Cyclopropylidene intermediate

This process is fully exploiting the high strain energy of bicyclopropylidene 288 and cyclopropylidene intermediate 297 as well as stability of arylthio-Pd bond to facilitate the CO insertion to carbon-Pd(SAr) bond. 

Novel preparative possibilities arise when cyclopropylidene intermediates are generated from functionalized dihalocyclopropanes. Thus Baird has shown that bicyclic ethers may be formed in high yield by an intramolecular insertion from cyclopropyl alkyl ethers [120],... 

De Meijere, A., Faber, D., Heinecke, U., Walsh, R., Muller, T., Apeloig, Y. On the question of cyclopropylidene intermediates in cyclopropene-to-allene rearrangements - tetrakis(trimethylsilyl)cyclopropene, 3-alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes, and related model compounds. Eur. J. Org. Chem. 2001,663-680. 

In refluxing cyclooctene, exo-7-bromo-c t/f>-7-(trimethyistannyl)bicyclo[4.1.0]heptane (20) reacted via the corresponding cyclopropylidene intermediate to afford spiro[bicyclo[4.1.0]hep-tane-7,9 -bicyclo[6.1.0]nonane] (21) in 76% yield. However, this reaction is not preparatively interesting and not applicable to a-bromo-a-(trimethylstannyl)cyclopropanes in general, because the cyclopropylidenes from the latter gave ring-opened allenic products. ... 

Cyclonona-1,2-diene (1), with its allenic chromophore bent ca. 10 ° from linearity, is the smallest unsubstituted cyclic allene which is kinetically stable at ambient temperature. " Benzene sensitized irradiation of this cyclic allene yielded tricyclo[4.3.3.0 ]nonane (3) (cj) = 0.17 in the vapor phase), while direct irradiation afforded four isomers, one of which was believed to be the tricyclononane 3. " The remaining products were not identified. In what appeared to be a general reaction of both linear and cyclic allenes, a cyclopropylidene intermediate 2 was proposed. Subsequently, the formation of cycloadducts 4 and 5 from the irradiation of 1 in benzene solution was reported. " ... 

Concomitant formation of a C-H and a C-C bond to the same carbon atom in a cyclopropane ring can be achieved in a number of ways. The best established method involves treatment of 1,1-dibromocyclopropanes with a strong base, usually methyllithium or butyllithium, which generates cyclopropylidene intermediates that can undergo intramolecular C-H insertion if the substrate structure and the reaction conditions are right. These reactions have been discussed thoroughly elsewhere (Houben-Weyl, Vol. E19b). 

Insertion with complete regio- and stereoselectivity occurs when steroids 2 are treated with methyllithium to give 3 the Cl 1 to HI la bond is exclusively attacked by the cyclopropylidene intermediate. The clean reaction with the dichloride is noteworthy. 

Both a cyclopropylidene intermediate and the recently suggested high-energy direct 7r-insertion are compatible with a direct attack of a recoiling carbon atom on a double bond. 

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