Chemists make superstar reaction more accessible

Jul 12, 2006

If a chemical reaction can enjoy superstar status, then the process known as metathesis just might qualify. The reaction, involving a swap of atom groups that has been compared to changing partners in a square dance, made news when three scientists shared the 2005 Nobel Prize in Chemistry for figuring out how the reaction works and how to make it more efficient and simpler to use.

Their work with metathesis in olefins—compounds that contain double bonds between carbon atoms—paved the way for developing new molecules that can be used in pharmaceuticals, advanced plastics and other applications.

Doing a similar trick with alkynes—molecules that contain at least one triple bond between carbon atoms—also can produce plenty of useful materials, but widespread use of alkyne metathesis has been stymied up until now, said Marc Johnson, University of Michigan assistant professor of chemistry. Like olefin metathesis, alkyne metathesis happens with the help of special catalyst molecules, but easily-synthesized catalysts are typically active only under harsh conditions, and making highly active catalysts has proven challenging.

Now, however, Johnson and graduate student Robyn Gdula have found an easy way to make an exceptionally active catalyst for alkyne metathesis. Their work is published online today in the Journal of the American Chemical Society.

Metathesis revolves around the bonds that link carbon atoms together to form compounds. All living things are based on carbon compounds, and such compounds also can be made artificially in the lab. Breaking carbon-carbon bonds is notoriously difficult, but managing to do so opens up host of possibilities—such as converting ring shaped molecules into chains or blending two compounds to come up with new, potentially useful compounds.

In metathesis, double or triple bonds between carbon atoms are broken and rearranged, taking with them any attached atom groups. While the process can happen by itself, it does so only under harsh conditions, so in order to harness it for making useful compounds, chemists need to find ways of making it happen predictably, under regular lab conditions. That's where catalysts are handy.

In their search for an easier way of making catalysts for alkyne metathesis, Johnson and Gdula first tinkered with molecules called metal nitrides, which contained the metal molybdenum linked by triple bonds to nitrogen. The idea was to use these complexes as stepping stones to making active catalysts that contain molybdenum triple bonded to carbon.

Once the researchers determined that their altered metal nitrides were in fact more reactive than the versions they had started with, they combined the reactive nitrides with an alkyne and applied heat. The resulting compound was a known, highly active molybdenum-based catalyst that could then be isolated and used in alkyne metatnhesis.

The whole process was surprisingly easy, said Johnson. "Now that it's so easy to make these catalysts we hope to find a partner in industry who will market them so that anyone who wants to do alkyne metathesis for making a complex organic molecule—to be used in pharmaceuticals or light-emitting diodes or liquid crystal displays or other applications—will be able to just purchase the catalyst instead of having to make it. Alkyne metathesis has been underutilized until now because it's been so difficult to get one's hands on a highly active catalyst, but this should make things much easier."

Source: University of Michigan

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