Tuning the Steric Properties of a Metathesis Catalyst for Copolymerization of Norbornene and Cyclooctene toward Complete Alternation

We have recently published a mechanistic concept for the olefin metathesis reaction with ruthenium catalysts that explains the independent control of chemo- and stereoselectivity by substitution in two orthogonal planes. The basic structure from which we started the structural modifications for improved stereoselectivity had been optimized substantially for chemoselectivity, as compared to the prototype that had been published. We designed the catalyst according to a concept in which the factor that governs alternation is directly related to the size difference of the substituents on the bidentate phosphine/phenolate ligand 2a−e and in which the origin of chemoselectivity arises from diastereomeric site control. The most selective catalyst (11d) outperforms our prototype (3) in selectivity as well as reactivity. Synthetic problems, i.e., the undesired formation of 2:1-complexes (bidentate ligand-to-ruthenium ratio) (8) by reaction of the bidentate phosphine/phenolate ligands 2a and 2b with the first-generation Hoveyda−Grubbs catalyst 4, were solved by introduction of a larger carbene unit, which not only favors the formation of 1:1-complexes but also results in increased initiation rates. The work is supported by NMR data and X-ray crystallography, which give insights into the steric properties of the investigated system.