When it comes to asymmetric olefin metathesis, molybdenum catalysts are generally better than their ruthenium homologues. However, chiral ruthenium metathesis catalysts have also been designed for asymmetric metathesis, with some success. Two main examples are the C2 symmetric catalyst 1 developed by Grubbs and the Hoveyda complex 2 featuring a bidentate ligand.1,2
Building upon those two models, Blechert recently designed a chiral second generation Hoveyda (pre)catalyst featuring an unsymmetrical NHC with two different N-aryl groups, and a unique monosubstitition pattern of the backbone.3 This highly stable complex was tested in asymmetric ring-closing metathesis, and in asymmetric ring-opening cross-metathesis (AROCM) with promising results. Indeed, excellent enantioselectivities (up to 93% ee) were obtained in AROCM and more interestingly, the catalyst is also highly E selective in this process (>30:1).
I look forward to further developments of this new family of complexes, and I am particularly curious to see what E/Z selectivities can be reached in cross-metathesis.
1 Funk, T. W.; Berlin, J. M.; Grubbs, R. H. Highly active ruthenium catalysts for asymmetric ring-closing olefin metathesis. J. Am. Chem. Soc. 2006, 128, 1840 – 1846.
Berlin, J. M.; Goldberg, S. D.; Grubbs, R. H. Highly active chiral ruthenium catalysts for asymmetric cross- and ring-opening cross-metathesis. Angew. Chem. Int. Ed. 2006, 45, 7591 – 7595.
2 Van Veldhuizen, J. J.; Campbell, J. E.; Guidici, R. E.; Hoveyda, A. H. A readily available chiral Ag-based N-heterocyclic carbene complex for use in efficient and highly enantioselective Ru-catalyzed olefin metathesis and Cu-catalyzed allylic alkylation reactions. J. Am. Chem. Soc. 2005, 127, 6877 – 6882.
3 Tiede, S.; Berger, A.; Schlesiger, D.; Rost, D.; Lühl, A.; Blechert, S. Highly active chiral ruthenium-based metathesis catalysts through a monosubstitution in the N-heterocyclic carbene. Angew. Chem. Int. Ed. 2010, 49, 3972 – 3975.