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

Abstract

The ring-closing metathesis (RCM) of enynes [1!2, Eq. (1)] was first reported by Katz and Sivavec in 1985. [1]A tungsten-alkylidene precatalyst was employed and, on the basis of mechanistic studies on alkene/alkyne metathesis copolymerization, a Chauvin-type [2] mechanism involving an "yne-then-ene" sequence was proposed (Scheme 1, cycle A).

Chemodivergent Metathesis of Dienynes Catalyzed by Ruthenium–Indenylidene Complexes: An Experimental and Computational Study

“…Mechanistic studies: In enyne metathesis, the mechanism [6b] at play is still a matter of debate. The question asked is whether an "ene-then-yne" [34] or a "yne-then-ene" pathway is involved. [35] In order to shed more light on the matter, we considered both pathways with the dienyne substrates shown in Scheme 1.…”

5.27 Ene–Yne Metathesis

“…First, an elegant mechanistic study was conducted by Lloyd-Jones. 54 In this investigation, the authors showed that 13 C-labeled ethylene produced vinyl carbene turnover, showing that ethylene turns over the vinyl carbene by what the authors termed a 'second cycle,' via reaction with the auxiliary alkene, ethylene. In other words, ethylene assists the turnover of the intermediate vinyl carbene (see Scheme 3, where R¼CH 2 ).…”

Transition metals in organic synthesis: Highlights for the year 2005

“…A mechanism involving an ene-thenyne metathesis of tethered enynes was supported by labeling experiments [1261]. Ruthenium-catalyzed ene-yne RCM reactions of cyclopropane substituted 1,6-enynes forming either bicyclic or monocyclic compounds depending on the catalyst (Eq.…”