A deuterium labeling study was undertaken to determine the mechanism of olefin isomerization during the metathesis reactions catalyzed by a second-generation Grubbs catalyst (2). The reaction of allyl-1,1-d 2 methyl ether with 2 at 35 °C was followed by 1H and 2H NMR spectroscopy. The evidence of deuterium incorporation at the C-2 position of the isomerized product, trans-propenyl methyl ether, led to the conclusion that a metal hydride addition−elimination mechanism was operating under these conditions. Consequently, complex 8, an analogue of 2 bearing deuterated o-methyl groups on the aromatic rings of the NHC ligand, was synthesized to investigate the role of the NHC ligand in the formation of hydride species. Thermal decomposition of benzylidene 8 and methylidene 8‘ was monitored by 2H NMR spectroscopy; no deuteride complex was detected in either case. The decomposition mixtures were tested for isomerization activity with benchmark 1-octene but did not match the isomerization rates observed with 2 under similar metathesis conditions. Reaction of complex 8 with various olefinic substrates not only confirmed the formation of a deuteride complex but also revealed the existence of a competitive H/D exchange process between the CD3 groups on the NHC ligand and the C−H bonds of the substrate. We propose that the exchange is promoted by a ruthenium dihydride intermediate whose formation is closely related to the methylidene decomposition.

A kinetic study of three ruthenium carbene catalysts, (H 2IPr)(PCy3)(Cl)2RudCHPh, 3 (investigated extensively by Mol), (H2IMes)(Cl)2RudCH(o-iPrOC6H4), 4 (Hoveyda's catalyst), and (H2-IPr)(Cl)2RudCH(o-iPrOC6H4), 5 (a new catalyst structure), was conducted under ADMET polymerization conditions. The kinetic behavior of these catalysts was compared to the classical first-and secondgeneration Grubbs' complexes at 30, 45, and 60 °C. Complex 3 exhibits the highest initial ADMET rate (80 DP s -1 ) of any phosphine complex to date and efficiently promotes metathesis even at temperatures as low as 0 °C. Complex 4 alone does not polymerize 1,9-decadiene in the bulk; however, addition of a polar solvent induces polymerization. Combining elements of catalysts 3 and 4 yielded the new complex 5. This complex results in higher polycondensation rates than previous Hoveyda-type structures and exhibits an increased stability over its parent phosphine complex. The new catalyst polymerizes 1,9decadiene in the bulk to high polymer (M n ) 40 000 g/mol) using low catalyst loadings (0.1 mol %). The isomerization chemistry induced by complexes 3 and 5 was investigated using a model compound, 1-octene.

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