Para-Xylene acylation

Table 5.27. Catalytic Action of Brdnsted or Lewis Acids in the Acylation of para-Xylene by Benzoyl Chloride347...

The remarkably low reactivity of tetrafluoroparacyclophane (29) to Friedel-Crafts acylation in comparison with para-xylene indicates the deactivation of the nonfluorinated aromatic ring of 29 because of electron-deficiency due to a through-space polarization (Scheme 1.33) [26]. The tt-tt stacking interaction between arenes and perfluoroarenes has been employed for modification of the liquid crystals [27]. 

In the preparation of mixed carboxylic-perfluoroalkane sulfonic anhydrides from the corresponding acids, water formahon is responsible for the low yields of the acylation product. By removing water from the reaction mixture by azeotropic distillation and adsorption onto molecular sieves, the yield of benzoylahon of para-xylene with BAC in the presence of perfluorobutanesulfonic acid (10% mol) is improved to 90%. 

The direct 1,4-dimethylanthraquinone 17 synthesis via double acylation of para-xylene with phthalic anhydride can be performed in 15% yield at 138°C, but an equimolecular amount of triflic acid is necessary (Scheme 3.3). Better results are achieved by using phthaloyl dichloride under triflic acid catalysis (5% mol) at the same temperature anthraqui-none 17 can be synthesized in 52% yield. Moreover, product 17 is obtained in 89% yield when the same reaction is carried out in 1,2-dichloroethane and a higher amount of triflic acid (10% mol). 

The preparation and use of indium trichloride, gallium trichloride, and zinc chloride supported on MCM-41 as Lewis acids in the Friedel-Crafts acylation of aromatics with acyl chlorides was investigated. The support itself shows no catalytic activity in the benzoylation of benzene with BC, whereas the highest activity is showed by the supported indium trichloride. The order for acylation activity of the supported metal chloride (indium trichloride > gallium trichloride zinc chloride) is quite similar to that of the redox potential of the metals [E , +/, (-0.34 V) > E°Ga /Ga ( 0.53 V) > E 2n +/zn ( 0.74 V)] and confirms a possible relationship between the redox potential and the catalytic activity of the supported metal chloride. The reaction can be efficiently applied to a variety of aromatic compounds, including toluene, para-xylene, mesitylene, anisole, and 2-MN (70%-90% yield), confirming the moisture insensitivity of the catalyst. ... 

In a kinetic study of the acylation of toluene, with p-xylene and the corresponding perdeutero compounds with aroyl triflates, correlation was found between the primary kinetic isotope effect and the ortho para ratio.35 Different conformations of the bent cr complexes36 for the two isomers resulted in a much higher rate of deprotonation and rearomatization for the para isomer. By appropriately selecting reaction conditions and thereby affecting the ratio of the two conformations, unusually high amounts of ortho products may be obtained.37,38... 

Pioneering and extensive work in this field was realized in 1985 [8] by use of exchanged Ce Y-zeolite as the catalyst for the Friedel-Crafts acylation of toluene (Eq. 3) and xylene with carboxylic acids. Different aspects of this initial work are of interest. Firstly, it shows that the mild acidity of zeolites is sufficient to effect the reaction, and, secondly, it shows that this reaction can be conducted with carboxylic acids and not the corresponding acid chlorides. Only the more lipophilic acids were found reactive whereas no acetylation occurs with acetic acid. The reaction with toluene (Table 1) is extremely para selective and more selective than a conventional aluminum chloride homogeneous process. The different reactivities reported in this paper are essentially because of differences between preferential adsorption of the substrates on the catalyst and not their intrinsic activity. 

Para-acetylation of anisole with AAN is performed in the presence of ytterbium triflate (20% mol). The reaction proceeds smoothly even when the catalyst is employed in a small amount (5% mol), and the desired acylation product is obtained in 79% yield. Several substituted benzenes are subjected to ytterbium-triflate-catalyzed acetylation. Though acetylation of benzene does not occur, introduction of a methylthio or dimethylamino group on the aromatic ring gives the acylation products in high yields. The presence of a methyl group is less effective, and mete-xylene is acy-lated to 2,4-dimethylacetophenone in only 25% yield. Ytterbium triflate can be recovered from the aqueous layer by simple extraction and reused for two additional runs in the model acetylation of anisole, affording para-acetylanisole with almost the same yield as in the first use. 

In a series of valuable studies, the activity of various cation-exchanged Y zeolites in the acylation of toluene and xylenes with aliphatic carboxylic acids was investigated In a model reaction between toluene and octanoic acid, the activity of rare earth-, transition metal-, and alkaline earth-cation-exchanged Y zeolites was considered. CeY zeolite exhibits the highest activity (3 yield = 75%) in the para-acylation (Scheme 4.3) in agreement with the results published in an early study qj., contrary, unmodified Y zeolite shows a lower activity (3 yield < 40%), and transition metal and alkaline-earth-exchanged Y are nearly inactive. 

Analysis of products obtained with para- and mefa-xylene shows that all the catalysts are more active for the acylation than the alkylation reaction. Indeed, indanones 58 and 59 (Figure 4.11) are obtained in 65% and 79% yield. 

Geneste and coworkers in 1986 reported that zeolites catalyzed Friedel-Crafits acylations in presence of a Ce + exchanged zeolite Y catalyzed the acylation of toluene and xylenes with carboxylic acids (Table 11.2) [42]. They also mentioned that only mild acidity is sufficient for Friedel-Crafts acylations which can be induced by means of zeolites catalysts. The reaction exhibited a very high para-selectivity. 

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