A ketene is an organic compound of the form R′R″C=C=O, where R and R' are two arbitrary monovalentchemical groups. The name may also refer to the specific compound ethenone, 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 they are produced.
History
Ketenes were first studied as a class by Hermann Staudinger before 1905. Ketenes were systematically investigated by Hermann Staudinger in 1905 in the form of diphenylketene. Staudinger was inspired by the first examples of reactive organic intermediates and stable radicals discovered by Moses Gomberg in 1900.
Preparation
Ethenone, the simplest ketene can be generated by pyrolysis of acetone: This reaction is called the Schmidlin ketene synthesis. Other ketenes can be prepared from acyl chlorides by an elimination reaction in which HCl is lost: 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 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. The N-pyridylamines are prepared via a condensation with R-malonates with N-amino and DCC as the solvent. 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. 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–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 carbeneprecursors, nucleophiles and imines.
Ketenes react with ammonia to primary amides: The reaction of ketenes with primary amines produces secondary amides: Ketenes react with secondary amines to give tertiary amides:
Hydrolysis
By reaction with water, carboxylic acids are formed from ketenes
Enolacetates are formed from ketenes with enolisablecarbonyl compounds. The following example shows the reaction of ethenone with acetone to form a propen-2-yl acetate:
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. AKD's react with the hydroxyl groups on the celluose via esterification reaction. They will also undergo 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. 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 and bis-ketenes yield polyesters with a repeat unit of. 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.
