Equilibrium Ring-Closing Metathesis

Ring-closing metathesis (RCM) of dienes is one of the most important methodologies used for the assembly of cyclic organic compounds. The synthetic efficiency of RCM is limited by the competition between intramolecular ring-closing and intermolecular oligomerization reactions. The increasing incidence of equilibrium RCM is growing in dominance of ruthenium metathesis catalysts, because these are less susceptible to decomposition by trace air and water. The kinetic proclivities of the catalysts themselves are more important toward intramolecular vs intermolecular reactions. The difference between the first- and second-generation catalysts is the kinetic bias of the former toward RCM and of the latter toward oligomerization, even in dilute solution. Formation of unwanted oligomers during RCM does not imply failure of the reaction but the need to optimize reaction conditions. Under appropriate conditions, the ring-chain equilibrium can be exploited to recycle unwanted oligomers and cyclic oligomers into the desired cyclic species.