Silica anisole acylation

The activity of 42%STA/silica catalysts for the acylation of related aromatic reactants with iso-butyric anhydride was investigated. In the presence of anisole and veratrole, 100% anhydride conversion was observed, leading to the expected para-acylation products. No reaction was observed in the presence of chlorobenzene and other deactivated aromatic systems. 

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... 

Nafion-silica nanocomposite catalysts have also been tested in the Friedel-Crafts acylation of aromatics with acyl chlorides.191,194,371 Anisole, toluene, and... 

A mixture of Yb(OTf)3 (620 mg, 1 mmol), anisole (1, 540 pL, 5 mmol), and acetic anhydride (940 pL, 10 mmol) in nitromethane (5mL) was stirred at 50 °C for 4h. After dilution with water (10 mL), the mixture was extracted with dichlorometh-ane. The organic layers were combined and dried over NaS04. After filtration and evaporation of the solvents, the crude mixture was purified by column chromatography on silica gel to afford 4-methoxyacetophenone (2). The aqueous layer was concentrated in vacuo to give a crystalline residue, which was heated at 190 °C for 4h in vacuo to afford 576.6 mg (93 %) Yb(OTf)3 as colorless crystals. The recovered Yb(OTf)3 was reused in the next acylation reaction. All products of the acylation of aromatic compounds shown in this chapter are known compounds and are commercially available. 

In our study of the acylation of anisole with carboxylic acids [24], and the Nafion-silica composite as the catalyst, no leaching was detected during reaction conditions. This was also not observed in the study of Botella et al. [22] in the alkylation of 2-butene by isobutane. 

Corma s group then published very interesting work on the activity of zeolites in the acylation of anisole with phenyacetyl chloride (Eq. 4) [9]. HY and WP Zeolite are active in this reaction, and modification of catalyst parameters has been examined. The amount of zeolite Na" exchanged and the silica-to-alumina ratio were examined. It was found that the rate of formation of the reaction product correlates linearly with Na exchange, indicating that all the acid sites are active in the reaction. A material does not, therefore, need to have very strong acid sites to have high activity. 

Interesting reactivity has recently been found in the use of Nafion/silica composite material [30]. Nafion resins are active in the acylation of anisole with acetyl chloride, but entrapping highly dispersed nanosized Nafion particles in a silica matrix leads to a much more active catalyst, as indicated in Table 5. 

Gallium(lll) oxide supported on MCM-41 mesoporous silica shows high catalytic activity with little or no moisture sensitivity in the acylation of aromatics wifh acyl chlorides. The cafalysf is utilized in 1,2-dichloro-ethane af 80°C for 3 h wifh differenf aromatic compounds, and aromatic as well as aliphatic acyl chlorides, giving ketones in 54%-82% yield. The activity order of fhe aromatic subsfrafes is benzene (43% yield) < toluene (50% yield) < mesifylene (71% yield) < anisole (79% yield), in agreement with the electrophilic substitution trend previously observed. This acylation reaction follows a probable redox mechanism similar to thaf described in Scheme 4.26. ... 

The acylation of anisole wifh AAN was examined in the presence of silica-supporfed PW.i The supporfed catalysfs are prepared by impregnating silica (surface area 300 m x g i) or mesoporous silica MCM-41 (surface area 1250 m x g i) wifh a mefhanol solution of PW. The acylations are carried out in liquid phase in a glass reactor charged with aromatic substrate and AAN, the substrate taken in excess over the acylat-ing agent no solvent is used. 

A detailed analytical study of fhe acfivity of some solid acid catalysts, including mesoporous silica-supported Nation, in the acylation of anisole with AAN allows the conclusion that catalyst deactivation is caused by the primary ketone product and/or multiple acetylated products in the micropores of Nation catalyst aggregates. i Experiments were performed with a commercially available silica-supported Nation catalyst in a continuous-mode slurry operation by using carbon-dioxide-expanded liquids (nitromethane or nitrobenzene) as solvents. At 90°C, 80% AAN conversion is observed with a TOS of 2 h, but the catalyst rapidly deactivates, and 27% conversion is evaluated after 6 h TOS with a TON value of about 400. The catalyst can, however, be completely regenerated upon nitric acid treatment. These results confirm that silica-supported Nation catalysts are promising alternatives for the traditional aluminum chloride homogeneous Lewis acid catalyst. 

H. M. C. 2004. Friedel-Crafts acylation of anisole with acetic anhydride over silica-supported heteropolyphosphotungstic acid (HPW/SiOj). /. Mol. Catal. A Chem. 209 189-197. 

Izumi et al. pioneered the use of heteropoly acids as catalysts for aromatic acylation. Silica-supported acids H4[SiWi204o] and H3[PWi204o] were found to effectively catalyse the acylation of p-xylene with benzoyl chloride. Cs2.5Ho.5[PWi204o] showed high efficiency in the acylation of activated arenes, such as p-xylene, anisole, mesitylene, etc., by acetic and benzoic anhydrides and acyl chlorides. This catalyst provided higher yields of acylated arenes than the parent acid H3[PWi204o], the latter being partly soluble in the reaction mixture. ... 

Our recent study ° showed that bulk and silica-supported H3[PWi204o] exhibit a very high activity in the acylation of anisole (Eq. 4) with acetic anhydride in liquid phase, yielding up to 98% para and 2 - 4% ortho isomer of methoxyacetophenone (MOAP) at 70 - 110"C and an anisole to acetic anhydride molar ratio AN/AA = 10-20 (Table 1). 

Obtained by Friedel-Crafts acylation of anisole with cyclohexanecarbonyl chloride on mesoporous silica catalyst MCM-41 (73%) [1756]. 

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