Cycloaddition of allenes

Lately a third type of transition state has been favored for [2 + 2] cycloadditions forming carbocyclic and heterocyclic four-membered rings. The experimental data on the addition of diarylketenes to arylethylenes are well accommodated by the [ 2s + 2s + 2s] process proposed by Baldwin (70JA4874). The steric effects on the cycloaddition of allenes to ketenes also favor this mechanism (76JA7698). 

Transition Metal-Catalyzed Addition/Cycloaddition of Allenes 925... 

As exemplified in Eq. 8.38, thermal [2 + 2] cycloadditions of 4-vinylidene-2-oxazoli-dinone 287 and alkynes such as phenylacetylene result in the formation of 3-phenyl-substituted methylenecyclobutene 288 [149]. The authors confirmed by NMR analysis that only the Z-configuration isomer was formed. It is worth noting that the [2 + 2] cycloaddition of allenes 287 is not restricted to alkynes even olefins such as acrylic esters or silyl enol ethers furnish the corresponding methylenecyclobutanes... 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

Intramolecular [2 + 2]-cycloadditions of allenes and enones occur with fair to good site- and regioselectivities. Thus, cyclohexenone 40 bearing an allenic side-chain at the C3 position was irradiated and a single product 41 was isolated in 95% yield [45],... 

The cycloaddition of allenic lactone 124 with cyclopentadiene was carried out under thermal conditions and also in the presence of a Lewis add catalyst [102]. In both cases, the exo-adduct was formed predominantly. 

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