Methylbis(methylthio)sulfonium Hexachloroantimonate1

[56648-69-2]  · C3H9Cl6S3Sb  · Methylbis(methylthio)sulfonium Hexachloroantimonate  · (MW 475.78)

(methylthiolating agent for several functional groups;1 can induce cyclization of functionalized alkenes and alkynes1)

Physical Data: mp 123-125 °C (dec).

Solubility: sol CH2Cl2, SO2; reacts with H2O, protic solvents, acetone, and other enolizable carbonyl derivatives.

Analysis of Reagent Purity: 1H NMR spectrum at -50 °C in CD2Cl2 of the pure compound shows two signals at d 3.6 and 3.0 with an intensity ratio 1:2.2 At higher temperature in this solvent, as well as in other solvents (SO2,3 CD3CN4), a single coalescence line is observed. This behavior is due to trace amounts of dimethyl disulfide impurities.3

Preparative Methods: to an ice cooled solution of Antimony(V) Chloride (7.85 mmol) in anhydrous CH2Cl2 (5 mL), a solution of Methanesulfenyl Chloride (7.85 mmol) and Dimethyl Disulfide (7.85 mmol) in CH2Cl2 (10 mL) is added dropwise. The product partially precipitates from the solution. Addition of n-pentane causes complete precipitation of the hexachloroantimonate salt. The product (95% yield) is isolated by filtration in a dry box and can be used without any further purification.3 Alternatively, to a well-stirred solution of dimethyl disulfide (0.1 mol) in dry CH2Cl2 (50 mL) is dropped at 0 °C a solution of SbCl5 (0.1 mol) in dry CH2Cl2 (50 mL). After completion of the addition the salt starts crystallizing as yellowish needles. Addition of dry Et2O completes the precipitation of the product (92% yield).4

Purification: crystallization by solution in dry CH2Cl2 and addition of dry n-pentane or Et2O.

Handling, Storage, and Precautions: stable for some months if stored in the refrigerator; sensitive to moisture, protic solvents, and enolizable carbonyl compounds.

Functional Group Methylthiolation.

The main feature of methylbis(methylthio)sulfonium hexachloroantimonate (Me3S3SbCl6) is the easy transfer of a methylsulfenylium ion to a variety of functional groups more nucleophilic than the sulfur of dimethyl disulfide. The low nucleophilicity of the disulfide and of the hexachloroantimonate ion allows the isolation, or the characterization, of positively charged reaction products.

The methylthiolation of dialkyl or cyclic sulfides with Me3S3SbCl6 proceeds smoothly at 0 °C in CH2Cl2 and gives methylthiosulfonium salts in good yields (60-80%).5 Methylthiosulfonium salts can also be prepared by methylation of disulfides with trimethyloxonium salts.6 However, the latter method cannot be applied to the synthesis of methylthiosulfonium salts derived from cyclic sulfides (eq 1). The reaction of Me3S3SbCl6 with diphenyl sulfide gives an almost equimolar mixture of 4-methylthiodiphenyl sulfide and 4,4-bis(methylthio)diphenyl sulfide (eq 2).2 Other examples of methylthiolation of aromatic substrates have been reported.7

The reaction of Me3S3SbCl6 with alkenes8-12 and alkynes13-16 gives thiiranium and thiirenium ions respectively. Depending on the structure of the unsaturated derivatives and reaction conditions, the three-membered ring sulfonium ions can be spectroscopically characterized or isolated as stable hexachloroantimonate salts. Some examples are shown in eqs 3-6. It is important to note the formation of only trans,trans-1-methylthiiranium salts from (Z)-alkenes8,10,11 (eq 3) and the rearrangement of cis,trans-1-methyl-2,3-di-t-butylthiiranium ion to a thietanium ion (eq 4).10,11 Dialkylacetylenes give thiirenium ions which are stable at low temperature in SO2 solution. The presence of bulky alkyl residues makes these ions stable enough to be isolated at rt as hexachloroantimonate salts (eq 5).14,16 Arylalkylacetylenes undergo addition of the Me3S3 residue to the triple bond (eq 6).17 The characteristic reactivity of thiiranium and thiirenium ions implies attack at ring carbons by nucleophiles with ring opening.1 Nucleophiles also attack the sulfonium sulfur;1 however, this reaction is masked sometimes by its reversibility.

Cyclofunctionalization of Alkenes and Alkynes.

The nucleophilic ring opening of thiiranium and thiirenium ions has been exploited for the synthesis of functionalized heterocycles using Me3S3SbCl6 and suitably substituted alkenes and alkynes (eqs 7-13).7,18-23

1. (a) Capozzi, G. PAC 1987, 59, 989. (b) Capozzi, G.; Modena, G. In Studies in Organic Chemistry 19. Organic Sulfur Chemistry. Theoretical and Experimental Advances; Elsevier: New York, 1985; Chapter 5. (c) Capozzi, G.; Modena, G.; Pasquato, L.; The Chemistry of Sulfenic Acids and Their Derivatives; Wiley: New York, 1990; Chapter 10.
2. Tsuchida, E.; Yamamoto, K.; Nishide, H.; Yoshida, S.; Jikei, M. Macromolecules 1990, 23, 2101.
3. Capozzi, G.; Lucchini, V.; Modena, G.; Rivetti, F. JCS(P2) 1975, 900.
4. Weiss, R.; Schlierf, C. S 1976, 323.
5. Capozzi, G.; DeLucchi, O.; Lucchini, V.; Modena, G. S 1976, 677.
6. (a) Helmkamp, G. K.; Cassey, H. N.; Olsen, B. A.; Pettitt, D. J. JOC 1965, 30, 933. (b) Smallcombe, S. H.; Caserio, M. C. JACS 1971, 93, 5826.
7. Capozzi, G.; Lucchini, V.; Marcuzzi, F.; Modena, G. JCS(P1) 1981, 3106.
8. Capozzi, G.; DeLucchi, O.; Lucchini, V.; Modena, G. TL 1975, 2603.
9. Minkwitz, R.; Gerhard, V.; Krause, R.; Prenzel, H.; Preut, H. Z. Anorg. Allg. Chem. 1988, 559, 154 (CA 1989, 110, 7620a).
10. Lucchini, V.; Modena, G.; Pasquato, L. JACS 1988, 110, 6900.
11. Lucchini, V.; Modena, G.; Pasquato, L. JACS 1991, 113, 6600.
12. Lucchini, V.; Modena, G.; Pasquato, L. CC 1992, 293.
13. Capozzi, G.; DeLucchi, O.; Lucchini, V.; Modena, G. CC 1975, 248.
14. Capozzi, G.; Lucchini, V.; Modena, G.; Scrimin, P. TL 1977, 911.
15. Lucchini, V.; Modena, G.; Valle, G.; Capozzi, G. JOC 1981, 46, 4720.
16. Capozzi, G.; Lucchini, V.; Modena, G. Rev. Chem. Intermed. 1979, 2, 347.
17. Capozzi, G.; DaCol, L.; Lucchini, V.; Modena, G. JCS(P2) 1980, 68.
18. Capozzi, G.; Caristi, C.; Gattuso, M.; Stagno D'Alcontres, G. TL 1981, 22, 3325.
19. Capozzi, G.; Caristi, C.; Gattuso, M. JCS(P1) 1984, 255.
20. Capozzi, G.; Ottana', R.; Romeo, G.; Valle, G. JCR(S) 1986, 200.
21. Capozzi, G.; Ottana', R.; Romeo, G. H 1986, 24, 583.
22. Capozzi, G.; Ottana', R.; Romeo, G. H 1987, 26, 39.
23. Capozzi, G.; Menichetti, S.; Nicastro, M.; Taddei, M. H 1989, 29, 1703.

Giuseppe Capozzi, Stefano Menichetti, & Cristina Nativi

Università di Firenze, Italy

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