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Thermally Stable, Latent Olefin Metathesis Catalysts
Renee M Thomas1, Alexey Fedorov, Benjamin K Keitz
1The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125.
Organometallics
|January 28, 2012
Summary
New ruthenium catalysts with N-heterocyclic carbene (NHC) ligands exhibit excellent thermal stability and latent behavior for olefin metathesis. These catalysts are inactive at room temperature but initiate reactions upon heating, offering enhanced control.
Area of Science:
- Organometallic Chemistry
- Polymer Chemistry
- Catalysis
Background:
- Olefin metathesis is a crucial reaction in organic synthesis and polymer chemistry.
- Developing latent catalysts that activate only at elevated temperatures remains a significant challenge.
- Second-generation ruthenium catalysts often lack robust latent behavior.
Purpose of the Study:
- To design and synthesize highly thermally stable N-aryl,N-alkyl N-heterocyclic carbene (NHC) ruthenium catalysts for latent olefin metathesis.
- To investigate the structural features influencing catalyst latency in cross-metathesis and ring-opening metathesis polymerization (ROMP).
- To evaluate the stability and lifetime of these novel catalysts.
Main Methods:
- Synthesis of novel NHC ruthenium complexes with varying N-substituents and halide ligands.
- Evaluation of catalyst activity and latency in cross-metathesis reactions at different temperatures.
- Assessment of catalyst performance in ROMP of norbornene-derived monomers.
- Stability studies under ambient and elevated temperatures, as well as exposure to air and moisture.
Main Results:
- Designed ruthenium catalysts exhibited excellent latent behavior, being inactive at ambient temperature and initiating at elevated temperatures.
- A sterically hindered N-tert-butyl substituent on the NHC ligand induced latency in cross-metathesis.
- Exchange of chloride for iodide ligands was essential for achieving latency in ROMP.
- Iodide-based catalysts showed no ROMP activity at 25 °C but rapid polymerization at 85 °C.
- Catalysts demonstrated high stability to air, moisture, and temperatures up to 90 °C, with long lifetimes in solution.
Conclusions:
- Highly thermally stable NHC ruthenium catalysts with tunable latent behavior have been developed.
- Structural modifications, including N-tert-butyl substitution and iodide ligands, are key to achieving controlled initiation.
- These catalysts offer significant advantages for olefin metathesis applications requiring precise temporal control.
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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.
Ruthenium-based Grubbs catalyst is the most commonly used catalyst for olefin metathesis polymerization. Grubbs catalyst consists of a...
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