Ketene

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General formula for a ketene.

A ketene is an organic compound of the form R′R″C=C=O, where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule).[1] The name may also refer to the specific compound ethenone H
2
C=C=O
, the simplest ketene.

Although they are highly useful, most ketenes are unstable. When used as reagents in a chemical procedure, they are typically generated when needed, and consumed as soon as (or while) they are produced.[1]

History[edit]

Ketenes were first studied as a class by Hermann Staudinger before 1905.[2]

Ketenes were systematically investigated by Hermann Staudinger in 1905 in the form of diphenylketene (conversion of -chlorodiphenyl acetyl chloride with zinc). Staudinger was inspired by the first examples of reactive organic intermediates and stable radicals discovered by Moses Gomberg in 1900 (compounds with triphenylmethyl group).[3]

Preparation[edit]

Ethenone, the simplest ketene can be generated by pyrolysis (thermal cracking) of acetone:[4]

CH3−CO−CH3 → CH2=C=O + CH4

This reaction is called the Schmidlin ketene synthesis.[5][6]

Other ketenes can be prepared from acyl chlorides by an elimination reaction in which HCl is lost:

Formation of a ketene from an acyl chloride.

In this reaction, a base, usually triethylamine, removes the acidic proton alpha to the carbonyl group, inducing the formation of the carbon-carbon double bond and the loss of a chloride ion:

Synthesis of Ketene

Ketenes can also be formed from α-diazoketones by Wolff rearrangement.

Another way to generate ketenes is through flash vacuum thermolysis (FVT) with 2-pyridylamines. Plüg and Wentrup developed a method in 1997 that improved on FVT reactions to produce ketenes with a stable FVT that is moisture insensitive, using mild conditions (480 °C). The N-pyridylamines are prepared via a condensation with R-malonates with N-amino(pyridene) and DCC as the solvent.[7]

A more robust method for preparing ketenes is the carbonylation of metal-carbenes, and in situ reaction of the thus produced highly reactive ketenes with suitable reagents such as imines, amines, or alcohols.[8] This method is an efficient one‐pot tandem protocol of the carbonylation of α‐diazocarbonyl compounds and a variety of N‐tosylhydrazones catalysed by Co(II)–porphyrin metalloradicals leading to the formation of ketenes, which subsequently react with a variety of nucleophiles and imines to form esters, amides and β‐lactams. This system has a broad substrate scope and can be applied to various combinations of carbene precursors, nucleophiles and imines.[9]

Reactions and applications[edit]

Due to their cumulated double bonds, ketenes are very reactive.[10]

Formation of carboxylic acid esters[edit]

By reaction with alcohols, carboxylic acid esters are formed:

Ketene Reaktion1 V1.svg

Formation of carboxylic anhydrides[edit]

Ketenes react with a carboxylic acids to form carboxylic acid anhydrides:

Ketene Reaktion2 V1.svg

Formation of carboxylic acid amides[edit]

Ketenes react with ammonia to primary amides:

Ketene Reaktion3 V1.svg

The reaction of ketenes with primary amines produces secondary amides:

Ketene Reaktion5 V1.svg

Ketenes react with secondary amines to give tertiary amides:

Ketene Reaktion4 V1.svg

Hydrolysis[edit]

By reaction with water, carboxylic acids are formed from ketenes

Ketene Reaktion6 V1.svg

Formation of enol acetates[edit]

Enolacetates are formed from ketenes with enolisable carbonyl compounds. The following example shows the reaction of ethenone with acetone to form a propen-2-yl acetate:

Ketene Reaktion7 V3.svg

Dimerisation[edit]

At room temperature, ketene quickly dimerizes to diketene, but the ketene can be recovered by heating:

Ketene Reaktion8 V1.svg

[2+2]-Cycloaddition[edit]

Ketenes can react with alkenes, carbonyl compounds, carbodiimides and imines in a [2+2] cycloaddition. The example shows the synthesis of a β-lactam by the reaction of a ketene with an imine (see Staudinger synthesis):[11][12]

Staudinger-Synthese ÜV6.svg

Applications[edit]

Ketenes are generally very reactive, and participate in various cycloadditions. One important process is the dimerization to give propiolactones. A specific example is the dimerization of the ketene of stearic acid to afford alkyl ketene dimers, which are widely used in the paper industry.[1] AKD's react with the hydroxyl groups on the celluose via esterification reaction.

They will also undergo [2+2] cycloaddition reactions with electron-rich alkynes to form cyclobutenones, or carbonyl groups to form beta-lactones. With imines beta-lactams are formed. This is the Staudinger synthesis, a facile route to this important class of compounds. With acetone, ketene reacts to give Isopropenyl acetate.[1]

A variety of hydroxylic compounds can add as nucleophiles, forming either enol or ester products. As examples, a water molecule easily adds to ketene to give 1,1-dihydroxyethene and acetic anhydride is produced by the reaction of acetic acid with ketene. Reactions between diols (HO−R−OH) and bis-ketenes (O=C=CH−R′−CH=C=O) yield polyesters with a repeat unit of (−O−R−O−CO−R′−CO).

Ethyl acetoacetate, an important starting material in organic synthesis, can be prepared using a diketene in reaction with ethanol. They directly form ethyl acetoacetate, and the yield is high when carried out under controlled circumstances; this method is therefore used industrially.

See also[edit]

References[edit]

  1. ^ a b c d Miller R, Abaecherli C, Said A, Jackson B (2001). "Ketenes". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a15_063. ISBN 978-3527306732.
  2. ^ Staudinger H (1905). "Ketene, eine neue Körperklasse" [Ketenes, a new class of substances]. Berichte der Deutschen Chemischen Gesellschaft. 38 (2): 1735–1739. doi:10.1002/cber.19050380283.
  3. ^ Thomas T. Tidwell, The first century of Ketenes (1905-2005): the birth of a family of reactive intermediates, Angewandte Chemie, Int. Edition, Band 44, 2005, S. 5778–5785
  4. ^ Weygand C (1972). Hilgetag G, Martini A (eds.). Weygand/Hilgetag Preparative Organic Chemistry (4th ed.). New York: John Wiley & Sons, Inc. pp. 1031–1032. ISBN 978-0471937494.
  5. ^ Hurd CD, Kamm O (1941). "Ketene in Organic Syntheses". Organic Syntheses. Collective Vol. 1. p. 330.
  6. ^ Schmidlin J, Bergman M (1910). "Darstellung des Ketens aus Aceton" [Preparation of ketene from acetone]. Berichte der Deutschen Chemischen Gesellschaft (in German). 43 (3): 2821–2823. doi:10.1002/cber.19100430340.
  7. ^ Carsten Plüg ,Hussein Kanaani and Curt Wentrup (12 February 2015). "Ketenes from N-(2-Pyridyl)amides". Australian Journal of Chemistry. 68 (4): 687. doi:10.1071/CH14714.
  8. ^ Paul ND, Chirila A, Lu H, Zhang XP, de Bruin B (September 2013). "Carbene radicals in cobalt(II)-porphyrin-catalysed carbene carbonylation reactions; a catalytic approach to ketenes". Chemistry. 19 (39): 12953–8. doi:10.1002/chem.201301731. PMC 4351769. PMID 24038393.
  9. ^ Chirila A, van Vliet KM, Paul ND, de Bruin B (2018). "[Co(MeTAA)] Metalloradical Catalytic Route to Ketenes via Carbonylation of Carbene Radicals" (PDF). European Journal of Inorganic Chemistry. 2018 (20–21): 2251–2258. doi:10.1002/ejic.201800101. ISSN 1099-0682.
  10. ^ Siegfried Hauptmann (1985), Organische Chemie: mit 65 Tabellen (in German), Leipzig: Deutscher Verlag für Grundstoffindustrie, pp. 410–412, ISBN 3871449024
  11. ^ Jie Jack Li (2006), Name reactions. A collection of detailed reaction mechanisms (in German) (3 ed.), Berlin: Springer-Verlag, pp. 561-562, doi:10.1007/3-540-30031-7, ISBN 9783540300304
  12. ^ Hermann Staudinger (1907), "Zur Kenntnis der Ketene. Diphenylketen", Justus Liebigs Annalen der Chemie (in German), John Wiley & Sons, Inc., 356 (1–2), pp. 51–123, doi:10.1002/jlac.19073560106

External links[edit]

  • Media related to Ketenes at Wikimedia Commons