Ytterbium salts

To avoid excessive acid waste, lanthanide(III) triflates are used as recyclable catalysts for economic aromatic nitration. Among a range of lanthanide(III) triflates examined, the ytterbium salt is the most effective. A catalytic quantity (1-10 mol%) of ytterbium(III) triflate catalyzes the nitration of simple aromatics with excellent conversions using an equivalent of 69% nitric acid in refluxing 1,2-dichloromethane for 12 h. The only by-product of the reaction is water, and the catalyst can be recovered by simple evaporation of the separated aqueous phase and reused repeatedly for further nitration.12... 

The ytterbium salt was prepared from commercially available lithium triflamide (3M chemical company). A representative procedure is given below (Ref. 29)... 

Lanthanide triflates were found to be effective for the activation of formaldehyde water solution. The effect of ytterbium salts was also investigated (Table 3) [19]. While the Yb salts with less nucleophilic counter anions such as OTf or C104 effectively catalyzed the reaction, only low yields of the product were obtained when the Yb salts with more nucleophilic counter anions such as CL, OAc, N03, and S042 were employed. The Yb salts with less nucleophilic coun-... 

In contrast to thallium salts, ytterbium salts are not known to be toxic. 

In comparison to samarium and ytterbium salts, there were few examples for cerium, praseodymium, and other the rare earth metals catalyzed aldol reaction (214,215). In 2000s, Samarium salts, especially Sml2, have been used in versatile aldol reactions, for example, direct aldol of aldehydes and substituted oxi-ranyl ketones (216), nitro aldol reaction (217,218), intramolecular aldol reaction (219), and other aldol reaction of special carbonyl compounds (220-222). However, catalytic asymmetric samarium-catalytic aldol reaction was not reported so far. In the asymmetric version of the aldol reaction, ytterbium exhibited promising enantioinduction. In the first example of the asymmetric ytterbium-catalyzed aldol reaction, moderate levels of enantioselectivities were achieved (Scheme 56) (223). Subsequently, Mlynarski and co-workers improved enatioinduction ability of the ytterbium-catalyzed aldol reaction by using catalytic amount of Yb(OTf)3... 

For multicomponent synthesis of homoallylic amines using all-yltributylstannane instead of allyltrimethylsilane and catalyzed hy different species, see Polymer-bound-Yb salts (polyfluoro-alkyl sulfonic acids) (a) Y. Yin, G. Zhao, G.-L. Li, Tetrahedron 2005, 61, 12042-12052. Synthesis of polystyrene-bound per-fluoroaUcyl sulfonic acids and the application of their ytterbium salts in multicomponent reactions (MCRs). (Bromodimethyl) suhbnium bromide (Me S BrBr) (b) B. Das, B. Ravikanth, P Thirupathi, B. Vittal Rao, Tetrahedron Lett. 2006, 47, 5041-5044. (Bromodimethyl)sulfonium bromide catalyzed efficient multicomponent one-pot synthesis of homoaUyhc amines. 

Kobayashi et al. have reported the use of a chiral lanthanide(III) catalyst for the Diels-Alder reaction [51] (Scheme 1.63, Table 1.26). Catalyst 33 was prepared from bi-naphthol, lanthanide triflate, and ds-l,2,6-trimethylpiperidine (Scheme 1.62). When the chiral catalyst prepared from ytterbium triflate (Yb(OTf)3) and the lithium or sodium salt of binaphthol was used, less than 10% ee was obtained, so the amine exerts a great effect on the enantioselectivity. After extensive screening of amines, ds-1,2,6-... 

In 1997 the application of two different chiral ytterbium catalysts, 55 and 56 for the 1,3-dipolar cycloaddition reaction was reported almost simultaneously by two independent research groups [82, 83], In both works it was observed that the achiral Yb(OTf)3 and Sc(OTf)3 salts catalyze the 1,3-dipolar cycloaddition between nitrones 1 and alkenoyloxazolidinones 19 with endo selectivity. In the first study 20 mol% of the Yb(OTf)2-pyridine-bisoxazoline complex 55 was applied as the catalyst for reactions of a number of derivatives of 1 and 19. The reactions led to endo-selective 1,3-dipolar cycloadditions giving products with enantioselectivities of up to 73% ee (Scheme 6.38) [82]. In the other report Kobayashi et al. described a... 

The compounds of the rare earth elements are usually highly colored. Neodymium s compounds are mainly lavender and violet, samarium s yellow and brown, holmium s yellow and orange, and erbium s rose-pink. Europium makes pink salts which evaporate easily. Dysprosium makes greenish yellow compounds, and ytterbium, yellow-gold. Compounds of lutetium are colorless, and compounds of terbium are colorless, dark brown, or black. 

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

The salts of ytterbium are paramagnetic, which exhibit weaker magnetic fields than do iron magnets. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

The metal dissolves in dilute and concentrated mineral acids. Evaporation crystallizes salts. At ordinary temperatures, ytterbium, similar to other rare earth metals, is corroded slowly by caustic alkalies, ammonium hydroxide, and sodium nitrate solutions. The metal dissolves in liquid ammonia forming a deep blue solution. 

The spectra of scandium and ytterbium were first studied by Tobias Robert Thalen (22, 32). Although scandium salts possess no visible... 

In dilute solutions of perchlorate salts, at room temperature. Ytterbium triflate. c Estimated from quantum yield. 

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