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September 1, 2009
Another Iron Reaction Hits The Mat
Beware of iron! That's the lesson that's being hammered home these days in synthetic chemistry. I wrote recently about the discovery that a series of iron-catalyzed couplings were actually being caused by trace amounts of copper compounds. Now there's another re-examination of some similar iron couplings that were reported last year.
If you click on that last link, you'll see that there was already trouble with the original work. The authors themselves appear to have had a hard time repeating it, and earlier this year they retracted the paper. This latest publication (from other workers) details their own attempts to reproduce the original iron-catalyzed work. In most cases, they got nothing at all, but once (and only once) they had a wonderful spot-to-spot reaction take place with para-bromoacetophenone, which must have been just the sort of thing that excited the original researchers.
But it could never be reproduced. The best guess is that this one reaction may have been catalyzed by trace amounts of palladium. That's plausible, because, as it turns out, the coupling can be run at high conversion with one ten-thousandth of a per cent of palladium acetate. Yes, a substrate-to-catalyst ratio of one million to one is sufficient, and that's the kind of activity that makes it very, very hard to assume that trace amounts of palladium salts aren't doing the work.
It also makes you wonder why anyone would use anything else, at least for activated systems like para-Br acetophenone. In the future, anyone trying to come up with a non-palladium coupling protocol had better stick with the tough reactions that don't work well anyway. That will keep this sort of thing from happening again - and those are the kinds of reactions we need help with, anyway. A new catalyst for coupling red-hot electron-poor aryl bromides, on the other hand, will be greeted with yawns, and with suspicion as well.
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