Polymer-supported catalysts Lewis acids

Keywords acrylates, acrylamides, fumarates, a, -unsaturated ketones, vinyl ethers, vinyl sulphoxides, chiral dienophiles, chiral dienes, chiral catalysts polymer-supported chiral Lewis acids... 

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

The Diels-Alder reaction of a diene and a dienophile has become one of the most powerful carbon-carbon bond-forming processes [81]. In normal Diels-Alder reactions of an electron-poor dienophile with an electron-rich diene, the main interaction is between the HOMO of the diene and the LUMO of the dienophile. Coordination of a Lewis acid to the dienophile reduces its frontier orbital energies, and this increases the rate of the reaction. Regio- and stereoselectivity are also markedly affected by the Lewis acid. Recent extensive studies on the design of chiral Lewis acids have led to fruitful results in the control of the stereochemistry of a variety of pericyclic reactions. Several chirally modified Lewis acids have been developed for the asymmetric Diels-Alder reaction [82,83] and spectacular advances have recently been achieved in this area. Various kinds of polymer-supported chiral Lewis acid have also been developed. Polymer-supported A1 Lewis acids such as 62 have been used in the Diels-Alder reaction of cyclopentadiene and methacrolein (Eq. 20) [84] as has polymer-supported Ti alkoxide 63 [84]. These Ti catalysts are readily prepared and have high activity in the Diels-Alder reaction. 

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. 

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... 

Supported Lewis acids are an interesting class of catalysts because of their operational simplicity, filterability and reusability. The polymer-bound iron Lewis-acid 53 (Figure 3.8) has been found [52] to be active in the cycloadditions of a, S-unsaturated aldehydes with several dienes. It has been prepared from (ri -vinylcyclopentadienyl)dicarbonylmethyliron which was copolymerized with divinylbenzene and then treated with trimethylsilyltriflate followed by THF. Some results of the Diels-Alder reactions of acrolein and crotonaldehyde with isoprene (2) and 2,3-dimethylbutadiene (4) are summarized in Equation 3.13. 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

Rare earth metal triflates are recognized as a very efficient Lewis acid catalysts of several reactions including the aldol reaction, the Michael reaction, allylation, the Diels-Alder reaction, the Friedel-Crafts reaction, and glycosylation [110]. A polymer-sup-ported scandium catalyst has been developed and used for quinoline library synthesis (Sch. 8) [111], because lanthanide triflates were known to be effective in the synthesis of quinolines from A-arylimines [112,113]. This catalyst (103) was readily prepared from poly(acrylonitrile) 100 by chemical modification. A variety of combinations of aldehydes, amines, and olefins are possible in this reaction. Use of the polymer-supported catalyst has several advantages in quinoline library construction. 

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. 

Keywords Lewis acids, Polymer-supported catalysts, Rare earth triflate, Combinatorial synthesis, Carbon-carbon bond-forming reactions... 

A Microencapsulated Rare Earth Lewis Acid (A New Type of Polymer-Supported Catalyst)... 

Thus, a micro encapsulation technique has been shown to be quite effective for binding catalysts to polymers. Utilizing this technique, unprecedented polymer-supported, microencapsulated rare earth Lewis acids have been prepared. The catalysts thus prepared have been successfully used in many useful carbon-carbon bond-forming reactions. In all cases, the catalysts were recovered quantitatively by simple filtration and reused without loss of activity. This new technique for binding nonpolymer compounds to polymers will be applicable to the preparation of many other polymer-supported catalysts and reagents. 

Polymer supported catalysts have advantages because of the ease of catalyst recovery and the opportunity for simultaneously using otherwise incompatible catalytic systems. Indeed, the immobilization of several catalysts onto a polymer matrix is a unique way of avoiding antagonistic reactions between them, and of lowing reagents to participate in a cascade of reactive processes. For example, polymer-supported catalysts have been used as the Lewis acid catalysts in the carbocationic polymerization of isobutylene. After the reaction, polyisobutylene is obtained by simply filtering the supported catalyst. The reaction cycle can be repeated many times. 

The enantioselective Diels-Alder reaction is another main motif in chiral Lewis acid catalysis. In 1996, Itsuno and coworkers reported an asymmetric Diels-Alder reaction using polymer-supported catalysts under flow conditions. Immobilized chiral oxazoboloridune (34) was prepared from a copolymer of N-sulfonylvabne and borane having styrene moiety, affording the Diels-Alder adduct in an enantioselective manner (up to 71% yield) [126], The authors used a gravity-fed-type column for the flow reaction. Ti-TADDOL-functionalized monolithic resins (35) were developed by Altava and Luis for the asymmetric Diels-Alder reaction (Scheme 7.30). 

In a first series of trials, trimethylsUyl cyanide (TMSCN) was used as the cyanide source and polymer-supported (ethylenediaminetetraacetic acid) ruthenium(lll) chloride as the Lewis acid catalyst (Scheme 23). After the optimisation of the conditions on a model reaction, a small library of compounds was produced, proving the concept by obtaining 100% yields in 2.5 h reaction time. Using flow rates of... 

A very interesting technique that has been used widely in the MTO-catalyzed olefin oxidation reaction is the microencapsulation technique. This technique uses poly(4-vinylpyridine) (PVP), either 2% or 25% cross-linked with divinylbenzene (PVP-2% or PVP-25%, Fig. 4), as well as poly(4-vinylpyridine-/V-oxide) (PVPN-2%, Fig. 4). In addition, 2% cross-linked PS (PS-2%, Fig. 4, X = CH2) and a mixture of PS-2% and PVP-2% (5 1, Fig. 4, X = N) have been used as support polymers. This approach is based on the physical envelopment of the Lewis-acidic MTO by the PS polymer, enhanced by interactions of the 7t-electrons of the phenyl rings with MTO. In the case of the pyridine-containing polymers, Lewis acid-Lewis base interactions between the pyridine moiety and MTO obviously play an important role. In the case of the PVP and PVPN polymers, MTO can be incorporated in the support matrix by mixing the polymer and MTO in ethanol to obtain the desired immobilized catalyst. 

Solid-phase synthesis is of importance in combinatorial chemistry. As already mentioned RuH2(PPh3)4 catalyst can be used as an alternative to the conventional Lewis acid or base catalyst. When one uses polymer-supported cyanoacetate 37, which can be readily obtained from the commercially available polystyrene Wang resin and cyanoacetic acid, the ruthenium-catalyzed Knoevenagel and Michael reactions can be performed successively [27]. The effectiveness of this reaction is demonstrated by the sequential four-component reaction on solid phase as shown in Scheme 11 [27]. The ruthenium-catalyzed condensation of 37 with propanal and subsequent addition of diethyl malonate and methyl vinyl ketone in TH F at 50 °C gave the adduct 40 diastereoselectively in 40 % yield (de= 90 10). 

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