Synthesis of Conjugated Linear and Cyclic Polyynes by Selective Alkyne Metathesis.

The formation of polyynes with an odd number of conjugated triple bonds is synthetically demanding, particularly for complex architectures, since most established methods rely on irreversible C-C bond-forming reactions and operate under kinetic control. To address this problem, we have developed the synthesis of triynes via molybdenum-catalyzed alkyne metathesis of diynes, which allows thermodynamic control through reversible cleavage and formation of carbon-carbon triple bonds. We demonstrate the potential of this method through the synthesis of challenging, more complex products, including triyne precursors to [7]cumulenes, a pentayne, a heptayne, and dehydrobenzannulene macrocycles containing triynes. The key to the success of the proposed methodology is achieving a balance between the selectivity and reactivity of the molybdenum catalyst with the diyne precursors.