DFT study on the mechanism of bimetallic Pd–Zn-catalyzed cycloaddition of alkynyl aryl ethers with internal alkynes

Abstract

The reaction mechanism of bimetallic Pd–Zn-catalyzed cycloaddition of alkynyl aryl ethers with internal alkynes has been studied theoretically. Besides cycloaddition reaction, the dimerization of alkynyl aryl ethers was also considered. Both C₆H₅OCCSiⁱPr₃ and C₆H₅OCCSiMe₃ were chosen as the substrates. The reactions involve C–H activation of the substrate, acetic acid rotation, H transformation, MeCCMe or substrate insertion into the Pd–phenyl bond and reductive elimination steps. It is found that cycloaddition is favored for C₆H₅OCCSiⁱPr₃, while dimerization is preferred for C₆H₅OCCSiMe₃, because the steric repulsion between two bulky SiⁱPr₃ groups is relatively large and the steric repulsion between two small SiMe₃ groups is relatively small. In addition, besides C₆H₅OCCSiⁱPr₃, four other substrates C₆H₅CH₂CCSiⁱPr₃, C₆H₅C(O)CCSiⁱPr₃, C₆H₅SCCSiⁱPr₃ and C₆H₅N(H)CCSiⁱPr₃ have been calculated as the substrates for cycloaddition reaction with MeCCMe. Among the five substrates, C₆H₅OCCSiⁱPr₃ has the lowest energy barrier (29.9 kcal mol⁻¹), consistent with the experimental observation that C₆H₅OCCSiⁱPr₃ is the appropriate substrate for successful cycloaddition.