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Org. Synth. 1984, 62, 202
DOI: 10.15227/orgsyn.062.0202
α-ALLENIC ESTERS FROM α-PHOSPHORANYLIDENE ESTERS AND ACID CHLORIDES: ETHYL 2,3-PENTADIENOATE
[2,3-Pentadienoic acid, ethyl ester]
Submitted by Robert W. Lang1 and Hans-Jürgen Hansen2.
Checked by William F. Burgoyne and Robert M. Coates.
1. Procedure
A 1-L, three-necked, round-bottomed flask is equipped with a nitrogen inlet, a 250-mL, pressure-equalizing dropping funnel fitted with a gas outlet, and a Teflon-coated magnetic stirring bar. The flask is charged with 300 mL of dichloromethane (Note 1) and 34.8 g (0.10 mol) of ethyl (triphenylphosphoranylidene)acetate (Note 2) and flushed with nitrogen. The yellow solution is stirred at 25°C as a solution of 10.1 g (0.10 mol) of triethylamine (Note 3) in 100 mL of dichloromethane is added dropwise over 5 min. After 10 min, 9.25 g (0.10 mol) of propionyl chloride (Note 4) in 100 mL of dichloromethane is added dropwise to the vigorously stirred solution over 15 min (Note 5). Stirring is continued for an additional 0.5 hr (Note 6), after which the clear, yellow-tinted mixture is evaporated on a rotary evaporator at reduced pressure using a water bath maintained at 25°C (Note 7). A 500-mL portion of pentane (Note 8) is added to the semisolid residue, and the slurry is allowed to stand for 2 hr while it is shaken periodically to facilitate solidification and to complete the extraction of the product. The precipitate is removed by filtration through a coarse, sintered-glass Büchner funnel, and the filter cake is washed with a 50-mL portion of pentane. The filtrates are combined and concentrated at reduced pressure to approximately one-fourth of the original volume using a water bath maintained at 25°C. The mixture is filtered again to remove triphenylphosphine oxide, and the remaining solvent is then evaporated. Rapid distillation of the residual liquid in a short-path distillation apparatus under reduced pressure (Note 9) affords a small forerun amounting to 0.5 mL or less and 7.8–8.1 g (62–64%) of ethyl 2,3-pentadienoate, bp 57–59°C (12–14 mm) (Note 10) and (Note 11).
2. Notes
1. Dichloromethane was purified by percolation through Woelm activity grade 1 basic alumina and stored under nitrogen.
2. Ethyl (triphenylphosphoranylidene)acetate is available from Fluka AG and Tridom Chemical Inc. under the name (ethoxycarbonylmethylene)triphenylphosphorane and from Aldrich Chemical Company, Inc. under the name (carbethoxymethylene)triphenylphosphorane. The reagent may be prepared from triphenylphosphine and ethyl bromoacetate by the following procedure.3
A 1-L, two-necked, round-bottomed flask fitted with a dropping funnel and a mechanical stirrer is charged with 131.0 g (0.5 mol) of triphenylphosphine (Fluka AG, purum) and 250 mL of benzene (Merck, pro analysi). The solution is stirred vigorously while 83.5 g (0.5 mol) of ethyl bromoacetate (Fluka AG, practical-grade) is added dropwise at a rate that maintains the reaction mixture at, or slightly above, room temperature. After a total of 2 hr the reaction is complete and the colorless phosphonium salt is filtered. The salt is washed with 300 mL of cold benzene and 200 mL of pentane and then dissolved in 3 L of water at room temperature. Some further organic impurities are removed by extraction with ether, after which 2 drops of 2% alcoholic phenolphthalein are added. The aqueous solution is stirred vigorously and cooled in an ice bath as 2 M aqueous sodium hydroxide is added slowly until the pink endpoint is reached (pH 8–10). The crystalline phosphorane is collected by filtration, washed thoroughly with cold water, and dried, first with a rotary evaporator under reduced pressure at 60°C and then overnight in a drying oven at 180 mm and 70°C. The white to cream-colored crop of ethyl (triphenylphosphoranylidene)acetate, mp 124–126°C, weighs 150–156 g (86–90%) and may be used for the preparation of α-allenic esters without further purification.
3. Triethylamine was supplied by Fluka AG and Aldrich Chemical Company, Inc.
4. Propionyl chloride was purchased from Fluka AG and Aldrich Chemical Company, Inc. and was freshly distilled at 78–80°C (760 mm) prior to use.
5. The checkers maintained the temperature of the reaction mixture at ca. 25°C by cooling with a water bath during the addition of propionyl chloride.
6. The progress of the reaction may be followed by analytical thin-layer chromatography on alumina. The submitters used polygram precoated plastic sheets (Alox N/UV254) purchased from Macherey-Nagel, Inc. The plates were developed with 1 : 1 hexane : ether and stained with basic permanganate. The retardation factor of the product is 0.56.
7. For the isolation of relatively volatile α-allenic esters such as ethyl 2,3-pentadienoate, the submitters recommend that the rotary evaporation be carried out with cooling in an ice bath. When this precaution was taken, the submitters obtained 8.5–9.5 g (67–75%) of product after distillation.
8. The checkers dried the pentane over sodium wire prior to use.
9. The checkers stirred the distilling liquid rapidly with a magnetic stirrer and maintained a bath temperature of 75–85°C throughout the distillation.
10. Ethyl 2,3-pentadienoate has the following spectral properties: IR (thin film) cm−1: 1965, 1720, 1410, 1250, 1025, 865, 790; 1H NMR (CCl4) δ: 1.26 (t, 3 H, J = 7, OCH2CH3), 1.78 (m, 3 H, CH3), 4.11 (q, 2 H, J = 7, OCH2CH3), 5.28–5.68 (m, 2 H, at C-2 and C-4).
11. On 0.01-mol scale the yield of ethyl 2,3-pentadienoate is 0.79–0.93 g (64–74%). The product was purified by bulb-to-bulb distillation with a Kugelrohr apparatus at 12–14 mm with an oven temperature at 75–85°C.
3. Discussion
The acylation of Wittig reagents provides the most convenient means for the preparation of allenes substituted with various electron-withdrawing substituents.4 The preparation of α-allenic esters has been accomplished by the reaction of resonance-stabilized phosphoranes with isolable ketenes5,6,7,8,9,10 and ketene itself11 and with acid chlorides in the presence of a second equivalent of the phosphorane.6 The disadvantages of the first method are the necessity of preparing the ketene and the fact that the highly reactive monosubstituted ketenes evidently cannot be used. The second method fails when the α-carbon of the phosphorane is unsubstituted.12
TABLE I
PREPARATION OF α-ALLENIC ESTERS BY THE WITTIG REACTION13

R1

R2

R3

R4

Solvent

Procedurea

Yield (%)


CH3

H

H

H

CH2Cl2

A

40

C2H5

H

(CH3)3C

H

CH3CN

B

55

CH3

H

C6H5

H

CH3CN

B

23

C2H5

CH3

H

H

CH2Cl2

A

59

C2H5

CH3

CH3

H

CH2Cl2

A

74

C2H5

CH3

CH3

CH3

CH2Cl2

B

39

CH3

CH3

(CH3)3C

H

CH3CN

B

66

C2H5

CH3

C6H5

H

CH2Cl2

A

70


aThe reaction times varied from 10 min to 18 hr. Procedure: A—the corresponding phosphonium salt was used with the addition of 2 mol of triethylamine; B—the corresponding phosphorane was used with the addition of 1 mol of triethylamine.

The present procedure affords a general method for preparing α-allenic esters (Table I) that avoids the limitations of the previous methods.12 Thus, α-allenic esters unsubstituted at C-2 are now available in generally satisfactory yields. Ethyl 2,3-pentadienoate, the title compound, had not been prepared prior to the development of this procedure by the submitters. The mild conditions (i.e., room temperature for relatively short times), avoid the base-catalyzed isomerization of the conjugated allenes to acetylenes.14 The corresponding phosphonium salts may also be used directly in the reaction provided two equivalents of triethylamine are employed, obviating the lengthy process for drying the phosphorane.15 Dichloromethane and acetonitrile have been used as solvents for the reaction.12 The α-allenic esters are usually obtained in analytically pure form after bulb-to-bulb distillation. They may also be purified by column chromatography on alumina with 9 : 1 hexane : ether as eluant.14
The submitters have shown that these reactions proceed by dehydrochlorination of the acid chloride to the ketene, which is then trapped by reaction with the phosphorane. The resulting betaine decomposes to the allenic ester via an oxaphosphetane. In contrast, the reaction of acid chlorides with 2 equiv of phosphoranes involves initial acylation of the phosphorane followed by proton elimination from the phosphonium salt.5
This preparation is referenced from:

References and Notes
  1. Zentrale Forschungslaboratorien, CIBA-Geigy AG, Postfach CH-4002, Basel, Switzerland;
  2. Zentrale Forschungseinheiten, F. Hoffmann-La Roche & Co. AG, Postfach, CH-4002 Basel, Switzerland. Work done at the Institute of Organic Chemistry, University of Fribourg, CH-1700 Fribourg, Pérolles, and supported by the Swiss National Science Foundation.
  3. Isler, O.; Gutmann, H.; Montavon, M.; Rüegg, R.; Ryser, G.; Zeller, P. Helv. Chim. Acta 1957, 40, 1242–1249.
  4. Traylor, D. R. "The Chemistry of Allenes," Chem. Rev. 1967, 67, 317–359; Sandler, S. R.; Karo, W. "Organic Functional Group Preparations"; Academic Press: New York, 1971; Vol. 2, pp. 1–40; Murray, M. "Allene bzw. Kumulene" in "Methoden der Organischen Chemie" (Houben-Weyl), 4th ed., Müller, E., Ed.; Thieme: Stuttgart, 1977; Vol. 5/2a, pp. 963–1076; "The Chemistry of Allenes," Landor, S. R., Ed.; Academic Press: London, 1982; Vols. 1–3; Schuster, H. F.; Coppola, G. M.; "Allenes in Organic Synthesis," J. Wiley & Sons, New York, 1984.
  5. Wittig, G.; Haag, A. Chem. Ber. 1963, 96, 1535–1543.
  6. Bestmann, H.-J.; Hartung, H. Chem. Ber. 1966, 99, 1198–1207.
  7. Aksnes, G.; Frøyen, P. Acta Chem. Scand. 1968, 22, 2347–2352.
  8. Andrews, S. D.; Day, A. C.; Inwood, R. N. J. Chem. Soc. (C) 1969, 2443–2449.
  9. Orlov, V. Y.; Lebedev, S. A.; Ponomarev, S. V.; Lutsenko, I. F. J. Gen. Chem. U.S.S.R. (Engl. transl.) 1975, 45, 696.
  10. Gompper, R.; Wolf, U. Liebigs Ann. Chem. 1979, 1388–1405, 1406–1425.
  11. Hamlet, Z.; Barker, W. D. Synthesis 1970, 543–544.
  12. Bestmann, H.-J.; Arnason, B. Chem. Ber. 1962, 95, 1513–1527; Bestmann, H.-J. Angew. Chem. 1965, 77, 651–666.
  13. Lang, R. W.; Hansen, H.-J. Helv. Chim. Acta 1980, 63, 438–455.
  14. Cymerman Craig, J.; Moyle, M. J. Chem. Soc. 1963, 5356–5360; Busby, R. J. Quart. Rev. Chem. Soc. 1970, 24, 585–600.
  15. Lang, R. W.; Hansen, H.-J. Helv. Chim. Acta 1979, 62, 1025–1039.

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

alumina

Benzene (71-43-2)

ether (60-29-7)

acetonitrile (75-05-8)

sodium hydroxide (1310-73-2)

nitrogen (7727-37-9)

sodium wire (13966-32-0)

Pentane (109-66-0)

dichloromethane (75-09-2)

phenolphthalein (77-09-8)

propionyl chloride (79-03-8)

Ethyl bromoacetate (105-36-2)

hexane (110-54-3)

triethylamine (121-44-8)

triphenylphosphine (603-35-0)

triphenylphosphine oxide (791-28-6)

betaine (107-43-7)

phosphorane (7723-14-0)

Ethyl 2,3-pentadienoate,
2,3-Pentadienoic acid, ethyl ester (74268-51-2)

ethyl (triphenylphosphoranylidene)acetate,
(ethoxycarbonylmethylene)triphenylphosphorane,
(carbethoxymethylene)triphenylphosphorane (1099-45-2)

oxaphosphetane