Chemistry

Sizing Rings

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Science  22 Jul 2005:
Vol. 309, Issue 5734, pp. 537
DOI: 10.1126/science.309.5734.537b

Over the past hundred years, chemists have developed numerous methods to squeeze molecules into tight, small rings, despite the inherent strain this places on bond angles. Large rings have little or no strain, but their synthesis poses a different challenge—the ends of long strands must be coaxed to form a loop, instead of linking end-to-end to yield linear oligomers.

Hori et al.used π-stacking interactions to help achieve this goal. They prepared a precursor resembling a double key-chain: An oligomer of -OCH2CH2O- was capped at both ends by palladium centers complexed to cyclic ligands comprising seven aromatic groups. Adding water to a solution of this compound in dimethyl sulfoxide led to its dimerization, presumably driven by stacking of the large aromatic rings. After they had been brought together, the cyclic ligands became catenated by means of their reversible coordination to Pd, resulting in a very large ring of 238 atoms. — JSY

Angew. Chem. Int. Ed. 10.1002/anie.200501559 (2005).

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