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Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)

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Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring...

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Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

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Recently, the development of olefin metathesis polymerization advanced the field of polymer synthesis. Simply put, the reorganization of substituents on their double bonds between two olefins in the presence of a catalyst is known as the olefin metathesis reaction. The use of metathesis reaction for polymer synthesis is called olefin metathesis polymerization.
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.

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Ring-closing metathesis reactions: interpretation of conversion-time data.

Vasco Thiel¹, Klaus-Jürgen Wannowius, Christiane Wolff

¹Organometallic Chemistry, Petersenstrasse 18, TU Darmstadt, 64287 Darmstadt (Germany).

Chemistry (Weinheim an Der Bergstrasse, Germany)

|October 16, 2013

Summary

Slowly activated Hoveyda-type precatalysts generate more ring-closing metathesis (RCM) product by maintaining active ruthenium species, despite slower initial rates. This kinetic model clarifies RCM reaction dynamics and catalyst behavior.

Keywords:

homogeneous catalysis kinetics reaction mechanisms ring-closing metathesis

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Area of Science:

Organometallic Chemistry
Catalysis
Organic Synthesis

Background:

Ring-closing metathesis (RCM) is a crucial reaction in organic synthesis for forming cyclic olefins.
Hoveyda-type ruthenium precatalysts are widely used but their activation and deactivation kinetics can influence reaction outcomes.
Understanding the interplay between precatalyst activation, active species formation, and deactivation is key to optimizing RCM reactions.

Purpose of the Study:

To investigate the kinetic behavior of various Hoveyda-type precatalysts in RCM reactions.
To develop and apply a kinetic model to analyze conversion-time data and determine rate constants.
To elucidate the factors influencing active species concentration and catalyst deactivation pathways.

Main Methods:

Recording conversion-time data for RCM reactions using different Hoveyda-type precatalysts.
Deriving a kinetic model involving precatalyst activation (k(act)), catalytic conversion (k(cat)), and active species deactivation (k(dec)).
Analyzing experimental data (substrate concentration and conversion rate) to calculate pseudo-first-order rate constants.

Main Results:

Slowly activated precatalysts yield more RCM product compared to rapidly activated ones, albeit with slower initial rates.
The kinetic model revealed that active ruthenium species (Acat) are generally quasistationary, with decomposition occurring via unimolecular and bimolecular pathways.
Electron-deficient precatalysts exhibit faster deactivation rates; slowly initiating precatalysts act as reservoirs for active species.

Conclusions:

The identity and activation profile of the precatalyst significantly impact RCM reaction efficiency and product yield.
The derived kinetic model accurately describes the complex interplay of activation, catalysis, and deactivation in RCM.
Understanding these kinetic parameters allows for rational selection of precatalysts to control olefin metathesis outcomes.