Diels-Alder reactions scandium triflate

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. ... 

Fukuzawa et al. [99] found analogous scandium(III)triflate/ Pr-PyBOx complex as efficient catalyst for the asymmetric Diels-Alder reaction between cyclopentadiene or acyclic dienes and acyl-l,3-oxazohdin-2-ones with up to 90% ee. They latter described the same reaction in super critical CO2 in the presence of MSdA [ 100] that proceeded more rapidly than in CH2CI2 leading to the expected product with analogous selectivity. 

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 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]. 

Kobayashi reported an asymmetric Diels-Alder reaction catalyzed by a chiral lanthanide(III) complex 24, prepared from ytterbium or scandium triflate [ Yb(OTf)3 or Sc(OTf)3], (Zf)-BINOL and tertiary amine (ex. 1,2,6-trimethylpiperidine) [30], A highly enantioselective and endose-lective Diels-Alder reaction of 3-(2-butenoyl)-l,3-oxazolidin-2-one (23) with cyclopentadiene (Scheme 9.13) takes place in the presence of 24. When chiral Sc catalyst 24a was used, asymmetric amplification was observed with regard to the enantiopurity of (/ )-BINOL and that of the endoadduct [31 ]. On the other hand, in the case of chiral Yb catalyst 24b, NLE was affected by additives, that is, when 3-acetyl-l,3-oxazolidin-2-one was added, almost no deviation was observed from linearity, whereas a negative NLE was observed with the addition of 3-pheny-lacetylacetone. 

Lanthanide triflates catalyse Diels-Alder reactions, with the scandium complex as the most effective catalyst, and, again, the catalyst can be recovered and reused, being just as effective in subsequent runs. 

Rare earth metal triflates are recognized as a very efficient Lewis acid catalysts of several reactions including the aldol reaction, the Michael reaction, allylation, the Diels-Alder reaction, the Friedel-Crafts reaction, and glycosylation [110]. A polymer-sup-ported scandium catalyst has been developed and used for quinoline library synthesis (Sch. 8) [111], because lanthanide triflates were known to be effective in the synthesis of quinolines from A-arylimines [112,113]. This catalyst (103) was readily prepared from poly(acrylonitrile) 100 by chemical modification. A variety of combinations of aldehydes, amines, and olefins are possible in this reaction. Use of the polymer-supported catalyst has several advantages in quinoline library construction. 

Simple imines undergo Diels-Alder reactions in the presence of suitable catalysts. Lanthanide triflates, which are stable in water, are especially effective. Thus in the presence of 10 mol% of ytterbium or scandium triflate, Danishefsky s diene 4 reacts with benzylideneaniline in acetonitrile at 0 °C to give the dihydropyridone 76 quantitatively (equation 50) analogous products are obtained from 4 and furylideneaniline, benzyli-denebenzylamine and pentyIidenebenzylamine. In a one-pot version of the reaction, a mixture of an aldehyde, an amine and the diene 4 in acetonitrile containing magnesium sulfate is treated with 10 mol% ytterbium triflate to afford the dihydropyridone in ca 80% yield. Even phenylglyoxal monohydrate can be employed. ... 

In another attempt. Song et al. reported that ionic liquids [bmim]PF act as powerful media in scandium triflate catalyzed Diels-Alder reactions not only for the facilitating of catalyst recovery but also for the accelerating of the reaction rate and improving of selectivity (Scheme 17.5). Various dienes and dienophiles provided excellent yields and selectivity at room temperature in 4 h [47]. 

Scheme 17.5 Scandium triflate catalyzed Diels-Alder reaction under [bmim]PF ...

Song CE, Shim WH, Roh El, Lee S, Choi JH (2001) Ionic liquids as powerful media in scandium triflate catalyzed Diels-Alder reactions signiflcant rate acceleration, selectivity improvement and easy recycling of catalyst. Chem Commun 1122-1123... 

Methylrhenium trioxide (CH3Re03) has proved to be an excellent catalyst in organic solvents, and in water when the dienophile is an a, -unsaturated ketone (or aldehyde). Nearly exclusively one product isomer was formed, the same one that usually predominates [47]. Likewise, scandium triflate [48] and indium trichloride [49] were found to catalyze the Diels-Alder reaction in a tetrahydro-furan/water mixture and in pure water, respectively. 

Yadav, J.S., Reddy, B.V.S., Gayathri, K.U. and Prasad, A.R. (2002) Scandium triflate immobilized in ionic liquids A novel and recyclable catalytic system for hetero-Diels-Alder reactions. Synthesis-Stuttgart, 17, 2537-2541. 

A recent improvement in the rate of the aqueous Diels-Alder reaction came with the use of Lewis acid in aqueous media. The first study deals with the Diels-Alder reaction between cydopentadiene and a bidentate dienophile. A large acceleration can be achieved by the combined use of copper(II) nitrate as a catalyst and water as a solvent [10, 34], Lanthanide and scandium triflates [9, 35] as well as indium trichloride [36] were found to catalyze the Diels-Alder reaction in water. 

A variation in this technique has recently been reported, although not necessarily using the high pressures described above. Diels-Alder reactions can be done under pressure in supercritical carbon dioxide., although the synthetic application may be limited by slow reaction rates. Addition of a Lewis acid appears to overcome this problem, as in Kobayashi s use of scandium triflate in the reaction of methyl vinyl ketone with 2-methyl-1,3-butadiene to give a 93 7 mixture of 137/138. " 7a Rayner and co-workers had previously reported the use of scandium triflate in supercritical and observed that the maximum selectivity was obtained... 

Diels-Alder reactions. Enones substituted at the a -position with an acetal group form cyclic vinyloxocarbenium ions. The enhanced dienophilic reactivity of such species is synthetically useful, and diasteroselective processes have been developed in which the ionization is mediated by fluoroboric acid. Scandium(III) triflate provides lower diastereoselectivity, and BFj OEtj is useless, as extensive decomposition of the substrates occurs. 

Further examples also show particularly impressive selectivity. Kiindig has used the cationic iron catalyst (8.83) in the Diels-Alder reaction to give cycloadduct (8.85). The ruthenium (BINAP) catalyst (8.86) effects the Diels-Alder reaction of cyclopentadiene with 2-substituted acrylates with ees up to 99%. Kobayashi has shown that lanthanide and scandium triflate complexes of BINOL with 1,2,6-trimethylpiperidine, formulated as complex (8.87), are effective with oxazohdinone-based substratesand gives good selectivity in the formation of the Diels-Alder product (8.52). 

The ability of ionic liquids to dramatically improve the yields and selectivi-ties of reactions is clearly shown by the scandium-triflate-catalyzed Diels-Alder reaction.This cycloaddition usually has low turnover (TON < 10-20). Song studied this reaction in ionic liquids, which offered the possibility of recovering the catalyst. The cycloaddition between 1,4-naphthoquinone and 1,3-dimethylbutadiene catalyzed by a very small loading (0.2mol%) of Sc(OTf)3 in the standard [bmim][PF6] ionic liquid was used as a test reaction (Scheme 43). 

Diels-Alder reactions constitute one of the most important methodologies for the constructuction of a cyclic molecular framework. Lanthanide Lewis acid catalyzed Diels-Alder reaction was pioneered by Danishefsky et al., who revealed that NMR shift reagent Eu(hfc)3 served as chiral catalyst in hetero Diels-Alder reaction of silyloxydiene and aldehydes [32]. Later, although Yb(OTf)3 was first introduced for Diels-Alder reactions as an effective catalyst among lanthanide triflates, scandium triflates (Sc(OTf)3), classified as rare earth metal triflate, has gained popularity as a superior catalyst for Diels-Alder reactions [11, 33]. This section highlights several examples of the reactions where lanthanide triflates displayed preferable performance over scandium triflates. 

Song, C.E., Shim, W.H., Roh, E.J., Lee, S.C. Choi, J.H. (2001). Ionic Liquids as Powerful Media in Scandium Triflate Catalysed Diels-Alder Reactions Significant Rate Acceleration, Selectivity Improvement and Easy Recycling of Catalyst, Chem. Commun., pp. 1122-1123... 

Thus, the reaction in aqueous Cu(N03)2 solution proceeded about 800 times faster than in water alone and 250000 times faster than in acetonitrile. This Lewis-acid-catalyzed aqueous Diels-Alder reaction presumably occurs via a transition state such as 2.50 with bidentate complexation to the metal as shown. In a related study, Kobayashi reported that scandium triflate can be used as a water-tolerant Lewis-acid catalyst for a Diels-Alder reaction in (9 1) tetrahydrofuran-water (THF-H2O) [30], though this mixed solvent system cannot take advantage of the hydrophobic effect observed in pure water or highly aqueous mixtures. 

In recent years some work has been done to link oleochemicals with petrochemicals via oligomerization. One possibility is the Dids-Alder reaction of linoleic acid esters with dienophiles, for instance with quinones or ,/Tun saturated aldehydes and ketones [80]. Using scandium or copper triflates as catalysts the reaction can be carried out at very mild temperature conditions (25-40°C) with good yields (< 94%). For the first time in oleochemistry it was possible to carry out Diels-Alder cycloadditions with low catalyst concentrations instead of stoichiometric amounts of Lewis acids. The most successful way to recycle the catalyst was the successive extraction of the triflates with water. After removing the water and drying in vacuum the catalyst was used three times without any loss of yield. 

Non-chloroaluminate ILs, which are in general poor nucleophiles, have proven to be attractive alternative media for Lewis acid catalyzed reactions. ILs may have a reaction rate accelerating effect, and they may improve selectivity and facilitate catalyst recovery. This is the case for scandium triflate catalyzed Diels-Alder cycloaddition [8,9], three-component (aldehyde, aniline, triethylphosphite) synthesis of a-aminophosphonates [10], Claisen rearrangement and cyclization reactions [11], or Friedel-Crafts reactions [12, 13]. 

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