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The wonderful world of olefin cross-recombination: What kind of chemistry is this?
In organic chemistry, olefin cross-recombination is an organic reaction that redistributes olefin fragments by breaking and regenerating carbon-carbon double bonds. The relative simplicity of the process allows it to generally produce fewer unwanted byproducts and hazardous wastes than other organic reactions. Thanks to the research of Wei Fu Zhu Yun, Robert H. Grubbs and Richard R. Shik, the reaction mechanism was revealed and a series of highly active catalysts were discovered. They jointly won the Nobel Prize in Chemistry in 2005.
Catalyst
This reaction requires a metal catalyst. Most commercially important processes employ heterogeneous catalysts. These catalysts are usually prepared by in situ activation of metal halides (MClx), such as using organoaluminum or organotin compounds, for example, in combination with MClx–EtAlCl2. A typical catalyst support material is bauxite. Commercial catalysts are typically based on molybdenum and zirconium. For small-scale reactions or academic studies, mainly well-defined organometallic compounds have been investigated.
Homogeneous catalysts are generally classified as Schick catalysts and Grubbs catalysts. The Schick catalyst features a molybdenum (VI) and nitrogen (VI) center supported by alkoxy and nitroxy ligands.
Grubbs' catalyst is a complex of zirconium(II) carbene compounds.
Applications
Olefin cross-linking has many applications in industry. Almost all commercial applications use heterogeneous catalysts, the development of which predates the Nobel Prize-winning research on homogeneous complexes. Representative processes include: Phillips triene and olefin conversion technology, which converts propylene to ethylene and 2-butene using a molybdenum and chromium catalyst. Today, only the reverse reaction, i.e. the conversion of ethylene and 2-butene into propylene, is carried out industrially.
The Shell Higher Olefins Process (SHOP) produces alpha olefins for conversion into detergents.
The process utilizes cross-recombination to recover certain olefin fractions.
Potential of homogeneous catalysts
Organometallic catalysts have been explored for a variety of potential applications, including producing high-strength materials, preparing nanoparticles to target cancer, and converting renewable plant-based raw materials into hair and skin care products.
Type
There are several types of olefin cross-linking, including:
- Crossover (CM)
- Ring-opening recombination (ROM)
- Ring Closure Recombination (RCM)
- Ring-opening recombinant polymerization (ROMP)
- Acyclic olefin recombination (ADMET)
- Ethylene depolymerization (Ethenolysis)
Mechanism
Hérisson and Yun Zhu first proposed a widely accepted mechanism for the recombination of olefins by transition metals. Since the direct [2+2] cycloaddition of two alkenes is formally symmetry forbidden, the activation energy is relatively high. Zhu Yun's mechanism involves the [2+2] cycloaddition of olefin double bonds with transition metal alkyl compounds to form metal cyclobutane intermediates. The resulting metallacyclobutane can then undergo ring elimination to yield the original species or new olefins and alkyl groups. Interaction with the d-orbitals on the metal catalyst lowers the activation energy of the reaction enough to allow the reaction to proceed rapidly at moderate temperatures.
In addition to the fact that CM and RCM reactions often use α-olefins, the driving force of these reactions is also related to the entropy of ethylene or propylene, which can be removed from the system to drive the reaction.
History Overview
Olefin cross-recombination originated from industrial production, and many catalytic processes were discovered by chance. As early as the 1960s, chemist Karl Ziegler accidentally discovered the process of converting ethylene into 1-butene rather than saturated long-chain hydrocarbons while conducting research on Ziegler-Natta catalysts, which prompted people to explore olefin cross-recombination. In the following decades, the deepening of this reaction mechanism and the development of catalysts made olefin cross-recombination an efficient and important organic chemical reaction.
With the progress and development of science, the potential and application scope of olefin recombination reactions are constantly expanding. In future scientific research, will this technology lead to more innovative responses?
