Regio-controlled intramolecular reductive cyclization of diynes

Metal promoted intramolecular reductive cyclization of diynes, useful for construction of cyclic diene systems, is formally classified into three types with respect to the orientation of two acetylene moieties to the newly formed ring. While the exo-exo mode of reductive cyclization reactions have been well known to be promoted by transition metal two-electron reductants, the exo-endo mode has rarely been studied and the endo-endo mode had been virtually unknown. Presented herein are the first examples of the endo-endo mode cyclization of diynes promoted by lithium naphthalenide, in addition to some typical examples of nickel-catalyzed exo-exo mode cyclization of diynes and their applications to the synthesis of novel pi-conjugated oligomers and polymers containing silacyclopentadiene ring. The unusual electronic structures of the silole-containing pi-conjugated systems have been analyzed by molecular orbital calculations.