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