An enyne metathesis is an organic reaction taking place between an alkyne and an alkene with a metal carbene catalyst forming a butadiene.
The ring-closing step takes place when this center reacts with the alkene group to a metallacyclobutane 4.5 as in a regular olefin metathesis reaction.
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.