Getting Metathesis Started

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Science  29 Jul 2011:
Vol. 333, Issue 6042, pp. 500
DOI: 10.1126/science.333.6042.500-a

When olefin metathesis garnered the Nobel several years ago, the process was frequently analogized to a dance in which the participants switched partners. In this context, one question that continues to occupy researchers is how best to start the music, or activate the catalyst that steers olefinic carbons to and fro. A key step in activation of the widely used class of ruthenium-based catalysts is dissociation of a phosphine ligand, which creates a vacancy where the olefin can bind. Coordination of a strong donor ligand opposite the phosphine ought to accelerate this initial step, but in practice the observed acceleration is lower than expected. Yang et al. have now explored this puzzle using density functional theory calculations, and they find that the trouble arises because the phosphine dissociation is not an isolated process. Instead, it is coupled to rotation of the carbene ligand bound in the orthogonal plane (which will eventually couple to one of the olefin's carbons and get the dance started). This rotational motion upends the expected trends in expelling phosphine, but also facilitates subsequent olefin binding in a reactive orientation—the ultimately redeeming feature of using the stronger donor ligand. Such insights may facilitate design of more efficient next-generation catalysts.

Organometallics 30, 10.1021/om200529m (2011).

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