Scandium triflates

It turned out that the dodecylsulfate surfactants Co(DS)i Ni(DS)2, Cu(DS)2 and Zn(DS)2 containing catalytically active counterions are extremely potent catalysts for the Diels-Alder reaction between 5.1 and 5.2 (see Scheme 5.1). The physical properties of these micelles have been described in the literature and a small number of catalytic studies have been reported. The influence of Cu(DS)2 micelles on the kinetics of quenching of a photoexcited species has been investigated. Interestingly, Kobayashi recently employed surfactants in scandium triflate catalysed aldol reactions". Robinson et al. have demonshuted that the interaction between metal ions and ligand at the surface of dodecylsulfate micelles can be extremely efficient. ... 

Benzyl and allyl alcohols which can generate stabilized caibocations give Friedel-Crafts alkylation products with mild Lewis acid catalysts such as scandium triflate. ... 

Scandium triflate and lanthanide triflates also catalyze alkylation by secondary methane-sulfonates. ... 

These reactions are presumed to occur through aroyl triflate intermediates which dissociate to aiyl acylium ions. Lithium perchlorate and scandium triflate also promote acylation. ... 

Skraup/Doebner-von Miller-type reactions with lanthanide catalysts under microwave radiation are efficient for a variety of different anilines. For example, cyclisation of aniline 44 with acetone in the presence of scandium triflate gave the desired product 45 in excellent yield. 

Rare earth metals and scandium trifluoromethanesulfonates (lanthanide and scandium triflates) are strong Lewis acids that are quite effective as catalysts in... 

Scandium triflate [33] is a more active catalyst than the lanthanide triflates and the cycloadditions can also be carried out in aqueous media (Chapter 4). The catalyst is easily recovered from the aqueous layer after the reaction is completed, and can be reused. Some of the cycloadditions carried out in DCM and catalyzed by Sc(OTf)3 are summarized in Table 3.2. 

Scandium triflate is also an effective catalyst for dioxolane formation.246... 

Scandium triflate has been found to catalyze D-A reactions.39 For example, with 10 mol % Sc(03SCF3)3 present, isoprene and methyl vinyl ketone react to give the expected adduct in 91% yield after 13 h at 0°C. 

Allylations, allenylations, and propargylations of carbonyl compounds in aqueous media can also be carried out with preformed organic tin reagent, rather than the use of metals.86,87,88 For example, the allylation reaction of a wide variety of carbonyl compounds with tetraal-lyltin was successfully carried out in aqueous media by using scandium trifluoromethanesulfonate (scandium triflate) as a catalyst (Eq. 8.40).89 A phase-transfer catalyst (PTC) was found to help the allylation mediated by tin at room temperature without any other assistance.90... 

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. 

The addition of alkyl nitronate anions to imines in the presence of a Lewis acid proceeds in high yield with up to 10 1 diastereoselection favoring the anti isomer. This reaction is used for the stereoselective synthesis of 1,2-diamines (Eq. 4.121).167 Scandium triflate catalyzes the addition of 1-trimethylsilyl nitropropanoate to imines with a similar selectivity.35... 

The methoxymethyl ether protecting groups of 33 were then cleaved using triphenylphosphine and carbon tetrabromide. The resulting hydroquinone function was oxidized by palladium on carbon under an atmosphere of air to afford the quinone 52 (70 %). A two-step procedure was implemented to install the diazo function. First, the ketone function of 52 was condensed with N,N -bis( tert-butyldimethylsilyl)hydrazine in the presence of scandium triflate, which formed the Af-tert-butyldimethylsilyl hydrazone 53. The hydrazone (53) was then oxidized using difluoroiodobenzene to afford kinamycin C (3) in 35 % yield. 

Scandium triflate (Sc(OTf)3) was also found to be an effective catalyst in aldol reactions in aqueous media1131 In many cases, SdOTf)3 is more active than Yb(OTf)3, as expected from the smaller ionic radius ofSdffl). 

S. Kobayashi, T. Busujima, S. Nagayama, Scandium Triflate-Catalyzed Strecker-Type Readions of Aldehydes, Amines, and Tributyltin Cyanide in Both Organic and Aqueous Solutions. Achievement of Com-plde Recovery of the Tin Compounds toward Environmentally Friendly Chemical Processes Chem. Commun 1998, 981-982. 

Finally (pathway e, Scheme 5.56), triazoline 103 formed by cyclo addition of azide to glycal 1 can be photolytically converted into a 1,2-aziridine intermediate 104, from which 2-benzylamino-2-deoxy-P-glucosides can be formed on addition of an alcohol and catalytic scandium triflate [176]. 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

Table 9.39 Chiral scandium triflate-catalyzed additions oftrimethylsilyl-substituted-allenes to aldehydes.

Table 9.40 Chiral scandium triflate-catalyzed [3+2] cycloadditions of DPS-allenes to aldehydes.

The advantages of using ionic liquids as solvents for Diels-Alder reactions are exemplified by the scandium triflate catalysed reactions [14] in [bmim][PFg], [bmim][SbF6] and [bmim][OTf] for the reaction shown in Scheme 7.6. Whilst the nature of the anion seems to have little effect, all these solvents give rate enhancements for a range of Diels-Alder reactions compared to when the reactions are carried out in dichloromethane (DCM). Also, the selectivity towards the endo product is higher than in conventional solvents. As well as the enhanced rates and selectivities, the products can also be removed by extraction with diethyl ether and the ionic liquid and catalyst can immediately be reused. Experiments... 

Although the above demonstrated that product control could be achieved in scC02, the difference in selectivity was relatively small. However, later work using a Lewis acid catalyst, scandium triflate, on the Diels-Alder reaction of n-butyl acrylate and cyclopentadiene (Scheme 7.7) showed that the endo exo ratio was again found to rise to a maximum and then decrease again as the pressure, and hence density, was increased (Figure 7.3) [19]. 

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