Nafion Friedel-Crafts reaction

Another type of soHd supetacid is based on perfluorinated resin sulfonic acid such as the acid form of Du Font s Nafion resin, a copolymer of a perfluorinated epoxide and vinylsulfonic acid or soHd, high molecular weight petfluotoalkanesulfonic acids such as petfluotodecanesulfonic acid, CF2(CF2)qS02H. Such sohd catalysts have been found efficient in many alkylations of aromatic hydrocarbons (225) and other Friedel-Crafts reactions (226). 

Olah et al. (1999) have been able to realize selective cyclisation of o-benzoyl benzoic acid to anthraquinone using dichlorobenzene as a solvent and Nafion-H as a catalyst. This may lead to avoidance of the Friedel-Crafts reaction using a stoichiometric amount of aluminium chloride and resulting in a lot of wa,ste. Many other examples of similar reactions have been reported. 

Nafions in their acid forms are super-acidic, i.e., stronger than sulfuric add and so have high catalytic power. Nafion powders in acid form have therefore been used as catalysts in many types of reactions, snch as esterification and Friedel-Crafts reactions at low temperatures. 

In homogeneous solutions, fluorosulphonic acid combined with antimony pentafluoride is used as a superacid having some useful catalytic properties in organic chemistry. In its protonated form, perfluorosulphonic Nafion powders are used as the catalyst for a variety of organic syntheses. The catalytic power is higher than that of other solid phase superacid catalysts. It enables lower temperatures and pressures to be used and its specificity is higher. Nafion is used as the solid superacid catalyst for the gas phase alkalination of aromatic hydrocarbons and liquid phase esterification or Friedel-Craft reactions ... 

As catalysts Lewis acids such as AICI3, TiCU, SbFs, BF3, ZnCh or FeCl3 are used. Protic acids such as FI2SO4 or FIF are also used, especially for reaction with alkenes or alcohols. Recent developments include the use of acidic polymer resins, e.g. Nafion-Fl, as catalysts for Friedel-Crafts alkylations and the use of asymmetric catalysts. ... 

Solid Catalysts. Nafion-H is an active catalyst for acylation with aroyl halides and anhydrides.60,61 The reaction is carried out at the boiling point of the aromatic hydrocarbons. Yields with benzoyl chloride using 10-30% Nafion-H for benzene, toluene, and p-xylene are 14%, 85% and 82%, respectively. Attempted acylation with acetyl chloride, however, led to HC1 evolution and ketene formation. Nation resin-silica nanocomposite materials containing a dispersed form of the resin within silica exhibits significantly enhanced activity in Friedel-Crafts acylations.62,63... 

In discussing superacids as catalysts for chemical reactions, we will review both liquid (Magic Acid, fluoroantimonic acid, etc.) and solid (Nafion-H, etc.) acid-catalyzed reactions, but not those of conventional Friedel-Crafts-type catalysts. The latter reactions have been extensively reviewed elsewhere (see G. A. Olah, Friedel-Crafts Chemistry, Wiley, New York, 1972 G. A. Olah, ed., Friedel-Crafts and Related Reactions, Vols. I-IV, Wiley-Interscience, New York, 1963-1965). 

Olah and co-workers219 have applied Nafion-H in the benzylation of benzene with benzyl alcohols [Eq. (5.83)] and also reported the reaction of benzyl alcohol with substituted aromatics (toluene, xylenes, mesitylene) to yield diphenylmethanes. The reaction is performed under mild conditions and produces the corresponding dibenzyl ethers as byproducts (2-22%). The substrate and positional selectivity in competitive benzylation of benzene and toluene (1 1 molar ratio) was found to be almost the same as observed in solution-phase Friedel-Crafts benzylation with benzyl chloride (AICI3-CH3NO2). Cyclic products 56 and 57 resulting from cyclialkylation were isolated when Nafion-H-catalyzed benzylation was applied to 2-(hydroxymethyl) diphenylmethane and 3,4-dimethoxybenzyl alcohol, respectively. 

Friedel-Crafts alkylation of benzene,220 221 toluene,222para-xylene,220 and naphthalene223 with benzyl alcohols have been studied over Nafion-silica nano-composite catalysts, including the kinetics of alkylation.221,223 In most cases, 13% Nafion-silica showed the highest activity, testifying again to the much higher accessibility of the active sites. Complete conversion of para-xylene was found in the presence of triflic acid, and it was the only reaction when ether formation as side reaction did not occur. 

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. 

An impressive number of papers and books has been published and numerous patents have been registered on the aq lation of aromatic compounds over solid catalysts. Recently Sartori and Maggi [1] have written an excellent review with 267 references on the use of solid catalysts in Friedel-Crafts acylation. In one section of this review, namely acylation of aromatic ethers or thioethers, the authors report work on acylation by solid catalysts such as zeolites, clays, metal oxides, acid-treated metal oxides, heteropolyacids or Nafion. When examining in details these results, it appeared very difficult for us to build upon these experimental results as the reaction conditions differ drastically from one paper to the next. This prompted us to reinvestigate the scope and limitations of the Friedel-Crafts acylation using heterogeneous solids as catalysts, trying as much as we could to rationalize the observed effects. 

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. 

The activity of Nafion composites of greater surface area was investigated in different organic reactions, e. g. Friedel-Crafts alkylation and acylation, the Fries rearrangement, the dimerization of a-methylstyrene, esterification reactions, and isobutane alkylation. 

Yamato, T., Hideshima, C., Prakash, G. K. S., and Olah, G. A. 1991. Organic reactions catalyzed by solid superacids. 5. Perfluorinated sulfonic acid resin (Nafion-H) catalyzed intramolecular Friedel-Crafts acylation. /. Org. Chem. 56 3955-3957. 

Alvaro, M., Corma, A., Das, D., Fomes, V., and Garda, H. 2005. "Nafion"-functionalized mesoporous MCM-41 silica shows high activity and selectivity for carboxylic acid esterification and Friedel-Crafts acylation reactions. 7. Catal. 231 48-55. 

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. ... 

SCHEME 3.15 Fluorenone and related ketones synthesis via Friedel-Crafts acylation reaction with Nafion. 

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. 

The catalytic activities of polystyrene-bound tetrafluorophenylbis(triflyl)methane in several reactions (direct esterification, Friedel-Crafts acylation, acetalization, the Mukaiyama aldol reaction, and Sakurai-Hosomi allylation) were superior to those of Nafion SAC-13 which is perfiuororesinsulfonic acid (Table 2.3). 

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