Lewis acid scandium-based

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Since the discovery of Sc(OTf)3 as a water-compatible Lewis acid, several immobilized scandium catalysts that work efficiently in water have been developed. Polymer-supported scandium-based Lewis acid (7) worked well in several carbon-carbon forming reactions in water (Schemes 12.67-12.69) [168]. It was suggested that the spacer could help to form hydrophobic reaction environments in water. As expected, (7) was easily recovered and reused. 

Scandium-based Lewis acid catalysts, especially its trifluoromethanesulfonate (Sc(OTf)3), are versatile catalysts that can accelerate a variety of organic reactions even in water as an environmentally benign solvent [48]. Various kinds of polymer-immobilization techniques have been developed as shown below. 

Polymer-supported scandium-based Lewis acid also shows high activity in this reaction in water and can be easily recovered and reused [70]. Experiments carried out with 4-phenyl-2-butanone and tetraallyltin found that the reaction proceeded smoothly at room temperature to afford the... 

Polymer-supported scandium-based Lewis acids are also active for the Diels-Alder reaction of 3-acryloyl-l,3-oxazolidin-2-one with cyclopentadiene. This reaction proceeded smoothly in the presence in water (Equation (8.46)) affording the corresponding adduct quantitatively [70]. 

The Skraup cyclization is another reaction principle that provides rapid access to the quinoline moiety. Theoclitou and Robinson have published the preparation of a 44-member library based on the 2,2,4-trisubstituted 1,2-dihydroquinoline scaffold by the Lewis acid-catalyzed cyclization of substituted anilines or aminoheterocyc-les with appropriate ketones (Scheme 6.237) [420], The best results were obtained using 10 mol% of scandium(III) triflate as a catalyst in acetonitrile as solvent at... 

Under these conditions, base-sensitive substrates may undergo decomposition. To avoid this drawback, protic and Lewis acid can be utilized, such as p-toluenesulfonic acid, zinc chloride, cobalt chloride or scandium triflate. 

Chiral Lewis acids based on titanium [33, 36, 37] or scandium [38] have been used to catalyze the Diels-Alder reactions, and a high asymmetric amplification has been described by Narasaka et al. [36]. This occurs presumably because of... 

Recently, scandium triflate [Sc(OTf)3] was found to be stable in water and successful Lewis acid catalysis was carried out in both water and organic solvents [6-8]. Sc(OTf)3 coordinates to Lewis bases under equilibrium conditions, and thus activation of carbonyl compounds using a catalytic amount of the acid has been achieved [6,7]. In addition, effective activation of nitrogen-containing compounds such as imines, amino aldehydes, etc. has been performed successfully [8]. Encouraged by the characteristics and the usefulness of Sc(OTf)3 as a Lewis acid catalyst, a polymer-supported scandium catalyst was prepared. 

In Sects. 1 and 2, polymer-supported scandium Lewis acids based on Nation and a polyacrylonitrile derivative were discussed. While several useful reactions... 

An early example of a Diels-Alder reaction catalyzed by a polymer-supported Lewis acid involves the use of copper-loaded polystyrene-based polymers in the reaction of furan with 2-cyanoacrylonitrile [41]. Nafion-supported scandium... 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

The enantioselective conjugate addition of hydroxylamines to a range of a, 3-carbonyls occurs with moderate to good ee in the presence of Lewis acidic magnesium- and zinc-based bis-oxazo lines, and with high ee using the scandium-phosphate catalyst (11.124). ... 

Scandium-catalyzed hydroxymethylation reaction with aqueous formaldehyde in water proceeds without any organic cosolvents. In the presence of Sc(0S03Ci2H25)3, a representative LASC, both achiral and asymmetric hydrox-ymethylations proceed smoothly with high selectivities. Lewis acid-base interactions between the scandium catalyst and formaldehyde were suggested to be cracial on the basis of several experiments. This is the first example of a catalytic hydroxymethylation of silicon enolates with aqueous formaldehyde in water that does not require any... 

Less traditional Lewis acid catalysts have also been employed to mediate additions of silyl ketene thioacetals to imines and hy-drazones, including scandium(III) triflate and bismuth(III) triflate. In a significant recent advance, Mukaiyama and coworkers reported Lewis base catalyzed additions of the trimethylsilyl congener of the title reagent to iV-sulfonyl aldimines (eq 18). The reaction is tolerant of water and competent Lewis base activators include readily obtained salts such as lithium acetate and lithium benzamide. 

Chu J, Han X, Kefalidis CE, et al. Lewis acid tri ered reactivity of a Lewis base stabilized scandium-terminal imido complex C—H bond activation, cycloaddition, and dehydrofluorination. J m Chem Soc. 2014 136 10894-10897. 

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