Metalloporphyrins, catalysts for

Although impressive progress has been made in unraveling the mechanism of ORR catalysis by cofacial porphyrins, much remains to be learned before we can understand how this mechanism relates to those in heme enzymes and simple metalloporphyrins and use our mechanistic knowledge to rationally design improved metalloporphyrin catalysts for the ORR. 

E. Brule, Y. R. de Miguel, Supported metalloporphyrin catalysts for aUcene epoxidation, Org. Biontol. Ghent. 4 (2006) 599. 

Supported metalloporphyrin catalysts for alkene epoxidation 06OBC599. 

Brule E, De Miguel YR (2006) Supported metalloporphyrin catalysts for alkene epoxidation. Org Biomol Chem 4 599-609... 

The above-described structures are the main representatives of the family of nitrogen ligands, which cover a wide spectrum of activity and efficiency for catalytic C - C bond formations. To a lesser extent, amines or imines, associated with copper salts, and metalloporphyrins led to good catalysts for cyclo-propanation. Interestingly, sulfinylimine ligands, with the chirality provided solely by the sulfoxide moieties, have been also used as copper-chelates for the asymmetric Diels-Alder reaction. Amide derivatives (or pyridylamides) also proved their efficiency for the Tsuji-Trost reaction. 

In addition to their proven capacity to catalyze a highly efficient and rapid reduction of O2 under ambient conditions (e.g., cytochrome c oxidase, the enzyme that catalyzes the reduction of >90% of O2 consumed by a mammal, captures >80% of the free energy of ORR at a turnover frequency of >50 O2 molecules per second per site), metalloporphyrins are attractive candidates for Pt-free cathodes. Probably the major impetus for a search for Pt-free cathodic catalysts for low temperature fuel cells is... 

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. 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

Cobalt porphyrin derivatives were also reported129 to be active for electrochemical reduction of C02 to formic acid at an amalgamated Pt electrode. More recently, Becker et al have reported130 that Ag2+ and Pd2+ metalloporphyrins acted as homogeneous catalysts for C02 reduction in dry CH2C12 oxalic acid and H2 (its source was not clear) were produced, but no CO was detected. 

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

A new trend in the field of oxidations catalyzed by metalloporphyrin complexes is the use of these biomimetic catalysts on various supports ion-exchange resins, silica, alumina, zeolites or clays. Efficient supported metalloporphyrin catalysts have been developed for the oxidation of peroxidase-substrates, the epoxidation of olefins or the hydroxylation of alkanes. 

A chiral Ru porphyrin complex based, in part, on the Kodadek design for Rh has recently been reported to give good-to-high (80-91% ee) enantiocontrol for the cyclopropanation of styrenes and 1,1 -disubstituted alkenes [83]. The advantage here is the high trans. cis ratio, which seems to be characteristic of Ru, and, like other metalloporphyrin catalysts, turnover numbers are large. 

Metallosalen complex [salen = N, A-ethylenebis(salicyldeneaminato)] has a structure similar to metalloporphyrin, and these two complexes catalyze the epoxidation of olefins. For example, Kochi et al. have found that metallosalen complexes such as (salen )manganese(III) [25] and (salen)chromium(IIl) complexes [26] (hereafter referred to as Mn- and Cr-salen complexes, respectively) serve as catalysts for the epoxidation of unfunctionalized olefins by using iodosylbenzene [25] or sodium hypochlorite [27], In particular, cationic Mn-salen complex is a good catalyst for epoxidation of unfunctionalized olefins, which proceeds through an oxo(salen)manganese(V) species (Scheme 6B.14) [25,28], The presence of oxo-Mn(V)-salen... 

Metalloporphyrin complexes serve as catalysts for aziridination in the presence of PhI=NTs [73], Che et al. have reported the chiral version of metalloporphyrin-catalyzed aziridination (Scheme 6B.36) [81], The reaction of styrene derivatives with a D4-manganese(III) porphyrin complex 34 proceeds with fairly good enantioselectivity, up to 68% ee. This reaction is proposed to proceed through a Mn(IV)-PhINTs adduct 35 on the basis of EPR analysis. 

Messer A, Carpenter K, Forzley K, Buchanan J, Yang S, Razskazovskii Y, Cai Z, Sevilla MD (2000) Electron spin resonance study of electron transfer rates in DNA determination of the tunneling constant (1 for single-step excess electron transfer. J Phys Chem B 104 1128-1136 Meunier B (1992) Metalloporphyrins as versatile catalysts for oxidation reactions and oxidative DNA cleavage. Chem Rev 92 1411-1456... 

Fascinated by the oxygen turnover processes catalyzed by heme proteins, namely the peroxidases, cytochromes P-450, and catalase, thousands of researchers appear be looking for active and robust metalloporphyrin catalysts. 

Certain dinuclear iron complexes are found to be efficient catalysts for the oxidation of primary and secondary alcohols with hydrogen peroxide.214 Metalloporphyrins are used as peroxidase mimics in the oxidation of phenol with hydrogen peroxide. 

Catalysts for epoxide polymerisation of quite different characteristics comprise metalloporphyrins of aluminium and zinc, such as (5,10,15,20-tetraphe-nylporphinato)aluminium chloride [(tpp)AlCl], methoxide [(Mtpp)AlOMe] or 1-propanethiolate [(tpp)AlSPr] and (5,10,15,20-tetraphenyl-21-methylporphi-nato)zinc methoxide [(Mtpp)ZnOMe] [32 35] ... 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

As in the case of horseradish peroxidase, several synthetic metalloporphyrins in the presence of H2O2 have been found to be potent catalysts for the chemiluminescent oxidation of luminol or isoluminol. The microperoxidases, mainly MP8 and MPll, have been shown to act as functional peroxidase enzyme models. " However, they are readily inactivated within one min of catalytic turnover, and incorporation into a molecular sieve... 

Whereas important progress has been made regarding the use of metalloporphyrins as catalysts for alkene epoxidations and alkane hydroxyla-tions, work concerning the mechanism of hydroxylation of aromatic hydrocarbons has received only limited attention. In fact, the main problem encountered with the design of systems capable of performing such oxidative reactions is in the preparation of superstructured porphyrins for the selective complexation of aromatic compounds. 

B. Meunier, Metalloporphyrins as versatile catalysts for oxidation reactions and oxidative DNA cleavage, Chem. Rev. 92 (1992) 1411. 

The kinetics of product evolution in a typical reaction of adamantane hydroxylation showed an initial induction period followed by a fast, apparently zero-order phase with the maximum rate and highest efficiencies (Fig. 2). Deviation from linear behavior took place only after 90% oxygen donor and 80% of the substrate had been consumed. When Ru (TPFPP)(0)2, prepared by reaction of Ru"(TPFPP)(CO) with 3-chloroperbenzoic acid was used as the catalyst, no induction time was detected and zero-order kinetics were observed as well. The well defined and characteristic UV-vls spectra of metalloporphyrins provide an invaluable tool for the mechanistic studies. Thus, monitoring the state of the metalloporphyrin catalysts during the course of both model reactions by UV-vis spectroscopy revealed that the initial form of the catalyst remained the predominant one throughout the oxidation, i.e. in the Ru°(TPFPP)(CO) catalyzed reaction c.a. 80% of the porphyrin catalyst existed as Ru"(TPFPP)(CO) and in Ru (TPFPP)(0)2 catalyzed reaction more than 90% of... 

Meunier, B., Metalloporphyrins as Versatile Catalysts for Oxidation Reactions and Oxidative DNA Cleavage, Chem. Rev. 1992, 92, 1411 1456. 

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