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 sunstitution sites in the same molecule).[1] The name may also refers 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]

Preparation[edit]

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

Ethenone, the simplest ketene can be generated by pyrolysis (thermal cracking) of acetone:[3]

CH3−CO−CH3 → CH2=C=O + CH4

This reaction is called the Schmidlin ketene synthesis.[4][5]

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.

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.[6]

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.[7] 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.[8]

Reactions and 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. ^ 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.
  4. ^ Hurd CD, Kamm O (1941). "Ketene in Organic Syntheses". Organic Syntheses. Collective Vol. 1. p. 330.
  5. ^ 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.
  6. ^ 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.
  7. ^ 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.
  8. ^ Chirila A, van Vliet KM, Paul ND, de Bruin B (2018). "[Co(MeTAA)] Metalloradical Catalytic Route to Ketenes via Carbonylation of Carbene Radicals". European Journal of Inorganic Chemistry. 2018 (20–21): 2251–2258. doi:10.1002/ejic.201800101. ISSN 1099-0682.

External links[edit]

  • Media related to Ketene at Wikimedia Commons