Ring-closing metathesis, or RCM, is a widely used variation of olefin metathesis in organic chemistry for the synthesis of various unsaturated rings via the intramolecular metathesis of two terminal alkenes, which forms the cycloalkene as the E- or Z- isomers and volatile ethylene.The most commonly synthesized ring sizes are between 5-7 atoms; however, reported syntheses include 45- up to 90- membered macroheterocycles. These reactions are metal-catalyzed and proceed through a metallacyclobutane intermediate. It was first published by Dider Villemin in 1980 describing the synthesis of an Exaltolide precursor, and later become popularized by Robert H. Grubbs and Richard R. Schrock, who shared the Nobel Prize in Chemistry, along with Yves Chauvin, in 2005 for their combined work in olefin metathesis. RCM is a favorite among organic chemists due to its synthetic utility in the formation of rings, which were previously difficult to access efficiently, and broad substrate scope. Since the only major by-product is ethylene, these reactions may also be considered atom economic, an increasingly important concern in the development of green chemistry.There are several reviews published on ring-closing metathesis.