Acetylene

HCCH

[74-86-2]  · C2H2  · Acetylene  · (MW 26.04)

(ethynylation reagent,1 hydrosilylation,23 carbonylation,24-26 cycloadditions,31-33 cyclotrimerization34)

Alternate Name: ethyne.

Physical Data: bp -83 °C.

Form Supplied in: widely available as compressed gas. Drying: can be purified by passing through a trap at -80 °C followed by a column of sulfuric acid and then through a column of sodium hydroxide.2

Handling, Storage, and Precautions: flammable, colorless gas which possesses a garlic odor; use only with adequate ventilation; skin irritant. The low electrical conductivity of acetylene requires that care must be taken to eliminate any static charge buildup. Use in a fume hood.

Ethynylation.

Due to the sp hybridization of acetylene's s-bonds the methine proton has a pKa of about 25 and can be removed with strong bases to form the acetylide anion.3 Sodium acetylide is commonly prepared by deprotonation with sodium amide. Lithium Acetylide can be conveniently prepared using n-Butyllithium, while the corresponding Grignard reagent is commonly generated by treatment with Ethylmagnesium Bromide.4 Alkyl cuprates undergo nucleophilic addition to acetylene.5 The dianion of acetylene is available by using higher temperatures during deprotonation.6 A variety of other counter cations of the acetylide ion have also been formulated, usually starting from the acetylides previously mentioned.1a The acetylide anion undergoes nucleophilic addition to a wide range of electrophiles such as halides,7 alkyl halides,8 alkyl sulfonates,9 epoxides,10 aldehydes,11 ketones,12 esters,13 and imides.14 In certain cases the yields of addition may be improved by additives such as ethylenediamine15 or Copper(I) Iodide.16 When the electrophile is hindered, elimination may become the predominant pathway. This problem may be avoided with trialkylborane electrophiles (eq 1).17 After treatment with iodine, the intermediate borate yields the substituted alkyne via migration of one of the alkyl groups. This sequence is not as sensitive to steric effects as simple substitutions with halides. Higher yields can be obtained if Lithium (Trimethylsilyl)acetylide is used instead (see also Trimethylsilylacetylene).

The nucleophilicity of the acetylide anion is such that discrimination between several electrophilic centers within a substrate is possible (eq 2).18

The regioselectivity of acetylide additions are sensitive to the counter cation. For example, ethynyltriisopropoxytitanium acetylide, prepared from ethynyllithium and Chlorotitanium Triisopropoxide, gives a single addition product with pyrimidin-2(1H)-ones while the Grignard and lithium acetylides give mixtures of regioisomers (eq 3).19 There are also examples of diastereoselective additions of acetylide anions to ketones (eq 4).20

The acetylide anion is useful for the synthesis of enynes which have been used in palladium catalyzed cyclizations (eq 5).21

Ethynylation of aryl halides (Stephens-Castro coupling) with the copper acetylide is synthetically effective when one of the acetylenic methine protons is protected.22

Hydrosilylation.

Acetylene can be converted23 into a vinylsilane under appropriate hydrosilylation conditions using Pt, Rh, Ru, or Al catalysts, but the initial 1:1 product can react further to give a 1,2-disilylethane. Examples of these processes are shown in eq 6.

Reactions with CO and CO2.

Alkynes react with CO2 and secondary amines in the presence of ruthenium complexes to afford vinyl carbamates (eq 7).24 Mononuclear ruthenium complexes and [RuCl2(norbornadiene)]n or Ruthenium(III) Chloride have been shown to be the best catalysts for monosubstituted alkynes and acetylene, respectively.

Acetylene reacts25 with CO and O2 in presence of Palladium(II) Chloride to give dimethyl maleate in 90% yield. Organic acids have been formed by treating acetylene with CO and Ni or Pd catalysts.25a Acetylene reacts with CO in presence of Co2(CO)8 to give an (E)-bisbutenolide in aprotic solvents (eq 8), while the (Z)-isomer is obtained in the presence of tetramethylurea.26

Addition.

Acyl chlorides add across acetylene to give b-chloroenones in CCl4 in the presence of Aluminum Chloride (eq 9).27

The acid catalyzed hydration of acetylene to acetaldehyde followed by condensation to give crotonaldehyde is a well known reaction first studied by Berthelot in 1862.28 Although more efficient procedures have been developed since then, the mechanism of hydration of acetylene has not yet been unambiguously resolved.29 Kinetic data based on NMR experiments suggest that a vinyl cation is probably an intermediate.30

Cycloadditions.

Acetylene has low reactivity in Diels-Alder reactions and reacts with electron rich dienes only under severe conditions.31 The reaction of dienes with vinyl bromide and subsequent elimination of HBr from the adduct leads to the same product as direct addition of acetylene. In addition a number of compounds have been suggested as acetylene surrogates, such as 2-phenyl- and 2-Thiono-1,3-dioxol-4-ene,32 and phenyl vinyl sulfoxide.33 Vinylene thioxocarbonate and 2-phenyl-1,3-dioxol-4-ene, on reaction9 with anthracene in benzene at 170 °C for 16 h, gave the corresponding adducts in 60 and 65% yield, respectively. The [4 + 2] adducts yield the corresponding alkene upon treatment with trivalent phosphorus or n-butyllithium respectively. Similarly, heating anthracene and phenyl vinyl sulfoxide in chlorobenzene for 120 h afforded dibenzobarrelene in 83% isolated yield (eq 10).9

The following example (eq 11) involves in situ reductive extrusion of an oxygen atom. It has been suggested that the liberated PhSOH appears capable of deoxygenating isobenzofurans and related molecules.

Cyclotrimerization.

In addition to acetylene, many mono- or disubstituted alkynes undergo cyclotrimerization in the presence of transition metal complexes. Acetylene itself trimerizes to give benzene. Trimerization of unsymmetrically substituted alkynes gives rise mostly to benzenes in which the most sterically demanding substituents occupy positions 1, 2, and 4 around the ring and to a lesser extent a 1,3,5-substitution pattern is also obtained.34 Although numerous mechanistic pathways have been postulated, depending on the metal involved, a common feature involves intermediacy of metallocyclopentadienes and complexation with a third alkyne which inserts to give a transient metallocycloheptatriene, leading finally to reductive elimination of the metal and the benzene product.

Selective intermolecular co-cycloaddition has been achieved, exploiting the relative unreactivity of phosphine nickel carbonyls towards trimerization of internal alkynes.35,36 Thus two molecules of acetylene and one molecule of an internal alkyne give a mixture of the unsubstituted and substituted benzenes (eq 12).


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2. Skattebøl, L.; Jones, E. R. H.; Whiting, M. C. OSC 1963, 4, 792.
3. (a) Cram, D. J. Fundamentals of Carbanion Chemistry; Academic Press: New York, 1965; pp 1-45. (b) Dessy, R. E.; Kitching, W.; Psarras, T.; Salinger, R.; Chen, A.; Chivers, T. JACS 1966, 88, 460.
4. Jones, E. R. H.; Skattebøl, L.; Whiting, M. C. JACS 1956, 4765.
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6. Sudweeks, W. B.; Broadbent, H. S. JOC 1975, 40, 1131.
7. Brandsma, L.; Verkruijsse, H. D. S 1990, 984.
8. Campbell, K. N.; Campbell, B. K. OSC 1963, 4, 117.
9. (a) Ireland, R. E.; Highsmith, T. K.; Gegnas, L. D.; Gleason, J. L. JOC 1992, 57, 5071. (b) Crombie, L.; Heavers, A. D. JCS(P1) 1992, 1929.
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11. (a) Chiarino, D.; Fantucci, M. JHC 1991, 28, 1705. (b) Girard, S.; Deslongchamps, P. CJC 1992, 70, 1265.
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21. Trost, B. M.; Dumas, J.; Villa, M. JACS 1992, 114, 9836.
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23. Hiyama, T.; Kusumoto, T. COS 1991, 8, 769 and references cited therein.
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30. Lucchini, V.; Modena, G. JOC 1990, 55, 6291.
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Samit K. Bhattacharya, John E. Stelmach, & Jeffrey D. Winkler

University of Pennsylvania, Philadelphia, PA, USA



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