Rearrangement methylenecyclopropene

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

In the case of methylenecyclopropenes such as (315), the pyrazoline rearranges to a diazo-compound, which in turn cyclises at the original exocyclic double bond 250) ... 

Methylenecyclopropene (19) dimerizes to give (20) and (21) in a 92 8 ratio. Bicyclopropylidene (22) is converted in 35% yield to dimer (23) a competitive methylenecyclopropane rearrangement forming (24) is the major side reaction. 

Photolysis of matrix isolated diazo(2-furyl)methane led to the aldehyde (79) by stereospecific rearrangement of the carbene (80). " The corresponding 3-furyl compound gave the (5-Z)-methylenecyclopropene (81) by ring closure of the initially formed vinylcarbene to give cyclopropene (82) followed by ring opening of the furan. 

There is one substituted methylenecyclopropene for which the radical cation is well understood the 2,3-diphenyl, 4-hydroxy, 4-methoxy derivative (16). This ion is not formed by 1-electron oxidation or chemi-ionization from the methylenecyclopropene. Instead it arises from rearrangement of its tautomer, methyl 2,3-diphenylcyclopropene-l-carboxylate (17) in pro tic media . This represents another example of reversal of stability... 

Interesting reactions of lb are additions to acetylene HCCH and difluoroa-eetylene FCCF. The reaction with HCCH directly leads to the methylenecyclo-propene 13a, and no other intermediates are observed. [43] In contrast, the reaction with FCCF yields allenylcarbene 14b as the primary thermal product. Visible light irradiation is required to induce the rearrangement to the corresponding methylenecyclopropene 13b. This clearly demonstrates that the formation of the methylenecyclopropene is a two-step reaction with carbene 14b as an intermediate. 

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