DFT Study on the Mechanism of Bimetallic Pd−Zn-Catalyzed Cycloaddition of Alkynyl Aryl Ethers with Internal Alkynes
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 ether was also considered. Both C6H5OC≡CSiiPr3 and C6H5OC≡CSiMe3 were chosen as the substrates. The reactions involve C−H activation of substrate, acetic acid rotation, H transformation, MeC≡CMe or substrate insertion into Pd-phenyl bond and reductive elimination steps. It is found that the cycloaddition is favored for C6H5OC≡CSiiPr3, while dimerization is preferred for C6H5OC≡CSiMe3, because the steric repulsion between two bulky SiiPr3 groups is relatively larger and steric repulsion between two little SiMe3 groups is relatively small. In addition, besides C6H5OC≡CSiiPr3, other four substrates C6H5CH2C≡CSiiPr3, C6H5C(O)C≡CSiiPr3, C6H5SC≡CSiiPr3 and C6H5N(H)C≡CSiiPr3 have been calculated as the substrates for cycloaddition reaction with MeC≡CMe. Among the five substrates, C6H5OC≡CSiiPr3 has the lowest energy barrier (29.9 kcal/mol), consistent with the experimental observation that C6H5OC≡CSiiPr3 is the appropriate substrate for successful cycloaddition.