The ring closing metathesis (RCM) reaction has become a “standard” in olefin metathesis chemistry. However, there are a number of unobserved yet implied intermediates assumed to exist. As a consequence, little is known about the energetics of all the processes involved. Upon reading this recent report from the Piers’ group, I realized that I don’t spend nearly as much time thinking about all the processes involved in a typical RCM reaction as I probably should. Their paper highlights some direct observations of the implied intermediates and quantifies the energy barriers overcome during the completed RCM reaction.
There are a number of important observations to point out. First, at 223K, the degenerate exchange of unsubstituted ruthenacyclobutane, 1, is 100 times faster than the α,β-substituted ruthenacyclobutane, 2-an intermediate in the RCM reaction, Figure 1.
Second, a degenerate exchange of the methylidene protons was readily observed instead, Figure 2. Based on 2H-labeling studies and E-group variation studies, the ΔG‡ for α,β-degenerate exchange is about 20% higher than for methylidene degenerate exchange under comparable conditions. Interestingly, intermolecular exchange could be directly observed in 2 by 2D EXSY, but not in 1.
Third, over the course of kinetic studies with 2, a ring-opened product, 3, could be observed in the presence of ethylene, Figure 3. The authors suggest that a ring-opened, olefin chelating complex could exist, although not directly observed.
The observed intermediates, including 3, allowed for a completed energetic landscape of RCM of a common diene. Interestingly, an energy barrier of only 5 kJ/mol was found between the ring-opening versus ring-closing pathway from complex 2. Furthermore, the trapping of the Ru methylidene intermediate was largely indiscriminate between ethylene and cyclopentene with a ratio of 0.7.
Overall, these results represent a detailed observation of the intermediates involved in the RCM of diethyl diallylmalonate – a standard in olefin metathesis chemistry. Also, the authors have suggested that although the numbers ascertained from experiments were determined at necessary low temperature, the entropic contributions are similar and should transgress to normal conditions. Lastly, the energy barriers at 215K are lower than barriers observed in metathesis catalysts based on titanium and molybdenum, and raise important questions about quantifying catalyst activity.