(it—m CATALYSIS
RINGS MADE EASY Catalyst releases product of polymerization as a closed ring
F
ACILE ACCESS TO CYCLIC
polyethylenes has been realized with a cyclic ruthenium polymerization catalyst. The work could lead to new materials and may help solve some unanswered questions in polymer science. Robert H. Grubbs, a professor of chemistry at California Institute of Technology, and graduate students Christopher W. Bielawski and Diego Benitez have prepared high-molecular-weight cyclic polyethylenes through ring-opening metathesis polymerization of «>-cyclooctene, catalyzed by a cyclic ruthenium complex [Science, 297, 2041 (2002)}. The reaction forms a cyclic polyoctenamer, which readily precipitates after addition of acetone or methanol and is isolated simply by filtration. Hydrogenation of the product yields cyclic polyethylenes. Using this method, the Caltech chemists have prepared cyclic
TOTAL
polyethylenes with molecular weights ofup to 1,200 kilodaltons. And using various characteriza-
complete, side reactions are common, and linear impurities must be removed by elaborate procedures. Neither route easily applies to high-molecular-weight cyclic polyethylenes. How to make cyclic polyethylenes easily posed "an interesting question," Grubbs says. The key he adds, is using not only a cyclic monomer, which has no ends, but
ENDLESS Cyclic monomer is converted into a loop polymer by cyclic catalyst
•o Cy = cyclohexyl tion techniques, they have shown that the physical properties ofthe cyclic polymers are distinct from those of linear analogs. Ring polymers are not new, but they have been difficult to prepare. Typically, they are made by intramolecular cyclization of linear precursors at very low concentrations or by shifting certain polymerization reactions to favor cyclic products. With either route, ring closure is usually in-
also a cyclic—endless—catalyst. With cyclic polyethylenes at hand, Grubbs says, "we can now answer the fundamental question: Does a cyclic polymer behave like the linear version even when you get to really high molecular weights? And the answer is, no. Whether that translates into practical application is for somebody else to decide. We've got the material; now people can testit."-MAUREENR0UHI
n-1 Cyclic polyethylene
RESYNTHESIS
Lab Creates Correct Version Of Natural Product
C
hemistry professor K. C. Nicotaou, at Scripps Research Institute, and colleagues synthesized a rare natural product, using another group's reassignment of the compound's structure. Diazonamide A was first isolated from a sea invertebrate in 1991 by University of California, San Diego, chemistry professor William H. Fenical and colleagues, who then proposed its structure. Because of diazonamide A's striking potential anticancer properties, as well as the rarity of its natural source, numerous research groups attempted its total synthesis. The solution didn't come until last year, when Patrick G. Harran, associate biochemistry professor at the University of Texas Southwestern Medical Center, in
HTTP://PUBS.ACS.ORG/CEN
Nicolaou's lab then worked to syntheDallas, and colleagues announced that they not only had synthesized the proposed size the real compound, employing a number of strategies, including using diisobutylstructure but also had discovered that it lithium aluminum was wrong—what hydride to initiate clothey'd created wasn't sure of the molecule's the natural product 10th ring \Angew. Chem. [Angew. Chem. Int. Ed., Int. Ed., 41,3495 (2002)]. 40,4765 and 4770 "[Nicolaou's] work (2Q01)](C&EN,Dec17, continues to dazzle," 2001, page 11). says Joseph P. KonoAlthough the bulk of pelski, chairman of the Harran's molecule is Diazonamide A UC Santa Cruz chemsimilar—and indeed shows as much biological activity as the nat- istry and biochemistry department. ural product-^the real diazonamide A actu- "There's so much chemistry that's already ally contains a 10th ring, as well as a nitro- been developed because of this molecule, gen atom instead of an oxygen in the middle and I'm sure we're going to see much more/'-ELIZABETH WILSON of the molecule, they reported.
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