Core ConceptsIn this article, you will learn about the reagents, mechanisms, and application of the Olefin Metathesis. This reaction is a versatile and powerful tool in organic synthesis, enabling the construction of complex molecular architectures with precision.IntroductionWe can think of an alkene as two alkylidene groups (=CHR) held together by the double bond. Olefin metathesis is a chemical reaction in which carbon-carbon double bonds (olefins) break and reform in new configurations, resulting in the exchange of alkylidene groups between the reacting molecules. Metal-carbene complexes catalyze this reaction, which is widely used in organic synthesis for creating complex molecules, polymers, and fine chemicals due to its efficiency and versatility. Metathesis comes from the Greek words meta (change) and thesis (position), meaning that the alkylidene groups change their positions in the products.Catalysts for Olefin MetathesisScientists first observed olefin metathesis in the 1950s and used it in industry to convert propylene into a mixture of but-2-ene and ethylene.Around 1990, Richard Schrock developed versatile molybdenum and tungsten catalysts for olefin metathesis that tolerate a wide range of functional groups in the alkylidene fragments of the olefins. Schrock molybdenum metathesis catalysts are highly effective but face limitations due to their sensitivity to air and moisture and thermal instability.One of the Schrock molybdenum metathesis catalysts.In 1992, Robert Grubbs developed a ruthenium phosphine catalyst that is less sensitive to oxygen and moisture than the Schrock catalysts and tolerates even more functional groups in the alkylidene fragments of the olefins.One of the Grubbs ruthenium metathesis catalysts.Examples of ReactionsOlefin metathesis reactions are reversible, forming equilibrium mixtures of reactants and products unless driven toward desired products. A common method used in these reactions is the formation of ethylene gas. Ethylene bubbles off as it forms, effectively driving the reaction to completion. Let’s look at some examples:Several mechanisms were proposed for catalytic metathesis reactions, but the mechanism published by Yves Chauvin in 1971 is now widely accepted. Chauvin’s mechanism views an alkene as two alkylidene groups bonded together and considers the Schrock and Grubbs catalysts as metal atoms bonded to one alkylidene group. The symbol [M]=CHR represents the catalyst, where the brackets around [M] signify that the metal atom has other ligands that fine-tune its reactivity.Chauvin suggested that the metal-alkylidene catalyst creates an intermediate four-membered ring with an alkene. This ring then splits, either reverting to the original alkene and catalyst or forming a new alkene that has swapped an alkylidene group with the catalyst. This process enables the alkylidene groups to continuously exchange with the catalytic metal until they achieve thermodynamic equilibrium. High product yields can be obtained if there is an effective driving force, such as the formation of a gaseous by-product or the release of ring strain, to shift the equilibrium toward the desired products.The process is catalytic. Here is the catalytic cycle:The mechanism starts with an alkene and a metathesis catalyst reacting to form a four-member metallacycle (Step 1). This intermediate undergoes a rearrangement, breaking and forming new bonds to yield a new metal-carbene complex and a new olefin (Step 2). The carbene can then combine with a new equivalent of alkene (Step 3) followed by another reverse cycloaddition (Step 4), allowing for the continuous transformation of olefins.
