Chemists celebrate happy accident
A team of chemists in Israel has shed light on metathesis reactions involving alkynes, by accidentally synthesising a probable catalytic intermediate. This compound could now form the basis for a novel catalyst that might enable these reactions to be used more widely.
The simplest metathesis reactions (where atoms are interchanged between two molecules) tend to take place in several steps. Ionic compounds tend to form intermediate hydrated ions that then come together in a different combination. Even for reactions between two alkynes - hydrocarbons that contain a carbon-carbon triple bond - the precise reaction mechanism is still a mystery, although it is thought to involve the formation of an intermediate 'in-situ' catalyst.
trimolybdenum complex with carbon-carbon triple bond
One type of alkyne metathesis reaction, which involves treating a disubstituted alkyne with molybdenum hexacarbonyl (Mo(CO)6) and a phenol derivative, is used by industry to produce some natural products and semi-conducting polymers. But it needs anaerobic conditions and high temperatures and doesn't work in the presence of water. This has prevented its widespread use to produce other polymers and fine chemicals.
By chance, a team of chemists from the Hebrew University of Jerusalem has discovered the probable identity of the unknown intermediate in this type of alkyne metathesis reaction.
The chemists were exploring the reaction between Mo(CO)6 and acetic acid, which yields a stable trinuclear molybdenum complex. They discovered that when they dissolved this molybdenum complex in hydrogen bromide they were able to isolate a salt of the complex. They were also able to calculate the complex's structure using single-crystal x-ray diffraction.
The complex is sufficiently similar to the intermediate in-situ catalysts that have been proposed for alkyne metathesis reactions to be a highly plausible candidate. Furthermore, the researchers discovered it retains its stability in water, implying that any catalyst based on it would also work in water.
In a related article in Science, Uwe Bunz, a chemist from Georgia Institute of Technology, Atlanta, US, predicted this research could lead 'to the perfect alkyne metathesis catalyst'. Jon Evans
A Bino, M Ardon and E Shirman, Science, 2005, 308, 234