Alkenes, manganese -based

All of the examples described above involve stoichiometric imido transfer to the alkene substrate. A crucial advance in the area of catalytic oxidation was made by Mansuy, who disclosed the first example of catalytic aziridination of alkenes in 1984 [10]. Both iron- and manganese-based porphyrin complexes were found... 

Many routes for the chlorination of alkenes exist, and one new entry in recent years has been manganese-based chlorinating reagents. Many of these systems have been prepared by Bellesia and co-workers 410-414). These systems in general produce frans-chlorinated alkenes. In only one system has the proposed halogenating complex been structurally characterized (245). 

As previously noted, optically active trans-epoxides are not easily available through the (salen)Mn-catalyzed epoxidation of rrans-olefins. However, a modification in the conditions for cis-alkene epoxidation can provide access to trans-epoxides [94JA6937]. Addition of an cinchona alkaloid derivative such as 18 promotes a remarkable crossover in diastereoselectivity, such that the trans-epoxide 17 can be prepared in 90% de from cis-B-methylstyrene (16). It is not yet clear whether these chiral quaternary ammonium salts fundamentally change the nature of the manganese-based oxidant, or rather somehow prolong the lifetime of the radical intermediate, allowing rotation before collapse. 

Manganese Complexes for Alkene Oxidation Based on Pyridyl Ligands... 

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. 

Nishino H (2006) Manganese(III)-Based Peroxidation of Alkenes to Heterocycles. 6 39-76 Nishiwaki N, Ariga M (2007) Ring Transformation of Nitropyrimidinone Leading to Versatile Azaheterocyclic Compounds. 8 43-72... 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

A new stereoselective epoxidation catalyst based on a novel chiral sulfonato-salen manganese(III) complex intercalated in Zn/Al LDH was used successfully by Bhattacharjee et al. [125]. The catalyst gave high conversion, selectivity, and enantiomeric excess in the oxidation of (i )-limonene using elevated pressures of molecular oxygen. Details of the catalytic activities with other alkenes using both molecular oxygen and other oxidants have also been reported [126]. 

Generation of the carbon based radical in these processes involves the prior formation of a complex between manganese(lll) and the enol of the carbonyl reactant. Intramolecular electron transfer occurs within this complex. Addition to the olefin then takes place within the co-ordination sphere of manganese. When manganese is present in catalytic amount, the relative values of the equlibrium constants between manganese and both the carbonyl compound and the alkene arc important. If the olefm is more strongly complexed then no radical can form and reaction ceases. Reactions are usually carried out at constant current and the current used must correspond to less than the maximum possible rate for the overall chemical steps involved. Excess current caused the anode potential to rise into a region where Kolbe reaction of acetate can occur and this leads to side reactions [28]. 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

In the case of manganese porphyrin catalyzed epoxidations, the axial ligands have been used alone or together with other additives like carboxylic acids (Banfi and coworkers) and soluble bases (Johnstone and coworkers). For example, Mansny and coworkers showed that in the presence of imidazole, 2-methylimidazole or 4-imidazole chloromanganese(tetra-2,6-dichlorophenylporphyrin) catalyzes the epoxidation of varions aUtenes including 1-alkenes by Under these conditions alkene conversion... 

Manganese(III)-Based Peroxidation of Alkenes to Heterocycles H. Nishino... 

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. 

Figure 9.8 Catalytic cycle for the epoxidation of unfunctionalized alkenes with a chiral Schiff base complex of manganese as the catalyst.

A number of workers have shown that a small amount of nitrogenous base with manganese metalloporphyrins greatly increases the stereospecificity of alkene epoxidation 49-51 Figure 2.18 illustrates the dual behaviour of imidazole during the epoxidation of alkenes by hydrogen peroxide.52... 

Whilst the chiral manganese complexes can epoxidize alkenes with high enantioselectivity (> 90% e.e.), they are not particularly stable. This instability is probably due to the easily oxidizable imine and phenoxide ligands on the complex. Attempts are currently being made to immobilize Schiff-bases in order to increase their stability in a similar manner to the metalloporphyrins discussed earlier. 

Recently it has been shown that simple manganese sulfate in the presence of sodium bicarbonate is reasonably effective in promoting the epoxidation of alkenes with aqueous H202 using DMF or t-BuOH as solvents [69]. In this system peroxo-carbonate is formed in situ, thus minimizing the catalase activity of the Mn salt. Following this discovery, Chan and coworkers introduced an imidazole-based ionic... 

The catalytic asymmetric epoxidation of alkenes offers a powerful strategy for the synthesis of enantiomerically enriched epoxides and enantioselective oxidation reactions in ionic liquids have been summarised previously.[39] Complexes based on chiral salen ligands - usually with manganese(III) as the coordinated metal - often afford excellent yields and enantioselectivities and the catalytic cycle for the reaction is depicted in Scheme 5.5 J40 ... 

The relatively simple O.N, Schiff base salpn2- complex, dichloro[7V,7V (salicyli-dene)-l,3- propanediamino]manganese(IV), MnIV(salpn)Cl2, made by action of HC1 on Mnm(salpn)Cl in MeCN, is unusual in that it can halogenate alkenes.36... 

Manganese compounds of the type (22-XXXVIII) catalyze the epoxidation of alkenes with Phl=0, OCT, and similar oxidants. There is a debate about the mechanism, though a metallaoxirane (22-XXXIX) intermediate is likely with chiral Schiff base ligands the reaction is enantioselective.164... 

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