Acetic anhydride anisole para-acetylation

Amorphous and mesostructured Zr02 solid catalysts impregnated with various amounts of triflic acid were tested in the acylation of biphenyl356,357 and toluene358 (with benzoyl chloride and para-toluyl chloride, respectively, nitrobenzene solvent, 170°C and 130°C). All catalysts exhibited lower activity when compared with neat triflic acid. The mesoporous catalysts, however, showed complete selectivity in the formation of para-benzoylbiphenyl. A triflic acid-silica catalyst, in turn, prepared using an aminopropyl-modified silica, showed good characteristics in the solvent-less acetylation of anisole and 2-methoxynaphthalene with acetic anhydride.359,360 The activity of 1,1,2,2-tetrafluoroethanesulfonic acid, either neat or embedded in silica, was found to be similar to that of triflic acid in the acetylation of anisole.196... 

Anisole acetylation, which was one of the main reactions investigated, was first shown to be catalysed by zeolite ten years ago by Bayer (13), which was confirmed by Harvey et al. (14), then by Rhodia (15). Large pore zeolites and especially those with a tridimensional pore structure such as HBEA and HFAU were found to be the most active at 80°C, in a batch reactor with an anisole/acetic anhydride molar ratio of 5 and after 6 hours reaction, the yield in methoxyacetophenone (MAP) was close to 70% with HBEA and HFAU zeolites, to 30% with HMOR and 12% with HMFI. With all the zeolites and also with clays and heteropolyacids, the selectivity to the para-isomer was greater than 98%, which indicates that this high selectivity is not due to shape selective effects but rather to the reaction mechanism (electrophilic substitution). The lower conversion observed with HMOR can be related to the monodimensional pore system of this zeolite which is very sensitive to blockage by heavy secondary products. Furthermore, limitations in the desorption of methoxyacetophenone from the narrow pores of HMFI are probably responsible for the low activity of this intermediate pore size zeolite. 

Thus, the Rhodia company developed and applied an industrial process for fhe acylation of anisole with acetic anhydride over BEA zeolite to selectively give para-acetylanisole. Furthermore, the reaction of isobutyl-benzene with the same acylating agent leads with high yield and selectivity to para-acetyl isobutylbenzene, which is an intermediate in the synthesis of ibuprofen, an important anti-inflammatory drug. ... 

More recently, Gaare and Akporiaye (1996) studied the acylation of anisole by acetyl chloride and acetic anhydride. They observed that the activity of La-exchanged H-Y zeolite depends on the lantanum content. In fact, as can be seen in fig. 11, the highest yield to acylated compounds was obtained on catalysts with a 93% exchange. However, the activity to acylation reactions depends not only on the catalyst compositions but also on the acylating agents. Thus, it has been shown that activities for the acylation of anisole increases in the order benzoyl < acetyl < propionyl chloride, but, in all cases, a high selectivity to the para isomer was observed (Akporiaye et al. 1993). 

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