Rare earth metal silyls

Immobilization of Rare-Earth Metal (Silyl)amide Complexes... 

Keywords bis(trimethylsilylamides), hypersilyl, rare earth metal amides, rare earth metal silyls... 

Sc(OTf)3 is an effective catalyst in aldol reactions of silyl enol ethers with aldehydes.49 Compared with other typical rare-earth-metal (Y, Yb) trifiates, Sc(OTf)3 has the strongest activity in the reaction of 1-trimethylsiloxycyclohexane with benzaldehyde in dichloromethane. Although the reaction scarcely proceeded at —78°C in the presence of Y(OTf)3 or Yb(OTf)3, the aldol adduct was obtained in 81% yield in the presence of Sc(OTf)3 (Scheme 9). 

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. 

Kobayashi, S., Araki, M., Ishitani, H., Nagayama, S. and Hachiya, I., Activation of imines by rare earth metal triflates. Ln(OTf)3- or Sc(OTf)3-catalyzed reactions of imines with silyl enolates and Diels-Alder reactions of imines, Synlett, 1995, 233-234. 

A rare example of isospecific 3,4-polymerization of isoprene mediated by a constrained-geometry rare-earth metal initiator was reported by Z. Hou [270]. Binuclear silyl-linked cyclopentadienyl phosphido lanthanide dialkyl complexes were synthesized in good yields and activated with an equimolar amount of [Ph3C] [B(C6Fs)4] (Scheme 68). Cationic alkyl species were proposed as intermediates and an activation scenario was presented based on DFT calculations [270]. 

For example, aldol reactions of silyl enolates with aldehydes in the presence of Sc(OTf)3 (Tf = OSO2CF3) or the rare earth metal triflates LnfOTf), (Ln = Yb, Gd, Lu) proceed smoothly in both organic solvents and aqueous solution under mild reaction conditions in high yields [1, 3]. 

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. 

The silyl amide type ligands have been used extensively in rare earth chemistry, as well as in actinide and transition metal chemistry, to stabilize electronically unsaturated metal centers due to the available lone pair on the nitrogen donor atom. Because of the relatively larger steric encumbrance, the rare earth complexes with silyl amide type ligands often exhibit low coordination numbers. As a consequence, the large and electropositive rare earth metal centers are accessible to external reagents, which make them more active in many reactions. 

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),... 

The ability of organo-rare-earth metal complexes to undergo alkene or alkyne insertion provides the possibility to perform polyene cyclizations, producing metal-alkyl species which can then undergo o-bond metathesis with an appropriate reagent to produce a cyclic compound. Thus, termination via protonolysis (6) results in cycloalkane derivatives however, termination via silylation is more desirable as a functionalized cyclic framework is formed (Fig. 9). 

While Ln(OTf)3 are the first metal salts which were found to catalyze aldol reactions of aldehydes with silyl enol ethers efficiently in aqueous media, it has been difficult to realize asymmetric versions of Ln(OTf)3-catalyzed reactions in such media. Recently, the first example of this type of reaction using chiral bis-pyridino-18-crown-6 (Structure 4) has been developed (Eq. 8) [38]. In the reaction of benzal-dehyde 5 with water-ethanol (1/9), the cation size of rare earth metal triflates including Ln(OTf)3 strongly affected the diastereo- and enantioselectivities of the... 

The first use of rare earth metals in the aldol reaction began in the case of cerium enolate (198). Subsequently, Kagan and Kobayashi groups reported systematically the use of rare earth metalscatalyzed for the Mukaiyama aldol-type reaction of silyl enol ethers with aqueous formaldehyde solution (199,200). The efficiency of rare earth metals in a Mukaiyama aldol reaction of 1-trimethylsiloxycyclohexene with benzaldehyde was examined in aqueous THF (Scheme 52). Of the rare earth metal trifiates screened, catalytic efficiency was increased in the order of Yb (91%) > Gd (89%) > Lu (88%) > Nd (83%) > Dy (73%) > Er (52%) > Ho(47%) > Sm (46%) > Eu (34%) > Tm (20%) > La (8%) > Y (trace) (201,202). For different aldol or aldol-type reactions, every rare earth metal occupied its special position in the aldol reaction with distinctive catalytic activity. There were several reviews concerning the rare earth metals catalyzed aldol reactions (203,204). New progress in this context will be discussed herein according to rare earth metals catalysis especially for the past 10 years. 

Rare earth metal complexes witii stericaUy demanding tris(aryl)silyl-substituted binaphtholate ligands efficiently catalyze asymmetric hydroamination/cyclization of aminoalkenes and the kinetic resolution of a-substituted aminopentenes. The catalytic activities are comparable to... 

The Mannich and related reactions provide one of the most fundamental and useful methods for the synthesis of P-amino ketones or P-amino esters. Three-component Mannich-type reactions of aldehydes, amines, and silyl enol ethers have been developed. With the development of green solvent systems, this reaction was also examined in a fluorous phase using perfluorinated rare earth metal salts including Sc(0S02C8Fi7)3 [5]. A characteristic point of this system is that it can be reused many times without reloading a new catalyst. There are also many reports on other scandium-catalyzed Mannich reactions. For example, Sc(OTf)3 was found to be an efficient catalyst for the three-component Mannich-type reactions of aromatic aldehydes, ketones, and nitriles in the presence of acetyl chloride (Scheme 12.3) [6]. 

Aldol-type Reactions. In 1993, Kobayashi et al. first introduced Sc(OTf)3 as an effective catalyst in aldol reactions of silyl enol ethers with aldehydes in aqueous media. Since then, other rare earth metal triflates [RE(OTf)3] have been shown to be useful for the aldol reaction, including Y(OTf)3. In general, Y(OTf)3 is less active than Sc(OTf)3 for the aldol reaction. However, good yields of the direct aldol reaction can be obtained at room temperature when stoichiometric amounts of Y(OTf)3 and a tertiary amine are used (eq 1). ... 

Lewis acids are quite often used as catalysts in organic synthesis. Although most Lewis acids decompose in water, it was found that rare earth triflates such as Sc(OTf)3, Yb(OTf)3, etc. can be used as Lewis acid catalysts in water or water-containing solvents (water-compatible Lewis acids) [6-9]. For example, the Mukaiyama aldol reactions of aldehydes with silyl enol ethers were catalyzed by Yb(OTf)3 in water-THF (1 4) to give the corresponding aldol adducts in high yields [10, 11]. Interestingly, when the reactions were carried out in dry THF (without water), the yield of the aldol adducts was very low (ca. 10%). Thus, this catalyst is not only compatible with water but also is activated by water, probably due to dissociation of the counteranions from the Lewis acidic metal. Furthermore, the catalyst can be easily recovered and reused. 

Metal salts other than those derived from rare earth elements are also water-compatible Lewis acids. To find other Lewis acids that can be used in aqueous solvents and to determine the criteria for water-compatible Lewis acids, group 1-15 metal chlorides, perchlorates, and triflates were screened in the aldol reaction of benzaldehyde with silyl enol ether 2 in water/THF (1/9) (Scheme... 

Reagents obtained by lithiation of ort/io-haloarylphosphines have been used in routes to new substituted arylphosphines, e.g., (69), (70), and (71). C-lithiated phosphinocyclopentadienides, e.g., (72), have been used in routes to dissymmetric heteroannular-functionalised ferrocenylpolyphosphines, e.g., (73). The reagent PhP(CH2CH2SLi)2 has been applied in a route to new phosphathiamacrocycles, e.g., (74). Apart from their applications in synthesis, interest has also continued in the preparation and structural characterisation of various C-metallated complexes of borane-protected alkyl- and silylated alkyl-phosphines, ° C-metallated potassium-and lanthanum-complexes of l,2-bis(diphenylphosphino)methane, and also of alkaline earth and rare earth N-metallated complexes of a variety of phosphino-amides. ... 

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