Organic Syntheses Procedure

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Org. Synth. 1974, 54, 79

DOI: 10.15227/orgsyn.054.0079

PREPARATION OF VINYL TRIFLUOROMETHANESULFONATES: 3-METHYL-2-BUTEN-2-YL TRIFLATE

[Methanesulfonic acid, trifluoro-, 1,2-dimethyl-1-propenyl ester]

Submitted by Peter J. Stang¹ and Thomas E. Dueber.

Checked by Wayne Jaeger and Herbert O. House.

1. Procedure

A. Trifluoromethanesulfonic anhydride. A dry, 100-ml., round-bottomed flask is charged with 36.3 g. (0.242 mole) of trifluoromethanesulfonic acid (Note 1) and 27.3 g. (0.192 mole) of phosphorus pentoxide (Note 2). The flask is stoppered and allowed to stand at room temperature for at least 3 hours. During this period the reaction mixture changes from a slurry to a solid mass. The flask is fitted with a short-path distilling head and heated first with a stream of hot air from a heat gun and then with the flame from a small burner. The flask is heated until no more trifluoromethanesulfonic anhydride distills, b.p. 82–115°, yielding 28.4–31.2 g. (83–91%) of the anhydride, a colorless liquid. Although this product is sufficiently pure for use in the next step, the remaining acid may be removed from the anhydride by the following procedure. A slurry of 3.2 g. of phosphorus pentoxide in 31.2 g. of the crude anhydride is stirred at room temperature in a stoppered flask for 18 hours. After the reaction flask has been fitted with a short-path distilling head, it is heated with an oil bath, yielding 0.7 g. of forerun, b.p. 74–81°, followed by 27.9 g. of the pure trifluoromethanesulfonic acid anhydride, b.p. 81–84° (Note 3).

B. 3-Methyl-2-buten-2-yl triflate. A solution of 2.58 g. (0.0300 mole) of 3-methyl-2-butanone (Note 4) and 2.78 g. (0.0352 mole) of anhydrous pyridine (Note 5) in 10 ml. of anhydrous n-pentane (Note 6) is placed in a dry, 50-ml. Erlenmeyer flask, and the flask is stoppered with a rubber septum. After the solution has been cooled in an acetone–dry ice cooling bath, 9.72 g. (5.80 ml., 0.0395 mole) of trifluoromethanesulfonic anhydride is added with a hypodermic syringe, dropwise and with swirling over 2–3 minutes. The resulting mixture, from which a white solid separates initially, is allowed to warm to room temperature over 22–24 hours. During this period the reaction mixture becomes red in color (Note 7), and a viscous, red semisolid separates. The supernatant pentane solution is decanted, and the residual viscous semisolid is washed with two 10-ml. portions of pentane. While the combined pentane solutions are stored over anhydrous potassium carbonate, the remaining red semisolid is dissolved in 5 ml. of saturated aqueous sodium bicarbonate and extracted with three 5-ml. portions of pentane. The combined pentane solutions (including the solid potassium carbonate) are washed rapidly (Note 8) with 5 ml. of cold saturated aqueous sodium hydrogen carbonate and dried over anhydrous potassium carbonate. After the orange pentane solution has been concentrated to a volume of approximately 10 ml. with a rotary evaporator, it is transferred to a small distilling apparatus. The remaining pentane is removed by distillation at atmospheric pressure (Note 9). The residual liquid is fractionally distilled under reduced pressure, yielding 2.94–2.97 g. (45%) of a colorless, liquid fraction containing 3-methyl-2-buten-2-yl triflate and a lesser amount of 3-methyl-1-buten-2-yl triflate, b.p. 58–66° (22 mm.), n²⁵D 1.3832–1.3898 (Note 10). Fractional distillation of this mixture with a 40-cm. spinning band column separated higher boiling fractions, b.p. 45–47° (12 mm.), that contained (Note 10) 98% of the 3-methyl-2-buten-2-yl triflate as a colorless liquid, n²⁵D 1.3838 (Note 11).

2. Notes

1. Trifluoromethanesulfonic acid, purchased from Eastman Organic Chemicals, was used without purification. Trifluoromethanesulfonic acid in large quantities is available from 3M Company as Fluorocarbon Acid FC-24.

2. The phosphorus pentoxide should be protected from atmospheric moisture by weighing this reagent in a dry, stoppered flask. On a larger scale the yields are better and the reaction easier if the phosphorus pentoxide is premixed with an equal volume of Celite (filter aid).

3. This product exhibits one major GC peak (retention time 2.3 minutes, silicone fluid QF₁ on Chromosorb P) as well as one minor, unidentified, more rapidly eluted impurity. The product has strong IR absorption (CCl₄) at 1470, 1240, and 1130 cm.⁻¹.

4. 3-Methyl-2-butanone, purchased from Eastman Organic Chemicals, was used without purification. In general, commercially available ketones may be used without further purification.

5. A reagent grade of pyridine, purchased from Fisher Scientific Company, was dried over anhydrous potassium carbonate, distilled, and collected at 112–113°.

6. A commercial sample of n-pentane was distilled from calcium hydride to separate the pure solvent, b.p. 35–36°.

7. The intensity of color developed in the reaction mixture is an approximate indication of the extent of reaction. With reactive triflates a dark, almost tarry-looking mass develops in a few hours, while with the slower forming triflates several days at room temperature may be required for adequate color development. In no case was any product isolated when fairly dark color had not developed in the reaction mixture.

8. Only relatively stable vinyl triflates should be washed with water. More reactive triflates such as α-styryltriflate do not survive washing with water.

9. For the more volatile triflates, removal of the solvent should be accomplished by distillation to minimize loss of the volatile product. Also, care should be taken not to overheat the residual product since overheating can result in decomposition.

10. The fractions from this distillation may be analyzed by GC, employing a column packed with Carbowax 20 M suspended on Chromosorb P. The retention times for the various components (minutes) are: pentane (1.6), 3-methyl-2-butanone (4.2), 3-methyl-1-buten-2-yl triflate (6.7), and 3-methyl-2-buten-2-yl triflate (9.5).

11. The pure, more highly substituted olefin, n²⁵D 1.3840, could also be separated by preparative GC, and the unchanged ketone could be separated from the two triflate isomers by chromatography on a silica gel column with pentane as the eluant. The pure product has IR absorption (CCl₄) at 1700 (enol C=C), 1210 and 1140 cm.⁻¹ (SO₂) with end absorption in the UV (heptane, ε 700 at 210 nm) and the following broad singlets in the ¹H NMR spectrum (C₆H₆): δ 1.42 (3H, CH₃), 1.63 (3H, CH₃), and 1.82 (3H, CH₃). The mass spectrum of the product exhibits the following relatively abundant peaks, m/e (relative intensity): 218 (M⁺, 58), 69 (75), 57 (92), 43 (100), 41 (44), and 39 (24).

3. Discussion

Vinyl trifluoromethanesulfonates (triflates) are a new class of compounds, unknown before 1969, that have been used most extensively in solvolytic studies to generate vinyl cations²^,³^,⁴^,⁵^,⁶^,⁷^,⁸^,⁹ and unsaturated carbenes.¹⁰ Three methods have been used to prepare these sulfonic esters. The first, involving the preparation and decomposition of acyltriazines,¹¹ requires several steps to prepare the acyltriazines and is limited to the preparation of fully substituted vinyl triflates. The second method involves the electrophilic addition of trifluoromethanesulfonic acid to acetylenes¹²^,¹³^,¹⁴ and, consequently, is not applicable to the preparation of trisubstituted vinyl triflates and certain cyclic vinyl triflates. However, this second procedure is relatively simple and often gives purer products in higher yield than the discussed reaction with ketones. Table I lists vinyl triflates prepared by this procedure.

TABLE I
VINYL TRIFLATES PREPARED BY THE REACTION OF ACETYLENES WITH TRIFLUOROMETHANESULFONIC ACID
Substrate	Product	b.p.	Yield, %


CH₃C≡CH	CH₂=C(CH₃)OSO₂CF₃	25–27° (12 mm.)	60–80
C₂H₅C≡CC₂H₅	cis and trans C₂H₅CH=C(C₂H₅)OSO₂CF₃	68.5–69.5° (25 mm.)	40–60
(CH₃)₃CC≡CH	CH₂=C(OSO₂CF₃)C(CH₃)₃	45–50° (15 mm.)	40–60
C₆H₅C≡CH	CH₂=C(C₆H₅)OSO₂CF₃	44–45° (0.3 mm.)	20–60
CH₃(CH₂)₃C≡CH	CH₂=C(OSO₂CF₃)CH₂(CH₂)₂CH₃	67–69° (15 mm.)	70
(CH₃)₂CHC≡CH	CH₂=C(OSO₂CF₃)CH(CH₃)₂	37–40° (15 mm.)	62

The third procedure, illustrated by this preparation, involves the reaction of ketones with trifluoromethanesulfonic anhydride in a solvent such as pentane, dichloromethane, or carbon tetrachloride and in the presence of a base such as pyridine, lutidine, or anhydrous sodium carbonate⁴^,⁵^,⁶^,⁷^,¹⁴^,¹⁵ and, most recently, the sterically hindered, nonnucleophilic 2,6-di-tert-butyl-4-methylpyridine.¹⁶ This procedure, which presumably involves either acid- or base-catalyzed enolization of the ketone followed by acylation of the enol with the acid anhydride, has also been used to prepare other vinyl sulfonate esters such as tosylates⁸ or methanesulfonates.⁹ Vinyl tosylates (but not vinyl triflates) may also be prepared from the ditosylate derivatives of 1,2-diols.¹⁷

Examples of the use of this procedure to prepare vinyl triflates from ketones are provided in Table II. Often mixtures of cis and trans isomers as well as the various double bond isomers of vinyl triflates are obtained by this procedure, and the amounts of these isomers produced may vary with the base and solvent used.¹⁸ Also, small amounts of unchanged ketone may contaminate the initial crude product. Consequently, separation procedures such as preparative GC or efficient fractional distillation may be required to obtain a single vinyl triflate isomer.

TABLE II
VINYL TRIFLATES PREPARED FROM KETONES
Ketone	Reaction Conditions	Time, Days	Product	Yield, %


XC₆H₄COCH₃ (X = H, 4-Cl, 3-Cl, c-CF₃, 4-NO₂)	CH₂Cl₂, Na₂CO₃	3–21		15–45
	CH₂Cl₂, Na₂CO₃	14		28
	CCl₄, pyridine	5		54
C₆H₅CH(CH₃)COCH₃	CCl₄, pyridine	2–4	C₆H₅C(CH₃)=C(CH₃)OSO₂CF₃ (cis, 18%; trans, 51%) + C₆H₅CH(CH₃)C(OSO₂CF₃)=CH₂ (31%)	53
	CCl₄, P₂O₅	1	C₆H₅C(CH₃)=C(CH₃)OSO₂CF₃ (trans, 80%; cis, 20%)	32
	CH₂Cl₂, Na₂CO₃	10	(CH₃)₂C=C(CH₃)OSO₂CF₃ (1, 70%) + (CH₃)₂CHC(OSO₂CF₃)=CH₂ (2, 30%)	33
	CCl₄, pyridine	1	1 (90%) + 2 (10%)	58

This preparation is referenced from:

References and Notes

Department of Chemistry, the University of Utah, Salt Lake City, Utah 84112.
P. J. Stang, Acc. Chem. Res., 11, 107 (1978).
P. J. Stang, Z. Rappoport, M. Hanack, L. R. Subramanian, “Vinyl Cations,” Academic Press, New York, 1979.
W. D. Pfeifer, C. A. Bahn, P. v. R. Schleyer, S. Bocher, C. E. Harding, K. Hummel, M. Hanack, and P. J. Stang, J. Am. Chem. Soc., 93, 1513 (1971).
M. A. Imhoff, R. H. Summerville, P. v. R. Schleyer, A. G. Martinez, M. Hanack, T. E. Dueber, and P. J. Stang, J. Am. Chem. Soc., 92, 3802 (1970).
T. C. Clarke, D. R. Kelsey, and R. G. Bergman, J. Am. Chem. Soc., 94, 3626 (1972).
P. J. Stang and T. E. Dueber, J. Am. Chem. Soc., 95, 2683 (1973).
N. Frydman, R. Bixon, M. Sprecher, and Y. Mazur, J. Chem. Soc. D., 1044 (1969).
W. E. Truce and L. K. Liu, Tetrahedron Lett., 517 (1970).
P. J. Stang, Chem. Rev., 78, 383 (1978).
W. M. Jones and D. D. Maness, J. Am. Chem. Soc., 91, 4314 (1969).
P. J. Stang and R. H. Summerville, J. Am. Chem. Soc., 91, 4600 (1969).
A. G. Martinez, M. Hanack, R. H. Summerville, P. v. R. Schleyer, and P. J. Stang, Angew. Chem., Int. Ed. Engl., 9, 302 (1970).
R. H. Summerville, C. A. Senkler, P. v. R. Schleyer, T. E. Dueber, and P. J. Stang, J. Am. Chem. Soc., 96, 1100 (1974).
T. E. Dueber, P. J. Stang, W. D. Pfeifer, R. H. Summerville, M. A. Imhoff, P. v. R. Schleyer, K. Hummel, S. Böcher, C. E. Harding, and M. Hanack, Angew. Chem., Int. Ed. Engl., 9, 521 (1970).
P. J. Stang and W. Treptow, Synthesis, 283 (1980).
P. E. Peterson and J. M. Indelicato, J. Am. Chem. Soc., 90, 6515 (1968).
P. J. Stang, M. Hanack, and L. R. Subramanian, Synthesis, 85 (1982).

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

P₂O₅

α-styryltriflate

potassium carbonate (584-08-7)

sodium bicarbonate,
sodium hydrogen carbonate (144-55-8)

sodium carbonate,
Na₂CO₃ (497-19-8)

carbon tetrachloride,
CCl₄ (56-23-5)

pyridine (110-86-1)

lutidine (583-61-9)

Pentane,
n-PENTANE (109-66-0)

dichloromethane,
CH₂Cl₂ (75-09-2)

3-methyl-2-butanone (563-80-4)

calcium hydride (7789-78-8)

trifluoromethanesulfonic acid (1493-13-6)

Trifluoromethanesulfonic anhydride,
trifluoromethanesulfonic acid anhydride (358-23-6)

3-Methyl-2-buten-2-yl triflate,
Methanesulfonic acid, trifluoro-, 1,2-dimethyl-1-propenyl ester (28143-80-8)

3-methyl-1-buten-2-yl triflate

vinyl triflate

phosphorus pentoxide (1314-56-3)

2,6-Di-tert-butyl-4-methylpyridine (38222-83-2)

Notes

References/EndNotes

Article Compounds

Authors