Catalytic Friedel-Crafts acylation

For catalytic Friedel-Crafts acylation, several excellent catalysts other than RE have recently been reported [11], Although some are more active than RE, most are water-sensitive and cannot be recovered and reused after the reactions. 

Side Note 5.3. Catalytic Friedel-Crafts Acylations... 

Against this background it is important that—quite fitting in this still new millennium— the first catalytic Friedel-Crafts acylations of (still relatively electron-rich) aromatic compounds were reported (Figure 5.35). Trifluoromethane sulfonates ( triflates ) of rare-earth metals, e. g., scandium(III)triflate, accomplish Friedel-Crafts acylations with amounts of as little as 1 mole percent. Something similar is true of the tris(trifluoromethanesulfonyl)-methides ( triflides ) of rare-earth metals. Unlike conventional Lewis acids, the cited rare-earth metal salts can form 1 1 complexes with the ketone produced, but these are so unstable that the Lewis acid can re-enter the reaction. Whether this works analogously for the third catalytic system of Figure 5.35 is unclear. 

Hachiya, I., Moriwaki, M., Kobayashi, S. Catalytic Friedel-Crafts acylation reactions using hafnium triflate as a catalyst in lithium perchlorate-nitromethane. Tetrahedron Lett. 1995, 36,409-412. 

Alternative catalysts for this reaction are polymer-supported alkyl sulfonic acids [231], even if they show lower performances than the zeolite. Two of the problems in the reactions are the need to vaporize the reactant and the periodic regeneration of the rapidly deactivating zeolite catalysts. It was thus proposed recently that continuous catalytic Friedel-Crafts acylation can be performed in the biphasic medium of an ionic liquid and supercritical carbon dioxide [232]. 

Kawada, A., Mitamura, S., and Kobayashi, S. 1993. Lanthanide trifluoro-methanesulfonates as reusable catalysts catalytic Friedel-Crafts acylation. /. Chem. Soc., Chem. Commun. 1157-1158. 

As already underlined in the introduction to this book, catalytic homogeneous acylation reactions represent a remarkable improvement in the preparation of aromatic ketones because, in the conventional Lewis-acid-promoted reactions, formation of a stable complex between the ketone product and the catalyst implies that at least a stoichiometric amount of catalyst must be utilized. This drawback prompted a great number of studies aimed at setting up the experimental conditions to make catalytic Friedel-Crafts acylation reactions. Some positive results from fhe homogeneous catalytic Friedel-Crafts acylations are described here, with special attention to crucial economic and environmental advantages such as the recycling of expensive catalysts and the development of solvenf-free and highly selective synthetic processes. 

Harada, T., Ohno, T., Kobayashi, S., and Mukaiyama, T. 1991. The catalytic Friedel-Crafts acylation reaction and the catalytic Beckmann rearrangement promoted by gallium(Itl) or an antimony(V) cationic species. Synthesis 1216-1220. 

Fiirstner, A., Voigtlander, D., Schrader, W., Giebel, D., and Reetz, M. T. 2001. A "hard/soft" mismatch enables catalytic Friedel-Crafts acylations. Org. Lett. 3 417-420. 

Izumi, J. and Mukaiyama, T. 1996. The catalytic Friedel-Crafts acylation reaction of aromatic compounds with carboxylic anhydrides using combined catalysts system of titanium(IV) chloride tris(trifluoromethanesulfonate) and trifluoromethanesulfonic acid. Chem. Lett. 739-740. 

Kobayashi, S. and Iwamoto, S. 1998. Catalytic Friedel-Crafts acylation of benzene, chlorobenzene, and fluorobenzene using a novel catalyst system, hafnium triflate and trifluoromethanesulfonic acid. Tetrahedron Lett. 39 4697 700. 

Salehi, R, Khodaei, M. M., Zolfigol, M. A., and Sirouszadeh, S. 2003. Catalytic Friedel-Crafts acylation of alkoxybenzenes mediated by aluminum hydro-gensulfate in solution and solvent-free conditions. Bull Chem. Soc. Jpn. 76 1863-1864. 

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. 

In conclusion, although a great number of studies on the homogeneous and heterogeneous catalytic Friedel-Crafts acylations have been developed, a general solution of the efficient and environmentally acceptable large-scale production of aromatic ketones is still missing. 

Moreover, because of the great interest in catalytic Friedel-Crafts acylations, studies on this topic were continuously progressing, and consequently, it was very difficult to recognize the best results. 

Nevertheless, the even difficult comparative analysis of articles and patents allowed some exciting conclusions concerning, in particular, the feasibility of very efficient catalytic Friedel-Crafts acylation reactions. 

Hafnium triflate has been used for catalytic Friedel-Crafts acylation of aromatic rings [25]. The same catalyst has been found to be effective for heteroaromatics such as furan and pyrrole (58) (Equations 19 and 20) [26]. The reaction of furan with acetic anhydride gave 2-acetylfuran exclusively, whereas that of pyrrole (58) gave a mixture of 2- and 3-acetylpyrroles. 

Friedel-Crafts Acylation. Catalytic Friedel-Crafts acylation of benzene and unactivated benzenes, such as chlorobenzene and fluorobenzene, has been successfully carried out using FIf(OTf)4 and trifluoromethanesulfonic acid as catalysts. Both aromatic and aliphatic carboxylic acid chlorides reacted smoothly under these conditions to afford the corresponding aromatic ketones in good yields (eq 22). ... 

FIf(OTf)4 was found to be effective in the catalytic Friedel-Crafts acylation reactions of various substituted benzenes with acid anhydrides in lithium perchlorate-nitromethane (LiC104-MeN02) (eq 24). This same system was also used for... 

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