters (Scheme 28). Moderate to low stereoselectivities for 1,2-disubstituted alkenes were observed indicating the involvement of radical intermediate. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Figure 18.13 Chemical structures of selected cofacial strapped diporphyrins (a), pillared diporphyrins (h), and pillared porphyrin/corrole, dicorrole, and diphthalocyanine derivatives (c) whose metal complexes have heen studied as ORR catalysts. Conventional notations for the structures are also hsted (in bold). Other molecular architectures of cofacial porphyrins are known, hut the corresponding complexes have not yet been studied as ORR catalysts.

The seiectivities of metal complexes of cofacial porphyrinoids (porphyrins, corroles, and phthalocyanines) reported in the literature by mid-2007 are summarized in Fig. 18.15. The data are organized by the type of catalyst as well as in order of decreasing Mav Whereas ORR catalysis by certain cofacial porphyrins, such as (FTF4)Co2 and (DPY)Co2 (Y = a, B) has been smdied extensively by a number of groups, and the values of av are known with high degree of confidence, those for most other catalysts... 

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. 

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. 

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... 

Mn corroles have been isolated as Mn(III) [22, 60, 89, 90], Mn(IV) halogenato [91], and Mn(V) oxo [75, 92-94], imido [81, 95], and nitrido complexes [96, 97] (oxidation of (nitrido)Mn(V) correlates to Mn(VI) complexes has been reported as well). Mn(III) corroles represent rare examples of four-coordinate Mn(III) complexes. Mn(IV) corroles are unusually stable in fact, Mn(dpoec)(I) (dpoec = 5,15-diphenyl-2,3,7,8,12,13,17,18-octaethylcorrolate) was the first well-characterized Mn(IV)-I complex reported in the literature [98]. Meanwhile, the high-valent oxo, imido, and nitrido complexes have proven to be competent catalysts for... 

Jerome F, Gros CP, Tardieux C, Barbe JM, Guilard R (1998) Synthesis of a face-to-face porphyrin-corrole a potential precursor of a catalyst for the four-electron reduction of dioxygen. New J Chem 22 1327-1329... 

Kadish KM, Fremond L, Burdet F, Barbe JM, Gros CP, Guilard R (2006) Cobalt(IV) corroles as catalysts for the electroreduction of 02 reactions of heterobimetallic dyads containing a face-to-face linked Mn(III) or Fe(III) porphyrin. J Inorg Biochem 100 858-868... 

Luobeznova I, Raizman M, Goldberg I, Gross Z (2006) Synthesis and full characterization of molybdenum and antimony corroles and utilization of the latter complexes as very efficient catalysts for highly selective aerobic oxygenation reactions. Inorg Chem 45 386-394... 

Simkhovich L, Mahammed A, Goldberg I, Gross Z (2001) Synthesis and characterization of germanium, tin, phosphorous, iron and rhodium complexes of tris(pentafluorophenyl)corrole, and the utilization of the iron and rhodium corroles as cyclopropanation catalysts. Chem Eur J 7 1041-1055... 

Epoxidation of alkenes with iodosylbenzene can be effectively catalyzed by the analogous salen or chiral Schiff base complexes of manganese(in), ruthenium(II), or ruthenium(III). For example, the oxidation of indene with iodosylbenzene in the presence of (/ ,5)-Mn-salen complexes as catalysts affords the respective (15,2/ )-epoxyindane in good yield with 91-96% ee [704]. Additional examples include epoxidation of alkenes with iodosylbenzene catalyzed by various metalloporphyrins [705-709], corrole metal complexes, ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazoUnyl)pyridine [710,711]. 

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