Reaction Mechanism
The reaction mechanism for this reaction is outlined in scheme 4:
In the first catalytic cycle the alkyne group of enyne 4.1 forms a metallacyclobutene intermediate 4.3 with carbene 4.2 with R' and R' ' any organic group required to stabilized it. In the next step the metathesis step is reversed with formation of a new double bond and a new carbenic center in 4.4. 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.
Read more about this topic: Enyne Metathesis
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