If all goes well, I plan to write about Ring Closing Metathesis (RCM) quite a bit on this site, so I figured I needed a very broad RCM overview. Anyway, what kind of metathesis website would this be without the generic RCM scheme we’ve all seen a hundred times?
The mechanism involves a first metathesis event (not shown in detail below) to give metal carbene (species with a metal-carbon double bond) intermediate I. The pendant olefin then coordinates the metal and undergoes a formal [2+2] reaction to give an intermediate metallocyclobutane (III). This is the step in the mechanism that determines the olefin geometry, though of course a 5-membered ring can only form a cis olefin. A retro-[2+2] reaction in the reverse direction gives the product and regenerates the metal carbene catalyst.
RCM exploded on the organic synthesis scene with the advent of the well-defined Schrock and Grubbs catalysts, and in that decade-plus, many variations have been developed. Depending on the conditions (and the substrates), you can make di-, tri-, or tetrasubstituted olefins. You can make polycyclic, heterocyclic, and aromatic rings. You can make medium rings, large rings, hindered rings, and strained rings.
Chemists like powerful reactions, but they love easy reactions. And in many cases RCM is both. If you want to make a ring by RCM, you first make a linear diene, dissolve it in solution, add some catalyst, and wait. If you watch, you can see the driving force behind RCM, the evolution of ethylene. When the ethylene evolution stops, your reaction is done. Because you’re irreversibly losing a gas, even rings that are thermodynamically disfavored can be trapped in their ring-closed form. Of course, not every reaction you can draw on paper will work in the lab, but if an RCM cyclization won’t work, there aren’t many other ones that will.