In recent years, the olefin metathesis reaction has attracted widespread attention as a versatile carbon-carbon bond-forming method. Many new applications have become possible because of major advances in catalyst design. State-of-the-art ruthenium catalysts are not only highly active but also compatible with most functional groups and easy to use. This Account traces the ideas and discoveries that were instrumental in the development of these catalysts, with particular emphasis on (PCy3)2Cl2Ru=CHPh and its derivatives. The discussion includes an analysis of trends in catalyst activity, a description of catalysts coordinated with N-heterocyclic carbene ligands, and an overview of ongoing work to improve the activity, stability, and selectivity of this family of L2X2Ru=CHR complexes.

The N-heterocyclic carbene-coordinated ruthenium benzylidene complex [(H 2 IMes)(PCy 3 )(Cl) 2 Ru¼CHPh] (1) is a highly active catalyst for a wide variety of olefin-metathesis reactions, [1] including those with sterically demanding [2] and electron-deficient olefins (Scheme 1). [3] In spite of recent advances, there are several processes that remain challenging, such as olefin cross metathesis (CM) with directly functionalized olefins. [4] For example, acrylonitrile CM has only been successful with Schrock©s arylimido molybdenum alkylidene catalyst [5] and the ether-tethered ruthenium alkylidene derivative [(H 2 IMes)(Cl) 2 Ru¼CH(o-iPrOC 6 H 4 )] (4). [6,7] Phosphane-ligated ruthenium catalysts have given poor results for this transformation, [3a, 5c, 6c, 8, 9] except for one report of efficient CM between purified acrylonitrile and 1-decene mediated by 1. [10] We have determined that dissociation rates of ligands are related to catalyst efficiency during CM with acrylonitrile. On this basis, we have developed a new, highly efficient ruthenium complex to perform acrylonitrile CM with unpurified acrylonitrile; this catalyst is the fastest initiator of any ruthenium-based catalyst reported to date. [11] Previous studies have shown that precatalysts of the type [L 2 X 2 Ru ¼ CHR] initiate by dissociating one L-type ligand before entering the catalytic cycle (Scheme 2); [12] in complexes 1±3, L is a phosphane (PR 3 ), and in complex 4, L is a tethered ether ligand (iPrO). Importantly, complexes 1±4 all provide the same propagating species (A and B) after a single turnover. [13] If either A or B is trapped by L, dissociation of L must occur before catalysis can continue. The relative affinity of A and/or B for the olefin in preference to L (i.e., favoring propagation) controls how long these species remain in the catalytic cycle. Consequently, the differences in activity between catalysts 1±4 depend on their rates of initiation and rebinding of L, both of which can be tuned by the nature of L. [12b, 14±16] Complexes 1±4 are all active for a variety of metathesis processes, such as CM, ring-closing metathesis (RCM), and ring-opening-metathesis polymerization (ROMP). However, the situation involving acrylonitrile CM is more complex; CM between acrylonitrile and allylbenzene proceeds efficiently with 4 (68 % yield) but not with 1±3 (21 %, 35 %, and 36 % yields, respectively; Table 1). [17] Clearly, this difference cannot simply be an issue of precatalyst initiation; 2 and 4 have roughly the same initiation rates and initiation is faster with 3 than 1, 2, or 4 Table 2. The difference is also not a result of COMMUNICATIONS

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This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.

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