Inorganic chemistry, aketene is anorganic compound of the formRR'C=C=O, where R and R' are two arbitrarymonovalentchemical groups (or two separatesubstitution sites in the same molecule).[1] The name may also refer to the specific compoundethenoneH2C=C=O, the simplest ketene.[2]
Although they are highly useful, most ketenes areunstable. When used asreagents in a chemical procedure, they are typically generated when needed, and consumed as soon as (or while) they are produced.
Ketenes were first studied as a class byHermann Staudinger before 1905.[3]
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 byMoses Gomberg in 1900 (compounds with triphenylmethyl group).[4]
Ketenes are highly electrophilic at the carbon atom bonded with the heteroatom, due to itssp character. Ketenes can be formed with different heteroatoms bonded to thesp carbon atom, such asO,S orSe, 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 1.160Å and the C=C bond is 1.314 Å. The angle between the twoH atoms is 121.5°, similar to the theoretically ideal angle inalkenes betweensp2carbon atoms and H substituents.[5]
Ketenes are unstable and cannot be stored. Absent nucleophiles with which to react, they dimerise (see§ Reactions).
Ethenone is produced on a commercial scale by thermal dehydration ofacetic acid. Substituted ketenes can be prepared fromacyl chlorides by anelimination reaction in whichHCl is lost:
In this reaction, a base, usuallytriethylamine, removes theacidicproton alpha to thecarbonyl group, inducing the formation of the carbon-carbon double bond and the loss of achloride ion:
Ketenes can also be formed from α-diazoketones by theWolff rearrangement, and fromvinylene carbonate byphosphorus(V) sulfide and irradiation.[6]
Another way to generate ketenes is throughflash 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) andDCC as the solvent.[7]
A more robust method for preparing ketenes is thecarbonylation ofmetal-carbenes, andin situ reaction of the thus produced highly reactive ketenes with suitable reagents such asimines,amines, oralcohols.[8] This method is an efficientone‐pot tandem protocol of the carbonylation of α‐diazocarbonyl compounds and a variety ofN‐tosylhydrazones catalysed by Co(II)–porphyrin metalloradicals leading to the formation of ketenes, which subsequently react with a variety ofnucleophiles and imines to formesters,amides andβ‐lactams. This system has a broad substrate scope and can be applied to various combinations ofcarbene precursors, nucleophiles and imines.[9]
Ethenone can be produced throughpyrolysis ofacetone vapours over a hot filament in an apparatus that was eventually developed into the "ketene lamp" or "Hurd lamp" (named for Charles D. Hurd).[10]
Due to theircumulated double bonds, ketenes are very reactive.[11] Thefree energy released in their saturation can power the formation of relatively strained rings.
Ketenes are strongacylating agents. They react with carboxylic acids to formcarboxylic acid anhydrides...
...withalcohols to formcarboxylic acid esters...
...withamines to giveamides...
...withwater to give carboxylic acids...
...and withenolisablecarbonyl compounds to giveenol esters. For example,ethenone reacts withacetone to form a propen-2-yl acetate:[1]
As first observed in 1908,[12] ketenes react with virtually anyelectron-rich[13] π bond to form 4-membered rings.[1] For example, in theStaudinger synthesis,[14][15] a ketene attacks animine to form aβ-lactam:
Ketenes also cyclize ontoenolic andenaminic alkenes,carbodiimides, and electron-richalkynes (the latter formingcyclobutenones).cis Alkenes react more easily thantrans alkenes.[16] Electron-withdrawing substituents on the ketene accelerate thereaction,[13] but disubstituted ketenes react slowly due to steric hindrance.[17]
Ketenes attackketones and aldehydes to give β-lactones, but only under Lewis acidcatalysis or when the carbonyl is electron-impoverished:[18]
Dienes generally react as two separate alkenes,andfulvenes typically react in the ring, leaving the exocyclic double bond intact:[19]
[2+2] cycloadditions proceed by a concerted, thermal mechanism, whichrequires suprafacial-antarafacial alignment. Ketenes, unlike most alkenes, can alignantarafacially with respect to other alkenes.[20] The uniquetransition state geometry has the interesting consequence that the bulkier substituent on the ketene will tendto 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 theendo position when attacking cyclic alkenes.[21]
The use of chiral amine catalysts has allowed access to cycloaddition products in high enantiomeric excess.[22]
In rarer cases, ketenes may undergo [3+2], and [4+2] cycloadditions.[23]
[3+2] Cycloadditions may take place with1,3-dipoles. This process appears to be concerted, buteither ketenic double-bond can react.[24]
Michael acceptors often react in a[4+2] fashion:[25]
Conjugated ketenes may act as 4π partners in [4+2] cycloadditions as well.[26] Examples in which a vinylketene serves as the 4π partner are rare, but occur with some ketene-conjugatedheterodienes:[27]
Ketenes autodimerize to give various products. The parent reacts acylates itself to formdiketene, aβ-lactone, whereas disubstituted ketenes undergo [2+2] cycloaddition to a substituted cyclobutadione:[28]
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.
Dimerization ofstearic ketene affordsalkyl ketene dimers, widely used in the paper industry.[1] AKD's react with the hydroxyl groups on the cellulose viaesterification reaction.
Likewise,diols (HO−R−OH) and bis-ketenes (O=C=CH−R'−CH=C=O) react to yieldpolyesters with a repeat unit of (−O−R−O−CO−R'−CO).
TheStaudinger synthesis is used to synthesizeβ-lactam antibiotics.[1]
Ethyl acetoacetate, an organic synthesis feedstock, is prepared industrially fromdiketene inethanol.[citation needed]
Media related toKetenes at Wikimedia Commons| International | |
|---|---|
| National | |
| Other | |
