Catalysis of Organic..

Griffin, Johnston, Prétôt and van der Schaaf 349As expected, the para-acylated ketone 3 is the major reaction product, with onlyminor yields of the ortho-acylated product formed. Reaction is very rapid andcomplete conversions are obtained. Catalyst TOF numbers are >19000h -1 under theseconditions. Enolisation and further reaction with the anhydride results in formationof the vinyl ester. Minor amounts of diacile and side products are formed. Similarperformances are observed with both supported and non-supported STA (atequivalent STA loading in reactor). Use of non-supported STA resulted inagglomeration and deposition of the material onto the reactor wall, whereas the silicasupported material could be readily removed by filtration.As the loading of STA on the catalyst support is decreased, incompleteanhydride conversion is observed and significant hydrolysis of the anhydride to formiso-butyric acid is observed (Table 2). Use of silica supported phosphoric acid resultsin lower ketone yields and significant hydrolysis of the iso-butyric anhydride. Blankreactions (catalyst and anhydride, 90 o C, 30 min) indicates that hydrolysis ofanhydride is observed in the presence of these catalysts and may result from eitherdehydroxylation of the silica support or residual water in the catalyst. However thisreaction is slow (42%STA/silica, 44% conversion and 70%H 3 PO 4 /silica, 86%conversion respectively).Table 2. Acylation of thioanisole with iso-butyric anhydride. Conditions: 90 o C,10wt% catalyst loading.Catalyst Conv. 2/ %Ketone 3/ %Acid 4/ %Side prods/ %42% STA/SiO 2 100 70.4 27.6 2.029% STA/SiO 2 100 69.8 26.9 3.317% STA/SiO 2 97.0 64.9 29.2 8.29% STA/SiO 2 84.5 56.5 41.6 1.970%H 3 PO 4 /SiO 2 91.3 52.6 44.7 2.7Incorporation of alumina into the silica supports as silica-alumina mixed oxidesor use of alumina as support results in materials exhibiting poor activity and poorselectivity to desired product, as shown in Table 3. Details of the support materials isgiven in Table 6. Increasing the alumina content of the silica-alumina supportsresults in an increase in Lewis acid sites as the alumina is largely located at thesurface of the oxide. These sites appear to favour hydrolysis of the iso-butyricanhydride to acid over thioanisole acylation. The poor activity of STA/alumina mayresult from partial neutralisation of the STA at the basic sites on the support.Phosphotungstic acid, partially neutralised with Cs (Cs 2.5 H 0.5 PW 12 O 40 ) supported onclay has been reported to show no activity in the acylation of thioanisole with aceticanhydride (10). Non-supported Cs 2.5 H 0.5 PW 12 O 40 displayed high activity in theacylation of benzene with benzoic anhydride (11).