Graphite aromatic acylation

Table 4.24 Aromatic acylation with acyl halides catalyzed by graphite...

Table 4.25 Aromatic acylation with carboxylic acids in the presence of graphite-para-toluenesulfonic acid mixture...

Tab. 7.8 Acylation of aromatic compounds in the presence of a small amount of graphite and under the action of MW irradiation [27, 66].

Comparison with previous FC acylations, the above processes are clean, without aqueous workup, and therefore without effluents ( green chemistry ). The graphite is, moreover, inexpensive and can be safety stored or discarded. Its activity is, however, limited to activated aromatic compounds. 

Acylations. Catalyzed by graphite the Friedel-Crafts acylation as well as the transformation of ethers to esters by acyl halides in refluxing 1,2-dichloroethane are realized. Although aliphatic chlorides are inferior to aromatic halides this method is quite general as shown by the synthesis of benzoates (e.g., allyl benzoate from allyl benzyl ether and methyl benzoates from methyl f-butyl ether). 

To overcome tbe problem of tbe bigb amount of graphite utilized and mainly to avoid tbe use of tbe expensive and pollutant acyl balides and benzene, graphite can be coupled with para-toluenesulfonic acid and utilized to activate the more ecocompatible carboxylic acids toward electrophilic acylation. i With this catalyst, not only a solvent-free process can be developed but also high yields with not activated aromatic substrates and with unactivated carboxylic acids (i.e., para-nitrobenzoic acid) can be obtained (Table 4.25). 

Sarvari, M. H. and Sharghi, H. 2005. Solvent-free catalytic Friedel-Crafts acylation of aromatic compounds with carboxylic acids by using a novel heterogeneous catalyst system p-toluenesulfonic acid/graphite. Helv. Chim. Acta 88 2282-2287. 

Graphite combined with methanesulfonic acid represents an effective catalyst for the acylation of phenols and naphthols, wifh carboxylic acids giving a good yield of ortho-hydroxy aryl ketones (Table 5.4). The reactions seem to be faster with activated aromatic carboxylic acids. It must be emphasized that, in the absence of graphite, in the model reaction between mefa-cresol and BAC, the product is isolated in lower yield (20% versus 81%). 

Graphite can promote the Friedel-Crafts acylation reaction of active aromatic compounds such as anisoles and polymethylbenzenes with acyl halides to afford the corresponding aromatic ketones [101]. In a typical experiment, graphite is added to a mixture of anisole and benzoyl bromide in benzene, and the mixture is heated under reflux for 8h to afford para-methoxybenzophenone in 80% yield. Different acyl halides and several anisoles and polymethylbenzenes are utilized to give the corresponding products in high yields (60-92%). 

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