Samarium oxidation-reduction

Calciothermic reduction of samarium oxide, in the presence of cobalt powder, yields samarium-cobalt alloys in the powder form. The process is popularly known as reduction diffusion. Samarium oxide, mixed with cobalt powder and calcium hydride powder or calcium particles, is heated at 1200 °C under 1 atm hydrogen pressure to produce the alloys. Cobalt oxide sometimes partly replaces the cobalt metal in the charge for alloy preparation. This presents no difficulty because calcium can easily reduce cobalt oxide. A pelletized mixture of oxides of samarium and cobalt, cobalt and calcium, with the components taken in stoichiometric quantities, is heated at 1100-1200 °C in vacuum for 2 to 3 h. This process is called coreduction. In reduction diffusion as well as in coreduction, the metals samarium and/or cobalt form by reduction rather quickly but they need time to form the alloy by diffusion, which warrants holding the charge at the reaction temperature for 4 to 5 h. The yield of alloy in these processes ranges from 97 to 99%. Reduction diffusion is the method by which most of the 500 to 600 t of the magnetic samarium-cobalt alloy (SmCOs) are produced every year. 

Figure 4.11. Examples of redox-initiated radical reactions. Samarium diiodide reduction of the bromide gives a radical that cyclizes faster than the second reduction reaction. Manganese triacetate oxidation of the P-keto ester gives an enol radical that is not further oxidized by the manganese reagent. The IBX oxidizes anilides to the corresponding radicals. Hexamethylphosphoramide = HMPA and Tetrahydrofuran = THE.

Nano-grained Ni/ZrOj and Ni/ZrOj-Sm Oj catalysts were prepared from amorphous Ni-Zr and Ni-Zr-Sm alloys by oxidation-reduction treatment. Their catalytic activity for methanation of carbon dioxide was examined as a function of precursor alloy composition and temperature. The addition of samarium is effective in enhancing the activity of the nickel-rich catalysts, but not effective for the zirconium-rich catalysts. The surface area and hydrogen uptake of the nickel-rich catalysts are increased by the samarium addition. In addition, tetragonal zirconia, the formation of which is beneficial to the catalytic activity, is stabilized and formed predominantly by the addition of samarium to the nickel-rich catalysts, although monoclinic zirconia is also formed in the zirconium-rich catalysts. As a consequence, the higher conversion of carbon dioxide is obtained on the Ni-Zr-Sm catalysts with relatively high nickel contents. 

The synthesis of sphingosine from 2,4- -benzylidene-E>-threose was hampered by ftilure to achieve ara /no-selectivity in the addition of tetradecosylmagnesium bromide as well as in the oxidation/reduction of the resulting mixed alcohols by all methods tried except the samarium iodide-assisted Tishchenko reaction. ... 

The reduction diffusion process has also been used for the production of powders of the magnetic neodymium-iron-boron alloy (Nd15Fe77B8). The reaction involves use of a powder mix of neodymium oxide, iron, ferroboron and calcium. The reaction is conducted by heating the powder charge mixture at 1200 °C for 4 h under vacuum. Neodymium-iron-boron alloys are much more prone to oxidation than samarium-cobalt alloys and a proprietary leaching procedure is used for the separation of the alloy and calcium oxide. 

Low-valent lanthanides represented by Sm(II) compounds induce one-electron reduction. Recycling of the Sm(II) species is first performed by electrochemical reduction of the Sm(III) species [32], In one-component cell electrolysis, the use of sacrificial anodes of Mg or A1 allows the samarium-catalyzed pinacol coupling. Samarium alkoxides are involved in the transmet-allation reaction of Sm(III)/Mg(II), liberating the Sm(III) species followed by further electrochemical reduction to re-enter the catalytic cycle. The Mg(II) ion is formed in situ by anodic oxidation. SmCl3 can be used in DMF or NMP as a catalyst precursor without the preparation of air- and water-sensitive Sm(II) derivatives such as Sml2 or Cp2Sm. 

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

Metallic samarium is obtained by heating the oxide, Sm203 with lanthanum turnings or cerium in slight excess amounts in a tantalum crucible under high vacuum. The metal is recovered by condensation of its vapors at 300 to 400°C. The metal cannot be obtained by reduction of its halides, SmFs or SmCls, or by heating with calcium or barium. In such reduction, trihalides are reduced to dihalides, but not to the metal. 

Barbier reaction Samarium(II) iodide, 270 Benzoannelation Chromium carbene complexes, 82 Dicarbonylcyclopentadienylcobalt, 96 Ethyl (Z)-3-bromoacryIate, 130 Grignard reagents, 138 Methyl acrylate, 183 Methyllithium, 188 Ruthenium(III) chloride, 268 Benzoin condensation Benzyltriethylammonium chloride, 239 3-EthyIbenzothiazolium bromide, 130 Benzoylation (see also Acylation) Cadmium, 60 Dibutyltin oxide, 95 Birch reduction Birch reduction, 32... 

Chiral induction was also observed in lanthanide(III)-alkoxide-mediated MPV reductions. The optically active ligand (Fig. 35C) was used in enantioselec-tive samarium-catalyzed MPV reductions of arylmethyl ketones (Scheme 30) [253], The resulting mixed alkoxide-iodide complex shows higher reactivity than (rBuO)SmI2. It was pointed out that the tridentate, secondary alkoxide ligand is not oxidized under the reaction conditions and that tridendate ligands... 

Active catalyst species or catalysis intermediates can often be trapped by stoichiometric reactions of the precatalyst with the substrate. The following example describes the successful isolation of such an intermediate with participation of Ln-O cr-bonds. Reduction processes mediated by low oxidation states of the lanthanide elements are of special interest in organic synthesis [256]. One of the most intensively studied reactions is the stoichiometric reduction of arylketones by rare earth metals ytterbium and samarium [277]. Thus formed dianions possess high nucleophilic character and excess lanthanide metal can even accomplish complete cleavage of the C-O double bond (Scheme 36). 

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