[63937-00-8] · - · Nafion-H · (MW ~1250 maximum)
(highly acidic solid; used as an acid catalyst for alkylation, aroylation, and nitration reactions; also used for ether, ester, and acetal synthesis1)
Alternate Name: Nafion ND50
Solubility: insol polymer; will swell with solvent, but will not dissolve.
Form Supplied in: available commercially (Nafion ND50 in 10-35 mesh beads) as a white powder.
Handling, Storage, and Precautions: irritant; hygroscopic.
Nafion-H is a perfluorinated resinsulfonic acid, and its primary use in organic synthesis is as a superacid catalyst. The acidity of the surface acid groups on Nafion-H is comparable to that of 96-100% Sulfuric Acid,1 and as such will promote a wide variety of acid-catalyzed reactions.
Gas-phase alkylations of aromatic hydrocarbons over Nafion-H catalyst give superior yields and cleaner products than the usual Friedel-Crafts alkylations in solution.2 Several methods of alkylation are available.
It should be noted that alkylation with alkenes (eq 1) results in a slow reduction in the catalytic activity of Nafion-H due to surface polymerization, which deactivates the catalytic sites.1
The life of the catalyst is improved with this method.3 The best example of this case is the methylation of phenol with methyl alcohol (eq 2).4
Unlike Friedel-Crafts alkylation in which polyalkylation is a problem, the gas-phase alkylation reaction over the Nafion-H catalyst shows a high selectivity for monoalkylation (eq 3).5
This may be performed in either the gas or liquid phase over Nafion-H.6 Unlike the Friedel-Crafts method which may produce both alkylated and acylated products, the Nafion-H catalyst shows high selectivity for alkylation (eq 4).7
Nafion-H may be used to catalyze the acylation of benzene with aroyl chlorides and anhydrides (eq 5).8 A great advantage of the Nafion-H catalyst over the typical Friedel-Crafts catalyst is that only 10-30% of the Nafion-H catalyst is needed, unlike acylations typically performed in solution where more than a molar equivalent of the catalyst is required.8
Nitration of substituted benzenes and other aromatic hydrocarbons may be performed with n-butyl nitrate,9 acetone cyanohydrin nitrate,10 or Nitrogen Dioxide.10 The advantage of nitration with Nafion-H catalyst over electrophilic aromatic substitution of nitrate using Nitric Acid in solution is that no aqueous workup is required. Also, the nitro compounds produced may be easily isolated by filtration of the catalyst.10
Ethers are commonly synthesized by an acid-catalyzed mechanism, but require basic workup procedures in typical methods. Nafion-H, however, provides a method for ether synthesis that does not require basic workup, but only filtration of the solid catalyst and evaporation of the solvent.11 Dimethoxymethane may be reacted with primary and secondary alcohols over the Nafion-H catalyst to produce good yields of methoxymethyl ethers,11 which are commonly used as a means for protecting hydroxyl groups (eq 6).12 Tertiary alcohols give only the dehydration products.11
Cyclization of 1,4-butanediols to the corresponding tetrahydrofurans may also be accomplished by simply heating the 1,4-butanediol over the Nafion-H catalyst (eq 7). No workup is required, high yields are obtained (~90%),13 and product isolation is very easy. This method is far superior to the aqueous mineral-acid version of the same reaction, which does require elevated reaction temperatures and aqueous basic workup.13
Acid-catalyzed acetal synthesis is possible with Nafion-H, particularly in the reaction of trimethyl orthoformate with ketones or aldehydes (eq 8).14
Nafion-H is capable of catalyzing the reaction of ketones with Paraformaldehyde to form 1,3-dioxanes in high yield (eq 9).15
Nafion-H has a particular advantage as a catalyst for the Diels-Alder reaction. Nafion-H may be used in catalytic amounts and provides high yields of product with no diene polymerization in many cases.16 Also, product isolation is easy as it involves only filtration of the catalyst.16 Lewis acid catalysts may be used, but these cause polymerization of the diene. Two molar equivalents of the Lewis acid are usually necessary, and Lewis acid complexes with reactants and products may form. All of this causes difficulties in the workup.17,18 As an example of the effectiveness of Nafion-H catalysis, the uncatalyzed reaction of 1,3-cyclohexadiene with Acrolein gives only 25% of the Diels-Alder product when heated to 100 °C for 3.5 h. However, Nafion-H catalysis gives 88% yield after stirring for 40 h at 25 °C (eq 10).16
Nafion-H catalyzes the condensation of Acetone to mesitylene with a selectivity of greater than 99%.19 Unlike other acid-catalyzed condensations which may produce other condensation products such as propene,19 isobutene,19 and pentenes,19 the Nafion-H-catalyzed mechanism is superior in its selectivity for mesitylene (eq 11). It should be noted, however, that overall percentage yields are lower for the Nafion-H-catalyzed mechanism than for other acid-catalyzed mechanisms.19
Nafion-H catalyzes the azidobromination of alkenes to form b-bromoalkyl azides.20 N-Bromosuccinimide and Azidotrimethylsilane are reacted over the Nafion-H surface in a solvent that is 3:1 in methylene chloride-nitromethane. One postulated mechanism is outlined by eqs 12 and 13.20
The combination of the acid catalyst Nafion-H with the Palladium on Carbon hydrogenation catalyst in the presence of hydrogen is useful for performing an aldol condensation followed by hydrogenation of the condensation product.21 Pittman and Liang have effected one-step conversions of acetone to methyl isobutyl ketone,21 benzaldehyde and acetone to 4-phenyl-2-butanone,21 and 2,5-hexanedione to 2,5-dimethyltetrahydrofuran.21 The reaction is simply carried out by reacting the anhydrous starting materials over a mixture of 30-60 mesh Nafion-H into which dry Pd/C catalyst has been mixed. The reacting system is pressurized with H2 gas, and is heated to promote the reaction (eq 14). In the example of the condensation of acetone to methyl isobutyl ketone, Nafion-H acts as a simple acid catalyst to cause condensation to mesityl oxide.
Overall production of mesityl oxide is limited by the equilibrium between it and the original starting materials. If mesityl oxide were isolated and then hydrogenated, yields of methyl isobutyl ketone would be only about 20%. However, the sequential condensation/hydrogenation that may be performed by the Nafion-H and Pd/C catalyst combination results in a yield of methyl isobutyl ketone that is about twice as high. This is because mesityl oxide is continuously removed from the equilibrium by hydrogenation, which pulls the equilibrium to the right.21
Yahya El-Kattan & Jeff McAtee
Emory University, Atlanta, GA, USA