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Updated: May 29, 2026

The Synthesis, Characterization and Reactivity of a Series of Ruthenium N-triphosPh Complexes
Published on: April 10, 2015
Probing the origin of degenerate metathesis selectivity via characterization and dynamics of ruthenacyclobutanes
Benjamin K Keitz1, Robert H Grubbs
1Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States.
Abstract:
The preparation of new phosphonium alkylidene ruthenium metathesis catalysts containing N-heterocyclic carbenes (NHCs) that result in a preference for degenerate metathesis is described. The reaction of the catalysts with ethylene or substrates relevant to ring-closing metathesis (RCM) produced ruthenacyclobutanes that could be characterized by cryogenic NMR spectroscopy. The rate of α/β methylene exchange in ethylene-only ruthenacycles was found to vary widely between ruthenacycles, in some cases being as low as 3.97 s(-1) at -30 °C, suggesting that the NHC plays an important role in degenerative metathesis reactions. Attempts to generate RCM-relevant ruthenacycles resulted in the low-yielding formation of a previously unobserved species, which we assign to be a β-alkyl-substituted ruthenacycle. Kinetic investigations of the RCM-relevant ruthenacycles in the presence of excess ethylene revealed a large increase in the kinetic barrier of the rate-limiting dissociation of the cyclopentene RCM product compared with previously investigated catalysts. Taken together, these results shed light on the degenerate/productive selectivity differences observed for different metathesis catalysts.
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