3. Alkyne metathesis

Alkyne metathesis has also proved extremely useful for synthesizing complex molecules. Catalyzed by organometallic complexes, the alkyne metathesis reaction creates new triple bonds, according to the Katz and Sivavec mechanism involving metallacyclobutanes (figure 56 ). Various processes can be carried out, such as cross-metathesis (figure 56 , eq. 1), ring closure (figure 56 , eq. 2) or polymerization (eqs. 3 and 4). ROAMP and ADIMET-type polymerization processes will not be dealt with here, but only cycle closure and cross-metathesis will be illustrated.