Polymer-supported Lewis Acids

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 

Most recent research has been focused on the application of polymers as chiral auxiliaries in enantioselective Lewis-acid-catalyzed reactions. Studies of Itsuno and co-workers [44] culminated in the development of a polymer-supported catalyst containing a chiral oxazaborolidinone with oxyethylene crosslinkages which gave the Diels-Alder adduct of cyclopentadiene and methacrolein in 88 % isolated yield with an exotendo ratio of 96 4 and 95 % e. e. for the exo adduct. A variety of polymer-supported chiral Lewis acids was also investigated by Mayoral et al. [45]. Some supported catalysts were more active than their homogeneous analogs, but enantioselectivity was always lower. 

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Itsuno et al. explored the possibility of using polymer-supported chiral Lewis acids in a model Diels-Alder reaction of methacrolein with cyclopentadiene [23a]. By using an insoluble polymer-supported Lewis acid [23], prepared from borane with cross-linked polymers with a chiral moiety such as an A-sulfonylamino acid, the Diels-Alder adduct is obtained in good yield with almost perfect exo selectivity and moderate enantioselectivity (Eq. 23). 

Catalyzed enantioselective Mukaiyama-aldol reactions have been developed extensively [101] and chiral polymer-supported Lewis acids are the catalysts of choice. Polymer-supported chiral A(-sulfonyloxazaborohdinones 86 and 87, prepared by copolymerization of styrene, divinylbenzene, and chiral monomers derived from L-valine and L-glutamic acid, respectively, have been used for aldol reactions [102]. The rates of reaction using the polymeric catalysts were slow and enantioselectivity was lower than was obtained by use of the low-molecular-weight counterpart (88). The best ee obtained by use of the polymeric catalyst was 90 % ee with 28 % isolated yield in the asymmetric aldol reaction of benzaldehyde with 89 (Eq. 27). 

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-... 

Kobaysahi, S. Nagayama, S., A Microencapsulated Lewis Acid. A New Type of Polymer-Supported Lewis Acid Catalyst ofWide Utility in Organic Synthesis. / Am. Chem. Soc. 1998, 120,2985. 

Polymer-supported Lewis acids represent an important target as they can be applied for the catalysis of a number of different organic reactions of interest for the preparation of fine chemicals. From an applied point of view, those materials have several advantages associated with their easier separation and the potential of their use in continuous processes [1]. Additionally, the presence of the polymeric matrix can modify the activity and selectivity of the reaction under study. 

Scheme 6.20 The use of polymer-supported Lewis acid catalyst 76 for the enantioselective synthesis of the cyanohydrin 77, using EOF as the pumping mechanism.

Abstract. Three types of polymer-supported rare earth catalysts, Nafion-based rare earth catalysts, polyacrylonitrile-based rare earth catalysts, and microencapsulated Lewis acids, are discussed. Use of polymer-supported catalysts offers several advantages in preparative procedures such as simplification of product work-up, separation, and isolation, as well as the reuse of the catalyst including flow reaction systems leading to economical automation processes. Although the use of immobilized homogeneous catalysts is of continuing interest, few successful examples are known for polymer-supported Lewis acids. The unique characteristics of rare earth Lewis acids have been utilized, and efficient polymer-supported Lewis acids, which combine the advantages of immobilized catalysis and Lewis acid-mediated reactions, have been developed. 

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]... 

Kobayashi, S. and Nagayama, S. 1998. A microencapsulated Lewis acid. A new type of polymer-supported Lewis acid catalyst of wide utility in organic synthesis. /. Am. Chem. Soc. 120 2985-2986. 

For practical reasons, polystyrene is used as polymer support in many cases. In most cases of the polymer-supported Lewis acids polystyrene-based cross-linked polymers have still been utilized as support materials mainly due to their easy synthesis and chemical modiflcations. Some other examples of polyethylene (PE) fibers, PE glycols and silica-supported Lewis acids were also developed. Microenvironments within network structme are different in individual support materials. 

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... 

Scheme 7.30 Polymer-supported Lewis acids for a flow asymmetric Diels-Alder reaction.

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