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June 30, 2010
Another Zero-Palladium Delusion?
As mentioned here before, there have been several episodes where people have thought to have discovered a new metal-catalyzed coupling reaction that uses some metal not known for such things. But closer examination often reveals that ridiculous trace amounts of palladium, copper, or other more reactive metals are still in the system and responsible for all the results.
The most recent candidate is been a series of gold-catalyzed reactions. Gold complexes have been quite fashionable in recent years, after a long period where they were considered next to useless. But perhaps things have gone a bit too far. A new paper in Organic Letters examines some gold-catalyzed couplings and finds, well. . .
Experimental reports claim that Au(I) is selective and very active, for instance, toward cross coupling of aryl halides with acetylenes (“Pd-free Sonogashira” for example), in the presence of mild bases. Surprisingly, this intriguing process has not been investigated mechanistically. We decided to set out experiments that would explain mechanistically the Pd-free cross-coupling catalysis with gold, but in fact what we are reporting is our failure to find a plausible mechanism. Furthermore, our experiments suggest that the presence of adventitious Pd might explain the positive “Pd-free Sonogashira” catalysis reported. . .
It's the oxidative addition step (the first one in the cycle) that makes things go off the rails. Gold complexes (at least the ones reported) just don't seem to be able to do it. On the other hand, as the authors mention, even high-quality gold often has a bit of palladium in it, and that bit is all it takes.
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