The success of olefin metathesis has spurred the intense investigation of new catalysts for this transformation. With the development of many different catalysts, however, it becomes increasingly difficult to compare their efficiencies. In this article we introduce a set of six reactions with specific reaction conditions to establish a standard for catalyst comparison in olefin metathesis. The reactions were selected on the basis of their ability to provide a maximum amount of information describing catalyst activity, stability, and selectivity, while being operationally simple. Seven of the most widely used rutheniumbased olefin metathesis catalysts were evaluated with these standard screens. This standard is a useful tool for the comparison and evaluation of new metathesis catalysts.

The decomposition of a series of ruthenium metathesis catalysts has been examined using methylidene species as model complexes. All of the phosphine-containing methylidene complexes decomposed to generate methylphosphonium salts, and their decomposition routes followed first-order kinetics. The formation of these salts in high conversion, coupled with the observed kinetic behavior for this reaction, suggests that the major decomposition pathway involves nucleophilic attack of a dissociated phosphine on the methylidene carbon. This mechanism also is consistent with decomposition observed in the presence of ethylene as a model olefin substrate. The decomposition of phosphine-free catalyst (H 2IMes)(Cl)2RudCH(2-C6H4-O-i-Pr) (H2IMes ) 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) with ethylene was found to generate unidentified ruthenium hydride species. The novel ruthenium complex (H2IMes)-(pyridine)3(Cl)2Ru, which was generated during the synthetic attempts to prepare the highly unstable pyridinebased methylidene complex (H2IMes)(pyridine)2(Cl)2RudCH2, is also reported.

Highly enantioselective addition reactions between silyl ketene acetals and N-Boc aldimines are catalyzed by the thiourea-based catalyst 1c. Extraordinary scope is observed in this methodology with regard to the imine substrate, with aryl and heteroaromatic derivatives generally affording nearly quantitative yields of beta-amino ester product in up to 98% enantioselectivity.

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The reaction of the phosphonium alkylidene [(H2IMes)RuCl2=CHP(Cy)3)]+ BF4– with propene, 1-butene, and 1-hexene at –45 °C affords various substituted, metathesis-active ruthenacycles. These metallacycles were found to equilibrate over extended reaction times in response to decreases in ethylene concentrations, which favored increased populations of α-monosubstituted and α,α’-disubstituted (both cis and trans) ruthenacycles. On an NMR timescale, rapid chemical exchange was found to preferentially occur between the β-hydrogens of the cis and trans stereoisomers prior to olefin exchange. Exchange on an NMR timescale was also observed between the α- and β-methylene groups of the monosubstituted ruthenacycle (H2IMes)Cl2Ru(CHRCH2CH2) (R = CH3, CH2CH3, (CH2)3CH3). EXSY NMR experiments at –87 °C were used to determine the activation energies for both of these exchange processes. In addition, new methods have been developed for the direct preparation of metathesis-active ruthenacyclobutanes via the protonolysis of dichloro(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(benzylidene) bis(pyridine)ruthenium(II) and its 3-bromopyridine analog. Using either trifluoroacetic acid or silica-bound toluenesulfonic acid as the proton source, the ethylene-derived ruthenacyclobutane (H2IMes)Cl2Ru(CH2CH2CH2) was observed in up to 98% yield via NMR at –40 °C. On the basis of these studies, mechanisms accounting for the positional and stereochemical exchange within ruthenacyclobutanes are proposed, as well as the implications of these dynamics towards olefin metathesis catalyst and reaction design are described.