Bis(trimethylsilyl) Sulfate-Silica Catalysed Thioacetalisation of Carbonyl Compounds

0040-4039193$6.00 + .OO PergamcnPress Ltd Tetrahedron Letters.Vol. 34. No. 44. Pp. 7127-7128.1993 Printedin Gut Britain Bis(trimethylsily1) sulfate-Silica Catalysed of Carbonyl Compounds Thioacetalisation Harish K. Patney Departmwu Chemistry. School of F’bysicd Sciences of Universityof Technology. Sydney, P 0 Box 123, Broadway Sydney Abstract: NSW 2007.Attsunlii Bis@imethylsilyl) sulfatesilica has beat shown to be an effdrtt carbonyl canpounds at mom lemperatme. reagent system for promoting thioacetalisatiott of 1,3-Dithiolanes have long been used as carbonyl protecting gtoupsQ and as masked methylene functions3 in organic synthesis. They am generally obtained by protic acid or Lewis acid catalysed condensation of carbonyl More recently, anhydrous lanthanum trichloride4. tellurium tetrachlorides, compounds with thiolst. tetrachlorosilane6, polyphosphoric acid trimethylsilyl ester’ and supported reagents such as silica gel treated with thionyl chlorides, anhydrous iron (III) chloride dispersed on silica gels, sulfonated charcoalto and zeolitestt have also been developed for thioacetalisation. However, many of these methods suffer from harsh conditions, expensive reagents and poor yields when applied to hindered and aromatic ketones; consequently there is a continuing need to develop milder and mom efficient reagents for this reaction. (BTS), in the presence of silica gel, is a very mild and We now report that bis(trimethylsily1) sulfate 1 13.14 2 efficient reagent for the conversion of a variety of carbonyl compounds into their respective thioacetals. Both reaction conditions and work-up procedure are very simple and convenient. When a mixture of a carbonyl compound and 1,2-ethanedithiol in dry dichloromethane is treated with BTS and silica, the reaction proceeds smoothly at room temperahue and dithioacetals are obtained in excellent yields (see Table 1). The high reactivity of the BTS-silica reagent system is clearly demonstrated in that the less reactive aromatic ketones also reacted at room temperature and gave the corresponding dithioacetals in high yields (Table 1, entry 13- 16). The efficiency of the BTS-promoted thioacetalisation is not surprising in view of its high Lewis acidityIs. Presumably the presence of silica makes this system even more effective, as it acts as a water scavenger. It should be noted that in the absence of silica the reaction required forcing conditions, boiling benzene with azeotropic removal of water. In conclusion, BTS-silica has been demonstrated to be a highly efficient and mild reagent system for thioacetalisation of carbonyl compounds at room temperature. Further synthetic applications of this reagent system are currently under investigation and will be mported in due course. General Procedwe. To a stirred solution of carbonyl compound (10 mmol) and 1,2-ethanedithiol (lO-20 mmol, see Table 1) in anhyd. dichloromethane (15 mL) was added bis(trimethylsily1) sulfate’s (l-2 mmol), followed by silica (4-8 g, 230400 mesh, dried overnight 100eC). The progress of the reaction was followed by GLC, IR and NMR spectroscopy. Upon completion of the reaction, the mixture was filtered and silica washed with fresh dichloromethane (- 60 mL). The combined dichloromethane layer was washed with a 10% aqueous NaOH solution (2 X 15 mL), water (15 mL), brine (15 mL) and dried (NazS04). 7127 Evaporation of the solvent 7128 under mduced pressure gave the crude product which was purified by chromatography over neutral alumina. Table 1. Tbioacetalisation Entry of Carbonyl Compounds Substrate using BTS/Silica BTS (mmol) 1 1 2 Reagent 1,3-Dithiolanes1.b Reaction Yield (%) time 95 15 min -Ydc Benzaldehyde 1 97 15 mill 3 1 97 15 mill 4 p-Nitmbenzaldehyde 1 89 3Omin 5 Cinnamaldehyde 1 90 15 mill 6 Cyclohexanone 1 93 30min 7 2Methylcyclohexanone 1 96 lh 8 2-Adamantanone 2-Hexanone 1 92 2h 9 1 98 1 hc 10 Cyclopentanone 2 99 lh 11 3Pentanone 2 95 3h 12 2-Gctanone 2 97 3h 13 Acetophenone 2 96 8 h“ 14 9-Fluorenone 2 1.3~Diphenyl-2-propanone 2 95 94 5 h=C 15 16 Benzophenone 2 94 2 dayscd 17 Camphor 2 750 2 day& 8 hd a) Yield of isolatedproductcltarachsed by physicaland spectral data; b) PurityL 98%by GLC: c) 20 mm01of 1.2-ethanedithiol used; d) 8 g of silica gel used; e) 75%convmion by GLC. AckttowZedgntent.The author wishes to thank Mr B. McQuillan for the GLC work and MS L. Ambrose for the technical assistance. References and Notes ’ 1. Greene, T.W.; Wuts, P.G.M. 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