Olefin Metathesis Mechanism

Introduction

How can a catalyst cut two C=C bonds and rejoin the pieces? In 1971 Yves Chauvin proposed that metathesis runs through a metal–carbon double bond, a metal alkylidene, and a four-membered metallacycle. His idea explained why product distributions in early experiments did not match a simple pairwise swap. Well-defined catalysts later confirmed the mechanism, and it remains the basis for designing new metathesis catalysts.

Core explanation

The Chauvin mechanism. The active catalyst is a metal alkylidene, M=CHR, an electrophilic or nucleophilic carbene-type ligand discussed earlier in this unit. The cycle has two key steps:

1. [2+2] Cycloaddition. An alkene, R′CH=CHR′, coordinates to the metal and then combines with the M=CHR bond to form a metallacyclobutane : a ring of M, C(R), C(R′) and C(R′). 2. [2+2] Cycloreversion. The ring splits the other way, cleaving the original C=C and M=C bonds and forming a new alkene, RCH=CHR′, and a new alkylidene, M=CHR′.

The new alkylidene reacts with another alkene in the same way, so the metal carries a fragment from one alkene to the next. Every step is reversible, which is why metathesis reaches equilibrium unless a driving force intervenes.

Why not a pairwise mechanism? Early proposals suggested two alkenes pair up on the metal and exchange partners directly in a cyclobutane-like intermediate. Crossover experiments with mixtures of labelled and unlabelled dienes gave statistical scrambling of labels even at low conversion, which only a chain mechanism with alkylidene carriers explains.

Schrock catalysts. These are high-oxidation-state Mo(VI) or W(VI) imido alkylidene complexes such as Mo(=NAr)(=CHCMe₂Ph)(OR)₂. They are highly active, react even with hindered and electron-poor alkenes, and are the basis for Z -selective and enantioselective catalysts. They are sensitive to air, moisture and protic or strongly Lewis basic functional groups, because the electron-poor metal binds oxygen strongly.

Grubbs catalysts. These are Ru(II) alkylidene complexes, RuCl₂(=CHPh)L₂. The first generation has two PCy₃ ligands; the second generation replaces one with an N-heterocyclic carbene (NHC). Ru prefers soft alkenes to hard oxygen donors, so Grubbs catalysts tolerate alcohols, acids, aldehydes and water and can be handled in air as solids. Hoveyda–Grubbs variants replace the phosphine with a chelating isopropoxybenzylidene ligand for greater stability.

Initiation in Grubbs catalysts. The 16-electron RuCl₂(=CHPh)(PCy₃)L precatalyst first loses PCy₃ to give a 14-electron species, which binds the alkene. This dissociative step explains why the second-generation catalyst, whose NHC is a stronger donor, is more active in propagation even though phosphine loss itself is slower. After the first turnover releases styrene, the propagating species carries a fragment of the substrate rather than benzylidene.

Decomposition pathways. A metallacyclobutane can undergo β-hydride elimination instead of cycloreversion, and alkylidenes can couple bimolecularly. These routes limit turnover numbers and can generate isomerisation-active metal hydrides, which move double bonds as a side reaction.

Step-by-step reasoning

To follow one metathesis turnover:

1. Start with M=CHR and the substrate alkene. 2. Form the metallacyclobutane by joining M to one alkene carbon and the alkylidene carbon to the other. 3. Break the ring across the opposite pair of bonds. 4. Write the released alkene and the new alkylidene. 5. Use the new alkylidene as the starting point of the next turnover.

Visual explanation

Draw M=CHR above R′CH=CHR′, aligned so the four atoms form a square. Draw the square as a metallacyclobutane with single bonds all round. Then break the two bonds that were not originally double bonds' partners — M–C(R) stays with M as M=CHR′ on one side, and RCH=CHR′ leaves on the other.

Real-world analogy

Picture a square dance. The metal dancer holds one partner's hand, briefly forms a square with a new couple, then leaves with a new partner while the old partner pairs with someone else. The metal is always holding one partner, carrying them from couple to couple.

Real-world example

Grubbs catalysts are used industrially to make pheromones for insect control and in the synthesis of macrocyclic drugs. Schrock-type molybdenum catalysts are used where high activity or Z -selectivity is needed, and Ru catalysts cure ROMP resins in moulding processes.

Why?

Why do ruthenium catalysts tolerate water and alcohols while Mo and W catalysts do not? Ru(II) is a soft, relatively electron-rich metal that binds C=C bonds more strongly than hard oxygen donors. High-oxidation-state Mo(VI) and W(VI) are hard, oxophilic Lewis acids that are attacked by protic and oxygen-containing groups.

Common misconception

"The two alkenes swap fragments directly with each other on the metal." The accepted mechanism involves a metal alkylidene that reacts with one alkene at a time through a metallacyclobutane; the fragments are carried by the metal, not exchanged pairwise.

Worked example

Question: A catalyst Ru=CHPh reacts with propene. Give the two possible metallacyclobutane cycloreversion products that release a new alkene.

Reasoning: Propene (CH₂=CHCH₃) can add with either end towards Ru. If CH₂ bonds to Ru, cycloreversion releases PhCH=CHCH₃ and forms Ru=CH₂. If CHCH₃ bonds to Ru, it releases PhCH=CH₂ and forms Ru=CHCH₃.

Answer: Either 1-phenylpropene with Ru=CH₂, or styrene with Ru=CHCH₃.

Quick check

1. Name the four-membered intermediate formed in the Chauvin mechanism of olefin metathesis. Answer: A metallacyclobutane, containing the metal and three carbon atoms.

Exam focus

Draw the Chauvin mechanism showing [2+2] cycloaddition and cycloreversion. Compare Schrock (Mo/W, high oxidation state, very active, air-sensitive) and Grubbs (Ru, tolerant of functional groups) catalysts. Explain phosphine dissociation as the initiation step for Grubbs catalysts.

Advanced insight

Metallacyclobutanes have been observed directly by low-temperature NMR spectroscopy for both Ru and W systems. In stereoselective catalysts, the orientation of substituents on the metallacycle decides E or Z outcome; bulky aryloxide ligands on Mo, or cyclometallated NHCs on Ru, force substituents to the same side, giving Z alkenes. Computational studies reproduce these preferences with good accuracy.

Summary

Olefin metathesis proceeds by the Chauvin mechanism: a metal alkylidene undergoes [2+2] cycloaddition with an alkene to give a metallacyclobutane, which cycloreverts to a new alkene and a new alkylidene. Schrock Mo/W catalysts are highly active but sensitive; Grubbs Ru catalysts are tolerant of air, water and many functional groups and initiate by phosphine loss.

Practice questions

1. What evidence ruled out a pairwise mechanism for metathesis? Answer: Crossover experiments gave statistical scrambling of labelled fragments even at low conversion, consistent only with a chain mechanism. 2. What is the oxidation state of Mo in a typical Schrock imido alkylidene catalyst? Answer: +6. 3. What is the first step when a first-generation Grubbs catalyst initiates? Answer: Dissociation of a PCy₃ ligand to give a 14-electron ruthenium alkylidene. 4. Give one decomposition route of metathesis catalysts. Answer: β-Hydride elimination from the metallacyclobutane (or bimolecular coupling of alkylidenes).