A recent report out of the Grubbs’ research group describes a series of novel Ru-(pre)catalysts prepared by the selective substitution of one of the standard chloride ligands of a 2nd generation Grubbs/Hoveyda complex with sterically encumbering sulfonate or phosphate anions. It was hypothesized that the larger steric nature of the alternative X-type ligand may affect the conformation of the metallacyclobutane intermediate and lead to increased levels of cis-olefin products.
A protocol has been established to determine E/Z selectivity for various olefin metathesis catalysts as described in 2006 by the Grubbs group.1The procedure involves performing the cross metathesis reaction of allyl benzene with 2 equivalents of cis-1,4-diacetoxy-2-butene, and monitoring the E/Z ratio of the hetero-coupled cross-product versus product formation. First-generation catalysts typically have E/Z ratios on the order of ~ 5 that stay relatively constant throughout the reaction. On the other hand, second-generation catalysts have E/Z ratios of ~ 3 at low reaction conversions, but as the reaction progresses, the E/Z ratios increase dramatically (~ 10 @ 80% conversion).This is a result of the greater efficiency of the 2nd generation catalysts to promote secondary metathesis events and isomerize the product to the more thermodynamically favored trans-product.
The new catalysts in this report supply the product with E/Z ratios of ~ 3 at product conversions around 80%.The best E/Z ratio reported was 2.7 (@ 78% conversion), obtained with a catalyst comprising the DIPP NHC and a mesitylsulfonate ligand. Currently, the results obtained with the new Ru-catalysts do not provide the same level of selectivity as some molybdenum and tungsten catalysts reported recently by Schrock and Hoveyda,2 however, it definitely is a step in the right direction. Hopefully some further modification of the catalyst architecture will lead to Ru-systems that favor the formation of cis olefins.
1 Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H. “A Standard System of Characterization for Olefin Metathesis Catalysts.” Organometallics, 2006, 25, 5740-5745.
2 Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. M. “Highly Z-Selective Metathesis Homocoupling of Terminal Olefins.” J. Am. Chem. Soc., 2009, 131, 16630-16631.