2-Butynoic Acid1

[590-93-2]  · C4H4O2  · 2-Butynoic Acid  · (MW 84.07)

(dianion used in a- and g-alkylation;2 used in conjugate addition reactions and Diels-Alder reactions)

Physical Data: mp 78-80 °C.3

Preparative Methods: from 3-methyl-2-pyrazolin-5-one via bromination followed by basic hydrolysis;3 via carboxylation of propynylsodium or Propynyllithium.4,5

Purification: sublimation (60 °C/0.02 mm Hg).3

Handling, Storage, and Precautions: use in a fume hood.

Selective a- and g-Alkylation.

The dianion of butynoic acid undergoes selective alkylation at the a- or g-position depending on the reaction conditions (eq 1).2 The original Pitzele procedure using Lithium Diisopropylamide in THF-HMPA or Lithium 2,2,6,6-Tetramethylpiperidide is not regioselective and leads to mixtures in favor of the alkynic derivative.6 Reaction of aldehydes under the latter conditions also gives mixtures of alkynic and allenic derivatives.7,8

Conjugate Addition.

2-Butynoic acid undergoes conjugate addition with Phenylcopper or Lithium Diphenylcuprate to yield the (E) derivative with high stereoselectivity (eq 2).9 Stereospecific syn addition is also observed with esters of 2-butynoic acid.10


2-Butynoic acid undergoes Diels-Alder reactions with Cyclopentadiene (neat, 1 h, 185 °C; 68%).12 The intramolecular version with a sorbyl ester is also known (eq 3).13

Related Reagents.

3-Butyn-2-one; 3,3-Diethoxy-1-propyne; Methyl Propiolate; Propargyl Aldehyde; 3-Tetrahydropyranyloxy-1-propyne; 3-Tetrahydropyranyloxy-1-trimethylsilyl-1-propyne.

1. Fieser, L.; Fieser, M. FF 1977, 6, 345; 1980, 8, 308.
2. Shen, C. C.; Ainsworth, C. TL 1979, 83.
3. Simmross, F. M.; Weyerstahl, P. S 1981, 72.
4. Kauer, J. C.; Brown, M. OSC 1973, 5, 1043.
5. Smith, W. N.; Kuehn, E. D. JOC 1973, 38, 3588.
6. (a) Pitzele, B. S.; Baran, J. S.; Steinman, D. H. JOC 1975, 40, 269. (b) Pitzele, B. S.; Baran, J. S.; Steinman, D. H. T 1976, 32, 1347.
7. Meyer, H. H. LA 1978, 337.
8. Appendino, G.; Hoflack, J.; De Clercq, P. J. T 1988, 44, 4605.
9. (a) Klein, J.; Turkel, R. M. JACS 1969, 91, 6186. (b) Klein, J.; Levene, R. JCS(P2) 1973, 1971.
10. See e.g.: Miginiac, P.; Daviaud, G.; Gérard, F. TL 1979, 20, 1811.
11. Bastide, J.; Henri-Rousseau, O. In The Chemistry of the Carbon-Carbon Triple Bond; Patai, S., Ed.; Wiley: New York, 1978; Part 2, pp 448-517.
12. (a) Simmross, F. M.; Weyerstahl, P. LA 1981, 1089. (b) Kirmse, W.; Hartmann, M.; Siegfried, R.; Wroblowsky, H.-J.; Zang, B.; Zellmer, V. CB 1981, 114, 1793.
13. White, J. D.; Sheldon, B. G. JOC 1981, 46, 2273.

Pierre J. De Clercq & Frank Nuyttens

Universiteit Gent, Belgium

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