Nafion Diels-Alder adducts

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. 

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-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. 

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]. 

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. 

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