What is Olefin Metathesis?
In chemical reactions, the bonds between different atoms are broken and new bonds are formed. This typically requires the use of a catalyst that enables a chemical reaction to occur at a faster rate or at a lower temperature than is otherwise possible. The catalyst technology used by Elevance is olefin metathesis, words meaning, “to change places,” and is based on the work of Nobel Laureates, Dr. Robert H. Grubbs and Dr. Richard R. Schrock. In olefin metathesis (olefin is another name for alkene, a chemical compound with at least one carbon-carbon double bond), the carbons attached with a double bond change places with other alkene carbons—enabling new chemical compounds and manufacturing processes previously impossible, with compelling economics.
Traditionally, metathesis had only been used with hydrocarbons (molecules with only hydrogen and carbon) in a high-pressure, high-temperature process. A simple way of viewing the metathesis reaction is to consider the reaction with ethylene and butylene. In this metathesis reaction, one of the CH2 groups on the ethylene molecule is exchanged with the CH3CH group in the butylene molecule. The result is two propylene molecules. The real breakthrough came with the discovery of highly efficient metathesis catalysts that could accommodate the complexity of plant oils as feedstocks to make new materials, such as unsaturated methyl esters and renewable oil-derived olefins at industrial scale with low temperatures and pressures and thus attractive economics.
Schrock's First Practicable Catalysts
Richard R. Schrock started research on new alkylidene complexes in the early 1970s. First he had to determine which metals were the best to use in order to make stable alkylidene compounds. He found that compounds could be made containing metals such as tantalum, tungsten and molybdenum. He gradually developed an understanding of which metals could be used in the catalysts and how they functioned. For Schrock, molybdenum and tungsten soon appeared to be the most suitable metals. Some catalysts were produced with those metals, but there was still uncertainty as to what groups would bind to the metal to give stable yet active alkylidene complexes. A breakthrough came in 1990 when Schrock and co-workers reported the preparation of a group of very active, well-defined molybdenum catalysts (Fig. 1).
With this discovery chemists began to realize that olefin metathesis could be used for general purposes in organic synthesis. Metathesis gained increasing attention among researchers active in synthetic chemistry. It turned out that metathesis can replace a number of traditional synthesis methods. At the same time, it permits entirely new approaches to the synthesis of organic molecules. Molybdenum catalysts often are sensitive to oxygen and water but, with the right treatment, are very powerful tools in organic synthesis.
General Catalysts Developed by Grubbs
Yet another breakthrough in the development of metathesis catalysts came in 1992 when Robert H. Grubbs and his co-workers published their discovery of a catalyst with the metal ruthenium. It was stable in air and demonstrated higher selectivity but lower reactivity than Schrock's catalysts. The new catalyst also had the ability to initiate metathesis in the presence of alcohols, water and carboxyl acids (compare with Fig. 2). Grubbs continued to improve his catalysts and, in Fig. 3, one of his effective metathesis catalysts that are easy to synthesize is shown.
Grubbs' ruthenium catalysts have become the first well-defined catalysts for general metathesis applications in ordinary laboratories. Compound 2 in Fig. 3 is generally called a first-generation Grubbs catalyst, and has become a standard by which all new catalysts are compared. The general applicability of Grubbs catalyst has given rise to uses in organic synthesis. Grubbs has continued developing new catalysts as well as carrying out detailed mechanistic studies on how the catalysts work, and finding interesting applications for the catalysts in making pharmaceuticals and polymers. He has continued development of ruthenium-based metathesis catalysts into yet more powerful tools for synthesis, including that of polymers with special properties.