Allenes formation from cyclopropylidenes

Interestingly, the alkene to allene conversion can be carried out directly without isolation of the intermediate dihalocyclopropane. This process involves the treatment of the alkene with 1 equiv. of carbon tetrabromide and 2 equiv. of methyllithium in ether at -65 °C.163 Ultrasonic irradiation facilitates the formation of cyclopropylidenes, and therefore the allenes, from dihalocyclopropanes under the influence of Li, Na or Mg.1 The reactions are usually complete in 5-15 min. A report165 on the use of n-butyllithium complexed with the chiral tertiary amine (-)-sparteine, leading to optically active allenes, seems to be of questionable value. 

The generation and chemistry of cyclopropylidene has been extensively reviewed Of the unimolecular processes, the conversion of cyclopropylidene to allene represents a synthetically important type of reaction. However, this transformation is accompanied by competitive intra-and intermolecular processes such as C-H insertions and spiro[2.2]pentane formation from insertion to alkenes. Furthermore, vinylcyclopropylidenes undergo a concerted ring expansion to cyclopentenylidene (Skattebol rearrangement). Although cyclopropylidene to allene rearrangements can occur at temperatures below — 50 C, activation enthalpies of competing processes may be lower and therefore temperature effects can play an important role in the maximization of yields. 

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. " ... 

Although cyclopropylidenes have been generated by a number of independent methods which include cycloaddition of atomic carbon to alkenes, decarbonylation of cyclo-propylideneketenes, decarboxylation of oxaspiropentanes, from a preparative viewpoint, the a-elimination reaction of cyclopropane derivatives is most practical because of the mild reaction conditions and the availability of the starting materials. These methods are illustrated by the formation of allenes from atomic carbon/alkene reactions in Table 1 and in the preparation of 3/4, and from cyclopropanes in the preparation of 5/6. Examples showing allenes from decarbonylation of cyclopropylideneketenes are shown in the preparations of compounds 7/8, 9, 10, " and 11. ... 

The diazotization route is frequently accompanied by products derived from solvolysis of the initially formed cyclopropylidene or the rearranged cyclopentenylidene (Skattebol rearrangement) in the case of vinylcyclopropylidene with alcohol solvent, although allenes still account for the major products in the case of vrc-disubstituted cyclopropylidenes. It is noteworthy that the stereochemistry of the ring substituents (Table 2, entries 2 and 3) is an important factor in affecting yields of allenes. gcw-Disubstitution of vinylcyclopropanes diminishes formation of allenes in favor of products from the Skattebol rearrangement (entry 6). 

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