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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 Cn H2n-4 Because of the linear nature of the C-CΞC-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. 〔''Can cyclopropyne really be made?'' Paul Saxe and Henry F. Schaefer J. Am. Chem. Soc.; 1980; 102(9) pp 3239 - 3240; 〕 Cyclooctyne (C8H12) is the smallest cycloalkyne capable of being isolated and stored as a stable compound.〔(cycloalkyne (chemical compound) - Britannica Online Encyclopedia )〕 Despite this, smaller cycloalkynes can be produced and trapped through reactions with other organic molecules or through complexation to transition metals.
==Background==
Due to the significant geometric constraints imposed by the R-CΞC-R functionality, cycloalkynes smaller than cyclodecyne (C10H16) result in highly strained structures. While the nine (C9H14) and eight (C8H12) membered cycloalkyne analogues 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.〔''Angle Strained Cycloalkynes'' Adolf Krebs and Jurgen Wilke Topics in Current Chemistry; 1983; 109 pp 189 - 233; 〕 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.〔''Cyclobutyne: the ligand. The synthesis and molecular structure of osmium cluster Os3(CO)9(.mu.3-.eta.2-C2CH2CH2)(.mu.-SPh)(.mu.-H)" Richard D. Adams , Gong Chen , Xiaosu Qu, Wengan Wu and John H. Yamamoto J. Am. Chem. Soc.; 1992; 114(27) pp 10977 – 10978; 〕 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.〔''Zur Existenz niedergliedriger Cycloalkine, 1'' Georg Wittig and Adolf Krebs Chem. Ber.; 1961; 94(12) pp 3260 - 3275; 〕 Stable small ring cycloalkynes have subsequently been isolated in complex with various transition metals such as nickel, palladium and platinum.〔''Metal Complexes of Small Cycloalkynes and Arynes'' Martin A. Bennett and Heinz P. Schwemlein Angew. Chem.; 1989; 28(10) pp 1296 - 1320; 〕 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. 〔''Arynes and Cyclohexyne in Natural Product Synthesis'' Christian M. Gampe and Erick M. Carreira Angew. Chem.; 2012; 51(16) pp 3766 - 3778; 〕〔''Strained Cycloalkynes as New Protein Sulfenic Acid Traps''Thomas H. Poole, Julie A. Reisz, Weiling Zhao, Leslie B. Poole, Christina M. Furdui and S. Bruce King J. Am. Chem. Soc.; 2014; 136(17) pp 6167 - 6170; 〕
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