C-C versus C-H bond activation of propyne by Y: A DFT study

The reaction of ground state Y with propyne has been investigated using DFT methods. It was found that H2 elimination more likely proceeds via initial methyl C-H bond activation followed by the activation of another methyl C-H bond or α-C-H bond. The concerted H2-elimination pathways were found to be barrierless, whereas the stepwise pathways have notable activation barriers. Three isomers, Y(HCCCH), Y(CCCH2) and Y-CCCH2 were identified for the experimentally observed H2-elimination product YC3H2. For the elimination of CH3, a new pathway was identified. This pathway can be described as: π-complex → TS → HY(CH2CCH) → TS → (CH2)-HY-(CCH) → TS → H3C-Y-CCH → CH3 + YC2H. This novel mechanism can also rationalize the formation of CH3 + YC2H in the reaction of Y with allene. The previously proposed direct sp-sp3 C-C bond insertion mechanism was also confirmed, but this pathway is energetically less favorable.