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 NMR spectroscopy. The rate of a/� methylene exchange in ethylene-only ruthenacycles was found to vary widely, suggesting that the NHC plays an important role in degenerative metathesis reactions. Attempts to generate RCM-relevant ruthenacycles resulted in the formation of a previously unobserved species, which we assign to be a �-alkyl substituted ruthenacycle. Kinetic investigations of the RCM-relevant ruthenacycles revealed a large increase in the kinetic barrier of the rate-limiting dissociation of the cyclopentene RCM product compared to previously investigated catalysts. These results shed light on the degenerate/productive selectivity differences observed between different metathesis catalysts. Figure 1. <img src='https://acs-archive-assets2.s3.amazonaws.com/239nm/uploads/15728_image.jpg' alt='' />

Probing the origin of degenerate metathesis selectivity via characterization and dynamics of ruthenacyclobutanes containing variable NHCs

243rd National Meeting (2012)

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