Alkynoxy-Directed C-H Functionalizations: Palladium(0)-Catalyzed Annulations of Alkynyl Aryl Ethers with Alkynes

Palladium(0)-catalyzed insertion/annulation sequence between aryl silylethynyl ethers and internal alkynes was found to proceed through activation of ortho-C-H bonds assisted by alkynoxy groups and gave stereoselectively (Z)-2-silylmethylenechromenes. These products could be easily converted into 2,2-alkylated 2H-chromene derivatives, an important structural motif in medicinal chemistry and materials science. Various aryl silylethynyl ethers and alkynes can be transformed under the reaction conditions, and a wide range of chromenes is thus accessible. When unsymmetric alkynes are employed, a regioselective annulation takes place, especially those containing aryl and/or bulky substituents. Catalytic systems based on palladium(0), such as Pd(OAc)(2)/PCy3/Zn, [Pd(dba)(2)]/PCy3, or Pd(PCy3)(2) exhibit excellent catalytic activity, and the best performance is observed for Pd(PCy3)(2) in combination with Zn(OAc)(2) as an additive. Substituents on the aryl group in the alkynyl aryl ethers rarely affect the reaction rate. Deuterium-labeling experiments. suggest that the ortho-hydrogen atom migrates to the 2-methylene position in the chromene products. The cleavage of the C-H bond is considered to be the rate-determining step in these reactions.