Catenane (Trickier than they look)
31st July 2006
A catenane is a topologically locked two-ring system. Put more plainly, it is a molecular chain (with interlocking links). I used to doodle molecules like this before I knew they existed. As I was going through organic chemistry in undergrad, I always figured they were impossible to make.
After all, once you have a ring, it’s going to behave very much like a macroscopic ring; no passing through the side or anything. And to get two rings to loop together, they need to be oriented properly when the loop is made. This seemed unlikely to me, so I moved on to doodling other things.
Fortunately, someone had tried. If I remember correctly, the first catenanes were made by a plain old ring-forming reaction (i.e., one that forms monomers). The reasoning was that if you had a big enough ring, a high enough concentration that the ring-formers were pretty crowded, etc., you had to get SOME molecules oriented just right and make some catenane (as opposed to monomeric rings). Sure enough, it worked (although the yield was, predictably, quite poor).
Nowadays, the field of supramolecular chemistry is growing prodigously. As our knowledge of intermolecular interactions progresses, we are gradually getting better at better at orienting molecules in dilute solution and assembling unlikely-seeming shapes. A Self-assembly and pre-organization strategy is often used to pursue catenanes today.
See you tomorrow.
August 2nd, 2006 at 4:58 am
The catenane shown above is impossible, you need a ring with at least 24 atoms in it before you can thread another molecular chain through it…
August 2nd, 2006 at 10:17 pm
Ah, I was worried I’d get a comment like that. I had no idea what size of things actually could be made into catenanes. Is this an experimental thing, or has someone worked it out mathematically? Thanks for the correction!
August 3rd, 2006 at 5:43 am
You know, that’s a really good question..! It was just one of the things I took for granted when I did my PhD. I worked with crown ethers and secondary dialkylammonium ions, and to thread one of the latter through the former, you need to use dibenzo[24]crown-8 or a similar derivative, the next lower homologue is 21-crown-7 and that’s too small - making one with 23 atoms in the ring is a little more tricky as you lose the nice O-CH2-CH2-O repeat unit. I think there’s a discussion of ring size in a Chem. Eur. J. paper from 1996, pages 709-728 (out of the Stoddart group). As for the smallest catenane, I think that prize goes to David Leigh (now Edinburgh) who interlocked two 26-membered rings (Angewandte 1995 I think…). That’s ignoring some organometallic magnesium catenanes in which it is just the magnesium atom of a cyclic-dialkylmagnesium that sits inside a crown ether-type ring. There’s also been plently of ’slippage’ studies where different end-groups on molecular chains have been pulled through macrocycles in-computero… For a good general overview of the field, Wiley published a book by Jean-Pierre Sauvage on molecular catenanes, rotaxanes and knots… very expensive I imagine…