Method Makes Z Alkenes

June 30, 2011
Catalyst reportedly provides unprecedented levels of reactivity and selectivity.

A joint team of scientists from Boston College, Chestnut Hill, Mass., and Massachusetts Institute of Technology (MIT), Cambridge, Mass., have developed a new catalytic method for the synthesis of a large class of carbon-carbon double bonds. It enables production of the higher energy or Z isomer of alkenes.

Amir Hoveyda, the Joseph T. and Patricia Vanderslice Millennium Professor of Chemistry at Boston College, says these higher energy carbon-carbon double bonds are incredibly important to organic synthesis and materials research. For instance, Z alkenes serve as starting materials for some common chemical transformations.

Hoveyda and his team rely on a molybdenum-based catalyst to spur a Z-selective "cross metathesis" reaction — in which two different alkene-containing molecules combine, with only ethylene as a byproduct. Removing the generated olefin by running the reactions in a vacuum significantly improves the desired process and yields unprecedented levels of reactivity and selectivity.

Z isomers require a sufficiently active catalyst to prompt the chemical reaction while still maintaining the cis olefin's U-shape geometry.

"The trick here was to come up with a catalyst that is active enough to promote Z-alkene formation but not too active to also want to react with the product. So, in a way, we had to walk on a very tight rope. Olefin metathesis is a reversible reaction and you always run the risk of going back and forth between product and starting material, which forces you to end up with a lower energy and less desirable isomer. What we have found are catalysts that are sufficiently active to promote this difficult reaction but are also discriminating enough not to go after the product and cause it to isomerize."

The team demonstrated the new catalytic method through synthesis of a potent antioxidant plasmalogen phospholipids. Further details appear in a recent article in Nature.

For future research, there are a number of possible steps to take, says Hoveyda, as well as other cross-metathesis processes that must be developed.

"It is already our experience that each case has its own nuances, which is important, as it will likely lead to further catalyst development. Then there is the issue of tri- and even tetra-substituted olefin synthesis by cross-metathesis. It is our approach that all such developments are target-driven: we develop new methods based on their degree of relevance to facile and efficient preparation of various classes of important biologically active molecules," he says.

Like any highly active catalyst, the molybdenum-based catalyst is sensitive to air and moisture; however, Hoveyda and his team are working towards designing air-stable variants.

"Once this phase of our research is completed, and considering the effectiveness of the catalysts that we have and the significance of cross-metathesis reactions, the technology will be a formidable force in chemical synthesis," he believes.

Such work could last several years, depending on what the researchers determine as the desired outcome.

"If the eventual goal is to develop an efficient version for all possible catalytic stereoselective cross-metathesis reactions, then we are looking at least at a decade. But we already have very promising leads on four new classes," Hoveyda adds.

While all catalysts are designed and developed by the team at Boston College and MIT, Switzerland-based XiMo, AG, has licensed the technology and is working with the researchers to further develop the process. XiMo aims to use the available catalysts and technologies, as well as develop new ones for large-scale use.

"So far, there has been a great deal of interest and as we develop more catalysts, concepts and methods, there is no doubt this interest will grow," he concludes.

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