Ketene dimer, ring-opening

In light of the above results it is interesting to note that the reaction of diphenylcyclo-propenone dimer spirolactone with ironenneacarbonyl yields a mixture of ring-opened vinyl carbene and -vinylketene complexes, and these interconvert under addition (or removal) of CO (equation 225) . A possible pathwav to vinylketene Fe-complexes, prepared earlier from cyclopropenes and ironcarbonyls " , may thus involve initial f -coordination, followed by ring cleavage to vinyl carbene and finally carbonylation to the ketene iron // -complexes. An analogous // -manganese complex is prepared similarly by the reaction of CpMn(CO),THF with 3,3-dimethylcyclopropene complex (equation 226) . ... 

The 4-methylene-l,3-dithietane-2-ones lose carbon oxysulfide to give thio-ketenes that dimerize to 2,4-dimethylene-l,3-dithietanes, as exemplified by the flash-vacuum thermolysis of 551. Reaction of several methylene 1,3-dithietane-2-ones with secondary amines also appears to proceed via loss of carbon oxysulfide followed by addition of the amine to the thioketene. An exception to the loss of carbon oxysulfide is the reaction of a guanidine derivative with 551 to give the ring-opened product 552. ... 

Ketene dimer (XXII), which is an analog of the 2-methylene-oxetane (XXI), will tndergo a higher extent of ring opening on polymerization to give copolymers with different structures depending on whether the reaction is run in a sealed tube or open to the air. 

Diketene (4-methyleneoxetane-2-one) is formed by dimerization of ketene which in turn is prepared by pyrolysis of acetone or acetic acid. The compound is an industrial intermediate. It ring-opens with ethanol to give ethyl acetoacetate the nucleophile attacks the C-atom of the carbonyl group. 

Scheme 9.121. A representation of the formation and subsequent decomposition of methyl 3-ketopropanoate. It is formed by the ring opening of ketene dimer on reaction with methanol. Decomposition in the presence of aqueous acid and aqueous base, separately, produces ketone in the former and fragmentation in the latter.

Apparently the intermediate radical undergoes ring opening to give the carbonyl radical. In the sealed tube only a little decarbonylation takes place and the copol3mier contains mostly the g-diketone structure. When the reaction is carried out at atmospheric pressure, much more decarbonylation takes place and the copolymer consists of mostly the simple ketone units. Since ketene dimer is an article of commerce, this monomer appears to be an excellent start-x ing material for the introduction of keto groups into an addition polymer. 

Once more the synthetic utility of this process was exhibited via a series of ring-opening/fiinctionalization reactions of the ketene dimers to produce, among others, 1,3-diketones and 3-hydroxy ketones in good yield and without erosion of enantiointegrity. 

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