Catalysis of Organic..

348Heteropoly AcidsSOOO O+ +OS1 2 3 4OHHeteropoly acids are strong Brønsted acids containing a heteropoly anion andproton cations. In the case of silicotungstic acid (STA), used in this study, the anionconsists of a heteroatom (silicon) surrounded by 12 metal atoms (tungsten) which areheld together by oxygen bridges. The complex has the formula H 4 SiW 12 O 40 .nH 2 O ina Keggin structure. Heteropoly acids, in either supported form or in bulk form havebeen widely used as acid- as well as oxidation catalysts (3-6). Previous studiesdescribing the use of heteropoly acids in the acylation of anisole with aceticanhydride have been reported (7-9). Phosphotungstic acid was found to have slightlyhigher activity than silicotungstic acid under the conditions studied; this was ascribedto its stronger acid strength and resulted in the para- monoacylation product in highyield.Results and DiscussionThe activity and selectivity of silicotungstic acid as catalyst has been evaluated in theacylation of thioanisole with iso-butyric anhydride. Reactions were performed usinga 3 fold molar excess of thioanisole. The performance of silicotungstic acid wasevaluated in both supported (42%STA/silica) and bulk form. The results are shownin Table 1.Table 1. Acylation of thioanisole with iso-butyric anhydride. Conditions: 60 o C,7.5wt% catalyst loading.Catalyst Reaction Conv. 2 Product selectivity / %timemin % o-ketonep-ketone 3VinylesterSideproducts42%STA/SiO 2 0.5 92.7 1.44 94.33 0 4.2342%STA/SiO 2 1 94.1 1.40 94.30 0 4.3042%STA/SiO 2 5 98.8 1.42 95.26 0 3.3242%STA/SiO 2 7.5 99.4 1.29 94.95 0 3.7642%STA/SiO 2 10 100 1.26 93.95 0.49 4.3042%STA/SiO 2 20 100 1.28 94.77 0.43 3.52Bulk STA 10 99.4 1.17 91.97 0.47 6.39Bulk STA 20 99.7 1.18 92.16 0.51 6.15