According to the Swedish academy, olefin metathesis is a "great step forward for 'green chemistry.' " That statement reinforces the message that the best chemists in the world are doing green chemistry and that green chemistry is simply part of doing good chemistry, says Paul T. Anastas, director of the American Chemical Society's Green Chemistry Institute.
Olefin metathesis is "an example of how important basic science has been applied to the benefit of man, society, and the environment,"; the academy adds. Yet,"basic research is not recognized as much as it should be today," said Schrock at an MIT press conference last week. He and Grubbs, he said, "had faith that we were doing something new and that applications would develop, and they have."
For years, the chemistry community has recognized the importance and utility of olefin metathesis. Now, the has chosen to recognize it, too: Last week, the academy awarded the to three chemists who developed the reaction—Yves Chauvin of the French Petroleum Institute, Rueil-Malmaison, France; of California Institute of Technology; and of Massachusetts Institute of Technology. They will share equally the $1.3 million prize.
A reaction such asin which the cations and anions exchange partners is calledmetathesis. In actual fact, the chemistry takes place in several steps.
The Swedish academy's choice "confirms what is generally agreed upon in the synthetic chemistry community—that olefin metathesis is a very useful catalytic reaction that has a broad scope," comments Harvard University chemistry professor and Nobel Laureate .
Below is a catalytic cycle for the self-metathesis of propene to give 2-butene and ethylene. The starting point for the catalytic cycle is metal carbene I. This reacts with propene to give metallacyclobutane intermediate II. This four-membered ring then fragments in the opposite direction to release ethylene and create a new metal carbene III, which reacts with another equivalent of propene. Fragmentation of the resulting metallacyclobutane IV produces 2-butene and regenerates the initial metal carbene which re-enters the catalytic cycle.
Secondary metathesis reactions (controlled by catalyst choice and reaction conditions) also affect the product distribution. Recoordination of an alkene on the growing polymer chain with the catalyst can lead to cyclic oligomers through a ring-closing metathesis reaction (“backbiting”). Chain transfer (cross metathesis) between a growing polymer unit and an adjacent polymer alkene also leads to broadened molecular weights. Chain transfer can also be used to improve processability of the resulting polymer – addition of an acyclic olefin (chain-transfer agent) can limit chain molecular weights and introduce terminal functional groups.
Careful balance of catalyst, monomer, and other factors can offer excellent control of the polymer structure. In terms of homogeneous catalysts, most tungsten and molybdenum catalysts (Schrock catalysts) have rapid initiation rates and can produce “living” polymerizations with excellent control of polydispersity and chain tacticity, but the low functional group tolerance limits the monomers available. Ruthenium metathesis catalysts (Grubbs catalysts) tend to have slower initiation rates, often leading to higher polydispersities, but their air stability and greater tolerance for functional groups makes them “user friendly” and enables use of a wide range of functional monomers and additives.
It would take all day to go into all the details (that we know so far) about the olefin metathesis reaction mechanism. There have been a number of intense experimental and computational studies on various catalyst systems, and there’s still more work to be done to figure it all out. But the basics are widely accepted and relatively simple to understand.
Ring-opening metathesis polymerization (ROMP) uses metathesis catalysts to generate polymers from cyclic olefins. ROMP is most effective on strained cyclic olefins, because the relief of ring strain is a major driving force for the reaction – cyclooctene and norbornenes are excellent monomers for ROMP, but cyclohexene is very reluctant to form any significant amount of polymer. Norbornenes are favorite monomers for ROMP, as a wide range of monomer functionalities are easily available through Diels-Alder reactions.
Olefin metathesis is a chemical reaction in which two carbon-carbon double bonds (olefins) come together and exchange with one another, forming new olefinic products in the process.
Olefin metathesis catalysts and technology are available from Materia, a company based in Pasadena, Calif., that was founded by Grubbs and for which both Grubbs and Schrock are scientific advisers.