Direct observation of Ru-alkylidene forming into ethylene in ring-closing metathesis from hyperpolarized ¹H NMR

Abstract

Ring-closing metathesis was monitored using real-time NMR of 1H hyperpolarized olefins at room temperature. By applying a selective saturation to an observable intermediate, its protons were found to transfer to ethylene. The intermediate was thus identified as a Ru-alkylidene species, which appears in the ethylene formation pathway.

NMR methods for the analysis of mixtures

“…D-DNP is used extensively for kinetic studies based on the injection of hyperpolarised compounds in a mixture containing other reactants and/or catalysts, or in cell cultures. [92][93][94][95][96][97][98][99][100][101] In such applications, hyperpolarisation methods make it possible to acquire 13 C NMR spectra with a time resolution of a few seconds, which would be impossible to achieve at thermal equilibrium. They also make it possible to observe intermediate species that would be invisible with conventional NMR methods.…”

“…For example, Hilty and co-workers have described a series of studies of polymerisation reactions using DNPhyperpolarised monomers. 93,95,97 They used time-series of 1D 13 C NMR spectra at natural abundance to measure and compare reaction rates for different monomers, and also observe polymer products and side products, during a time of 10 to 30 s after the start of the reaction. Lerche, Meier and co-workers have used DNP-hyperpolarised 13 C-labelled substrates to track the metabolic fate of molecules such as glucose and fructose in biological systems such as cancer cell or yeast cultures.…”

Kinetics of initiation of the third generation Grubbs metathesis catalyst: convergent associative and dissociative pathways

“…The co-product from initiation, most frequently styrene or a derivative, 8 is non-innocent, able to undergo competitive metathetic reincorporation at ruthenium to regenerate the benzylidene complex. 9 Exquisite detail regarding catalyst turnover has been elucidated using techniques such as heteronuclear NMR magnetisation transfer, 10 dynamic nuclear polarisation labelling, 11 and single-molecule uorescence. 12 However, much of the work in the primary phases of the development of new ruthenium precatalysts, and the investigation of their relative efficiency and activity, focussed on the initiation event.…”

Structural Investigations of Polymer Materials by Dynamic Nuclear Polarisation Solid-state NMR

“…18,20 In particular, these authors used monomeric solutions hyperpolarized by D-DNP to detect key intermediates or highlight the polymer active site during polymerisation reactions, 31 hereby illustrating the potential of the technique as a precious strategy to obtain real-time mechanistic and kinetic data on metallocene catalysis or ring-opening polymerisation. 32,33 Second, Han and coworkers at the University of California, Santa Barbara, have strongly contributed to the use of the so-called low-field Overhauser liquid-state DNP (ODNP) technique as a way to probe and quantify the dynamics of hydration water locally (at the sub-nanometre scale). 34 In ODNP, a DNP polarising agent -usually referred to as a spin probe in this specific context -is added to the aqueous solution of interest, and its EPR transitions are saturated with microwaves to promote the DNP transfer from the unpaired electrons of the spin probe to the 1 H nuclei of the water molecules.…”