Lanthanide salts, Lewis acids

A recent notable finding in this field is Mukaiyama aldol reactions in aqueous medium (THF H20 = 9 1) catalyzed by metal salts. Lewis acids based on Fe(II), Cu(II), and Zn(II), and those of some main group metals and lanthanides are stable in water. Remarkably, the aldol reaction shown in Sch. 29 occurs more rapidly than the hydrolysis of the silyl enol ether [137]. In the presence of surfactants (dodecyl sulfates or dodecane sulfonate salts), reactions of thioketene silyl acetals with benzaldehyde can be performed in water [138]. 

The lanthanide salts are unique among Lewis acids in that they can be effective as catalysts in aqueous solution.61 Silyl enol ethers react with formaldehyde and benzaldehyde in water-THF mixtures using lanthanide triflates such as Yb(03SCF3)3. The catalysis reflects the strong affinity of lanthanides for carbonyl oxygen, even in aqueous solution. 

These reactions can be catalyzed by Lewis acids such as bw-alkoxytitanium dichlorides61 and lanthanide salts.62... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Kobayashi has found that scandium triflate, Sc(OTf)3,36 and lanthanide triflate, Ln(OTf)3, are stable and can be used as Lewis catalysts under aqueous conditions. Many other Lewis acids have also been reported to catalyze Diels-Alder reactions in aqueous media. For example, Engberts reported37 that the cyclization reaction in Eq. 12.7 in an aqueous solution containing 0.010 M Cu(N03)2 is 250,000 times faster than that in acetonitrile and about 1,000 times faster than that in water alone. Other salts, such as Co2+, Ni2+, and Zn2+, also catalyze the reaction, but not as effectively as Cu2+. However, water has no effect on the endo-exo selectivity for the Lewis-acid catalyzed reaction. 

By systematic screening of the effects of Lewis acids on the competitive reactions of benzaldehyde and A-bcnzylidcncanilinc with propiophenone enolate in CH2CI2 or CH3CN, it has been found that addition of a small amount of the lanthanide salt... 

Bismuth(m) salts, such as BiCl3, BiBr3, Bi(OCOR)3, Bi(N03)3, Bi(OTf)3, and Bi(NTf2)3, have been used as Lewis acid catalysts to mediate a variety of carbon-carbon bond-forming reactions.85 In some cases, true catalysts differ from the bismuth salts initially added. The most effective and frequently used catalyst is Bi(OTf)3, which is obtained as a hydrated or dehydrated form depending on the preparation methods.86,86a,86b Like lanthanide triflates, Bi(OTf)3 is water stable and reusable. 

Dipolar cycloaddition of alkenes with carbonyl ylides generated in situ is a versatile method for tetrahydrofuran synthesis. The synthetic potential of such transformations has been reviewed <2005JOM(690)5533, 2003BMI6-253>. In addition, the stereoselective [3 + 2] annulation of allyl silanes has become a reliable protocol for the synthesis of tetrahydrofurans as demonstrated in several total syntheses . Such a [3 + 2] annulation, for example, affords the tetrahydrofuran product 11 as a single stereoisomer (Scheme 15) <2002OL2945>. Lanthanide salts serve as efficient Lewis acid catalysts in similar [3 + 2] cycloaddition reactions . 

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. 

In the context of replacing conventional Lewis acids in organic synthesis it is also worth pointing out that an alternative approach is to use lanthanide salts [39] that are both water soluble and stable towards hydrolysis and exhibit a variety of interesting activities as Lewis acids (see later). 

The lanthanide triflate remains in the aqueous phase and can be re-used after concentration. From a green chemistry viewpoint it would be more attractive to perform the reactions in water as the only solvent. This was achieved by adding the surfactant sodium dodecyl sulfate (SDS 20 mol%) to the aqueous solution of e.g. Sc(OTf)3 (10 mol%) [145]. A further extension of this concept resulted in the development of lanthanide salts of dodecyl sulfate, so-called Lewis acid-surfactant combined catalysts (LASC) which combine the Lewis acidity of the cation with the surfactant properties of the anion [148]. These LASCs, e.g. Sc(DS)3, exhibited much higher activities in water than in organic solvents. They were shown to catalyze a variety of reactions, such as Michael additions and a three component a-aminophosphonate synthesis (see Fig. 2.44) in water [145]. 

Glycals react with nucleophiles such as allylsilanes and alcohols under Lewis-acid catalysed conditions. The reaction is usually highly stereoselective in favour of the a-enantiomer. Several lanthanide salts, Dy(OTf)3, Sc(OTf)3 and Yb(OTf)3 have been successfully used as catalysts... 

Lanthanide salts serve as efficient Lewis acid catalysts in the [3+2] cycloaddition of methylenecyclopropanes with activated aldehydes or ketones (Equation 100) <2003TL3839>. 

It is a commonplace to say that there has been explosive growth in the use of lanthanides in organic chemistry. For many years, the use of cerium(iv) compounds as oxidants was widespread, but more recently a whole range of other compounds have made their appearance. Thus samarium(ii) compounds are now routinely used as one-electron reducing agents and the use of trifluoromethanesulfonate ( triflate ) salts of scandium and the lanthanides as water-soluble Lewis acid catalysts is widespread. Beta-diketonate complexes and alkoxides have also come into use there are even applications of mischmetal in organic synthesis. 

The polycondensation of BHET to PET proceeds in the melt at temperatures of 270-305 °C, under vacuum (< 1 mbar absolute pressure) and in the presence of Lewis acid metal compounds, such as titanium alkoxides, dialkyltin oxide, gallium oxide, germanium oxide, thallium oxide, lanthanide salts, and most commonly, antimony oxide [1,2, 22-26]. Under polymerization reaction conditions, these catalysts are generally converted to their alkoxides with ethylene glycol. Typical of such alkoxides is antimony(III) glycolate, the active catalyst for the majority of the world s PET production [27] (cf. Structure 1). 

In addition to lanthanides, yttrium and scandium salts, which appear in the same column of Mendelejew periodic table, were also shown to act as Lewis acid... 

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