Diphenylsulfonium Methylide1

Ph2SR
(R = =CH2)

[40411-90-3]  · C13H12S  · Diphenylsulfonium Methylide  · (MW 200.32) (R = -Me+BF4-)

[10504-60-6]  · C13H13BF4S  · Methyldiphenylsulfonium Tetrafluoroborate  · (MW 288.14)

(methylene transfer reagent1,3)

Physical Data: Ph2SMe+BF4-: white powder, mp 63-66 °C; positive SIMS (secondary ion mass spectra) in glycerol have been reported.4

Solubility: Ph2SMe+BF4-: sol CH2Cl2, THF, DMF, DMSO, alcohols, H2O.

Form Supplied in: Ph2SMe+BF4-: white powder; commercially available.

Analysis of Reagent Purity: the purity of Ph2SMe+BF4- can be determined by HPLC analysis using a Hypersil 5 SAS column with 80/20 acetonitrile/low UV PIC B8 + 0.1% triethylamine as a mobile phase (document of analysis provided by Lancaster).

Preparative Methods: Ph2SMe+BF4- is usually prepared by direct methylation of diphenyl sulfide.2 Concentrated H2SO4 is added slowly to a mixture of MeOH (which serves as solvent as well as reactant) and Ph2S. After 18 h at 80 °C, the solution is cooled to 0 °C and washed with Et2O. After phase separation, tetrafluoroboric acid is then introduced to the reaction mixture at rt followed by addition of H2O. The reaction mixture is extracted with CH2Cl2. The combined organic layer is then washed with saturated NaHCO3, dried over MgSO4 and concentrated to give the crude product which is purified by recrystallization from MeOH/Et2O in 65% yield.2 Ph2SMe+BF4- can also be prepared by the reaction of diphenyl sulfide with excess methyl iodide in nitromethane at rt in the absence of light. Silver tetrafluoroborate is then added to precipitate the iodide ion. After the reaction is complete, the silver iodide salt is removed by filtration.5-9 The filtrate is then condensed to give the crude product as a pale yellow oil. Stirring of the oily residue in Et2O overnight provides a white powder. After filtration and drying under vacuum, diphenylmethylsulfonium tetrafluoroborate is isolated in 71% yield. Diphenylsulfonium methylide is generated by treatment of Ph2SMe+BF4- with a strong base such as NaH, LDA, n-BuLi, or t-BuLi1,10 under an inert atmosphere. Solvents such as DME and THF are generally used in the reaction to generate the ylide. Alternatively, Ph2S=CH2 can be prepared in situ by the reaction of benzyne with PhSMe.11 Ph2S=CH2 is usually prepared and used immediately without purification.

Handling, Storage, and Precautions: Ph2SMe+BF4- can be stored in a desiccator at rt for months or stored in closed vessels in a dry place away from sources of heat. It is incompatible with strong oxidizing agents.

Introduction.

Like Dimethylsulfonium Methylide and Dimethylsulfoxonium Methylide, diphenylsulfonium methylide is a methylene transfer reagent. Although it has not been as commonly used as dimethylsulfonium and dimethylsulfoxonium methylide in the preparation of epoxides and cyclopropanes, it has been reported to offer the advantage of avoiding sulfur-containing byproducts resulting from the Sommelet rearrangement.12

Epoxidation.

Treatment of 2 equiv of diphenylsulfonium methylide with a heterocycle possessing a leaving group produces a new sulfonium ylide which can be subsequently reacted with a ketone or aldehyde to generate an epoxide. The resultant epoxide can be converted to an acyl derivative by reacting with Lithium Diethylamide (eq 1).3

Cyclopropanation of Double Bonds.

Unlike many other sulfur ylides, diphenylsulfonium methylide can react directly with unactivated alkenes to produce cyclopropane derivatives. Copper(II) Acetylacetonate catalyzed cyclopropanation of alkenes with diphenylsulfonium methylide was reported in 1974. This transition metal induced methylene transfer might involve an intermediate metal-carbene complex.1 Better yields of cyclopropanes are achieved when copper bis(h-3-pentylacetylacetonate) is used instead of Cu(acac)2 in the methylenation of alkenes (eq 2).13

Cyclopropanation of Enones.

Diphenylsulfonium methylide is not commonly used in the cyclopropanation of enones. Reaction of diphenylsulfonium methylide with the enedione resulting from benzoquinone-cyclopentadiene Diels-Alder addition gives the corresponding cyclopropane derivative in only 9% yield.14

Methylation.

Diphenylmethylsulfonium tetrafluoroborate is a powerful alkylation reagent. Alkylation of PhCO2K with diphenylmethylsulfonium tetrafluoroborate in acetonitrile at 20 °C for 7 h provides the corresponding methyl ester in 66% yield.15 Methyl transfer to pyridine-d5 by diphenylmethylsulfonium tetrafluoroborate is reported to be 1300 times faster than that using s-ammoniomethylsulfurane.16 Diphenylmethylsulfonium tetrafluoroborate has also been used in the methylation of nitrogen heterocyclic compounds such as indole, benzimidazole, thymine, and uracil. These reactions are carried out in CH2Cl2 or DMF with bases such as aqueous NaOH, aqueous KOH, or powdered KOH (eq 3).2

O-Methylation of amides such as DMF, DMA, and caprolactam can readily be accomplished with diphenylmethylsulfonium tetrafluoroborate in excellent yields.17 Alkylation of b-keto esters with diphenylmethylsulfonium tetrafluoroborate is also reported to provide the C-alkylated products in excellent yields under aprotic conditions.18

Other Applications.

Diphenylmethylsulfonium tetrafluoroborate has been utilized in the presence of Aluminum Chloride to prepare a low temperature molten salt electrolyte.19,20 It has also been used as a cationic photoinitiator in polymer chemistry.21


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4. Duffin, K. L.; Busch, K. L. Int. J. Mass Spectrom. Ion Processes 1986, 74, 141 (CA 1987, 106, 165 211q).
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10. Taylor, E. C.; Martin, S. F. U.S. Patent 3 989 691, 1976 (CA 1977, 86, 43 883h).
11. Ando, W. Yuki Gosei Kagaku Kyokai Shi 1971, 29, 899 (CA 1972, 77, 74 788z).
12. Lancaster Synthesis 89/90; Lancaster Synthesis Ltd., 1988; p 447.
13. Cimetiere, B.; Julia, M. SL 1991, 271.
14. Russell, G. A.; Dodd, J. R.; Ku, T.; Tanger, C.; Chung, C. S. C. JACS 1974, 96, 7255.
15. Badet, B.; Julia, M.; Ramirez-Munoz, M.; Sarrazin, C. A. T 1983, 39, 3111.
16. Ohkata, K.; Takee, K; Akiba, K. TL 1983, 24, 4859.
17. Julia, M.; Mestdagh, H. T 1983, 39, 433.
18. Winkler, J. D.; Finck-Estes, M. TL 1989, 30, 7293.
19. Jones, S. D.; Blomgren, G. E. U.S. Patent 4 764 440, 1988 (CA 1978, 109, 173 599z).
20. Jones, S. D.; Blomgren, G. E. J. Electrochem. Soc. 1989, 136, 424 (CA 1989, 110, 181 565w).
21. Land, J. M. U.S. Patent 4 694 029, 1987 (CA 1988, 108, 39 778h).

John S. Ng & Chin Liu

Searle Research and Development, Skokie, IL, USA



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