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.
Ketenes were systematically investigated by Hermann Staudinger in 1905 in the form of diphenylketene (conversion of <math>\alpha</math>-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).<ref>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</ref>
Properties
Ketenes are highly electrophilic at the carbon atom bonded with the heteroatom, due to its sp character. Ketenes can be formed with different heteroatoms bonded to the sp carbon atom, such as O, S or Se, respectively called ketenes, thioketenes and selenoketenes.
Ethenone, the simplest ketene, has different experimental lengths for each of its double bonds; the C=O bond is 1.160 Å and the C=C bond is 1.314 Å. The angle between the two H atoms is 121.5°, similar to the theoretically ideal angle in alkenes between sp2carbon atoms and H substituents.<ref>Template:Cite journal</ref>
Ketenes are unstable and cannot be stored. Absent nucleophiles with which to react, they dimerise (see Template:Slink).
In this reaction, a base, usually triethylamine, removes the acidicproton alpha to the carbonyl group, inducing the formation of the carbon-carbon double bond and the loss of a chloride ion:
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.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Ethenone can be produced through pyrolysis of acetone vapours over a hot filament in an apparatus that was eventually developed into the "ketene lamp" or "Hurd lamp" (named for Charles D. Hurd).<ref>Template:Cite journal</ref>
Reactions
Due to their cumulated double bonds, ketenes are very reactive.<ref name=":1">Template:Citation</ref> The free energy released in their saturation can power the formation of relatively strained rings.
As first observed in 1908,<ref>Frances Chick and Norman Thomas Mortimer Wilsmore (1908) "Acetylketen: a polymeride of keten," Journal of the Chemical Society, Transactions, 93 : 946-950.</ref>
ketenes react with virtually any electron-rich<ref name=":0" /> π bond to form 4-membered rings.<ref name="Ullmann" />
For example, in the Staudinger synthesis,<ref name=":2">Template:Citation</ref><ref name=":3">Template:Citation</ref> a ketene attacks an imine to form a β-lactam:
Ketenes also cyclize onto enolic and enaminic
alkenes, carbodiimides, and electron-rich alkynes (the latter forming cyclobutenones).
cis Alkenes react more easily than trans alkenes.<ref>Rey, M.; Roberts, S.; Dieffenbacher, A.; Dreiding, A. S. Helv. Chim. Acta1970, 53, 417.</ref>
Electron-withdrawing substituents on the ketene accelerate the
reaction,<ref name=":0">Isaacs, N. S.; Stanbury, P. F. J. Chem. Soc., Chem. Commun.1970, 1061.</ref> but disubstituted ketenes react slowly due to steric hindrance.<ref>Huisgen, R.; Mayr, H. Tetrahedron Lett.1975, 2965.</ref>
Ketenes attack ketones and aldehydes to give β-lactones, but only under Lewis acid
catalysis or when the carbonyl is electron-impoverished:<ref>Metzger, C.; Borrmann, D.; Wegler, R. Chem. Ber.1967, 100, 1817.</ref>
Dienes generally react as two separate alkenes,
and fulvenes typically react in the ring, leaving the exocyclic double bond intact:<ref>Stadler, H.; Rey, M.; Dreiding, A. S.Helv. Chim. Acta1984, 67, 1854.</ref>
[2+2] cycloadditions proceed by a concerted, thermal mechanism, which requires suprafacial-
antarafacial alignment. Ketenes, unlike most alkenes, can align antarafacially with respect to other alkenes.<ref>Moore, H. W.; Wilbur, D. S. J. Org. Chem.1980, 45, 4483.</ref> The unique transition state geometry has the interesting consequence that the bulkier substituent on the ketene will tend
to end up on the more sterically hindered face of the cyclobutanone ring. In the transition state for cyclization, the small substituent points toward the alkene.
Ketenes place the larger substituent in the endo position when attacking cyclic alkenes.<ref>England, D. C.; Krespan, C. G. J. Org. Chem.1970, 35, 3300.</ref>
The use of chiral amine catalysts has allowed access to cycloaddition products in high enantiomeric excess.<ref>Wynberg, H.; Staring, E. J. J. Am. Chem. Soc.1982, 104, 166.</ref>
[3+2] Cycloadditions may take place with 1,3-dipoles. This process appears to be concerted, but
either ketenic double-bond can react.<ref>Texier, F.; Carrié, R.; Jaz, J. J. Chem. Soc., Chem. Commun.1972, 199.</ref>
Michael acceptors often react in a
[4+2] fashion:<ref>Mosti, L.; Menozzi, G.; Bignardi, G.; Schenone, P. Il Farmaco (Ed. Sci.)1977, 32, 794 [C.A. 1978, 88, 62262n].</ref>
Conjugated ketenes may act as 4π partners in [4+2] cycloadditions as well.<ref>Staudinger, H. Die Ketene, Verlag von Ferdinand Enke, Stuttgart, 1912.</ref>
Examples in which a vinylketene serves as the 4π partner are rare, but occur with some ketene-conjugated
heterodienes:<ref>Jäger, G.; Wenzelburger, J. Justus Liebigs Ann. Chem.1976, 1689.</ref>
Ketenes autodimerize to give various products. The parent reacts acylates itself to form diketene, a β-lactone, whereas disubstituted ketenes undergo [2+2] cycloaddition to a substituted cyclobutadione:<ref>Tenud, L.; Weilenmann, M.; Dallwigk, E. Helv. Chim. Acta1977, 60, 975.</ref>
Monosubstituted ketenes can afford either the ester or diketone dimer.
Although many polar solvents and catalysts accelerate many reactions using ketene, such reactions are normally performed in nonpolar media to prevent dimerization.
Applications
Dimerization of stearic ketene affords alkyl ketene dimers, widely used in the paper industry.<ref name="Ullmann" /> AKD's react with the hydroxyl groups on the cellulose via esterification reaction.
