Comparative investigation of kinetic consequences associated with long-range electronic effects on catalytic ruthenium-promoted ring-closing metathesis

Feasibility of associative mechanism in enyne metathesis catalyzed by grubbs complexes

The mechanism of enyne metathesis catalyzed by first and second generation Grubbs complexes has been computationally explored at the DFT level. The relative reactivity and the regioselectivity for the reaction of differently substituted alkenes and alkynes with model Ru complexes has been studied. The usually accepted dissociative mechanism for the alkene metathesis has been explored for alkynes, and compared with an associative pathway involving initial coordination of the alkyne to the 16-electron catalyst. Our results show that an associative mechanism would be the preferred pathway for the reaction of phosphine-based (first generation) Ru carbenes, at least for small phosphines such as PMe(3), whereas for the more reactive complexes containing a heterocyclic carbene as ligand (second generation catalysts), the dissociative process is far more favourable.

Relative rate profile for ring-closing metathesis of a series of 1-substituted 1,7-octadienes as promoted by a 4,5-dihydroimidazol-2-ylidene-coordinated ruthenium catalyst

[reaction: see text] This report details the kinetic responses of nine compounds of type 6 to ring-closing metathesis as promoted by 2 to give the identical product 7. The experimental observations have been subjected to Hammett analysis. The rho value for the composite aromatic derivatives (R = p-XC(6)H(4)-) differs from that of the aliphatic series, although both are negative because electron-donating groups accelerate the reaction.