Rate Enhancement by Ethylene in the Ru‐Catalyzed Ring‐Closing Metathesis of Enynes: Evidence for an “Ene‐then‐Yne” Pathway that Diverts through a Second Catalytic Cycle

Mixing it: A dual-substrate/dual isotopic labeling strategy has shown that the accelerating effect of ethylene in intermolecular ring-closing metathesis of enynes is best explained by an ene-then-yne mechanism rather than the commonly proposed yne-then-ene mechanism (see picture).