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

Abstract

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.

Probing the Origin of Degenerate Metathesis Selectivity via Characterization and Dynamics of Ruthenacyclobutanes Containing Variable NHCs

“…12,13 This information has allowed for the development of increasingly sophisticated catalysts. However, much less is known about the effect of ligand structure on processes that occur within a complex catalytic cycle such as RCM.…”

Why is RCM Favoured Over Dimerisation? Predicting and Estimating Thermodynamic Effective Molarities by Solution Experiments and Electronic Structure Calculations

“…Ethylidene 8 was identified at the end of these high [1 c] 0 reactions as a quartet in the 1 H NMR spectrum (d H (CDCl 3 ) = 18.6 ppm (J = 5.5 Hz)) with a single cross-peak in the 2D [ 1 H, 1 H] CO-SY NMR spectrum to a doublet signal at d H = 1.6 ppm. [24] It is likely that 8 is a product of the metathesis of isomerised dienes (for example, 1,7-nonadiene RCM would yield cyclohexene 3 b and ethylidene 8).…”

Catalysis of Selective Hydrogen/Deuterium Exchange at Allylic Positions Using Deuterium Oxide

“…Similar deuteration at only C1 and C3 was seen when 4-allylanisole was heated with 1 and D 2 O at 70°C. The retention of H at C2 using 1 renders alkene isomerization by metal hydride formation, alkene insertion and b-hydride elimination unlikely [24][25][26][27] (Fig. 1).…”