Activated manganese metal

Benzylic organomanganese reagents prepared by direct insertion of activated manganese metal display the same behavior (Scheme 2.60) [131]. Excellent results are also obtained for 1,4-additions of organomanganese reagents to unsaturated esters in the presence of CuCI (3 mol%) [127]. 

The inhibition of lipid peroxidation by metalloporphyrins apparently depends on metal ions because only compounds with transition metals were efficient inhibitors. Therefore, the most probable mechanism of inhibitory effects of metalloporphyrins should be their disuniting activity. Manganese metalloporphyrins seem to be more effective inhibitors than Trolox (/5o = 204 pmol I 1) and rutin (/50 112 pmol I 1), and practically equal to SOD (/50= 15 pmol I 1). The mechanism of inhibitory activity of manganese and zinc metalloporphyrins might be compared with that of copper- and iron-flavonoid complexes [167,168], which exhibited enhanced antiradical properties due to additional superoxide-dismuting activity. 

A similar hybrid type of radical/anionic reactions can be effected, when manganese metal, activated by catalytic amounts of lead dichloride and trimethylchlorosi-lane, is employed instead of zinc, which makes the original process synthetically more reliable and attractive by reducing the amounts of reagents (RX and ketone) needed to a 1.5 molar excess over the alkenes (Scheme 6.36) [57]. 

The bifunctional cyclopropane 156 was also prepared by modified Simmons-Smith cyclopropanation 85) of 2-trimethylsilyl-2-propen-l-ol 157 84) followed by oxidation of the cyclopropylcarbinol 158 with activated manganese dioxide 88 >, in 72% overall yield, Eq. (50) 86,89). Coupling of the aldehyde 156 with 2,6-dimethylcyclohexenone 159 90) induced by the low valent titanium reagent from TiCl3 and zinc-copper couple (or lithium metal) provided the silylated cyclopropyldiene 160, in 50-60% yield, Eq. (51) 89 91>. 

There have in addition been numerous studies of Mn-substituted proteins. In these, Mn is substituted for a metal found naturally in the protein (most often Mg, but also Zn Ca , or Fe ). This substitution is generally used to permit spectroscopic characterization of the native metal site, using Mn as the reporter. Often (although not always) the Mn-substituted protein is active. Manganese-substituted proteins are discussed briefly where they illustrate important properties of Mn protein interactions. 

Up to now, only a few catalyst systems based on organic polymers such as molybdenum compounds supported on benzimidazole, polystyrene, or poly(gly-cidyl methacrylate) [9] as well as micelle-incorporated manganese-porphyrin catalysts [66] have been tested in the epoxidation of propene. Molybdenum-doped epoxy resins were also employed in the epoxidation of propene with TBHP and propene oxide yields of up to 88% were obtained [65]. The catalysts were employed repeatedly in up to 10 reactions without significant loss of activity and metal leaching proved to be very low. 

The search is on for catalysts to replace those containing toxic heavy metals. The addition of hydrogen chloride to acetylene to form vinyl chloride is catalyzed by mercuric chloride. Rhodium (III) chloride on activated carbon works just as well and is much less toxic 97 It should be tried also in other addition reactions of acetylene as well as in trans-esteriflcation reactions of vinyl acetate. The reduction of 2 ethyl-2-hexenal to 2-ethylhexanol can be catalyzed by a mixture of copper, zinc, manganese, and aluminum oxides in 100% yield.98 This is said to be a replacement for carcinogenic copper chromite. In Reaction 4.15, the amount of toxic chromium(II) chloride has been reduced from stoichiometric to catalytic (9-15 mol% chromium(II) chloride) by the addition of manganese metal.99... 

Marshall, 1977). Fortunately, the active substances themselves are not persistent on the plant. Finally, it is important to keep the ETU content of the preparations to a minimum. Of the metal complexes, the coordination complex of zinc and manganese, mancozeb, is the most stable therefore, the preparations of this active substance contain the smallest amount of ETU (Blazquez, 1973). The potentiation of the activity of metal dithiocarbamates may allow the levels of ETU to be reduced by permitting a reduction in the amount of dithiocarbamate needed to control a fungal pathogen. Clifford and Bruyns-Haylett (1978) reported that zineb 1 1 complex with 2-(2-aminoethylamino)ethanol was shown to be more active than zineb in greenhouse and field trials against Plasmopara viticola. 

Finally, with the acknowledged similarities that currently exist between several manganese enzymes and counterparts utilizing other metal ions, it seems quite likely that other redox-active manganese enzymes will be discovered in the future. The discussions of data gathering on enzyme systems and model chemistry, both structural and functional, as presented in this article will provide a foundation for exploring current and as yet undiscovered manganese enzymes. 

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