Nafion Diels-Alder reaction

Recently Nafion-H was successfully used in the Diels-Alder reaction of olefin acetals with isoprene and cyclopentadiene (Scheme 4.27). The reactions work well in DCM at room temperature and Nafion-H did not cleave the acetal group [96]. The recovered Nafion-H was used four or five times without affecting the yield of the cycloadducts. 

Table 4.24 Nafion-H catalyzed Diels-Alder reactions in refluxing benzene...

Studies in Lewis acid and LiCi04 (or nafion-H) catalyzed ionic Diels-Alder reactions of chiral and achiral olefinic acetals respectively [96]... 

Vankar and co-workers709 have shown that Nafion-H can catalyze the hetero Diels-Alder reaction between the Danisefsky diene 164 and aromatic imines to form 2,3-dihydro-y-pyridones (Scheme 5.69). The reaction with aromatic aldehydes, however, yields only the Mukaiyama aldol condensation products. 

Olah et al.902 demonstrated in 1979 that Nafion-H is able to catalyze Diels-Alder reaction of anthracene with a number of dienophile (maleic anhydride [Eq. (5.329)], para-benzoquinone, dimethyl maleate, dimethyl fumarate) in chloroform or benzene... 

Nafion-H is a convenient acid catalyst for pinacolone rearrangements. Hydration of acetylenes can be conducted with Nafion-H impregnated with mercury(II) ions. Diels-Alder catalyst. This protic resin catalyzes Diels-Alder reactions, but longer reaction times are needed than in reactions catalyzed by Lewis acids. The reactions are generally conducted in refluxing benzene or chloroform. In the case of dienes that polymerize readily, the reaction is conducted at room temperature for 1-2 days.°... 

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. 

Indium chloride is also an excellent catalyst for ionic Diels-Alder reactions. Acyclic and cyclic olefinic acetals undergo reactions with isoprene and cyclopentadiene in the presence of 20 mol% IrifJ, to form the corresponding cyclic adducts in good yield with good selectivity (Scheme 8.103) [133]. With cyclopentadiene, the endole-xo ratio is fairly good and comparable with that for the LiClO4- and Nafion-H-based reactions. With other Lewis acids, e.g. Yb(OTf)3 or Sc(OTf)5, no cycloadduct is formed. 

Nafion resins have been used not only for the opening of epoxides but also for their isomerization to aldehydes or ketones [137]. Various other rearrangements and isomerizations are catalyzed by this solid acid, in some cases with selectivities higher than those obtained with other solid catalysts [138-140]. Other reactions that have been studied include the Peterson methylenation of carbonyl compounds [141], hetero-Michael additions to unsaturated ketones [142], the Koch-type carbon-ylation of alcohols to form carboxylic acids [143], dimerization of a-methylstyrene [144], addition of carboxylic acids to olefins [145] and Diels-Alder reactions [146]. Notably, in most cases, reutilization of the catalyst is considered but only after an appropriate washing protocol to regenerate its acidity/activity. 

Nafion-Sc was also found to be effective in some other reactions (Schemes 1-3). In typical Lewis acid-mediated reactions, such as Diels-Alder, Friedel-Crafts acylation, and imino Diels-Alder reactions, Nafion-Sc worked efficiently to afford the corresponding adducts in high yields. 

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... 

A variety of other heterogeneously catalyzed Diels-Alder reactions has been reported. Nafion-H, a perfluorinated resinsulfonic acid, catalyzed several Diels-Alder reactions and the isolated yield of the adducts was 80-95% [46]. We have found that a recently described Nafion-silica composite catalyst containing 13 % (w/w) Nafion [47] was approximately 30 times more active than the pure resin in the Diels-Alder reaction of 2,3-dimethylbutadiene with 1,4-naphthoquinone [48]. We also showed that another strong heterogeneous Brpnsted acid, tung-stophosphoric acid supported on silica gel, is a very active catalyst of Diels-Alder reactions of quinones [49] and other enones [50]. 

Polymeric catalysts. A very useful modification of the catalyst is obtained by ligand exchange with a (polyallyl)triflamide, forming polymer-bound scandium triflamide bistriflate. The catalyst has been used in the combinatorial synthesis of a tetrahydroquinoline library from anilines, aldehydes, and alkenes. A related catalyst prepared from ScClj-bHjO and Nafion is effective in several useful synthetic reactions, including allylation, Diels-Alder reaction, and Friedel-Crafts acylation. ... 

Diels-Alder reactions. The mechanism of Nafion-H catalytic... 

In general, excellent results were obtained. Even if the diene is easily polymerized, NAFION preferentially catalyzes the Diels-Alder reaction. As an example, isoprene and p-benzoquinone react at room temperature to give 80% of the adduct after 25 hours. 

Nafion-H is an efficent catalyst for Diels-Alder reactions (Table 3.39). The reactions of anthracene with maleic anhydride, dimethyl maleate, and dimethyl fuma late were carried out at 333 — 353 K in the presence of Nafion-H catalyst in either chloroform or benzene solvent. It should be noted that the reaction of dienophiles with very reactive dienes such as isoprene and 2,3-dimethylbutadiene can be carried out at room temperature to give the adduct in hi( yields. In usual systems, highly reactive dienophiles undergo polymerization during the desired reactions. In Diels-Alder reactions catalyzed by Friedel-Crafts Lewis acid catalysts, excess amounts of Lewis acid halides are required because of the formation of the complex between the halide and carbonyl oxygen atoms. Here again, Nafion-H catalysts allow easy and clean separation of products and the catalysts are not destroyed upon work up. 

Nafion-H (144), a perfluorinated resin-sulfonic acid, is an efficient Bronsted-acid catalyst which has two advantages it requires only catalytic amounts since it forms reversible complexes, and it avoids the destruction and separation of the catalyst upon completion of the reaction [94], Thus in the presence of Nafion-H, 1,4-benzoquinone and isoprene give the Diels-Alder adduct in 80% yield at 25 °C, and 1,3-cyclohexadiene reacts with acrolein at 25 °C affording 88 % of cycloadduct after 40 h, while the uncatalyzed reactions give very low yields after boiling for 1 h or at 100 °C for 3.5 h respectively [95], Other examples are given in Table 4.24. In the acid-catalyzed reactions that use highly reactive dienes such as isoprene and 2,3-dimethylbutadiene, polymerization of alkenes usually occurs with Nafion-H, no polymerization was observed. 

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