Group manganese complexes

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. 

The coordination chemistry of (phenoxyl)manganese complexes is rather more complicated because both metal- and ligand-centered electron-transfer processes are accessible in the normal potential range. The phenolato precursors are known to exist with manganesc(II), (III), and even (IV). In fact, three phenolato groups strongly stabilize the Mn(IV) oxidation state. 

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

Analogously, reaction of group 7 metal carbonyl anions M(CO)5 (M = Mn, Re) with MeC(CFFI), gave cyclopropylcarbinyl complexes. Flowever, whereas rhenium produces the normal (/-metal bonded cyclopropylcarbinyl complex, the manganese complex furnished (cyclopropylacetyl)Mn(CO)5, where carbonyl insertion into the manganese-alkyl bond occurred (equation 56)114. 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

In other examples, compounds in which a metal atom is already coordinated in a molecule can be used as a comonomer in an addition polymerization. Two examples involve the ferrocenes discussed in Chapter 6. The vinyl ferrocene molecule is shown in 7.14, and a similar vinyl manganese complex in 7.15.30 An alternative approach involves condensation polymerization. For example, if the R group in the ferrocene unit shown in 7.16 contains a hydroxyl group, it can be copolymerized with a diacid chloride. If it is an acid chloride, it can be copolymerized with a diamine. (This type of polymer is called a heteroannular chain if only one of the rings in the repeat unit is in the backbone, the polymer is called homoannular.)7 Similarly, the titanium complex shown in 7.17 is copolymerized with diacids or diols.30 Numerous other examples involving ferrocenes are discussed in Chapter 6. 

Group 7 complexes, surface chemistry on oxides manganese, 12, 531 rhenium, 12, 531... 

Group 10 complexes, 12, 537 Group 11 complexes, 12, 537 imprinted complexes on, 12, 810 manganese complexes, 12, 531 molybdenum complexes, 12, 527... 

Hexanuclear manganese complexes of polyhedral silsesquioxanes, supported on Si02, C, or A1203, evolve 02 from H20 when oxidized by Ru(bpy)33+ [166], The Mn ions (Figure 34) form a sandwiched ring structure and are octahe-drally coordinated by four equatorial x2-siloxy groups, one axial i6-chloridc ion, and one O from H20 or ROH. 

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