Chemistry

Make or Break

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Science  19 Feb 2010:
Vol. 327, Issue 5968, pp. 923
DOI: 10.1126/science.327.5968.923-c

In the past several years, increasingly selective chemical methods have emerged to append different groups to specific sites along the periphery of aromatic rings. Two recent studies focus instead on manipulating the ring framework itself. Donohoe and Bower show that a widely used olefin metathesis catalyst can direct the formation of furan rings by stitching together an enone and an allylic alcohol. An advantage of this process is the ease with which diverse functional groups can be introduced through substitution of the precursors. The immediate product of metathesis is pushed along to the final aromatic cycle by either a separate acid catalyst or a Heck protocol that appends an additional aryl substituent.

Sattler and Parkin work from the other end of the spectrum, showing that an intact quinoxaline falls rather dramatically apart on contacting a tungsten complex. Aromatic carbon bonds are among the strongest in organic molecular skeletons, yet in this product—formed at 90°C and characterized crystallographically—two carbon atoms previously bounding an edge of the ring are separated and linked independently to the tungsten center. Their hydrogen substituents are also lost in the process. Though the mechanism remains uncertain, the authors postulate that insertion of the tungsten into the C-H bonds precedes C-C scission.

Proc. Natl. Acad. Sci. U.S.A. 107, 10.1073/pnas.0913466107 (2010); Nature 463, 523 (2010).

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