Manganese complexes activation

Ordinary alkenes (without an allylic OH group) have been enantioselectively epoxidized with sodium hypochlorite (commercial bleach) and an optically active manganese-complex catalyst. Variations of this oxidation use a manganese-salen complex with various oxidizing agents, in what is called the Jacobsen-Katsuki... 

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. 

Chiral manganese complexes have been used to perform the enantioselective amidation of saturated C-H bonds.256-258,262 Cationic Mn(salen) 107 showed good catalytic activity and moderate enantioselectivity. Typical examples are shown in Equations (86)-(88). High enantioselectivity of 89% ee was obtained in the reaction of 1,1-dimethylindan (Equation (88)).258 Chiral manganese(m) porphyrin 106 was used in the enantioselective amidation as well nevertheless, the best enantioselectivity was only 54% (Equation (89)).256,257... 

In photosynthesis, water oxidation is accomplished by photosystem II (PSII), which is a large membrane-bound protein complex (158-161). To the central core proteins D1 and D2 are attached different cofactors, including a redox-active tyro-syl residue, tyrosine Z (Yz) (158-162), which is associated with a tetranuclear manganese complex (163). These components constitute the water oxidizing complex (WOC), the site in which the oxidation of water to molecular oxygen occurs (159, 160, 164). The organization is schematically shown in Fig. 18. 

Nitroarenes are reduced to anilines (>85%) under the influence of metal carbonyl complexes. In a two-phase system, the complex hydridoiron complex [HFe,(CO)u]2-is produced from tri-iron dodecacarbonyl at the interface between the organic phase and the basic aqueous phase [7], The generation of the active hydridoiron complex is catalysed by a range of quaternary ammonium salts and an analogous hydrido-manganese complex is obtained from dimanganese decacarbonyl under similar conditions [8], Virtually no reduction occurs in the absence of the quaternary ammonium salt, and the reduction is also suppressed by the presence of carbon monoxide [9], In contrast, dicobalt octacarbonyl reacts with quaternary ammonium fluorides to form complexes which do not reduce nitroarenes. 

Figure 38 Structural models of the manganese complex which constitutes the active site responsible for the water oxidation in WOC...

The synthesis of the Y zeolite-encapsulated manganese complex of the salen ligand has been reported recently [51]. It was found to have catalytic activity in the oxidation of cyclohexene, styrene, and stilbene with PhlO. Typically, 1 Mn(salen) is present per 15 supercages, resulting in catalytic turn-overs in the order of 60. The reactions investigated with the respective product yields are given in Scheme 5. Typical oxidation products are epoxides, alcohols and aldehydes. In comparison to the homogeneous case encapsulation seems to lower the reaction rate. From cyclohexene the expected oxidation product cyclohexene oxide is present in excess and is formed on the Mn(salen) site. 2-cyclohexene-l-ol is probably formed on residual Mn cations via a radical mechanism. 

Weiss, R. Gold, A. Trautwein, A. X. Terner, J. High-valent iron and manganese complexes of porphyrins and related macrocycles, The Porphyrin Handbook. Volume 4. Biochemistry and Binding Activation of Small Molecules , Eds. Kadish, . M. Smith, . M. Guilard, R. Academic Press San Diego, 2000, pp. 65-96. 

Although these cobalt species do not in themselves activate dioxygen, they do exhibit similarities to the proposed structures of manganese complexes that may be involved in photosynthetic water oxidation... 

The mechanism of the epoxidation of alkenes by the cytochrome P450 model, sodium hypochlorite-manganese(III) tetraarylporphyrins, involves rate-determining formation of an active species 234 from a hypochlorite-manganese complex 233 (Scheme 6) pyridine or imidazole derivatives, as axial ligands, accelerate this step by electron donation, although the imidazoles are destroyed under the reaction conditions368. 

Sumanont Y, Murakami Y, Tohda M, Vajragupta O, Matsumoto K, Watanabe H. 2004. Evaluation of the nitric oxide radical scavenging activity of manganese complexes of curcumin and its derivative. Biol Pharm Bull 27 170-173. 

Vajragupta O, Boonchoong P, Watanabe H, Tohda M, Kummasud N, Sumanont Y. 2003. Manganese complexes of curcumin and its derivatives Evaluation for the radical scavenging ability and neuroprotective activity. Free Radic Biol Med 35 1632-1644. 

Many manganese complexes decompose dihydrogen peroxide, but we limit our discussion to the functional dinuclear ones the catalase activity of bis-dinuclear (tetranuclear) photosystem II (PSII) models is discussed later. Furthermore, mainly model complexes reported in the last 5 years are discussed in detail since previous work is covered in several excellent reviews [1,2b,3a,5,8-12],... 

The manganese complex, Ph3SiMn(CO)5 has been used to promote hydrosilylation of olefins both thermally and photochemically. With HD4 and pentene, and 0.1 mol% of Ph3SiMn(CO)5, thermal activation gave a 20% yield of D4-D4 but none from photochemical activation.109 A few complexes of iron, Fe(CO)5,110a Fe(CO)2 P(OPh)3]2H(SiMe2Ph),110b and Fe(dppe)2(CH2=CH2)ul have been reported to couple tertiary silanes but few details are available. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

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