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June 30, 2010
Another Zero-Palladium Delusion?
Posted by Derek
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.
Comments (11)
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1. Student on June 30, 2010 2:03 PM writes...
I enjoyed the first line "In the past decade gold has been shown to promote a bewildering variety of metal-catalyzed processes to the point that it would seem that other transition metals are
Permalink to Commentbeing forced to retreat". It's rare that you see personality injected into scientific writing. Props to the authors.
2. InfMP on June 30, 2010 11:56 PM writes...
Yeah, when I read that first sentence I jizzed in my pants.
Did you know when he started his career in gold, due to funding restrictions, he dissolved his gold watch in nitric acid in order to get the metals needed for catalysts? this is a 100% true story.
Permalink to Comment3. Jose on July 1, 2010 12:55 AM writes...
You know, after the whole Leadbetter (wholly honourable) Pd-free Suzuki debacle, you'd really think this sort of thing would be on the forefront of many people's minds....
Permalink to Comment4. Andy on July 1, 2010 2:01 AM writes...
It makes me wonder if we ought to start quauratining glassware previously used for transition metal catalysed reaction....
Permalink to Comment5. GreedyCynicalSelfInterested on July 1, 2010 2:29 AM writes...
Gold comes from nature in two types of deposits. Either it is in placer deposits, native gold, or in lode deposits where it is dispersed throughout the rock in combination with other metals and elements. Most gold is mined and refined with the bullion market in mind. The refiners are interested mostly in removing the silver, copper and base metals from the metal. If there are traces of platinum metals in the gold, to 0.1%-0.01%, no one really cares as these are about equal in value to the gold. When someone wants to make gold compounds, they dissolve bullion, usually 999 fine or sometimes 999.9 fine in pure aqua regia and go from there. The palladium doubtless comes along for the ride. I suggest people doing gold catalysis work start with 999.999 fine gold. Good luck!
Permalink to Comment6. Jose on July 1, 2010 5:16 AM writes...
Well, Mr. Cynical, four nines won't be enough... Leadbetter tested to ppt; it was low ppm (or even ppb?) catalysis for his couplings.
Permalink to Comment7. A Nonny Mouse on July 1, 2010 5:27 AM writes...
#4
Permalink to CommentI pointed out the problems of Pd residues sticking in the glassware, even after going through industrial washers, in a publication in 1990. I would agree with your statement and say that new glassware should be used each time in this type of investigation (this statement was incorporated in the Org Syn prep of bis-TMS peroxide following my paper).
8. Rhenium on July 1, 2010 11:55 AM writes...
What a lovely readable paper.
As for Leadbetter, I don't know if it was quite "honourable", that would have involved retracting the ACIE paper, or even just admitting error, something not done in the JOC paper. But oh well, that's old history now...
Permalink to Comment9. Arvind on July 1, 2010 2:08 PM writes...
Do you think even aqua-regia wont work to clean glassware?
Permalink to Comment10. Jose on July 1, 2010 7:10 PM writes...
Re: Leadbetter, I was thinking more that his group made a reasonable effort to look at low levels before they published, and at least at a seminar I attended, he was honest and direct about the problems.
Permalink to Comment11. Jordan on July 2, 2010 10:22 AM writes...
The upside to these "Pd-free" reports (esp. the "Pd-free" Suzuki reaction) is that they demonstrate that these reactions can be made to work at extremely low catalyst loadings. Opens the door to a lot of industrial applications that would have previously been written off for cost reasons.
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