Microencapsulated Sc OTf

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-... 

Microencapsulated Sc(OTf)3 has shown considerable promise for both batch and flow reactions in those procedures in which the reaction was recirculated through columns containing the reagent. This reagent was found to be more reactive than the monomeric Lewis acid. One example is the TMS-mediated Michael reaction shown in Figure 3.25. Microencapsulated Sc(OTf)3 was also found to be useful for Mannich reactions, Michael additions, and Friedel-Crafts—like acylations [51]. 

FIGURE 3.25. TMS-mediated Michael reaction using microencapsulated Sc(OTf)3... 

Microencapsulated Sc(OTf)3 was used as the catalyst, allowing clean isolation of the product from the reaction mixture by extraction with diethyl ether. The ionic liquid containing the Lewis acid was repeatedly reused. ... 

Similarly, microencapsulated ytterbium triflate (MC Yb(OTf)3) has been prepared [38]. MC Yb(OTf)3 was found to be effective in the aza-Diels-Alder reaction using ethyl vinyl ether as a dienophile (Scheme 29). In the presence of MC Yb(OTf)3, N-benzylideneaniline reacted with ethyl vinyl ether to afford the corresponding tetrahydroquinoline derivative in a good yield. MCYb(OTf)3 could also be recovered quantitatively and could be reused. The yield of the 2nd and 3rd runs were comparable to that of the 1st run. In a similar reaction using Sc(OTf )3 or MC Sc(OTf )3 as a catalyst, ethyl vinyl ether polymerized rapidly and no desired adduct was obtained. 

The microencapsulation technique has been developed by Kobayashi for the immobilization of metal Lewis acid catalyst in polymer [57]. The catalysts were immobilized on to polymers using physical environment by polymer backbones and interaction between 71 electrons of benzene rings of the polystyrenes and vacant orbitals of the catalysts. Microencapsulated scandium trifiuoromethanesul-fonate was easily prepared from polystyrene and Sc(OTf)3 in cyclohexane, which was successfully used in several important Lewis acid catalyzed carbon-carbon bond-forming reactions [57]. 

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]. 

Scandium(III) trifluoromethanesulfonate (Sc(OTf)3) is extensively used in organic synthesis to catalyze a wide variety of carbon-carbon bond-forming reactions in aqueous media. The polymer microencapsulated triflate ME catalysts are recoverable and reusable, often reduce metal leaching, and have activity similar to that of their homogeneous counterparts [128]. 

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