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

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. A kinetic and thermodynamic analysis of the ruthenium-catalysed ring-closing metathesis reaction has been achieved through the study of key intermediates accessible, for the first time, from 14-electron phosphonium alkylidene catalyst precursors. High intrinsic activities and a thermodynamic preference for ring-closing versus ring-opening reactions is observed.