Cyclopropenones ring opening

Scheme 4.24 Ketene formation by cyclopropenone ring opening. H...

Cycloadditions of aziridines to diphenylcyclopropenone lead to 4-oxazolines (36) (70CJC89). A mechanism involving initial addition to the cyclopropenone carbonyl group followed by ring opening and recyclization was suggested. 

Whilst anhydrous hydrazine is reported124) to give products of nucleophilic ring opening and cyclization (7 62/7 63) with diphenyl cyclopropenone, hydrazine hydrochloride yielded the azine 16112s ... 

Ethyl phenyl cyclopropenone (14) on reduction with NaBH4 gave rise to prod ucts 289-291, which can be ascribed to a common cyclopropanone intermediate 288 ring-opened by further reduction or attack of solvent209 ... 

Reaction of 5-phenyl-3//-l,2,4-dithiazole-3-thione (84) with 1,2-diphenylcyclopropenone leads to the SS bridged ring-opened cyclopropenone product (85) (70%) and the trithiaheterapentalene bicycle (25 R = Ph)(7%) which is believed to result from thermal decomposition of (84) (Equation... 

Studies have defined the scope of the thermal reactions of cyclopropenone ketals which are characterized by their thermal, reversible ring opening to provide reactive intermediates best represented as delocalized singlet vinyl carbenes, three-carbon 1,1-/1,3-dipoles without octet stabilization, (eq 2). 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

The thermolysis of cyclopropenone acetals at 70-80 °C, generally in benzene solution leads to a ring opening which is formally described by the scheme below ... 

The comparison of the ionization potentials of identically substituted cyclopropenones, cyclopropenes and cyclopropanes is interesting, if not yet particularly informative to date. The ionization potentials of cyclopropane, cyclopropene and cyclopropenone are much closer, 9.86, 9.67 and 9.47 eV, than for their diphenyl derivatives. Diphenylcyclopropene has an adiabatic ionization potential of 7.45 eV while those of the cis and trans isomers of 1,2-diphenylcyclopropane (18) are 8.20 and 8.05 eV respectively. These latter values for the saturated species correspond to ring-opening to l,3-diphenylprop-l-yl-3-ium (19) (equation 24) a result corroborated by both experiment via solution phase chemi-ionization and ab initio calculations on the analogous divinylcyclopropane. (The... 

Until recently the isolation of organometallic complexes of cyclopropenones in which the ring is intact proved difficult because of ring-opening (a process subjected to... 

Ring-opening cross metathesis. Significant reactions belonging to this category are the formation of the cyclic acetals of l,4-alkadien-2-ones from cyclopropenone acetals and 1-alkenes, and the assemblage of 1,2-dialkenylcyclobutanes en route to complex 1,5-cyclooctadienes. 

Some thermally unstable cyclopropenes are known, which react with olefines at or below ambient temperature by ring-opening to give vinylcyclopropanes (Eq. 10)65). Cyclopropenone ketals react with electron deficient olefines in the same manner, but here an ionic mechanism is more likely 65e). 

The reaction is catalyzed both by Ni(0) and Pd(0) catalysts, the latter giving higher yields. This transformation may occur by ring-opening as observed with some 1,2-disubstituted cyclopropenones 104>, but the intermediacy of a tricyclop.l.O.O2,4]-hexane derivative cannot be fully excluded. 

Dibromo-3,3,6,6-tetramethylcycloheptanone, when allowed to react with a base, rearranged to an isolable cyclopropenone, 2,2,5,5-tetramethylbicyclo[4.1.0]hept-l-en-7-one (6), which then underwent ring opening to give 3,3,6,6-tetramethylcyclohex-l-enecarboxylic acid. ... 

The reaction may proceed by nucleophilic substitution of the 1- and 2-halogens by sulfinate, followed by hydrolysis to the cyclopropenone and then ring opening of this with subsequent decarboxylation. 

The reaction of trichlorocyclopropenylium ion 5 with acetylenic compounds such as 1,2-bis(trimethylsilyl)acetylene and l-(trimethylsilyl)propyne gave the methylenecyclopropene 6 and cyclopropenone 7 containing ethynyl groups by substitution, whereas the reaction with phenylacetylene or l-phenyl-2-(trimethylsilyl)acetylene afforded the ring-opened products 8 via addition reactions. 

In the reaction of 3,3-pentamethylenediaziridine 116 with diphenyl-cyclopropenone the C—N—N moiety can add either to the carbonyl group, to give 123, or to the ring-opened cyclopropenone to give 124. ... 

Haloaenation. At -25° bromine reacts with cyclopropenone to give a bromooxycyclopropenium salt which is reconverted to cyclopropenone by cyclohexene at 0° ring-opening ensues [110] ... 

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