Despite the huge advances in the area of olefin metathesis, better catalysts are still sought to increase the efficiency of ring closing metathesis (RCM) reactions and make them more applicable in industry1. Recently, several modified second-gen Grubbs-Hoveyda catalysts were prepared and tested for RCM by the Plenio group. The goal was to develop faster initiating catalysts but at the same time to keep the inherent stability of the parent complexes. The approach relied on modification of the Hoveyda ligand, but rather than introducing substituents in the phenyl ring, the donor ability of the hetero-atom was decreased significantly by the utilization of O-Ar2 and N-Ar3 donor groups. The resulting catalysts featured the excellent stability of the Hoveyda-type complexes but underwent full precatalyst initiation within a short period of time resulting in very fast RCM reactions.
The new 2-phenoxy-substituted Hoveyda complexes (e.g., 1)2 were tested for RCM reactions with various substrates at low catalyst loadings (100-15 ppm). The new catalysts were very efficient in the formation of N-heterocycles: 15-25 ppm 1 yielded >90% product in only 15 min!
The N-Ar Hoveyda complexes (e.g., 2a, b) 3 were very active and efficient for RCM reactions as well. In addition to simple di-substituted double bonds, they were also very efficient in reactions giving tri-substituted unsaturated nitrogen heterocycles with only 50 ppm catalyst loadings.
The new Hoveyda complexes described by Plenio are a good compromise between activity and stability and can perform RCM reactions very fast and at low catalyst loadings. These new developments in the area of more active and stable catalysts can make the industrial use of RCM more cost effective and attractive.
1For other examples of RCM with low catalyst loadings, see: Bieniek, M.; Michrowska, A.; Usanov, D. L., and Grela, K. Chem. Eur. J. 2008, 14, 806. Kuhn, K. M.; Bourg, J.-B.; Chung, C. K.; Virgil, S. C., and Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 5313. Gatti, M.; Vieille-Petit, L.; Luan, X.; Mariz, R.; Drinkel, E.; Linden, A., and Dorta, R. J. Am. Chem. Soc. 2009, 131, 9498. Kuhn, K. M.; Champagne, T. M.; Hong, S. H.; Wei, W.-H.; Nickel, A.; Lee, C. W.; Virgil, S. C.; Grubbs, R. H., and Pederson, R. L. Org. Lett. 2010, 12, 984. Urbina-Blanco, C. A.; Leitgeb, A.; Slugovc, C.; Bantreil, X.; Clavier, H.; Slawin, A. M. Z., and Nolan, S. P. Chem. Eur. J. 2011, 17, 5045. Kadyrov, R. Chem. Eur. J. 2013, 19, 1002.
2Kos, P.; Savka, R., and Plenio, H. Adv. Synth. Catal. 2013, 355, 439.
3Peeck, L. H..; Savka, R. D., and Plenio, H. Chem. Eur. J. 2012, 18, 12845.