Nafion Friedel Crafts alkylation

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. ... 

Frequently substantially more than catalytic amounts of a Lewis acid metal halide are required to effect Friedel-Crafts alkylation. This is due partly to complex formation between the metal halide and the reagents or products, especially if they contain oxygen or other donor atoms. Another reason is the formation of red oils. Red oils consist of protonated (alkylated) aromatics (i.e., arenium ions) containing metal halides in the counterions or complexed with olefin oligomers. This considerable drawback, however, can be eliminated when using solid acids such as clays,97 98 zeolites (H-ZSM-5),99,100 acidic cation-exchange resins, and perfluoro-alkanesulfonic acid resins (Nafion-H).101-104... 

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. 

Fujiwara et al.227 tested a nanocomposite material having Nafion immobilized in MCM-41 mesoporous silica in Friedel-Crafts alkylations with benzyl alcohol. Whereas Nafion-MCM-41 showed lower activity in the alkylation of toluene than 13% Nafion SAC-13 under identical conditions, it exhibited increased activity when used in the alkylation of para-xylene. 

Satisfactory results were obtained in the Nafion-H-catalyzed gas-phase alkylation of aromatic hydrocarbons with alkyl halides235 [Eq. (5.88)]. Alkylhalides are reactive Friedel-Crafts alkylating agents and give high conversions when alkylating benzene in the gas phase over Nafion-H catalyst. For example, in the alkylation of benzene with isopropyl chloride, conversions as high as 87% were achieved (Table 5.17, run 11). Conversions, however, were temperature and contact time dependent (Table 5.17). 

Isopropylation of toluene by isopropyl halo- and alkyl-sulfonates (equation 54) has been performed by Olah et al. A variety of cattdysts, such as AlCb, AlCb-MeNOa and Nafion-H were employed, and the isomer distribution (o, m, p) in the product was determined. Sartori et al have recently reported an unusual Friedel-Crafts alkylation of lithium phenolates with ethyl pyruvate in the presence of AlCb to afford a-(2-hydroxyphenyl)ethyl lactates (22), which are the precursors of 3-methyl-2,3-dihydrobenzo-furan-3-ols (23 Scheme 6). 

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. 

Nafion resins in the acid form have been successfully used as catalysts in Friedel-Crafts alkylations and acylations60 as well as in gas-phase esterifications. In contrast to the batch process, which is slow and produces a low yield, the solid heterogeneous H-Nafion resin catalyst was used in a flow reactor, resulting in close to quantitative yields at contact times of several seconds only61. 

Lewis acids supported on graphite as well as the perfluorinated resin sulfonic acids (Nafion-H) have been studied for their abilities to perform Friedel-Crafts alkylation of benzene and transalkylation of alkylbenzenes (Olah et aL, 1977). The graphite-supported materials were less stable than the perfluorinated resin sulfonic acid. Subsequent studies investigated the use of this latter resin in the methylation of phenols, benzene, and alkylbenzenes (Kaspi and Olah, 1978 Kaspi et aL, 1978), the nitration of aromatics (Olah et aL, 1978a), the rearrangement of allyl alcohols to aldehydes (Olah et al., 1978b), the pinacolone rearrangement (Olah and Meidar, 1978) and isomerizations of alkylbenzenes (Olah and Kaspi, 1978). 

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). 

Disproportionation (transalkylation) and positional isomerization usually take place simultaneously when either linear or branched alkylbenzenes are treated with conventional Friedel-Crafts catalysts or with Nafion-H. The reactivity of alkyl groups to participate in transalkylation increases in the order ethyl, propyl < isopropyl < tert-butyl.117 207 217... 

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