Catalysts Taking Turns

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Science  08 Jul 2005:
Vol. 309, Issue 5732, pp. 223
DOI: 10.1126/science.309.5732.223b

Enzymes can be highly selective in promoting reactions of just one enantiomer from a racemic mixture. In dynamic kinetic resolution, a second catalyst is added to rapidly interconvert the starting enantiomers, so that eventually the chiral catalyst guides every molecule in the mixture to a single enantiomer of product. Now van As et al.have adapted this technique to form chirally pure oligoesters from racemic monomers. They use a lipase enzyme to catalyze ring-opening polymerization of 6-methyl-ϵ-caprolactone. The ring-opening liberates an alcohol center that can open another lactone; however, the enzyme selects for attack of an (R)-alcohol on an (S)-lactone. A ruthenium catalyst then racemizes the ring-opened alcohol so that, as the reaction proceeds, the enzyme adds an (S)-center to the end of the growing chain, and the metal swaps the configuration of this center to enable further chain growth. Decomposition of the pentamer and chromatographic analysis revealed 92% selectivity for (R)-configurations in the backbone. The results are an important contribution toward generating a novel route toward enantiopure polyesters. — JSY

J. Am. Chem. Soc. 10.1021/ja052347d (2005).

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