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. ==Scope==