Cyclopropenone decarbonylation

The thermal cycloreversion of imines 268, i.e. formation of isocyanide and alkyne, which would be expected by analogy with cyclopropenone decarbonylation and in reversal of cyclopropenone imine formation (see p. 25), was found to be only a minor side-reaction203). 

Cyclopropenone formation should involve the bisketene 46 and its decarbonylation to the monoketene 47 (a valence tautomer of cyclopropenone 44), since photolysis in protic media like ethanol produces diethoxy diethyl tartrate 45) (meso and d,l). This method was also successful in the case of l,2-diphenyl-3,3-dichloro-cyclo-butene dione (48) giving rise to diphenyl cyclopropenone49 (but as for 44 only a moderate yield was produced) ... 

Application of the decarbonylation reaction to cyclohepteno cyclopropenone (J9)43) led to the intermediate formation of the highly strained cycloheptyne (246) as indicated by the formation of its cyclotrimerization product 247 (in analogy to... 

The 4-hydroxy aryl substituted cyclopropenone 251 was found by West193 to exhibit a remarkable cycle of decarbonylation and oxidation-reduction reactions ... 

The compound 251 decarbonylates on photolysis to bis(4-hydroxyaryl) acetylene 253, which is easily oxidized to the quinonoid cumulene 254. This is also obtained by thermal decarbonylation of the product of oxidation of cyclopropenone 251, the diquinocyclopropanone 252. Likewise, the blue derivative of 3-radialene 256 (a phenylogue of triketo cyclopropane) is formed from tris-(4-hydroxyaryl) cyclopropenium cation 255 by oxidation34. ... 

At temperature below the decarbonylation level cyclopropenones are preferentially transformed to stable dimers, which do not eliminate CO at higher temperatures. Thus, thermolysis of diphenyl cyclopropenone at temperatures above 160 °C gives mainly diphenyl acetylene, whilst heating in the molten state to 145— -... 

At the HF/6-31G level, ketenyl carbenes (1) were calculated to be intermediates in the decarbonylation of 1,2-bisketenes (2) to form cyclopropenones." At the MP2/6-31G and B3LYP levels, however, decarbonylation was predicted to form the cyclopropenones directly. The anfi-ketenyl carbenes were found to be 2.2-5.4kcalmoP higher in energy than the syn isomers (1). The mechanism of reaction of [l.l.ljpropellane with singlet dihalocarbene has been reported. ... 

When cyclopropenone, or its dichloro or diphenyl analogues are heated to ca. 100°C dimerization, rather than decarbonylation, takes place and spirocyclic compounds (316) are obtained. These most likely result from nucleophilic attack on the carbonyl carbon of one molecule by the oxygen atom of the second followed by 1,2-cleavage and cyclization ... 

Photolytic decarbonylation of cyclobutenediones 1 (see Sections 9A-C) produced cyclopropenones 2. 2-Alkoxy-3-alkyl- and 2,3-dialkoxycyclopropenones are labile to photolysis, thermolysis, and especially hydrolysis. Therefore, only compounds with bulky substituents were prepared. This reaction has only been used for the preparation of alkoxycyclopropenones. 

The photolytic decarbonylation of cyclopropenones proceeded smoothly to give the corresponding acetylenes... 

As well as formation of metallacyclobutenones, reactions of cyclopropenones with transition-metal compounds can lead to acetylene-derived products by decarbonylation, to quinones by a nickel-catalyzed cyclodimerization, and to cluster compounds involving a cleavage of the double bond of the cyclopropenone. ... 

The cyclopropenone (130) undergoes decarbonylation on flash photolysis in water.The resultant ynamine (131) is accompanied by the enamine (132). The enamine is formed from the carbene (133) and its trapping with water. The ynamine (131) is unstable and transforms into the ketenimine (134). The photo-... 

C-C bonds between carbonyls and other sp carbon centers are more activated still, and opening is possible even below room temperature in strained cases, for example of cyclopropenone by Pt(ll) at —35°C [150]. Likewise, cleavage of a bond between a carbonyl and an sp carbon is possible, as in the decarbonylation... 

Miscellaneous Reactions. A study of the photodissociation of ketene under jet-cooled conditions in the 193-215 nm wavelength range has shown that bond fission to afford H atoms is a principal reaction.Acryloyl chloride undergoes C-Cl bond fission on irradiation at 193 nm. The photodecarbon-ylation of the cyclopropenones (54) occurs quantitatively on irradiation in methanol solution to yield the corresponding alkynes. The quantum yield for decarbonylation for the alkyl substituted derivatives is in the range 0.2-0.3, while for the aryl derivatives = 0.7. A determination of the reaction dynamics for the photodissociation of diphenylcyclopropenone has shown that the dissociation occurs from the So state. 

Trost and Whitman have established a new route to diphenylcyclopropenone, using the photochemical decomposition of the aa -bisdiazoketone (161). The product (162) was stable to the reaction conditions when irradiation (436 nm) was carried out in methanol, but in toluene decarbonylation afforded a quantitative yield of diphenyl-acetylene. Application of the reaction to oea -bisdiazocyclohexanone gave only cyclopentane derivatives (Scheme 13), indicative of the transient intermediacy of a cyclopropenone. 

Scheme 7.10 (a) A quantum chain mechanism via radical cation. Immediately when formed an activated product (P ) or an intermediate transfer energy to another molecule causing a second reaction, (b) Elimination of CO from cyclopropenone. (c) Decarbonylation in S2 is followed by energy transfer and a second decarbonylation... 

Wilkinson s catalyst is useful for effecting intramolecular decarbonylation of aldehydes to hydrocarbons with retention of configuration (Walborsky and Allen, 1971). Decarbonylation of disubstituted cyclopropenones by iron, nickel, or cobalt carbonyls results in the formation of alkynes (Bird and Hudec, 1959). Tetraphenylethylene was obtained in 68% yield by reaction of diphenylketene with cobalt carbonyl (Hong et ai, 1968). 

Ketone decarbonylation reactions have been useful for the preparation of highly reactive molecules. A few examples include the preparation of ynols, yneamines, and strained alkynes by in situ photolysis of cyclopropenones 51 to 53 (Scheme 14). These high-energy compounds are short-hved transients that must be either detected by fast kinetic methods or kineticaUy stabilized in cryogenic matrices to facihtate their detection. Phenanthrocyclopropenone 53 was generated in situ by photolysis of a his(diazo)ketone or by photodecarboxylation of dicarboxyphenathrene anhydride. ... 

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