Biomimetic manganese catalysts

The most commonly used oxidants in the asymmetric epoxidation reactions are iodosylbenzene and sodium hypochlorite. Dihydrogen peroxide has been used in only a limited number of studies with chiral Mn-salen complexes and all turnover numbers reported are lower than those obtained with the porphyrin complexes 122). 

In the olefin epoxidation, the mechanistic scheme commonly proposed for the oxygen-transfer reaction consists of a two-step catalytic cycle (Pig. 21) 123). In the first step, an oxygen atom is transferred from the primary oxidant to the Mn -salen catalyst, which in the second step carries the activated oxygen to the olefinic double bond. The main problem in Mn-salen catalyzed epoxidation with H2O2 was the formation of HO radicals by the homolytic cleavage of the weak 0-0 bond, leading to indiscriminate oxidation 124). Addition of Lewis bases, such as imidazole, pyridine, or 

An alternative approach for the epoxidation of simple olefins has been developed by Pietikainen and co-workers and Katsuki and co-workers by using the Jacobsen-type catalysts (Fig. 23) and in situ addition of imidazole or N-methylimidazole (122,129). The highest cc-values obtained were 60 % for 

2-dihydronaphthalene oxide and 47% for trans- 3-methylstyrene oxide in the presence of N-methylimidazole (129). Under similar conditions, 6-aceta-mido-2,2-dimethyl-7-nitrochromene was converted into the corresponding epoxide of 95 % ee in 98 % yield based on substrate (122). 

With in situ generated peroxycarboxylic acids from urea-H202, carboxylic acid anhydrides and N-methylmorpholine N-oxide as an additive, chiral derivatives of Mn-salen complex I were found to oxidize effectively dihydronaphthalene and indene (126). The combination employing maleic anhydride has led to good results, up to 87 % ee and 71 % yield. Generally, lowering the temperature from 2 to —18°C had marginal positive effect on the yield and enantiomeric excess of the epoxides. 

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The present review has outlined the efforts to develop biomimetic non-heme iron and manganese catalysts for alkane hydroxylation, olefin epoxidation, and cis-dihydroxylation reactions. However, the examples reviewed here are mostly presented as reported in the literature, since the various reaction conditions involved in the catalytic oxidations hamper a direct comparison and critical evaluation of the data. The survey has not only illustrated a rich variety of iron and manganese complexes that lead to the successful structural modeling of important non-heme iron and manganese enzymes, but also significant features of the oxidation reactions catalyzed by these complexes in combination with dihydrogen peroxide. 

Consequently, in the present study, the proposed formalism is demonstrated for the rate-limiting chemical step in the OER, that is, 2[Mn=O Mn—0 ] Mn—O—O—Mn. Above, the force constants of the dioxo and peroxo species comprise fcreact and fcprod. respectively. By solving for Xjs and adding the equilibrium 0-0 bond lengths of the peroxo system, the interatomic 0-0 distance at the transition state is obtained. In what follows, the reactivity of a biomimetic binuclear molecular manganese catalyst will be contrasted by that of the binuclear Mn site supported on a magnesium oxyhydroxide rig. 

A simplihed model has been developed to investigate the simplest complete OER reaction cycle. It is inspired by the biomimetic manganese dimers used for studying the water oxidation reaction in biological systems. The interest in these systems is twofold since they offer candidates for possible future water oxidation catalysts [61]. 

To summarize, a handy procedme to estimate barrier heights for reactions involving avoided crossings has been described and evaluated for the dioxo to 0,-peroxo step on a biomimetic binuclear manganese catalyst and a similar system embedded into a MgOxfOH) rig has been discussed. Its applicability is determined by the ability of DPT to describe well-defined intermediates along a preselected reaction path. 

Selective Conversion of Hydrocarbons with H202 Using Biomimetic Non-heme Iron and Manganese Oxidation Catalysts... 

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. 

Najafpour MM, Ehrenberg T, Wiechen M, Kurz P. Calcium manganese(III) oxides (CaMn204 x H20) as biomimetic oxygen-evolving catalysts. Angew Chem Int Ed. 2010 49(12) 2233-7. 

In pursuit of biomimetic catalysts, metaUoporphyrins have been extensively studied in attempts to mimic the active site of cytochrome P450, which is an enzyme that catalyzes oxidation reactions in organisms. In recent decades, catalysis of alkene epoxidation with metaUoporphyrins has received considerable attention. It has been found that iron [1-3], manganese [4,5], chromium [6], and cobalt porphyrins can be used as model compounds for the active site of cytochrome P450, and oxidants such as iodosylbenzene, sodium hypochlorite [7,8], hydrogen peroxide [9], and peracetic acid [10] have been shown to work for these systems at ambient temperature and pressure. While researchers have learned a great deal about these catalysts, several practical issues limit their applicability, especially deactivation. 

Table II shows the results with C2, C3, and cycloCfi and manganese clusters 1-4 (Figure 1) with t-butyl hydroperoxide at room temperature in acetonitrile (methane did not react under the reaction conditions). The important observation of no catalyst decomposition upon continual addition of t-butyl hydroperoxide to again provide the initial turnover number is an extremely important characteristic of any biomimetic catalyst. It is interesting to note that this increase in catalyst lifetimes occurred in acetonitrile and not methylene chloride and shows the dramatic effect of a coordinating solvent We did not see any indication of acetonitrile activation.

SELECTIVE CONVERSION OF HYDROCARBONS WITH H2O2 USING BIOMIMETIC NON-HEME IRON AND MANGANESE OXIDATION CATALYSTS... 

Moghadam M, Mohammadpoor-Baltork 1, Tangestaninejad S, Mirkhani V, Kargar H, Zeini-Isfahani N. Manganese(lll) porphyrin supported on multi-wall carbon nanotubes a highly efficient and reusable biomimetic catalyst for epoxidation of alkenes with sodium periodate. Polyhedron 2009 28 3816-22. 

MANGANESE BIOMIMETIC CATALYSTS 2.1 The Crabtree-Brudvig Catalyst... 

Although these alternatives can be applied in specific systems, their overall performance does not match up to that of cobalt based driers the paint films usually remain too soft, whereas manganese also has a negative impact on the film colour. At A F a biomimetic approach is being followed to develop alternative cobalt free drying catalysts. 

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