 The butadiene group forms in the last step with expulsion of a new methylene carbene, initiating the next cycle but now with R' = H and R' ' = H.

This is the proposed "yne-then-ene" mechanism.Evidence for an "ene-then-yne" pathway is beginning to emerge, especially for ruthenium based catalytic systems.The driving force for this conversion is the formation of a thermodynamically stable conjugated butadiene.