Reaction mechanism of CH3M≡MCH3 (M=C, Si, Ge) with C2H4: [2+1] or [2+2] cycloaddition?

The mechanism of the cycloaddition reaction CH3M≡MCH3 (M=C, Si, Ge) with C2H4 has been studied at the CCSD(T)/6-311++G(d,p)//MP2/6-311++G(d,p) level. Vibrational analysis and intrinsic reaction coordinate (IRC), calculated at the same level, have been applied to validate the connection of the stationary points. The breakage and formation of the chemical bonds of the titled reactions are discussed by the topological analysis of electron density. The calculated results show that, of the three titled reactions, the CH3Si≡SiCH3+C2H4 reaction has the highest reaction activity because it has the lowest energy barriers and the products with the lowest energy. The CH3C≡CCH3+C2H4 reaction occurs only with difficulty since it has the highest energy barriers. The reaction mechanisms of the title reactions are similar. A three-membered-ring is initially formed, and then it changed to a four-membered-ring structure. This means that these reactions involve a [2+1] cycloaddition as the initial step, instead of a direct [2+2] cycloaddition.