Samarium halides

Divalent metals, ferrites of, 24 543 Divalent samarium halides, 24 649 Divalent zinc-silver oxide batteries, 3 454-455 Divergence, 24 656... 

Other catalytic uses of rare-earth compounds have not reached the same development. Neodymium salts are, however, used for mbber manufacturing (22). Divalent samarium halides are employed in organic synthesis (23). 

The thermal decomposition of trihalides is a good way to prepare divalent ytterbium and samarium halides (especially iodides). For example, Baernighausen (1961) has prepared excellent samples of Sm or Yb dihalide by the thermal decomposition of trihalide hydrates. Only a few preparations involve this type of reaction in the field of divalent organometallic compounds. 

Allylation of acyloyl-imidazoles and pyrazoles61 with allyl halide mediated by indium in aqueous media provides a facile regioselective synthesis of P, y-unsaturated ketones (Scheme 11.1), which has been applied to the synthesis of the monoterpene artemesia ketone. The same product can be obtained by indium-mediated allylation of acyl cyanide (Eq. 11.35).62 Samarium, gallium, and bismuth can be used as a mediator for the allylation of nitrones and hydrazones to give homoallylic hydroxylamine and hydrazides in aqueous media in the presence of Bu4NBr (Scheme 11.2).63 The reaction with gallium and bismuth can be increased dramatically under microwave activation. 

Scheme 168 Samarium-mediated Barbier-type reaction of carbonyl compounds with allyl halides.

Cerium, samarium, and other lanthanide halides promote addition of ketene silyl enol ethers to aldehydes.54 Imines react with ketene silyl acetals in the presence of Yb(03SCF3)3. Preferential addition to the imine occurs even in the presence of aldehyde... 

Samarium ore usually is digested with concentrated sulfuric or hydrochloric acid. The extraction process is similar to other lanthanide elements. Recovery of the metal generally consists of three basic steps. These are (1) opening the ore, (2) separation of rare earths first to various fractions and finally to their individual compounds, usually oxides or halides, and (3) reduc-... 

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. 

The divalent compounds of samarium primarily are halides, the reddish-brown crystalline dichloride, SmCb [13874-75-4] the dark-brown diiodide, Sml2 [32248-43-4] and the dark brown dibromide, SmBr2 [50801-97-3]. Samarium also forms a difluoride, SmF2 [15192-17-3]. The trivalent salts of these halogens are more stable than their divalent counterparts. 

Samarium(ii) triflate without halide ions, Sm(OTf)2(CH3CN)i,s, can be generated by treatment of samarium with... 

Arene(tricarbonyl)chromium complexes, 19 Nickel boride, 197 to trans-alkenes Chromium(II) sulfate, 84 of anhydrides to lactones Tetrachlorotris[bis(l,4-diphenyl-phosphine)butane]diruthenium, 288 of aromatic rings Palladium catalysts, 230 Raney nickel, 265 Sodium borohydride-1,3-Dicyano-benzene, 279 of aryl halides to arenes Palladium on carbon, 230 of benzyl ethers to alcohols Palladium catalysts, 230 of carboxylic acids to aldehydes Vilsmeier reagent, 341 of epoxides to alcohols Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281 of esters to alcohols Sodium borohydride, 278 of imines and related compounds Arene(tricarbonyl)chromium complexes, 19... 

The elucidation of the mechanism of the samarium Barbier reaction has proved particularly taxing (Scheme 10.26) and represents a classic example of how to investigate radical mechanisms [21, 22]. To complicate the investigation of the mechanism, both alkyl halides and ketones can be reduced by Sml2 (see above). Four feasible mechanisms can be postulated as shown in Scheme 10.26. Mechanisms 1 and 2 have reduction of the alkyl halide first to yield the alkyl radical, and mechanisms 3 and 4 have reduction of the ketone to the ketyl first. 

Samarium iodide catalyzes the reduction of halides in damp THF120-122 and yields may be significantly improved if HMPA is added to the reaction mixture121,123. a-Halocarbonyl compounds are readily hydrodehalogenated by the use of this and similar methods124-126. Bis-cyclopentadiene complexes of samarium catalyze the reduction of benzylic and ally-lie halides via an organosamarium complex intermediate127-129. This reaction may be controlled so that monodeuteration occurs if the reaction is carried out under dry aprotic conditions followed by addition of D20 (equation 12). 

The ability of Sml2 to reduce alkyl halides has been exploited in a number of carbon carbon bond-forming reactions. Radicals generated from the reduction of alkyl halides can be trapped by alkenes in cyclisation reactions to form carbocyclic and heterocyclic rings (see Chapter 5, Section 5.3), and the alkyl-samarium intermediates can be used in intermolecular and intramolecular Barbier and Grignard reactions (see Chapter 5, Section 5.4). The reduction of ot-halocarbonyl compounds with Sml2 gives rise to Sm(III) enolates that can be exploited in Reformatsky reactions (Chapter 5, Section 5.5) and are discussed in Section 4.5. 

In 1993, Molander found that in the presence of catalytic Fe(III) salts, Sml2 mediates intramolecular Barbier additions to esters to give cyclic ketones (or cyclic hemiketals, if they prove to be stable).135 Double addition to the ester is not observed, nor is reduction of the cyclic ketone product. This suggests that the tetrahedral intermediate, a samarium alkoxide of a cyclic hemiketal, is partially stable to the reaction conditions and the ketone group is not revealed until work-up. Molander found that both alkyl and allyl halides could be used in the additions (Scheme 5.83).135... 

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