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June 12, 2013
Product Inhibition, Or Grinding To A Halt
Here's a neat bit of reaction optimization from the Aubé lab at Kansas. Update: left the link out before - sorry!) They're trying to make one of their workhorse reactions, the intramolecular Schmidt, a bit less nasty by cutting down on the amount of acid catalyst. The problem with that is product inhibition: the amide that's formed in the reaction tends to vacuum up any Lewis acid around, so you've typically had to use that reagent in excess, which is not a lot of fun on scale.
By varying a number of conditions, they've found a new catalyst/solvent system that's quite a bit friendlier. I keep meaning to try some of these reactions out (they make some interesting molecular frameworks), and maybe this is my entry into them. But the general problem here is one that every working organic chemist has faced: reactions that, for whatever reason, stop partway through. In this situation, there's at least a reasonably hypothesis why things grind out, and there's always been a less-than-elegant way around it (dump in more Lewis acid).
I'm sure, though, that everyone out there at the bench has had reactions that just. . .stop, for reasons unknown, and can't be pushed forward by addition of more anything. I've always wondered what's going on in those situations (probably a lot of things, from case to case), and they're always a reminder of just how little we sometimes really understand about what's going on inside our reaction flasks. Aggregates or other supramolecular complexes? Solubility problems? Adsorption onto heterogeneous reactants? Getting a handle on these things isn't easy, and most people don't bother doing it, unless they're full-out process chemists in industry.
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