Metal triflate Lewis acids

Metal triflate Lewis acids can also be dispersed in ionic liquids for catalytic applications. Acetylation of alcohols with acetic anhydride and acetic acid has been reported with Cu(OTf)2, Yb(OTf)3, Sc(OTf)3, In(OTf)3, HfClq. (THF)2, and InCl3 in ionic liquids that consist of [BMIM] and the anions BF4, PF, or SbF 166). With lmol% acid, all the catalysts in [BMIMJPF showed >99% acetylation products in acetyl anhydride acetylation of benzyl alcohol. Sc(OTf)3 showed the best yield with recycling, with a 25% drop in yield after two cycles. A relatively long reaction period was needed to obtain a high yield (95-98%) for the acetylation of benzyl alcohol with acetic acid, indicating that the activities of the catalysts were... 

An example of a conventional cascade system with two catalysts comprising a high-valent metal triflate Lewis add and a supported Pd catalyst for the hydrogenolysis of ethers is illustrated in Fig. 8.24. Since primary C—O bonds are resistant to cleavage, in the presence of water, there is formation of the parent primary alcohol that further undergoes acid-catalyzed dehydration to alkene and rapid, irreversible metal-catalyzed hydrogenation to alkane. 

To achieve catalytic enantioselective aza Diels-Alder reactions, choice of metal is very important. It has been shown that lanthanide triflates are excellent catalysts for achiral aza Diels-Alder reactions [5]. Although stoichiometric amounts of Lewis acids are often required, a small amount of the triflate effectively catalyzes the reactions. On the basis of these findings chiral lanthanides were used in catalytic asymmetric aza Diels-Alder reactions. The chiral lanthanide Lewis acids were first developed to realize highly enantioselective Diels-Alder reactions of 2-oxazolidin-l-one with dienes [6]. 

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. ... 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

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... 

While the Lewis acid-catalyzed aldol reactions in aqueous solvents described above are catalyzed smoothly by several metal salts, a certain amount of an organic solvent such as THF had still to be combined with water to promote the reactions efficiently. This requirement is probably because most substrates are not soluble in water. To avoid the use of the organic solvents, we have developed a new reaction system in which metal triflates catalyze aldol reactions in water with the aid of a small amount of a surfactant, such as sodium dodecyl sulfate (SDS). 

In addition to Bronsted acid promoted Fischer-type glycosylations, Lewis acids have been investigated (Scheme 3.4). A variety of Lewis acids promote glycosylation under mild conditions, often in substoichiometric amounts. The earliest examples include ZnCl2 [18] and FeCl3 [19], although these readions were demonstrated only for preparation of trehalose-type disaccharides. Mukaiyama et al. have very recently developed metal triflate catalysts for the dehydrative glycosylation with... 

Glycosyl esters with remote functionality constitute a relatively new class of O-carbonyl glycosyl donors, which fulfill the prospect of mild and chemoselective activation protocols (Scheme 3.22). For example, Kobayashi and coworkers have developed a 2-pyridine carboxylate glycosyl donor 134 (Y = 2-pyridyl), which is activated by the coordination of metal Lewis acid (El+) to the Lewis basic pyridine nitrogen atom and ester carbonyl oxygen atom [324]. In the event, 2-pyridyl (carbonyl) donor 134 and the monosaccharide acceptor were treated with copper(II) triflate (2.2 equiv) in diethyl ether at —50 °C, providing the disaccharide 136 in 70% (a P,... 

Because of the high nucleophilicity and reactivity of diazoalkanes, catalytic decomposition occurs readily, not only with a wide range of transition metal complexes but also with Brpnsted or Lewis acids. Well-established catalysts for diazodecomposition include zinc halides [638,639], palladium(II) acetate [640-642], rhodium(II) carboxylates [626,643] and copper(I) triflate [636]. Copper(II)... 

The solubility of a Lewis acidic salt in an ionic liquid is dependent on the nature of both the salt and the ionic liquid. Hydrophobic ionic liquids have lower solubilities for such salts. For example, metal triflates such as Sc(TfO)3 readily dissolve... 

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. 

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. 

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. 

Traditionally Lewis acid catalysed reactions often utilise metal halides, however in recent years, metal trifluoromethanesulfonates or triflates having the general formula Mn+(S02CF3)n have been reported as a new and interesting type of Lewis acid4. These... 

Salicylaldehydes and 2-pyridine carboxaldehyde which cannot be normally used with Lewis acids because of their coordination to metal may be used as substrates with Lu triflate as a catalyst [156]. 

Another approach is to design homogeneous Lewis acids which are water-compatible. For example, triflates of Sc, Y and lanthanides can be prepared in water and are resistant to hydrolysis. Their use as Lewis acid catalysts in aqueous media was pioneered by Kobayashi and coworkers [144-146]. The catalytic activity is dependent on the hydrolysis constant (Kh) and water exchange rate constant (WERC) for substitution of inner sphere water ligands of the metal cation [145]. Active catalysts were found to have pKh values in the range 4-10. Cations having a pKh of less than 4 are easily hydrolyzed while those with a pKh greater than 10 display only weak Lewis acidity. 

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