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 degrees C was followed by H-1 and H-2 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 H-2 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.