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Polymer-bound chiral catalyst makes optically pure compounds
Traditionally tricky, the formation of carbon-carbon bonds is a much-sought-after goal of the compound-creating organic chemist. One extremely important synthetic route to this atomic marriage is olefin metathesis, in which two molecules--each with carbon-carbon double bonds--exchange carbons, bringing along any attached groups. The process, used to make everything from polymers to pharmaceutically important compounds, is usually accomplished with the help of an organometallic catalyst.
Now comes the first catalyst for this reaction that's not only supported and recyclable, but also chiral [Angew. Chem. Int. Ed., 41, 589 (2002)]. It was developed by chemistry professors Richard R. Schrock at Massachusetts Institute of Technology and Amir H. Hoveyda at Boston College and their postdoctorol fellow Kai C. Hultszch and graduate student Jesper A. Jernelius.
The fact that the molybdenum-based catalyst is supported--that is, sits in a layer on a chunk of some material, in this case a polymer--is important because it can be easily removed after the reaction. That means fewer impurities left behind, a problem that plagues reactions performed with homogeneous catalysts. The catalyst can also be reused several times. These properties make the catalyst attractive for use in combinatorial chemistry, Hoveyda says.
That the catalyst is chiral is even more important: It allows the chemists to make products that are highly enantiomerically selective. A number of optically pure compounds that are difficult to come by could be made with the catalyst, the authors say.
The group tested the catalyst in a number of asymmetric reactions, which they say were efficient and produced enantioselective products in high yields.
The catalyst, in which a chiral ligand is attached to the polymer by a rigid tether, is based on a family of achiral catalysts developed in Schrock's lab. The two groups developed the chiral version with the help of "modeling and intuition," Hoveyda says.
MO BETTER New Mo-based catalyst is supported and recyclable, and it produces enantioselective products.
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Copyright © 2002 American Chemical Society