title: Probing the origin of degenerate metathesis selectivity via characterization and dynamics of ruthenacyclobutanes containing variable NHCs.
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.
authors.affiliation: Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States.
authors.forename: Benjamin K
authors.initials: BK
authors.lastname: Keitz
authors.forename: Robert H
authors.initials: RH
authors.lastname: Grubbs
mesh_terms: D002073:Butanes / Q000737:chemistry*; D002384:Catalysis; D006571:Heterocyclic Compounds / Q000737:chemistry*; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D012428:Ruthenium / Q000737:chemistry*
chemical_list: D002073:Butanes; D006571:Heterocyclic Compounds; C030011:carbene; C046888:butane; D012428:Ruthenium; D008697:Methane