Mechanistic insights into the ruthenium-catalysed diene ring-closing metathesis reaction

Abstract

Ruthenium-catalysed ring-closing metathesis (RCM) is a powerful technique for the preparation of medium-to-large rings in organic synthesis, but the details of the intimate mechanism are obscure. The dynamic behaviour of an RCM-relevant ruthenacyclobutane complex and its reactivity with ethene were studied using low-temperature NMR spectroscopy to illuminate the mechanism of this widely used reaction. These kinetic and thermodynamic experiments allowed for mapping the energy surface of the key steps in the RCM reaction as mediated by Grubbs-type catalysts for alkene metathesis. The highest barrier along the RCM path is only 65 kJ mol(-1), which shows that this catalyst has extremely high inherent activity. Furthermore, this transition state corresponds to that connecting the intermediates in this reaction leading to ring opening of the cyclopentene product. This shows that ring closing is kinetically slightly favoured over ring opening, in addition to being driven by the loss of ethene.

The Pivotal Role of Symmetry in the Ruthenium‐Catalyzed Ring‐Closing Metathesis of Olefins

“…collapses into metallacycle 25 that, in agreement with experimental evidence, [45] is a rather stable free-energy sink along the reaction pathway.…”

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

“…of ethylene (Scheme 6). 15d,17 Unfortunately, under a variety of conditions, both 13 and 14 reacted to give the ethylene-only ruthenacycles 16 and 17 , respectively. Such an observation is consistent with the known preference of catalysts containing these NHCs to propagate as methylidene species in catalytic reactions (e.g.…”

“…15,16 By analyzing these intermediates and through a combination of kinetics and kinetic modeling, the Piers laboratory has been able to determine the activation energies for the fundamental steps along a productive RCM pathway. 17 …”

Mapping the potential energy surfaces for ring-closing metathesis reactions of prototypical dienes by electronic structure calculations

“…In a series of elegant experiments, Metzger and co-workers have recently used ESI-MS to compare the stabilities of various ions on the RCM pathway of simple dienes catalysed by 1a in the mass spectrometer, 8 while Piers and van der Eide have identified a series of key intermediates using low temperature NMR. 9, 10 There have been many computational studies; for example, the strength of ligand binding, 11 and the mode of coordination (bottom or side bound) have been explored extensively. 12- 15 Complex systems of special synthetic interest, 16, 17 are also described but studies of full systems 18, 19 of general interest and importance are rare.…”