Solid acids Friedel-Crafts acylations

The first example of SILP-catalysis was the fixation of an acidic chloroaluminate ionic liquid on an inorganic support. The acidic anions of the ionic liquid, [AI2CI7] and [AI3CI10], react with free OH-groups of the surface to create an anionic solid surface with the ionic liquid cations attached [72]. The catalyst obtained was applied in the Friedel-Crafts acylation of aromatic compounds. Later, the immobilisation of acidic ionic liquids by covalent bonding of the ionic liquid cation to the surface was developed and applied again in Friedel-Crafts chemistry [73]. 

Octahydrodibenzothiophene (73) has been prepared by the following sequence. Friedel-Crafts acylation of 4,5,6,7-tetra-hydrobenzo[6]thiophene with succinic anhydride (87%) or with the ester chloride of succinic acid followed by hydrolysis (80%), yields the keto acid (74). Huang-Minlon reduction of 74 followed by cyclization of the derived acid chloride with stannic chloride yields l-keto-1,2,3,4,6,7,-8,9-octahydrodibenzothiophene (53) (Section V,A). Reduction of 53 gives 73 as a solid (overall yield 32%). 

The Friedel-Crafts acylation of aromatic compounds is an important synthesis route to aromatic ketones in the production of fine and specialty chemicals. Industrially this is performed by reaction of an aromatic compound with a carboxylic acid or derivative e.g. acid anhydride in the presence of an acid catalyst. Commonly, either Lewis acids e.g. AICI3, strong mineral acids or solid acids e.g. zeolites, clays are used as catalysts however, in many cases this gives rise to substantial waste and corrosion difficulties. High reaction temperatures are often required which may lead to diminished product yields as a result of byproduct formation. Several studies detail the use of zeolites for this reaction (1). 

By in situ MAS NMR spectroscopy, the Koch reaction was also observed upon co-adsorption of butyl alcohols (tert-butyl, isobutyl, and -butyl) and carbon monoxide or of olefins (Ao-butylene and 1-octene), carbon monoxide, and water on HZSM-5 (Ksi/ Ai — 49) under mild conditions (87,88). Under the same conditions, but in the absence of water (89), it was shown that ethylene, isobutylene, and 1-octene undergo the Friedel-Crafts acylation (90) to form unsaturated ketones and stable cyclic five-membered ring carboxonium ions instead of carboxylic acids. Carbonylation of benzene by the direct reaction of benzene and carbon monoxide on solid catalysts was reported by Clingenpeel et al. (91,92). By C MAS NMR spectroscopy, the formation of benzoic acid (178 ppm) and benzaldehyde (206 ppm) was observed on zeolite HY (91), AlC -doped HY (91), and sulfated zirconia (SZA) (92). 

The synthesis of a pyrrole segment common to netropsin and distamycin is shown in Scheme 2ji°l Friedel-Crafts acylation of 1-methylpyrrole (1) followed by nitration at C4 provides 3 in 54% yield. After a haloform reaction, hydrogenolysis, N-protection with B0C2O, and saponification, the pyrrolecarboxylic derivative acid 7 was obtained in 30% overall yield from 3. This monomer is readily chain-extended to the pyrrole-imidazole derivative 9 (Scheme 3)J10 Furthermore, solid-phase synthesis with this and related pyrrole-containing building blocks leads to polyamides that have recently been used in the recognition of a 16 base-pair sequence in the minor groove of DNA.1" ... 

Attempts have recently been made to prepare solid acids by loading triflic acid into various inert oxides including silica,184 titania,185,186 and zirconia.187,188 Silica functionalized with anchored aminopropyl groups was also used to immobilize triflic acid.189 These new catalysts have been tested in a variety of organic transformations, such as alkane-alkene alkylation, Friedel-Crafts acylation, alkene dimerization, and acetalization. Silica nanoboxes prepared by dealumination of Na-X- and Ca-A-type zeolites were also loaded with triflic acid up to 32 wt%.190 The materials were thoroughly characterized but have not been tested as catalysts. 

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. 

Significant advances resulting from the use of aluminosilicate solids were made during the last few years [3-6] and the first industrial application of zeolites in large scale Friedel-Crafts acylations was reported very recently [7]. However, most of the efforts devoted so far focused on the acylation of aromatic compounds. To the best of our knowledge, recourse to heterogeneous aluminosilicate catalysts for the acylation of alkenes has not yet been reported. Conventional methods for alkene acylation [8] involve the use of Br0nsted or Lewis acids such as sulfuric acid [9], boron trifluoride [10], zinc chloride [11], or... 

Synthesis of fine chemicals and intermediates by using Friedel-Crafts acylations is important in organic chemical technology. In most of the cases very good yields and selectivity can be obtained with aluminium chloride as a catalyst in conjunction with nitrobenzene as a solvent. However, with the current environmental restrictions, replacement of aluminium chloride-nitrobenzene pair with solid acid catalysts has great industrial relevance. 

Maximum effort has been directed toward the use of solid acid catalysts. In fact, heterogeneous catalysts can be easily separated from the reaction mixture and reused they are generally not corrosive and do not produce problematic side products. Different classes of materials have been studied and utilized as heterogeneous catalysts for Friedel-Crafts acylations these include zeolites (acid treated), metal oxides, and heteropoly acids already utilized in hydrocarbon reactions. Moreover, the application of clays, perfluorinated resinsulfonic acids, and supported (fluoro) sulfonic acids, mainly exploited in the production of fine chemicals, are the subject of intensive studies in this area. 

The use of HPAs and multicomponenf polyoxometalates as catalysts in liquid-phase reactions was reviewed by Kozhevnikov. Moreover, an interesting minireview was published concerning the Friedel-Crafts acylation of arenes and the Fries rearrangement catalyzed by HPA-based solid acids. The results show that HPA-based solid acids, including bulk and supported heteropoly acids as well as heteropoly acid salts, are efficient and environmentally friendly catalysis for all reacfions analyzed. 

Olah, G. A., Malhotra, R., Narang, S. C., and Olah, J. A. 1978. Heterogeneous catalysis by solid superacids. 11. Perfluorinated resinsulfonic acid (Nafion-H) catalyzed Friedel-Crafts acylation of benzene and substituted benzenes. Synthesis 672-673. 

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. 

Hu, R.-J. and Li, B.-G. 2004. Novel solid acid catalyst, bentonite-supported polytrifluoromethanesulfosiloxane for Friedel-Crafts acylation of ferrocene. Catal. Lett. 98 43 7. 

Yadav, G. D. and Bhagat, R. D. 2005. Experimental and theoretical analysis of Friedel-Crafts acylation of thioanisole to 4-(methylthio)acetophenone using solid acids. /. Mol. Catal. A Chem. 235 98-107. 

Figure 4.3 Friedel-Crafts acylation with solid acid catalysts.

Alizadeh, A., Khodaei, M.M. and Nazari, E. 2007. Sihca sulfuric acid as an efficient solid acid catalyst for Friedel-Crafts acylation using anhydrides. Bull. Korean Chem. Soc. 28(10) 1854-1856. 

Hosseini-Sarvari M, Safary E (2011) Nano-sulfated titania (TiO) as a new solid acid catalyst for Friedel-Crafts acylation and Beckman rearrangement in solvent-free conditions. J Sulfur Chem 32(5) 463 73... 

D. Baudry-Barbier studied the catalytic properties of Sc, La and Nd modified KIO clays in Friedel-Crafts acylations of anisole with benzoyl chloride for yielding 4-methoxybenzophenone, and found La modified KIO clay exhibited the best catalytic activity for acylations of anisole. In comparison with the acid catalyzed reactions using liquid acids, the solid acid catalyzed process was non-polluting and also the final work-up didn t require any aqueous treatment, which exhibited wide prospect for the clean synthesis of building block and fine chemicals [195]. 

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). 

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. 

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