Advanced Inorganic Chemistry/Olefin Metathesis
Olefin Metathesis is an organic chemical reaction that uses a metal catalyst for the transfer of substituents between olefins, or alkenes by a 4-membered ring intermediate, also known as a Chauvin Mechanism. This efficient method does not only have a high yield, but it produces less byproducts and hazardous waste compared to other organic reactions. Yves Chauvin, Robert H. Grubbs and Richard R. Schrock won The Nobel Prize in Chemistry of 2005 for their contributions to olefin metathesis.
Herbert S. Eleuterio, an industrial chemist in the 1950s, found a propylene-ethylene copolymer from a propylene feed on a Mo-Al catalyst as a result of his experiment. He repeated the experiment with a cyclopentene and noticed that “the polymer looked like somebody took a pair of scissors, opened up cyclopentene, and neatly sewed it up again.” Other chemists were also getting similar results of the cleavage and reformation of the olefin double bonds. To his understanding, Grubbs suggested that the rearrangement of substituents occurred through a metallacyclopentane intermediate. Chauvin proposed that olefin metathesis is initiated by a metal carbene. Many scientists agreed with Chauvin that metal carbenes played a major role in the process of olefin metathesis and from there, it was further studied to understand the full mechanism.
The rearrangement of substituents on two olefins occurs through the formation of a 4-membered ring intermediate as in the following figure.
The initial alkene with substituent R1 forms an intermediate with an olefin metal carbene, allowing the metal element to be attached to the initial alkene. The product then reacts with a second alkene with substituent R2, forming another 4-membered ring intermediate and yields a final combined olefin with both R1 and R2 substituents while producing back the initial metal carbene. This type of 4-membered ring formation is called a [2+2] cycloaddition. A 4-membered ring is not the most stable and is known to cause strain on the molecule. As a result, the formation has a very high activation energy. Interaction with the metal lowers the activation energy of the ring formation, allowing the process to occur at moderate temperatures.
Olefin Metathesis requires the application of a metal catalysts. The most common catalysts used are Grubbs catalyst and Schrock catalyst.
Grubbs catalyst is a commercially available catalyst and is easy to handle as it is pretty stable against water, oxygen, and other small impurities. Although it has its advantages like high functional group tolerance, there is a drawback in that it has a lower reactivity compared to other catalysts like the Mo Imido Alkylidene Catalyst.
Upon interaction with an olefin, a 4-membered ring is formed cis to the carbene and trans to the Cl atoms.
Like the Grubbs catalyst, the Schrock catalyst is also commercially available. It has a higher reactivity than the Grubbs catalyst and is tolerant of multiple functional groups, but has many disadvantages such as needing to be handled under an inert atmosphere using dry solvents and substrates as well as being intolerant to protons on heteroatoms.
Basic Types of Metathesis
- Ring Opening Metathesis Polymerization (ROMP)
- Ring Closing Metathesis (RCM)
- Cross Metathesis (CM)
- Acyclic Diene Metathesis Polymerization (ADMET)
Olefin metathesis opened up new industrial pathways for petrochemicals, polymers, and so much more. In the petrochemical field, olefin metathesis is used for the Olefins Conversion Technology (OCT) Process and the Shell Higher Olefins Process (SHOP). ROMP has opened up an opportunity to generate useful polymers with special properties in many industrial methods. For future oleochemical advancements, metathesis of natural fats has opened a door of possibilities. Currently, the most important applications are used to make propene, detergent-range olefins, and polymers.