Chemistry
Principles of Ring-Closing Metathesis for Macrocycle Synthesis
Quick fact
Ring-closing metathesis can create macrocycles with ring sizes of 12 to over 20 atoms in high yield, a feat that is notoriously difficult with traditional cyclization methods. This was recognized by the 2005 Nobel Prize in Chemistry, awarded to Chauvin, Grubbs, and Schrock for developing metathesis.
Why this is interesting
You’ve probably seen rings in molecules like antibiotics and hormones, but how do chemists bend a long chain into a huge ring? The answer is a double-bond shuffle that closes the circle—ring-closing metathesis.
Read the full explanation
Understanding Principles of Ring-Closing Metathesis for Macrocycle Synthesis
Imagine a long, flexible piece of string with a small knot at each end. To form a ring, you bring the two ends together and tie them. But when the string is very long, its ends are unlikely to meet—they prefer to stay tumbling around in solution. Now imagine that each knot is a reactive double bond. A metathesis catalyst acts like a tiny machine that grabs both knots, swaps parts, and ties them together into a new double bond, thus closing the ring. The key is that the catalyst does this only when the two ends are close, so you need to keep the molecule at low concentration to avoid one chain reacting with another (which would create a polymer instead). Ring-closing metathesis (RCM) is exactly this: it uses a metal catalyst to exchange alkylidene fragments between two alkenes in the same molecule, forming a new C=C bond and releasing ethylene (or another alkene) as a byproduct. This reaction has become a standard tool because it works under mild conditions and tolerates many functional groups, making it far easier to construct large rings than older methods that required high dilution and harsh reagents.
A deeper explanation
The mechanism of RCM is a series of [2+2] cycloaddition/cycloreversion steps between the metal alkylidene and the substrate alkenes. The catalyst, typically a ruthenium carbene such as a Grubbs catalyst, first coordinates to a terminal alkene, forming a metallacyclobutane. This collapses to give a new metal alkylidene and releases ethylene. The metal then coordinates to the second alkene, forms another metallacyclobutane, and upon fragmentation yields the cyclic product and regenerates the catalyst. The thermodynamic driving force is the release of ethylene gas, which is irreversible and pulls the equilibrium toward the ring. However, the intrinsic ring strain of small rings (5-6 membered) helps, but for macrocycles, the entropic penalty of bringing the two chain ends together is large. To overcome this, chemists use high dilution or slow addition to minimize intermolecular reactions, and they often design the substrate with a conformational bias (e.g., using a template or rigid groups) to pre-organize the ends. The choice of catalyst also matters: ruthenium catalysts are robust and user-friendly, while molybdenum catalysts are more active but sensitive. Olefin geometry in the product is usually the more stable E-isomer, but some catalysts can control it. RCM has been used to synthesize epothilones, macrolide antibiotics, and many other bioactive macrocycles, demonstrating its power in complex molecule synthesis.