Cycloalkyne
In organic chemistry, a cycloalkyne is the cyclic analog of an alkyne. A cycloalkyne consists of a closed ring of carbon atoms containing one or more triple bonds. Cycloalkynes have a general formula CnH2n−4. Because of the linear nature of the C–C≡C–C alkyne unit, cycloalkynes can be highly strained and can only exist when the number of carbon atoms in the ring is great enough to provide the flexibility necessary to accommodate this geometry. Large alkyne-containing carbocycles may be virtually unstrained, while the smallest constituents of this class of molecules may experience so much strain that they have yet to be observed experimentally. Cyclooctyne (C8H12) is the smallest cycloalkyne capable of being isolated and stored as a stable compound. Despite this, smaller cycloalkynes can be produced and trapped through reactions with other organic molecules or through complexation to transition metals.
Due to the significant geometric constraints imposed by the R–C≡C–R functionality, cycloalkynes smaller than cyclodecyne (C10H16) result in highly strained structures. While the cyclononyne (C9H14) and cyclooctyne (C8H12) are isolable (though strongly reactive) compounds, cycloheptyne (C7H10), cyclohexyne (C6H8) and cyclopentyne (C5H6) only exist as transient reaction intermediates or as ligands coordinating to a metal center. There is little experimental evidence supporting the existence of cyclobutyne (C4H4) or cyclopropyne (C3H2), aside from studies reporting the isolation of an osmium complex with cyclobutyne ligands. Initial studies which demonstrated the transient intermediacy of the seven-, six- and five-membered cycloalkynes relied on trapping of the high-energy alkyne with a suitable reaction partner, such as a cyclic dienes or diazo compounds to generate the Diels–Alder or diazoalkane 1,3-dipolar cycloaddition products, respectively. Stable small-ring cycloalkynes have subsequently been isolated in complex with various transition metals such as nickel, palladium and platinum. Despite long being considered to be chemical curiosities with limited synthetic applications, recent work has demonstrated the utility of strained cycloalkynes in both total synthesis of complex natural products and bioorthogonal chemistry.
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