Lewis rare earth metal

Rare earth metals and scandium trifluoromethanesulfonates (lanthanide and scandium triflates) are strong Lewis acids that are quite effective as catalysts in... 

In the case of Lewis acids, protic solvents such as water or alcohol can strongly influence their reactivity, cause it to react via an alternative path to the one desired, or even cause decomposition. Recently, rare earth metal triflates were used to develop water tolerant Lewis acids that can be used in many organic reactions. ... 

Rare Earth Metal Trifluoromethanesulfonates as Water-Tolerated Lewis Acid Catiaysts Organic Synthesis," Kobavashi. Synlett, 1994, 679... 

In the last few years, asymmetric catalysis by means of chiral Lewis acids has led to highly enantioselective protocols for a variety of synthetic transformations, including important C-C bond formation processes. The most successful chiral Lewis acids for catalytic enantioselective C-C bond formation contain B(III), Al(III), Ti(IV), Sn(II), and rare earth metals. 

Lewis acids as water-stable catalysts have been developed. Metal salts, such as rare earth metal triflates, can be used in aldol reactions of aldehydes with silyl enolates in aqueous media. These salts can be recovered after the reactions and reused. Furthermore, surfactant-aided Lewis acid catalysis, which can be used for aldol reactions in water without using any organic solvents, has been also developed. These reaction systems have been applied successfully to catalytic asymmetric aldol reactions in aqueous media. In addition, the surfactant-aided Lewis acid catalysis for Mannich-type reactions in water has been disclosed. These investigations are expected to contribute to the decrease of the use of harmful organic solvents in chemical processes, leading to environmentally friendly green chemistry. 

Keywords Lewis Acids m Rare Earth Metal Triflate m Aldol Reactions m Aqueous Media... 

For these and similar reactions recently a variety of Lewis acidic aluminium, rare earth metals, and titanium alkoxides have been applied. Alkoxides have the additional advantage that they can be made as enantiomers using asymmetric alcohols which opens the possibility of asymmetric catalysis. Examples of asymmetric alcohols are bis-naphtols, menthol, tartaric acid derivatives [28], Other reactions comprise activation of aldehydes towards a large number of nucleophiles, addition of nucleophiles to enones, ketones, etc. 

Interestingly, salts other than tin(ll) bis-(2-ethylhexanoate) such as scandium and tin trifluoromethanesulfonate [41 3], zinc octoate [44, 45], and aluminum acetyl acetonate [45] were reported to mediate the ROP of lactones. As far as scandium trifluoromethanesulfonate is concerned, the main advantage is the increase of its Lewis acidity enabling the polymerization to be carried out at low temperatures with acceptable kinetics. Later, faster kinetics were obtained by extending the process to scandium trifluoromethanesulfonimide [Sc(NTf2)3] and scandium nonafluorobutanesulfonimide [Sc(NNf2)3] and to other rare earth metal catalysts (metal=Tm, Sm, Nd) [46]. 

First developments in the Friedel-Crafts alkylation were concentrated on the use of stoichiometric amounts of Lewis acids, such as A1C13, BF3 or TiCl4, to produce stoichiometric amounts of salt by-products [5-9]. However, in recent years more and more catalytic methods have been developed. In particular, rare earth metal triflates, including Sc(OTf)3, La(OTf)3 and Yb(OTf)3, have been extensively used as Lewis acid catalysts in various C-C and C-X bond forming reactions [10-13], Despite the benefit of their versatility for organic synthesis, these Lewis acids possess major drawbacks. They are expensive, rather toxic [14], and air- and moisture-sensitive. 

On the basis of these initial results, various rare earth metal triflates, including Sc(OTf)3, Hf(OTf)4 and Yb(OTf)3 were applied as catalysts [27-29]. Recently Beller and coworkers developed efficient Friedel-Crafts alkylations with catalytic amounts of Rh, W, Pd, Pt and Ir complexes [30] or FeCl3 [31-34] as Lewis acid catalysts. However, in the latter cases high catalyst loadings had to be applied. To overcome these major drawbacks, we decided to develop a Bi(III)-catalyzed Friedel-Crafts alkylation of arenes with benzyl alcohols. Although bismuth-catalyzed Friedel-Crafts acylations were well known at this time, Friedel-Crafts alkylations using benzyl alcohols had not been reported. 

Grignard reagents add with difficulty to imines derived from enolizable carbonyl compounds. The activation of the C=N bond can be achieved either by attachment of an electron-with drawing group or A-coordination with a Lewis acid . The use of a catalytic amount of the soluble rare-earth metal complex LnCl3 2LiCl allows the addition of... 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

An easy and efficient method to generate indolylnitroalkane (169) and the analogous pyrrolylnitroalkane in high yields using /3-nitrostyrene derivative (168) and indole/pyrrole at room temperature in the presence of iodine (50 mol%) has been reported as an alternative to the known Lewis acid or rare earth metal catalysts.197... 

Lappert and coworkers also found that such rare-earth metal aluminate complexes can be readily transformed into methyl derivatives by addition of equimolar amounts of donor Lewis base molecules such as pyridine or tetrahydrofuran (Lappert s concept of donor-induced aluminate cleavage, Scheme 1) [10]. Depending on the Lewis acidity of the Ln(III) center homo-... 

Against this background it is important that—quite fitting in this still new millennium— the first catalytic Friedel-Crafts acylations of (still relatively electron-rich) aromatic compounds were reported (Figure 5.35). Trifluoromethane sulfonates ( triflates ) of rare-earth metals, e. g., scandium(III)triflate, accomplish Friedel-Crafts acylations with amounts of as little as 1 mole percent. Something similar is true of the tris(trifluoromethanesulfonyl)-methides ( triflides ) of rare-earth metals. Unlike conventional Lewis acids, the cited rare-earth metal salts can form 1 1 complexes with the ketone produced, but these are so unstable that the Lewis acid can re-enter the reaction. Whether this works analogously for the third catalytic system of Figure 5.35 is unclear. 

The growing demand for efficient chemical transformations and catalysts has inspired a few research groups in recent years to develop rare earth metal catalysts for organic synthesis [1, 2]. Triflates of rare earth metals are strong Lewis acids, which are stable in aqueous solution. Rare earth metal alkoxides on the other hand are of interest as Lewis bases, e.g. in the catalysis of carbonyl reactions, because of the low ionization potentials (5.4-6.4 eV) and electronegativities (1.1-1.3) of the 17 rare earth elements. Rare earth metal-alkali metal complexes in contrast show both Brpnsted-basic and Lewis-acidic properties. Impressive applications of such catalysts are presented and discussed here. 

Traditional Lewis acids such as AICI3 or BFj OEt2 catalyze key steps in many reactions involving carbonyl compounds, leading to carbon-carbon bond formation. Because of their reactivity and instability these catalysts cannot be used in aqueous solution. For this area of application, rare earth metal catalysts open up new perspectives. 

Kobayashi et al. also published experiments in which they applied polymer-bound rare earth metal triflates [9], The performance of Lewis acid-catalyzed imino-aldol reactions at the solid phase was realized by linking the silyl enolethers u5-(4 -chloromethylphenyl)pentylpolystyrene[10]. 

With catalytic amounts of rare earth metal triflates, heterocarbonyl compounds, e.g. acylhy-drazones, are also successfully activated. From the latter and silyl enolates (Scheme 4), the coupling products are obtained directly or in a one-pot synthesis in the presence of 5 mol% of Sc(OTf)3 or Yb(OTf)3 in 45-96% yield. For example, compound 17 was isolated in 92 % yield and was subsequently cyclized with base to the corresponding pyrazolone (Scheme 4) [16]. In comparison with typical Lewis acids, such as SnCl4, (10% yield) and boron trifluoride etherate (42 % yield), Sc(OT03 proved to be superior. 

In the course of our investigations to circumvent the second drawback in the use of water (the decomposition problem), we have found that some metal salts such as rare earth metal triflates (triflate = trifluoromethanesulfonate) can be used as water-compatible Lewis acids [16,17]. Lewis acid catalysis has attracted much attention in organic synthesis [18]. Although various kinds of Lewis acids have been developed and many have been applied in industry, these Lewis acids must be generally used under strictly anhydrous conditions. The presence of even a small amount of water stops the reactions, because most Lewis acids immediately react with water rather than substrates. In addition, recovery and reuse of the conventional Lewis acids are formidable tasks. These disadvantages have restricted the use of Lewis acids in organic synthesis. 

Rare earth metal triflates are recognized as a very efficient Lewis acid catalysts of several reactions including the aldol reaction, the Michael reaction, allylation, the Diels-Alder reaction, the Friedel-Crafts reaction, and glycosylation [110]. A polymer-sup-ported scandium catalyst has been developed and used for quinoline library synthesis (Sch. 8) [111], because lanthanide triflates were known to be effective in the synthesis of quinolines from A-arylimines [112,113]. This catalyst (103) was readily prepared from poly(acrylonitrile) 100 by chemical modification. A variety of combinations of aldehydes, amines, and olefins are possible in this reaction. Use of the polymer-supported catalyst has several advantages in quinoline library construction. 

The f-transition metal catalysts were first described by von Dohlen [98] in 1963, Tse-chuan [99] in 1964 and later by Throckmorton [100]. In the 1980s Bayer [14] and Enichem [101] developed manufacturing processes based on neodymium catalysts. The catalyst system consists of three components [102] a carboxylate of a rare earth metal, an alkylaluminum and a Lewis acid containing a halide. A typical catalyst system is of the form neodymium(III) neodecanoate/diisobutylaluminum hydride/butyl chloride [103]. Neodymium(III) neodecanoate has the advantage of very high solubility in the nonpolar solvents used for polymerization. The molar ratio Al/Nd/Cl = 20 1 3. Per 100 g of butadiene, 0.13 mmol neodymium(III) neodecanoate is used. With respect to the monomer concentration, the kinetics are those of a first-order reaction. 

On the other hand, in the course of our investigations to develop new synthetic methods, we have found that rare earth metal triflates [Sc(OTf)3, Yb(OT03, ] [2] and some other metal salts can be used as water-stable Lewis acids for activation of C=0 and C=N groups in water-containing solvents. 

We also screened group 1-15 metal chlorides searching for Lewis acids stable in aqueous solvents (Table 14-3) [22], As a model, the reaction of benzaldehyde with (Z)-l-phenyl-l-(trimethylsiloxy)propene was selected. In the first screening, the chloride salts of Fe(II), Cu(II), Zn(II), Cd(II), Infill), and Pb(II) as well as the rare earth metals [Sc(III), Y(III), Ln(III)] gave promising yields. When the chloride salts of B(III), Si(IV), PflU), P(V), Ti(IV), V(III), Ge(IV), Zr(IV), Nb(V). Mo(V), Sn(IV), Sb(V), Hf(IV), Ta(V), W(VI), Re(V), and Tl(III) were used, decomposition of the silyl enol ether occurred rapidly and no aldol adduct was obtained. This is because hydrolysis of such metal chlorides is very fast and the silyl enol ether was protonated and then hydrolyzed to afford the corresponding ketone. On the other hand, no product or only a trace amount of the product was detected using the metal chloride salts of Li(l), Na(I), Mg(II), Al(III), K(I), Ca(Il),... 

Various metal salts such as rare earth metal triflates and copper triflate can function as Lewis acids in aqueous media. They can effectively activate aldehydes and imines in the presence of water molecules, and the first successful examples of Lewis acid-catalyzed reactions in aqueous solution have been demonstrated. Water-soluble aldehydes such as foimaldehyde could be employed directly in these reactions. Moreover, the catalysts could be easily recovered after the reactions were completed and could be reused. There are many kinds of Lewis acid-promoted reactions in industrial chemistry, and treatment of large amounts of the acids left over after the reactions have induced some severe environmental problems. From the standpoints of their catalytic use and reusability, the Lewis acids described in this chapter are expected to be new types of catalysts providing some solutions for these problems. 

This book contains four chapters in which part of the recent development of the use of molecular rare-earth metal compounds in catalysis is covered. To keep the book within the given page limit, not all aspects could be reviewed in detail. For example, the use of molecular rare-earth metal complexes as Lewis acidic catalysts is not discussed in this book. The first two chapters review different catalytic conversions, namely the catalytic o-bond metathesis (Chapter by Reznichenko and Hultzsch) and the polymerization of 1,3-conjugated dienes (Chapter by Zhang et al.). Within these chapters, different catalytic systems and applications are discussed. The final two chapters are more concentrated on recent developments of... 

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