Samarium carbonates

Samarium sesquioxide may be prepared by two methods (1) thermal decomposition of samarium carbonate, hydroxide, nitrate, oxalate or sulfate ... 

Cavell reported the first rare earth bis(iminophosphorano)methandiide complex 104 in 2000.54 Complex 104 was prepared from the reaction between the parent methane and the homoleptic complex samarium(III) tris(dicy-clohexylamide) under forcing conditions. The samarium-methandiide bond was ascribed double bond character although shorter single samarium-carbon bonds are known. No reactivity studies of 104 have ever been reported. 

Samarium phosphate was prepared by wet synthesis starting from samarium carbonate (Sm2(C03)3, originated RP). Precipitation of phosphate by phosphoric acid is conducted at 80°C by addition of a samarium carbonate suspension in a vessel containing phosphoric acid. After the end of the addition, the solid could be treated by ammonia at pH = 9. Cesium hydrogenophosphate is introduced by wetness impregnation of the dried (110°C) solid. 

For the preparation of samarium or europium carbonates, the rare earth(II) sulfate crystals were suspended in water, then concentrated (NH4)2C03 or K2CO3 solutions were introduced and allowed to contact the sulfate crystal. Some oxidation of the lanthanide occurs simultaneously (Asprey et al., 1964). Clifford et al. (1948) also reported preparation of samarium carbonate by metathesis of the dichloride. The water insoluble and unstable citrate was prepared in the same way (Clifford et al., 1948). 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

Samarium diiodide is another powerful one-electron reducing agent that can effect carbon-carbon bond formation under appropriate conditions.257 Aromatic aldehydes and aliphatic aldehydes and ketones undergo pinacol-type coupling with Sml2 or SmBr2. 

The fact that organosamarium allyl complexes of the type Cp 2Sm(CH2CH=CHR) can arise from the treatment of Cp 2Sm or [Cp 2Sm(/r-H)]2 with a variety of olefin and diene substrates makes samarium chemistry more intriguing. The reaction modes are illustrated in Scheme 18. These allylsamarium complexes 55 react with C02 to afford the carboxylate products 56, which participate in monometallic/bimetallic interconversions (Equation (10)). Carbon disulfide and 0=C=S also insert into carbon-samarium bonds, which form only monometallic species.29... 

Organoytterbium chemistry has been developed in the last 20 years, although the development rate is much slower than the other lanthanides like samarium or cerium. Dianionic complexes that are produced from the reaction of ytterbium with diaryl ketones react with various kinds of electrophiles including carbon-heteroatom unsaturated bonds.35 Phenylytterbium iodide, a Grignard-type reagent, is known to have reactivity toward carbon dioxide,36 aldehydes, ketones,37,37 and carboxylic acid derivatives38,3811 to form the corresponding adducts respectively. 

The mechanism of the MPVO reactions has been investigated and questioned on several occasions, and a variety of direct hydrogen-transfer pathways have been suggested (see Scheme 20.4) [31-35]. Recently, racemization of D-labeled 1-phenylethanol with deuterated samarium(III) isopropoxide (17) proved that the MPVO reaction occurs via a direct hydrogen transfer from the a-position of the isopropoxide to the carbonyl carbon of the substrate (Scheme 20.7) [31]. 

Burk et al. showed the enantioselective hydrogenation of a broad range of N-acylhydrazones 146 to occur readily with [Et-DuPhos Rh(COD)]OTf [14]. The reaction was found to be extremely chemoselective, with little or no reduction of alkenes, alkynes, ketones, aldehydes, esters, nitriles, imines, carbon-halogen, or nitro groups occurring. Excellent enantioselectivities were achieved (88-97% ee) at reasonable rates (TOF up to 500 h ) under very mild conditions (4 bar H2, 20°C). The products from these reactions could be easily converted into chiral amines or a-amino acids by cleavage of the N-N bond with samarium diiodide. 

Novel synthetic procedures for indolizidine alkaloids were developed via a samarium diiodide-promoted carbon-nitrogen bond cleavage as a key step. Application of the procedure led to the total synthesis of (+)-(8R, 8aR)-perhydro-8-indolizidinol <2006H193>. 

In the presence of samarium(II) iodide, A-(2-iodobenzyl)dialkylamines 347 react with electrophiles at an a-carbon atom to yield deiodinated products by way of intermediate samarium compounds 348. Thus TV-(2-iodobenzyl)diethylamine and pentan-3-one afford the hydroxy amine 349 and 7V-(2-iodobenzyl)pyrrolidine and propyl isocyanate give the amide 350390. 

Barium, Halocarbons, 0200 Beryllium, Halocarbons, 0220 f Bromomethane, Metals, 0429 Chloroform, Metals, 0372 Plutonium, Carbon tetrachloride, 4888 Samarium, 1,1,2-Trichlorotrifluoroethane, 4911 Tin, Carbon tetrachloride, Water, 4912 Titanium, Halocarbons, 4919 Uranium, Carbon tetrachloride, 4923 Zirconium, Carbon tetrachloride, 4928 See also HALOCARBONS METALS... 

Samarium salts are used in optical glass, capacitors, thermoionic generating devices, and in sensitizers of phosphors. The metal is doped with calcium fluoride crystals for use in lasers. It also is used along with other rare earths for carbon-arc lighting. Its alloys are used in permanent magnets. 

Samarium reduces several metal oxides to metals. Such metal oxides include iron, zinc, lead, chromium, manganese, tin, and zirconium. When heated with carbon monoxide, it forms samarium oxide and carbon. 

The oxide is reduced to metaUic samarium by heating with a reducing agent, such as hydrogen or carbon monoxide, at elevated temperatures ... 

Addition of HMPA to Sml2 in THE changes the reaction course of the benzaldehyde dimerization. Although samarium(ii) iodide promotes pinacol coupling of benzaldehyde, use of 2.8equiv. of HMPA leads to the formation of, in addition to the pinacol (10% yield), a dimer (60% yield) that results from the connection of a carbonyl carbon and a phenyl para-c3.rbon (Equation (31)). ... 

Guided by Marks s report of the samarium-catalyzed hydroboration of alkenes, Molander has developed a samarium-catalyzed protocol for the cyclization/hydroboration of unfunctionalized 1,6-dienes. In an optimized procedure, reaction of 1,5-hexadiene and l,3-dimethyl-l,3-diaza-2-boracyclopentane catalyzed by Gp 2Sm(THF) in toluene at room temperature for 18 h followed by oxidation gave hydroxymethylcyclopentane in 86% yield (Equation (70) R = H, n — ). The transformation was stereoselective, and Sm-catalyzed cyclization/hydroboration of 2-phenyl-1,5-hexadiene followed by oxidation formed /ra/ i--l-hydroxymethyl-2-phenylcyclopentane in 64% yield (Equation (70) R = Ph, n = ). The samarium-catalyzed reactions was also applicable to the synthesis of hydroxymethylcyclohexanes (Equation (70), n=X) but tolerated neither polar functionality nor substitution on the alkenyl carbon atoms. 

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