Triflates , rare earth

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. 

Lewis acids, especially rare earth triflates, are efficient catalysts of 1,3-dipolar (g) cycloadditions to imines, Ar-1CH=N-Ar2.67... 

On the other hand, rare-earth trifluoromethanesulfonates (rare earth triflate, RE(OTf)3) have been found to work efficiently as Lewis acids even in aqueous media or in the presence of amines [4], A catalytic amount of RE(OTf)3 enables several synthetically useful reactions, for example aldol, Michael, allylation, Mannich, Diels-Alder reactions, etc., to proceed. It has also been demonstrated that a small amount of RE(OTf)3 is enough to complete the reactions and that RE(OTf)3 can easily be recovered from the reaction mixture and can be reused. A key to accomplishing the catalytic processes was assumed to be the equilibrium between Lewis acids and Lewis bases, for example water, carbonyl compounds, and amines, etc. A similar equilibrium was expected between Lewis adds and aromatic ketones, and, thus, RE(OTf)3-catalyzed Friedel-Crafts acylation was investigated [5]. 

The intramolecular Friedel-Crafts cyclization of 3-(4-chlorophenylthio)butanoic acid to 6-chloro-2-methylthio-chroman-4-one is efficiently catalyzed by Bi and rare-earth triflates (Equation 195) <2003TL4007>. The cyclization of (3-arylthiopropanoic acids to thiochroman-4-ones by PPA is facilitated by microwave irradiation. Formation of these acids from the sodium salts of thiophenols and 3-chloropropanoic acid is similarly accelerated <2004JCM394>. 

Lanthanide(III) trifluoromethane sulfonates in the hydrated form can be prepared by reacting the rare earth oxide with triflic acid in aqueous solution. The product is hydrated salt which is heated under vacuum to produce an anhydrous rare earth trifluoromethane sulfonate. The anhydrous salts are used in organic reactions. Some of the advantages of using rare earth triflates over conventional Lewis acids are (i) they act as catalysts rather than stoichiometric reagents, (ii) they are effective catalysts in aqueous solutions and (iii) they can be easily recovered without any loss of catalytic activity for subsequent use. 

The rare earth triflates are used in three types of organic reactions, namely (i) carbon-carbon bond formation, (ii) carbon-oxygen bond formation and (iii) carbon-nitrogen bond formation. 

Lewis acid catalysts are used in these reactions to improve regioselectivity and endo addition. Rare earth triflates are used in this type of reaction. Typical data are given in Table 12.29 detailing the percent yield and endo and exo yields [161]. 

Rare earth triflates used as catalysis in Diels-Alder reactions [161]. 

Sc(OTf)3 is an effective catalyst in aldol reactions of silyl enol ethers with aldehydes [4,5]. The activities of typical rare earth triflates [Sc, Y, Yb(OTf)3] were evaluated in the reaction of 1-trimethylsiloxycyclohexene with benzaldehyde in dichloromethane (Table 1). Although the reaction scarcely proceeded at -78 °C in the presence of Yb(OTf)3 or Y(OTf)3 [3b], the aldol adduct was obtained in 81 % yield in the presence of Sc(OTf)3. Obviously, Sc(OTf)3 was more active than Y(OTf)3 or Yb(OTf)3 in this reaction. 

Efficient asymmetric Diels-Alder reactions catalyzed by chiral Lewis acids have recently been reported [34]. Although rare earth compounds were expected to be promising Lewis acid reagents, few asymmetric reactions catalyzed by chiral rare earth Lewis acids were reported [35], although rare earth triflates, especially Yb(OTf)3 and Sc(OTf)3, are good catalysts in the Diels-Alder reactions of a variety of dienophiles with cyclic and acyclic dienes (as mentioned in Section 19.2.6). 

Bickley, J. E., Hauer, B., Pena, P. C. A., Roberts, S. M., Skidmore, J. The semi-pinacol rearrangement of homochiral epoxy alcohols catalyzed by rare earth triflates. J. Chem. Soc., Perkin Trans. 1 2001,1253-1255. 

Kobayashi et al. have reported fhat Sc(OTf)3 works as an effective and reusable Lewis acid catalyst of fhe aldol and aldol-type reactions of aldehydes and acetals wifh silyl enolates in CH2CI2 [69]. The activities of Sc(OTf) ), and ofher rare earth triflates (Y(OTf)3 and Yl)(() l f) () were evaluated in the reaction of cyclohexanone TMS enolate wifh benzaldehyde (Scheme 10.21). The results clearly indicate that Sc(OTf)3 is more active fhan Y(OTf)3 and Yb(OTf)3. 

Rare earth triflates can also be used in the reaction of aldehydes with aromatic rings (6.15).59... 

Select a reaction that requires a stoichiometric amount of aluminum chloride as a catalyst. Try running it with a rare earth triflate. 

Using less complex forming salts than metal halides with aryl acetone in the same way as rare earth triflates (refs. 13, 14),... 

Keywords Lewis acids, Polymer-supported catalysts, Rare earth triflate, Combinatorial synthesis, Carbon-carbon bond-forming reactions... 

This reaction was discovered at the end of the nineteenth century, and despite intensive studies this drawback has not found a general solution [3]. Studies have been realized to find mild Lewis acids forming less stable complexes with the ketone and still able to catalyze the reaction. Some success has been obtained by use of rare earth triflates [4] or bismuth III salts [5]. These methods, if they prove to be catalytic still require substantial amounts of catalyst (a few percent) and recycling of the catalyst is not simple. 

This work incorporates all acid-catalyzed Friedel-Crafts-like acylation reactions. Thus, classic Lewis and Bronsted acid types are considered together with more innovative and advanced multicomponent superacid catalysts, ranging from rare earth triflates or triflimides and their... 

A recent report9 described the use of scandium triflate in conjunction with tributyltin cyanide, a more water stable cyanide source compared to TMSCN. These reaction conditions could be carried out in both organic and aqueous solutions. It was observed that rare earth triflates are stable Lewis acids in water. Thus, a variety of aldehydes 1 and amine 12 were converted to a-aminonitriles 13 in excellent yield. The spent reagent could be completely recovered and, along with the scandium reagent, recycled for subsequent use. 

In contrast, similar systems underwent semi-pinacol rearrangement promoted by rare earth triflates. Thus, treatment of epoxyalcohol 78 with 20 mol% of ytterbium triflate in methylene chloride cleanly produced the hydroxyketone 79 in 99% yield after three hours. Attempts to... 

Aldol-type Reactions. Sc(OTf)3 is an effective catalyst in aldol-type reactions of silyl enol ethers with aldehydes. The activities of typical rare-earth triflates [Sc, Y, Yb(OTf)3] were evaluated in the reaction of 1-trimethylsiloxycyclohexene with benzalde-... 

Takasu, A., Oishi, Y., lio, Y. et al. (2003) Synthesis of aliphatic polyesters by direct polyesterifica-tion of dicarboxylic acids with diols under mild conditions catalyzed by reusable rare-earth triflate. 

Arasa, M., Pethrick, R.A., Mantecon, A., Serra, A. New thermosetting nanocomposites prepared from diglycidyl ether of bisphenol and y-valerolactone initiated by rare earth triflate initiators. Eur. Polym. J. 46, 5-13 (2010)... 

A special case of control over the structure of copolymers may include the first stereoblock copolymers made by CRP. By applying either RAFT or ATRP to polymerization of acrylamides in the presence of rare-earth triflates such as Y(OTf)3 and Yb(OTf)3, it was possible to enhance isotacticity of A(A/-dmiethylacrylamide (DMA) from 50% meso to 90% meso dyads (170). At the same time control of molecular weights and polydispersity was preserved. Similar results were obtained for RAFT of A(-ispropylacrylamide (171). The ATRP and RAFT of DMA was applied to the first one-pot stereoblock synthesis by radical mechanism. RAFT or ATRP of DMA were started without Lewis acid to produce the first atactic block. Subsequently, the complexing agent was added at the desired conversion to continue the chain growth with the preferential isotactic placement (170). 

In addition to rare earth triflates, copper triflate was also found to be a stable Lewis acid in aqueous media. In a mixed aqueous solvent system (H20-EtOH-toIuene = 1 7 4), allylation of various aldehydes with tetraallyltin and aldol reactions with silyl enol ethers proceeded smoothly in the presence of Cu(OTf)2 (20 mol%) to give homoallylic alcohols and aldol adducts, respectively, in high yields (Schemes 3.9 and 3.10). 

As mentioned above, rare earth triflates, Cu(OTf)2, and AgOTf were foimd to act as Lewis acids in aqueous media. It would be of great importance to know which factors are key to the success of organic reactions in water. To address this issue, 1-15 metal chlorides, perchlorates, and triflates were screened in the aldol reaction of benzaldehyde with the silyl enol ether inwater-THF(l 9) (Scheme 3.13). This screening revealed that not only Sc(III), Y(lll), and Ln(III) but also Fe(II), Cu(II), Zn(II), Cd(II), and Pb(II) worked as Lewis acids in this medium to afford the desired aldol adducts in high yields. 

In an early stage of developing organic reactions in aqueous systems, rare earth triflates were used for aldol reactions in THF-water or ethanol-water, giving successful results. On the other hand, when the reactions were carried out in pure water, the corresponding aldol products were obtained only in low yields. " This was probably because solubility of organic substrates was low and decomposition of silyl enol ethers occurred faster than the desired aldol reactions in water. To address this issue, micellar catalysis in water was... 

The large radius of lanthanide(lll) and the specific coordination number also attracted us, and we started to investigate the use of lanthanide triflates as Lewis-acid catalysts for the reaction of amines with nitriles under anhydrous conditions [7a]. After starting this study, we found that scandium triflate (ScfOTfls) and yttrium triflate (YfOTfla) are also excellent water-tolerant Lewis acids, and we have developed new synthetic reactions using these rare-earth triflates as catalysts. 

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