Scandium trifluoromethanesulfonate (scan-dium triflate)

[144026-79-9]  · C3F9O9S3Sc  · (MW 492.16)

(a Lewis acid catalyst)

Physical Data: mp > 300 °C.

Solubility: soluble in H2O, alcohol, acetonitrile, and most polar organic solvents.

Form Supplied in: colorless solid.

Preparative Methods: 1scandium triflate is commercially available. On the other hand, it can also be prepared from the corresponding oxide (Sc2O3) and aqueous trifluoromethanesulfonic acid (TfOH). After filtration and concentration of the clear aqueous solution in vacuo, the resulting hydrated salt is dried in vacuo (< 1 mmHg) at 200 °C for 40 h to afford the anhydrous triflate, which is stored over P2O5.

Handling, Storage, and Precautions: the anhydrous triflate is fairly hygroscopic (not decomposed, forms a hydrate) and must be kept in a desiccator over P2O5 and freshly dried in vacuo (< 1 mmHg) at 200 °C for 1 h before using.

Scandium triflate [Sc(OTf)3] is a new type of Lewis acid that is different from typical Lewis acids such as AlCl3, BF3, SnCl4, etc.2 While most Lewis acids are decomposed or deactivated in the presence of water, Sc(OTf)3 is stable and works as a Lewis acid catalyst in water solution. Many reactions proceed smoothly when this reagent is used in catalytic quantities, while stoichiometric amounts of conventional Lewis acids are needed in the same reactions. Moreover, many nitrogen-containing compounds such as imines and hydrazones are also successfully activated by a catalytic amount of Sc(OTf)3 in both organic and aqueous solvents. Sc(OTf)3 can be recovered almost quantitatively after reactions are complete and can be reused. While lanthanide triflates [Ln(OTf)3] have similar properties, the catalytic activity of Sc(OTf)3 is higher than that of Ln(OTf)3 in several cases.

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 benzaldehyde in dichloromethane (1).3 While the reaction proceeds sluggishly at -78 °C in the presence of Yb(OTf)3 or Y(OTf)3, the aldol-type adduct is obtained in 81% yield in the presence of Sc(OTf)3. Obviously, Sc(OTf)3 is more active than Y(OTf)3 or Yb(OTf)3 in this case.

Sc(OTf)3 also catalyzes aldol-type reactions of silyl enolates with acetals. For example, the reaction of 3-phenylpropionaldehyde dimethyl acetal with the ketene silyl acetal of methyl isobutyrate proceeds at 0 °C to room temperature to give the desired adduct in 97% yield (2).2

Sc(OTf)3 is effective in the aldol-type reaction of silyl enolates with aldehydes in aqueous media (H2O-THF) without any significant decomposition of the water-sensitive silyl enolates. Thus, aldehydes available in aqueous-solution such as formaldehyde and chloroacetaldehyde can be directly used to afford the corresponding aldol adduct in high yield (3).2

The Sc(OTf)3-catalyzed aldol-type reactions of silyl enol ethers with aldehydes can be performed in micellar systems using a catalytic amount of a surfactant such as sodium dodecylsulfate (SDS).4,5 In these systems, reactions proceed smoothly in water without using any organic solvent.

Sc(OTf)3 is more soluble in water than in organic solvents such as dichloromethane. After the reaction is complete, the catalyst can be recovered almost quantitatively from the aqueous layer. The recovered catalyst is also effective in a second reaction, and the yield of the subsequent run is comparable to that of the first experiment (4).2

Aldol-type reactions of polymer-supported silyl enol ethers with aldehydes are also catalyzed by Sc(OTf)3.6 A 2-silyloxypyr-role and aldehydes undergo aldol-type reactions in the presence of 5 mol % Sc(OTf)3.7 The aldol-type reaction of silyl enol ethers with chiral h6-(benzaldehyde)chromium complexes has been rep-orted.8 Ketones unusually show higher reactivity than aldehydes in the presence of a Lewis acid such as Sc(OTf)3 or (C6F5)2SnBr2 due to the differentiated recognition of ketone and aldehyde carb-onyls.9 The cross-aldol reactions between aldehydes and scandium enolates prepared from ketones, Sc(OTf)3, and diisopropylethylamine have been reported,10 while the Sc(OTf)3/triphenyl-phosphine reagent is recognized to promote Reformatsky-type reactions between a-bromo carboxylic acid derivatives and alde-hydes.11

Related Sc(OTf)3-catalyzed reactions, i.e. alkylation of silyl enol ethers with sulfur dioxide adduct of 1-methoxybutadiene,12 that of lithium enolates with epoxides13, and benzopyran formation from O-hydroxybenzaldehydes and dimethoxypropane14 have also been reported.

Michael Reactions

The Michael reactions of silyl enol ethers or ketene silyl acetals with a,b-unsaturated carbonyl compounds are catalyzed by Sc(OTf)3 to give the corresponding 1,5-dicarbonyl compounds in high yields after acid work-up (5).2 When the crude adducts were worked up without acid, the synthetically valuable silyl enol ethers could be isolated. The catalyst can be recovered almost quantitatively and reused. Sc(OTf)3 also catalyzes 1,4-addition of PhMe2Si-ZnMe2Li to enones in the presence of 3 mol % of Me2Cu(CN)Li2.15

Mannich-type Reactions

The reactions of imines with ketene silyl acetals proceed smoothly in the presence of Sc(OTf)3 to afford the corresponding b-amino ester derivative in moderate yield (6).16 Sc(OTf)3 shows higher activity than Yb(OTf)3 does in this case. The catalyst can be recovered after the reaction is complete and reused. A Mannich-type reaction of N-(b-aminoalkyl)benzotriazoles with silyl enolates has also been developed.17 Mannich-type reactions of polymer-supported silyl enol ethers with imines18 or of polymer-supported a-iminoaceta-tes with silyl enolates19 are also catalyzed by Sc(OTf)3.

Four-component (silyl enolates, a,b-unsaturated thioesters, amines, and aldehydes) coupling reactions are catalyzed by Sc(OTf)3 to produce the corresponding amino thioester and g-acyl-d-lactam derivatives stereoselectively in high yields (7).20

Mannich-type reactions of aldehydes, amines, and vinyl ethers proceed smoothly in the presence of a catalytic amount of Sc(OTf)3 in aqueous media (8).21 Interestingly, dehydration accompanied by imine formation and successive addition of a vinyl ether proceed smoothly in aqueous solution.

The Sc(OTf)3-catalyzed three-component reactions of silyl enol ethers with aldehydes and aromatic amines can be performed in micellar systems using SDS without using any organic solvent (9).22

Polymer-supported silyl enol ethers (PSSEEs) can be employed in Sc(OTf)3-catalyzed reactions with aldehydes and aromatic amines.23 This process provides a convenient method for the construction of a b-amino alcohol library.

In the presence of a catalytic amount of Sc(OTf)3, benzoylhydrazones react with ketene silyl acetals to afford the corresponding adducts, b-N-benzoylhydrazino esters, in high yield (10).24 Several benzoylhydrazones including those derived from aromatic, aliphatic, a,b-unsaturated aldehydes, and glyoxylate work well. The reactions of polymer-supported hydrazones with silyl enolates are also catalyzed by Sc(OTf)3 to produce pyrazolone derivatives after base treatment.25

Allylation Reactions

The allylation reactions of carbonyl compounds with tetraallyltin proceed smoothly under the influence of a catalytic amount of Sc(OTf)3. The reaction can be carried out in aqueous media as exemplified by unprotected sugars that react directly to give the adducts in high yield (11).26

Related Sc(OTf)3-catalyzed allylation of aldehydes with allyltributyltin in nitromethane,27 allylation of aldehydes with allyl-germanes,28 allylation of acylsilanes,29 and intramolecular allenylation of propargyl silanes30 have also been reported. The allylation of aldehydes with tetraallyltin proceeds smoothly in micellar systems without using any organic solvents.31 Novel allylation of aldehydes with alkeneylepoxides is catalyzed by Sc(OTf)3 to produce d-hydroxy-a,b-unsaturated aldehydes.32

While allylation of imines is also catalyzed by Sc(OTf)3 in dichloromethane to give homoallylic amines in moderate yield,33 three-component reactions of aldehydes, amines, and allyltribut-yltin proceed in micellar systems to afford the corresponding homoallylic amines in good to high yield (12).34 It is suggested that imine formation from aldehydes and amines is very fast under these conditions, and that the selective activation of imines rather than aldehydes is achieved. Allylation of N-benzoylhydrazone or three-component reaction of aldehydes, benzoylhydrazine, and tetraallyltin are also catalyzed by Sc(OTf)3.35

Friedel-Crafts Acylation, Alkylation, and Related Reactions

While a stoichiometric amount of AlCl3 is needed in Friedel-Crafts acylations, a small amount of Sc(OTf)3 smoothly catalyzes the same reaction.36 In the acetylation of thioanisole and O- or m-dimethoxybenzene, a single acetylated product is formed in an excellent yield. In the benzoylation of anisole, both benzoic anhydride and benzoyl chloride are effective, while benzoic anhydride gives a slightly higher yield. Addition of lithium perchlorate (LiClO4) as a cocatalyst improves the yield dramatically (13).37

The Fries rearrangement of acyloxybenzene or naphthalene derivatives proceeds smoothly in the presence of a catalytic amount of Sc(OTf)3 (14).38 The direct acylation of phenol or naphthol derivatives with acid chlorides is also catalyzed to give the 2-acylated product in high yields.

Friedel-Crafts alkylations of arenes with mesylates,39 benzyl or allyl alcohols,40 aldehyde/diol combinations (reductive alkyla-tion),36b 1,3-dienes,41 or alkenes in an ionic liquid42 are also effectively catalyzed by Sc(OTf)3. Sc(OTf)3 works as an efficient catalyst for the condensation reaction of trimethylhydroquinone with isophytol to afford a-tocopherol.43 2-Aminoalkylation of phenols with a-iminoacetates (or glyoxylate/amine) is catalyzed by Sc(OTf)3 to produce amino acid derivatives.44 The Sc(OTf)3-catalyzed alkylations of indoles with a-hydroxy esters,45 aziri-dines,46 acetals,47 and aldehydes48 have been utilized as key steps of total syntheses as exemplified in 15.48

Sc(OTf)3 shows high catalytic activity for the nitration of arenes with nitric acid.49 Sequential Claisen rearrangement/cyclization of allyl phenyl ethers is efficiently catalyzed by Sc(OTf)3 at high temperature in an ionic liquid to give 2,3-dihydrobenzofurans.50

Diels-Alder Reactions and Related Cycloadditions

In the Diels-Alder reaction of methyl vinyl ketone (MVK) with isoprene, the adduct is obtained in 91% yield in the presence of 10 mol % Sc(OTf)3, while 10 mol % Y(OTf)3 or Yb(OTf)3 gives only a trace amount of the adduct.1c,51 Sc(OTf)3-catalyzed Diels-Alder reactions generally provide high yields with high endo selectivities. The present Diels-Alder reaction even proceeds in aqueous media. Thus, naphthoquinone reacts with cyclopentadiene in H2O-THF (9:1) at room temperature to give the corresponding adduct in high yield (100% endo) (16). Sc(OTf)3 also serves as an effective catalyst for Diels-Alder reactions in supercritical carbon dioxide (sc CO2).52

In the presence of 10 mol % Sc(OTf)3, N-benzylideneaniline reacts with 2-trans-1-methoxy-3-trimethylsiloxy-1,3-butadiene (Danishefsky's diene) to afford the corresponding aza Diels-Alder adduct, a tetrahydropyridine derivative, quantitatively (17).16 On the other hand, in the reaction of N-benzylideneaniline with cyclopentadiene under the same conditions, the tetrahydroquinoline derivative is obtained (18).16

Sc(OTf)3-catalyzed three-component coupling reactions of aldehydes, amines, and dienes have also been developed.53 In the presence of 10 mol % Sc(OTf)3 and magnesium sulfate, the reaction of benzaldehyde with aniline and Danishefsky's diene produces the tetrahydropyridine derivative in 83% yield, while the reaction with cyclopentadiene instead of Danishefsky's diene produces the tetrahydroquinoline derivative. Various combinations of aldehydes, amines, and alkenes are possible in these reactions to produce diverse tetrahydroquinoline derivatives in high yield. Moreover, the three-component coupling reactions proceed smoothly in aqueous solution, and commercial formaldehyde-water solution can be used directly (19). Sc(OTf)3-catalyzed three-component aza Diels-Alder cycloadditions also proceed smoothly in an ionic liquid.54

The three-component coupling reaction of benzaldehyde, N-benzylhydroxylamine, and N-phenylmaleimide proceeds smoo-thly in the presence of a catalytic amount of Sc(OTf)3, to afford the corresponding isoxazolidine derivative in good yield and with high diastereoselectivity (20).55

In the presence of a catalytic amount of Sc(OTf)3, imines and alkynyl sulfides undergo [2 + 2] cycloaddition and successive fragmentation to afford a,b-unsaturated thioimidates (21).56 Alky-nyl selenides57 or alkynyl silyl ethers58 undergo similar reactions in the presence of Sc(OTf)3.

Sc(OTf)3-catalyzed [5 + 2] cycloaddition of a methyleneoxindole and a h3-pyridinylmolybdenum complex59 and [4 + 3] cycloaddition of 2-silyloxyacrolein and furan60 have also been reported.

Asymmetric Catalysts Using Sc(OTf)3

The chiral Sc catalyst prepared from Sc(OTf)3, (R)-BINOL, and a tertiary amine in dichloromethane (22) serves as an asymmetric catalyst for the Diels-Alder reactions of an acrylic acid derivative with dienes (23).61 The highest enantioselectivities are observed when cis-1,2,6-trimethylpiperidine is employed as the amine. Even 3 mol % of the catalyst is enough to complete the reaction yielding the endo adduct in 92% ee. The structure of the chiral Sc catalyst is indicated by 13C-NMR and IR spectra.62 A similar Sc(OTf)3/(R)-BINOL/diisopropylethylamine complex serves as an effective catayst for the Diels-Alder reaction of N-benzyloxy-carbonyl-1-aminobutadiene with 3-acyl-1,3-oxazolidin-2-one.63

A chiral scandium catalyst prepared from Sc(OTf)3, (R)- BINOL, and DBU is effective in enantioselective aza Diels-Alder reactions (24).64 The reaction of N-alkylidene- or N-arylidene-2-hydroxyaniline with cyclopentadiene proceeds in the presence of the chiral catalyst and 2,6-di-tert-butyl-4-methylpyridine (DTBMP) to afford the corresponding 8-hydroxyquinoline derivatives in good to high yields with good to excellent diastereo selectivity and enantioselectivity.

In the presence of the chiral scandium catalyst, the 1,3-dipolarcycloaddition of benzylbenzylideneamine N-oxide with 3-(2-bute-noyl)-1,3-oxazolidin-2-one proceeds to yield the endo adduct (endo/exo 99/1) in 69% ee (25).65 Chiral scandium complexes prepared from Sc(OTf)3 and 2,2-bis(oxazolyl)-BINOL derivatives also serve as efficient catalysts.66

Sc(OTf)3/3,3-bis(aminomethyl)-BINOL complexes work as asymmetric catalysts for Michael-type reaction of 2-(trimethyl-silyloxy)furan to 3-(2-butenoyl)-1,3-oxazolidin-2-one.67

Sc(OTf)3/iPr-pybox complexes are found to catalyze asymmetric glyoxylate-ene reaction, though enantioselectivity is low.68

Miscellaneous Reactions

Sc(OTf)3 also catalyzes Meerwein-Ponndrof-Verley reductions,1d Tishchenko reductions,69 Baeyer- Villiger oxidations,70 acetalization reactions,71 acylal formation,72 b-selective glycosilation reactions with thioglycosides,73 acylation reactions of alcohols,74 chemoselective deprotection of silyl alkyl ethers,75 silyl ether protection of alcohols with allyl silanes,76 deprotection of benzylic poly(ethyleneglycol) ethers,77 guanidium formation reactions of carbodiimide with benzylamine,78 stereoselective radical reactions,79reactions of a-diazocarbonyl compounds,80 decarbonylation of aromatic aldehydes,81 dehydration reactions of aldoximes to nitriles,82 rearrangements of epoxides,83 ring expansions of cyclic ethers,84 Ferrier rearrange-ments,85 Prins-type cyclizations,86 additions of 1-trimethylsilyl nitropropanoate to imines,87 sequential carbonyl ene reaction/ace-tylations,88 three-component reactions of aldehydes/amines/tri-butyltin cyanide,89 Biginelli reactions,90 and a new three-compo-nent condensation of 2-aminopyridine/aldehydes/isonitrile.91 Interestingly, multiple reactions in one-pot (Diels-Alder reaction, allylation of aldehydes, and acetylation of alcohols) proceed smoothly in the presence of Sc(OTf)3 (26).92

Related Reagents.

Related polymer-supported scandium triflates, i.e. Nafion-Sc,93 MC Sc(OTf)3,94 PA-Sc-TAD,95 and a polymer-supported scandium that works efficiently in water.96 A Lewis acid-surfactant combined scandium catalyst, scandium tris(dodecylsulfate).97


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Masaharu Sugiura & Shu Kobayashi

The University of Tokyo, Japan



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