Scandium triflate catalyzed Diels-Alder reaction

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... 

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). 

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]. 

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. 

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. 

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. 

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. 

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. 

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. 

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