Unusual Cleavage of Ethers by Thiophenol on the Surface of Silica Gel Impregnated with Indium(III) Chloride under Microwave Irradiation: Efficient Procedure for the Synthesis of Thioethers through Transthioetherification

Abstract

Cyclic and open-chain benzylic ethers undergo cleavage by thiophenol on the surface of silica gel impregnated with indium(III) chloride under microwave irradiation to produce the corresponding di- and monothioethers.

Key words

cleavage of ether - thiophenol - indium(III) chloride - thioether - microwave - transthioetherification

References

• 1 Ranu BC. Hajra A. Jana U. Tetrahedron Lett.  2000,  41:  531 
• Reviews on cleavage of ethers :
• 3a Ranu BC. Bhar S. Org. Prep. Proced. Int.  1996,  28:  371 
• 3b Tiecco M. Synthesis  1988,  749 
• 3c Maercker A. Angew. Chem. Int. Ed. Engl.  1987,  26:  972 
• 4a Greene TW. Wuts PGM. Protective Groups in Organic Synthesis   3rd Ed.:  Wiley; New York: 1999.  p.454 
• 4b Schopfer U. Schlapbach A. Tetrahedron  2001,  57:  3069 ; and references cited therein

2

General Experimental Procedure. Representative one for the cleavage of THF by thiophenol. A mixture of THF (108 mg, 1.5 mmol) and thiophenol (110 mg, 1 mmol) was added to the surface of silica gel (1 g) impregnated with indium(III) chloride (66 mg, 0.3 mmol, 30 mol%) [The impregnation procedure is as follows: A solution of InCl₃ in a minimum amount of EtOH was added to silica gel HF₂₅₄ (E. Merck), activated by heating at 150 °C for 3 h under reduced pressure and the mixture was stirred for 5 minutes at room temperature. Evaporation of solvent under vacuum furnished the catalyst ready for use] under stirring for uniform mixing. The whole mixture in a pyrex round bottomed flask was then irradiated with microwave in a domestic microwave oven (manufactured by BPL-Sanyo, India; 2450 MHz) at 240 W (20% power) for a certain period of time (with a brief interval after every 5 minutes of heating) as required to complete the reaction. The reaction mixture was then eluted with ether and the ether extract was washed with 10% NaOH solution followed by brine and dried. Evaporation of solvent and purification of the crude product by column chromatography over silica gel (hexane-ether, 9:1) furnished the pure product (225 mg, 82%) as a colorless solid, mp 79-80 °C, IR: 2947, 1581, 1479, 1436, 731 cm^-1; ¹H NMR δ 7.34-7.15 (m, 10 H), 2.96-2.89 (m, 4 H), 1.82-1.76 (m, 4 H); ¹³C NMR δ 136.8 (2C), 129.6 (4C), 129.3 (4C), 126.3 (2C), 33.6 (2C), 28.5 (2C); MS: m/e = M⁺ 274(29), 165(95), 135(10), 123(100), 109(44). Anal. Calcd for C₁₆H₁₈S₂: C, 70.07; H, 6.57. Found: C, 70.01; H, 6.53.