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September 21, 2011
Pulling Molecules Apart, For Fun
Now here's an odd reaction, done in an odd way. Organic chemists will all be familiar with the azide/acetylene cycloaddition to form triazoles. In its copper-catalyzed variant, it's become a sensation, and is used as a convenient linker to do all kinds of interesting things. The reverse reaction, taking a triazole back to the starting materials, just isn't feasible. If you heat up one of the triazoles enough to get it to do anything, which takes some pretty serious heat, it just gives you a handful of decomposition products.
But what if you grabbed each side of the ring and just pulled on it? A paper in Science does just that, though having polymeric chains attached. If you subject that to ultrasound, the cavitation bubbles that form are violent enough to pull and jerk the molecular chains around - and when they try that on a triazole-linked molecule, they can see reversion to the acetylene and the azide. This only happens with long-chain polymers - the effect increases with polymer molecular weight, and small-molecule analogs aren't cleaved at all. It also appears that the effect works best when the triazole is near the midpoint of the polymer, not out towards one end. These are just what you would expect for this sort of "mechanosynthesis", and strong evidence for the proposed effect.
This could lead to some rather unusual reactions being discovered. Some sort of cleavable tether that stands up under sonication might allow you to put on "mechanosynthetic handles" that you could then take off again, as if they were protecting groups. Silyl ethers, maybe? Which functional groups can take the stress, and which will pull apart to give something new?
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