Polymer-Supported Sc Lewis Acids

Polymer-supported Lewis acid catalysts based on metals with high coordination numbers, such as Sc, Yb, and Ln, proved to be highly effective in promoting several organic transformations. Umani-Ronchi and coworkers developed polymer-supported indium (I II) Lewis acid [85]. The polymeric In (III) was easily prepared from Amberlyst-Na [86] and In(OTf)3 (Scheme 19.40). They tested the catalytic properties of the polymer-supported indium Lewis acid in the ring... 

Nafion is another choice of polymer support for Sc-based Lewis acids. Nafion-Sc catalyst is readily prepared by treatment of Nafion with ScCb 6H2O in acetonitrile under reflux [116]. Nafion-Sc catalyst has been found to be effective in several synthetic reactions including allylation of carbonyl compounds with tetraallyltin, Diels-Alder reaction, Friedel-Crafts acylation, and imino Diels-Alder reactions. The use of Nafion-Sc in flow systems has also been tested. 

A novel type of polymer-supported Lewis acid, a microencapsulated Lewis acid catalyst was investigated by Kobayashi [117]. Sc(OTf)3 was immobilized on to polystyrene by microencapsulation—Sc(OTf)3 is physically enveloped by polystyrene and stabilized by the interaction between the jr-electrons of benzene rings and vacant orbitals of the Lewis acid. This microencapsulated catalyst was used successfully in several Lewis acid-catalyzed carbon-carbon bond-forming reactions (imino aldol, aza Diels-... 

LLC networks containing catalytic headgroups have also been shown to be useful for heterogeneous Lewis acid catalysis. The Sc(III)-exchanged cross-linked Hu phase of a taper-shaped sulfonate-functionalized LLC monomer has been shown to be able to catalyze the Mukaiyama aldol and Mannich reactions [115] with enhanced diastereoselectivity. This Sc(III)-functionalized Hu network affords condensation products with syn-to-anti diastereoselectivity ratios of 2-to-l, whereas Sc(III) catalysts in solution or supported on amorphous polymers show no reaction diastereoselectivity at all. 

The throughput of the system was subsequently doubled as a result of employing polymer-supported scandium triflate [PS-Sc(OTf)2] (79) as the Lewis acid catalyst, under the aforementioned reaction conditions. Using this approach, the authors demonstrated the generality of the technique, synthesizing a 10 x 5 array of a-aminonitriles, derived from 10 aliphatic and aromatic aldehydes and five amines. The chemoselectivity of the technique was also demonstrated using the reaction of 4-acetylbenzaldehyde (80) and 2-phenylethylamine (81) (Scheme 6.22) whereby 2-(4-acetylphenyl)-2-(phenethylamino)acetonitrile (82) was obtained, in 99.8% yield, as the sole reaction product. 

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. 

Although the origin of the high activity of MC Sc(OTf)3 is not clear at this stage, it was reported that aldimine-Lewis acid complexes were stabilized by using a polymer-supported Lewis acid. Cf. [24]... 

Related Reagents. Related polymer-supported scandium tri-flates, i.e. Nafion-Sc, MC Sc(OTf)3, PA-Sc-TAD, and a polymer-supported scandium that works efficiently in water. A Lewis acid-surfactant combined scandium catalyst, scandium tris(dodecylsulfate). ... 

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. 

Kobayashi and coworkers further developed a new immobilizing technique for metal catalysts, a PI method [58-61]. They originally used the technique for palladium catalysts, and then applied it to Lewis acids. The PI method was successfully used for the preparation of immobilized Sc(OTf)3. When copolymer (122) was used for the microencapsulation of Sc(OTf)3, remarkable solvent effects were observed. Random aggregation of copolymer (122)-Sc(OTf)3 was obtained in toluene, which was named as polymer incarcerated (PI) Sc(OTf)3. On the other hand, spherical micelles were formed in THF-cyclohexane, which was named polymer-micelle incarcerated (PMI) Sc(OTf)3.. PMI Sc(OTf)3 worked well in the Mukaiyama-aldol reaction of benzaldehyde with (123) and showed higher catalytic activity compared to that of PI Sc(OTf)3 mainly due to its larger surface area of PMI Sc(OTf)3. This catalyst was also used in other reactions such as Mannich-type (123) and (125) and Michael (127) and (128) reactions. For Michael reactions, inorganic support such as montmorilonite-enwrapped Scandium is also an efficient catalyst [62]. 

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