Understanding Structural Isomerization during Ruthenium-Catalyzed Olefin Metathesis:  A Deuterium Labeling Study

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 1 H and 2 H 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 2 H 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 CD 3 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.