Since Daryl Allen wrote his post in 2011 on the development of the first cis-selective ruthenium catalysts with chelating NHC ligands, a lot of activity and exciting new developments occurred in the area which warrants an update. After the pivalate complex 1 was published1, it was found that the nitrate derivative 2 is more stable and easier to isolate. Catalyst 2 was found to be effective for the synthesis of homo- and hetero-cross metathesis products, polymers with high cis-content, a variety of insect pheromones and macrocyclic musks2,3.
Catalyst 3, due to the increased bulk in the N-Aryl moiety, showed improved selectivity and activity in both homodimerization of terminal olefins and cross metathesis reactions. TONs of up to 7400 and great cis-selectivity (>95%) were observed for catalyst 34 in homodimerization reactions of allylbenzene. It’s noteworthy that even with such an isomerization prone substrate as allylbenzene only a small amount of the styrene is formed under the reaction conditions.
Catalyst 3 was also evaluated in the homodimerization of 10-methyl 10-undecenoate and 4-pentenol as well as cross metathesis of 1-hexene and 8-nonenyl acetate to yield the Z-pheromone 8-tridecenyl acetate. Conversions and yields were good and in most cases no formation of the E-isomer was observed4.
Recently, an enantioenriched version of catalyst 2 was prepared and shown to be capable of highly Z-selective and enantioselective ring opening cross metathesis (ROCM)5. The yields and ee’s varied with different substituents on the norbornene ring and the cross partner, but overall they were very promising.
A different approach designing new ruthenium cis -selective catalysts based on substitution of the halogen groups with a bidentate anion was used by the Hoveyda group. Two of the new catalysts containing dithio-ligands were found to be very Z-selective in the ring-opening metathesis polymerization of norbornene and cyclooctadiene: both catalysts 4 and 5 showed >98% cis-selectivity6.
Catalyst 4 was also examined in ROCM of norbornenes and substituted cyclobutenes with styrenes and vinylcyclohexane. The transformations proceeded with good efficiencies and very high Z-selectivities. These results for the new type of Z-selective catalysts look very promising and I look forward to seeing further applications of the dithiolate catalysts.
The development of cis-selective ruthenium catalysts has been one of the last frontiers in olefin metathesis, but in recent years an increasing number of new results describing very selective and efficient catalysts have been published. I look forward to new developments from the talented researchers that are working on designing better, more selective and stable ruthenium catalysts and expanding the substrate scope and utility of Z-selective metathesis.
1K. Endo, and R. H. Grubbs, J. Am. Chem. Soc. 2011, 133, 8525.
2B. K. Keitz; K. Endo; P. R. Patel; M. B. Herbert, and R. H. Grubbs, J. Am. Chem. Soc. 2012, 134, 693.
3(a) B. K. Keitz; A. Fedorov, and R. H. Grubbs, J. Am. Chem. Soc. 2012, 134, 2040. (b) M. B. Herbert; V. M. Marx; R. L. Pederson, and R. H. Grubbs, Angew. Chem. Int. Ed. 2012, 52, 310. (c) V. M. Marx; M. B. Herbert; B. K. Keitz, and R. H. Grubbs, J. Am. Chem. Soc. 2013, 135, 94.
4L. E. Rosebrugh; M. B. Herbert; V. M. Marx; B. K. Keitz, and R. H. Grubbs, J. Am. Chem. Soc. 2013, 135, 1276.
5J. Hartung and R. H. Grubbs, J. Am. Chem. Soc. 2013, 135, 10183.
6R. K. M. Khan; S. Torker; and A. H. Hoveyda, J. Am. Chem. Soc. 2013, 135, 10258.
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