Z-Selective ethenolysis with a ruthenium metathesis catalyst: experiment and theory.

The Z-selective ethenolysis activity of chelated ruthenium metathesis catalysts was investigated with experiment and theory. A five-membered chelated catalyst that was successfully employed in Z-selective cross metathesis reactions has now been found to be highly active for Z-selective ethenolysis at low ethylene pressures, while tolerating a wide variety of functional groups. This phenomenon also affects its activity in cross metathesis reactions and prohibits crossover reactions of internal olefins via trisubstituted ruthenacyclobutane intermediates. In contrast, a related catalyst containing a six-membered chelated architecture is not active for ethenolysis and seems to react through different pathways more reminiscent of previous generations of ruthenium catalysts. Computational investigations of the effects of substitution on relevant transition states and ruthenacyclobutane intermediates revealed that the differences of activities are attributed to the steric repulsions of the anionic ligand with the chelating groups.

Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis.

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.

Kinetic Selectivity of Olefin Metathesis Catalysts Bearing Cyclic (Alkyl)(Amino)Carbenes.

The evaluation of ruthenium olefin metathesis catalysts 4-6 bearing cyclic (alkyl)(amino)carbenes (CAACs) in the cross-metathesis of cis-1,4-diacetoxy-2-butene (7) with allylbenzene (8) and the ethenolysis of methyl oleate (11) is reported. Relative to most NHC-substituted complexes, CAAC-substituted catalysts exhibit lower E/Z ratios (3:1 at 70% conversion) in the cross-metathesis of 7 and 8. Additionally, complexes 4-6 demonstrate good selectivity for the formation of terminal olefins versus internal olefins in the ethenolysis of 11. Indeed, complex 6 achieved 35 000 TONs, the highest recorded to date. CAAC-substituted complexes exhibit markedly different kinetic selectivity than most NHC-substituted complexes.