Tying Up Loose Ends, Chemically Speaking

At present, the practical applications of molecular knots are arguably few, but the challenge of creating such structures has intrigued synthetic chemists for years. The mental gymnastics it takes to tie the molecules is also a good way to get fledgling organic chemists to appreciate molecules in 3-D. Edward E. Fenlon, a chemistry professor at Franklin & Marshall College, in Lancaster, Pa., has been successful at snagging a few bright undergrads to conduct research on molecular knots.

Fenlon spoke about the progress he and undergraduate Matt Dietz have made toward preparing the world's smallest trefoil knot during a session sponsored by the Division of Organic Chemistry. Trefoil knots possess three crossings, making them the simplest of all knots. The smallest molecular trefoil knots deliberately synthesized to date have at least 80 backbone atoms.

Fenlon and Dietz have been working toward a C₆₃H₁₂₆ knot. If successful, they will have created the smallest knot yet and the only purely hydrocarbon knot ever prepared. The critical step uses olefin metathesis to intramolecularly link together the trefoil precursor's six arms. Two threading events must take place at a specific stage of the metathesis in order to tie the knot.

Upon hydrolysis of a key ester, Fenlon and Dietz were able to isolate three large triols—each of the same molecular weight—using preparatory thin-layer chromatography. Fenlon thinks these are a large macrocycle, a catenane, and the trefoil. Although preliminary analysis looks promising, Fenlon told C&EN he'd like to get unambiguous proof, such as an X-ray crystal structure, before declaring victory over the knot. He and Dietz are currently trying to derivatize the structure so it's more likely to crystallize.