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Alkyne Metathesis as a New Synthetic Tool: Ring-Closing, Ring-Opening, and Acyclic
Uwe H F Bunz1, Lioba Kloppenburg1
1Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208 (USA), Fax: (+1) 803-929-0267.
Abstract:
Conceptually and mechanistically (!) the metathesis of disubstituted alkynes in the presence of molybdenum or tungsten catalysts (see scheme) is related to alkene metathesis. With skillful experimentation and careful selection of substrate, alkyne metathesis can result in both ring opening/ring closing and the formation of high molecular weight polymers.
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The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.

