Olefin Metathesis

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This page is a draft outline. It is visible to admins and still needs a full lesson.

Klein 23.8. Grubbs catalysts exchange the ends of alkenes.

What metathesis means: two alkenes trade their alkylidene halves.
The outcome students must predict for a cross metathesis and for ring-closing metathesis.
Why ring-closing metathesis is a way to make a cycloalkene from a diene.
One cyclic example and one intermolecular example. The ruthenium carbene mechanism can be summarized, not fully derived.

Olefin Metathesis: Grubbs Catalysts

Olefin metathesis is one of the most powerful and elegant carbon-carbon bond-forming reactions in modern organic chemistry. Derived from Greek roots meaning "change places," metathesis allows two alkene molecules to literally swap their alkylidene fragments, catalyzed by well-defined transition metal carbene complexes developed by Robert Grubbs, Richard Schrock, and Yves Chauvin (awarded the 2005 Nobel Prize in Chemistry).

[ Insert Image: Overview of Olefin Metathesis Fragment Exchange ]

Figure 1. Conceptual cutting and stitching of alkene $\pi$-bonds via metathesis.

1. What Is Metathesis and How the Grubbs Catalyst Works

Metathesis is a catalytic reorganization of carbon-carbon double bonds where alkylidene units are exchanged:

The Catalyst: Grubbs catalysts (such as 1st and 2nd generation Grubbs ruthenium carbene complexes) feature a stable ruthenium-alkylidene ($\text{Ru}=\text{CHR}$) active site.
Mechanism Summary: The metal carbene adds across an alkene $\pi$-bond in a [2+2] cycloaddition to form a transient metallacyclobutane intermediate. This intermediate then cycloreverts in the opposite direction, releasing a new alkene and leaving behind a new ruthenium carbene species. Repeating this cycle rapidly equilibrates all alkene partners in the solution.

[ Insert Mechanism: Metallacyclobutane Intermediate and Grubbs Catalytic Cycle ]

Figure 2. [2+2] cycloaddition and cycloreversion via metallacyclobutane intermediates.

2. Cross Metathesis (Intermolecular)

In cross metathesis (CM), two separate alkene molecules react to exchange their substituents, forming a new unsymmetrical or symmetrical alkene:

Predicting the Outcome: When two terminal alkenes undergo cross metathesis, the reaction typically generates an internal alkene and releases ethylene gas ($\text{CH}_2=\text{CH}_2$).
Thermodynamic Drive: Because ethylene is a volatile gas that bubbles out of the reaction mixture, Le Chatelier's principle drives the equilibrium entirely forward to completion.

[ Insert Reaction: Intermolecular Cross Metathesis of Terminal Alkenes ]

Figure 3. Intermolecular cross metathesis driven by the evolution of gaseous ethylene.

3. Ring-Closing Metathesis (RCM) (Intramolecular)

When a single molecule contains two alkene units (a diene), metathesis can occur intramolecularly to form a cyclic structure:

Ring-Closing Metathesis (RCM): The catalyst snips the internal alkylidene fragments and stitches the two ends together, releasing a small volatile alkene (usually ethylene) as a byproduct.
Synthetic Power: RCM provides an exceptionally reliable way to construct 5-, 6-, 7-, and even large macrocyclic cycloalkenes from acyclic dienes—ring sizes that were historically difficult to assemble.

[ Insert Reaction: Ring-Closing Metathesis (RCM) of an Acyclic Diene ]

Figure 4. Intramolecular ring-closing metathesis generating a cycloalkene ring.

4. Worked Examples

Examine the following two practical applications of olefin metathesis in synthesis:

Example A: Intermolecular Cross Metathesis

Reaction: Treating 1-hexene with a Grubbs catalyst.
Prediction: The two terminal alkenes undergo self-metathesis. The alkylidene fragments combine to form dec-5-ene, while the terminal carbons are exhaled as volatile ethylene gas ($\text{CH}_2=\text{CH}_2$).

Example B: Ring-Closing Metathesis (RCM)

Reaction: Treating diallyl malonate (a bis-terminal diene) with a 2nd-generation Grubbs catalyst.
Prediction: The terminal $\pi$-bonds lock together in an intramolecular cyclization, yielding a 5-membered cyclic alkene (a cyclopentene derivative) along with gaseous ethylene.

[ Insert Reaction: Worked Examples of Cross Metathesis and RCM ]

Figure 5. Representative intermolecular cross metathesis and ring-closing metathesis pathways.