Solid acids Friedel-Crafts alkylations

Of the many methods which have been published so far for the substitution of existing crowns, probably the most straightforward are Friedel-Crafts alkylation or acylation reactions. Cygan, Biernat and Chadzynski have reported the successful di-t-butylation of dibenzo-24-crown-8 using t-butanol as alkylating agent s . The crown was heated at 100° for 4 h in the presence of excess t-butanol and 85% phosphoric acid. The product was obtained as a crystalline (mp 52—74°) solid in 93% yield. The alkylated crowns are presumably a mixture of isomers substituted once in each ring as illustrated in Eq. (3.14). 

Friedel-Crafts alkylations are among the most important reactions in organic synthesis. Solid acid catalysts have advantages in ease of product recovery, reduced waste streams, and reduction in corrosion and toxicity. In the past, people have used (pillared) clays (18), heteropolyacids (19) and zeohtes (20) for Friedel-Craft alkylations, with mixed success. Problems included poor catalyst stabihty and low activity. Benzylation of benzene using benzyl chloride is interesting for the preparation of substitutes of polychlorobenzene in the apphcation of dielectrics. The performance of Si-TUD-1 with different heteroatoms (Fe, Ga, Sn and Ti) was evaluated, and different levels of Fe inside Si-TUD-1 (denoted Fei, Fe2, Fes and Feio) were evaluated (21). The synthesis procedure of these materials was described in detail elsewhere (22). 

Cumene is an important intermediate in the manufacture of phenol and acetone. The feed materials are benzene and propylene. This is a Friedel-Crafts alkylation reaction catalyzed by solid phosphoric acid at 175-225 °C and 400-600 psi. The yield is 97% based on benzene and 92% on propylene. Excess benzene stops the reaction at the monoalkylated stage and prevents the polymerization of propylene. The benzene propylene ratio is 8-10 1. 

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... 

A novel mesoporous intercalate belonging to the class of mesostructured solid acids known as porous clay heterostructures (PCH) has been synthesized through the surfactant - directed assembly of silica in the two - dimensional galleries of saponite. The new saponite PCH, denoted SAP-PCH, exhibits a basal spacing of 32.9 A, a BET surface area of 850 m2/g and pore volume of 0.46 cm3/g. SAP-PCH is an effective catalyst for the condensed phase Friedel-Crafts alkylation of bulky 2,4-di-tert-butylphenol (DBP) with cinnamyl alcohol to produce a large flavan, namely, 6,8-di-tert-butyl-2,3-dihydro[4H]benzopyran. 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

C-Alkylations have been performed with both support-bound carbon nucleophiles and support-bound carbon electrophiles. Benzyl, allyl, and aryl halides or triflates have generally been used as the carbon electrophiles. Suitable carbon nucleophiles are boranes, organozinc and organomagnesium compounds. C-Alkylations have also been accomplished by the addition of radicals to alkenes. Polystyrene can also be alkylated under harsh conditions, e.g. by Friedel-Crafts alkylation [11-16] in the presence of strong acids. This type of reaction is incompatible with most linkers and is generally only suitable for the preparation of functionalized supports. Few examples have been reported of the preparation of alkanes by C-C bond formation on solid phase, and general methodologies for such preparations are still scarce. 

Friedel Crafts alkylation has been studied by Poliakoff in a continuous-flow reactor (Hitzler et al., 1998a). The reaction of mesitylene and anisole with propene or 2-propanol over a solid acid catalyst (based on a Deloxan support) in sc C02 provided exclusive formation of the monoalkylated products at 50% conversion. Use of the continuous-flow reactor prevents catalyst deactivation, and permits use of comparatively small reactors. The... 

Friedel-Crafts alkylation of mesitylene with 2-propanol Polysiloxane-supported solid acid Solvent tunability Selectivity enhancement Hitzler et al. (1998a, 1998b)... 

In comparison with molecular catalysts, solid catalysts can be isolated from the reaction mixtures by filtration or used in continuous processes this is both environmentally friendly and useful in laboratory-scale experiments. The most important reactions catalyzed by solid superbases are isomerization reactions and the alkylation of substituted arenes in the side chain (Scheme 2). They proceed at room temperature or below with high yield (typically >99%). The surperbase-cata-lyzed alkylation of aromatic compounds complements the acid-type Friedel-Crafts alkylation and acylation, because the latter results in ring alkylation, whereas the former results in side-chain alkylation. 

Sulfated zirconia is a good example of a structural Lewis acid which has been chemically treated to enhance acidity. It has been extensively studied as a solid acid catalyst for vapour phase reactions and we1112 and others14 have found that a mesoporous version of this material is a particularly effective catalyst for liquid phase Friedel-Crafts alkylation reactions and to a lesser extent Friedel-Crafts benzoylations. The commercial (MEL Chemicals Ltd) material SZ999/1 shows a nitrogen isotherm characteristic of a mesoporous solid (surface area 162 m2g, pore volume 0.22 cm3g )- Whereas microporous and mesoporous materials are capable of rapidly catalysing the alkylation of benzene with various alkenes (Table 1), on reuse only the mesoporous... 

Solid acid catalysts are, in principle, applicable to a plethora of acid-promoted processes in organic synthesis [27-29]. These include various electrophilic aromatic substitutions, e.g. nitrations, and Friedel-Crafts alkylations and acylations, and numerous rearrangement reactions such as the Beckmann and Fries rearrangements. 

HPAs, however, is their solubility in polar solvents or reactants, such as water or ethanol, which severely limits their application as recyclable solid acid catalysts in the liquid phase. Nonetheless, they exhibit high thermal stability and have been applied in a variety of vapor phase processes for the production of petrochemicals, e.g. olefin hydration and reaction of acetic acid with ethylene [100, 101]. In order to overcome the problem of solubility in polar media, HPAs have been immobilized by occlusion in a silica matrix using the sol-gel technique [101]. For example, silica-occluded H3PW1204o was used as an insoluble solid acid catalyst in several liquid phase reactions such as ester hydrolysis, esterification, hydration and Friedel-Crafts alkylations [101]. HPAs have also been widely applied as catalysts in organic synthesis [102]. 

Friedel-Crafts alkylation processes were traditionally operated at 65-70°C with AICI3 and at 40-60°C with HF. A variety of solid acid catalysts have been developed at the laboratory level, mainly based on zeolites, heteropolyacids or sulfated zirconia (zirconia treated with sulfuric acid). The most recent industrial achievement is the Detal process (UOP-CEPSA) which is based on silica-alumina impregnated with HF. The selectivity towards linear alkylbenzenes exceeds 95%. The cymene processes use AICI3 in the liquid phase or supported phosphoric acid as catalysts. 

Phenols undergo Friedel-Crafts alkylations with allylic chlorides or allylic alcohols over solid acid catalysts such as acidic KIO clay. For example, 2-buten-l-ol gives 3-aryl-1-butene and 1-ary 1-2-butene, albeit in low yields (12%) (equation 11). Allyl carbocations are involved as the reaction intermediates in these reactions. ... 

Friedel-Crafts alkylation 606-629, 676 Brpnsted acid catalyzed 612 Lewis acid catalyzed 607-611 solid acid catalyzed 612-621 stereoselective 621-626 under supercritical conditions 621 Fries rearrangement 472-478, 631, 685, 773, 774, 776, 778... 

Ideal kaolinites are ineffective as solid acid catalysts even after thermal and acid activation. The effect of impregnating ZnCU, FeCb, MnCl2, SnCL and AICI3 on the catalytic activity of a natural kaolinite and its activated form is examined. The process leads to catalysts with improved activity, the maximum being associated with FeCla when employed in the Friedel-Crafts alkylation of benzene with benzyl chloride. 1 g of supported FeCL catalyst (1 mmol/g clay) gave 86 mole% conversion to diphenylmethane with 100% selectivity when 2 mole benzerie is alkylated with 0.1 mole benzyl chloride. Supported AICI3 catalysts proved to be the least effective for the alkylation studied. 

Continuous Friedel-Crafts alkylation, with high selectivity, of mesitylene and ani-sole with propene or propan-2-ol in scCsHe or SCCO2 using a heterogeneous poly-siloxane solid acid catalyst (Deloxan, ASP 1/7) is described by Poliakoff et al. [52] (see Chapter 12). No comparison was made with the continuous alkylation in a conventional solvent and it is, therefore, diffiadt to judge the technical potential of this approach. 

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. 

Poliakoff et al. introduced the supercritical phase to the Friedel-Crafts alkylation reaction by using SCCO2 or by making propene, one of the reactants, the supercritical fluid [56]. This heterogeneous supercritical phase reaction was conducted continuously and the selectivity was very high if a solid acid Deloxan catalyst was utilized. 

Work on similar supported Lewis-acidic catalysts was further extended to a variety of Friedel-Crafts alkylation [7] and acylation [8] reactions (see Section 5.2.2.4). In these investigations the catalysts are simply prepared by the addition of a liquid to a solid support. A typical catalyst preparation [9] involves treating a previously dried... 

Continuous Friedel-Crafts alkylation in SCCO2 has been demonstrated using a fixed-bed flow reactor and Deloxan , a polysiloxane-based solid acid catalyst. ... 

Mesoporous Metal Oxide Solid Acids Three-dimensional porous metal oxides have been recently synthesized and applied to acid-catalyzed reactions. The use of mesoporous metal oxides is an interesting approach to develop a solid acid catalyst with enhanced activity. The mesopores in the oxide allow the reactants to access additional active acid sites in the pores, resulting in improved rates of acid catalysis. Mesoporous niobium oxides and tantalum oxides treated with phosphoric acid or sulfuric acid have been examined as solid acid catalysts [57-59]. These mesoporous oxides exhibited remarkable activity in Friedel-Crafts alkylation and 1-hexene isomerization in the liquid phase. For sulfated mesoporous tantalum oxides /m-TsL O ), the effect of pore size has been investigated using... 

Carbon-based solid acids exhibited remarkable activity for many acid-catalyzed reactions including esterification, Friedel-Crafts alkylation, hydrolysis, and hydration. Important reactions for biomass transformations, such as the transesterification of fatty acids with alcohols (biodiesel production) [64] and... 

The Friedel-Crafts alkylation of indoles could also be performed in water, as presented in Figure 21, with yields as high as 97% in the presence of Keggin heteropoly acids, solid superacid catalysts [40]. 

For the sake of completeness, it should be mentioned that the use of microreactors and miniaturized flow reactors for the Friedel-Crafts alkylation of aromatic compounds has also been documented by other authors. For example, the Friedel-Crafts alkylation ofbenzenewithcydohexene using H2SO4 as a catalyst has been described [7]. The reaction was conducted in a static micromixer giving 58% cyclohexylbenzene. PoUakofi and coworkers have carried out the Friedel-Crafts alkylation of anisole with n-propanol in supercritical CO2, testing five different Bronsted solid acid catalysts under systematic variation of process conditions such as temperature and pressure [8]. 

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