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March 4, 2007
Calcium: A Backwater
Posted by Derek
We've had a hundred years or so of nonstop love directed toward organomagnesium compounds (from Victor Grignard, patron saint of getting the reaction named after you and not your supervisor, right on down). So I've always found it interesting that there weren't more organocalciums out there.
Calcium is probably (from an organic chemist's viewpoint) one of the more underused elements in the first few rows of the periodic table. It's always overshadowed by its neighbors. I've never even seen pure calcium metal, as far as I can remember. OK, people distill some organic solvents from calcium hydride to dry them - at least they do in grad school, 'cause in many industrial labs no one distills solvents at all. And there's calcium sulfate as a drying agent (Drierite, by trade name), but people mostly use that for gas drying (calcium chloride, too, although I haven't seen a good old calcium chloride drying tube in a while). For drying liquids, a higher-volume trade, people reach for sodium or magnesium sulfate instead.
And while that's about as high-profile as calcium gets in many labs, those kinds of uses aren't exactly in the center ring. I recall seeing some old work with calcium metal in liquid ammonia, doing Birch chemistry, but I've never heard of anyone actually doing any of it. As far as real organocalcium compounds, the literature is mighty thin. One problem seems to be that the metal itself (unlike magnesium) doesn't just up and react with organic halides very well. Some Grignards, once they get going, have to be beaten down with frantic bucket runs to the ice machine, but not so with calcium.
Chemist Rueben Rieke has gotten around this problem in his usual fashion, by making insanely reactive calcium metal. His calcium work is about ten years old now, but I haven't seen too much follow-up. (One reason might be that Rieke's conditions can be rather painful to use, which difficulty he wisely exploited by forming his own company to do the stuff for other people). But I see that the latest Angewandte Chemie has an organocalcium article from a group of enterprising Germans, so perhaps this stuff might be working its way into the mainstream.
Once people have a reasonable way to get to these compounds, the hard part can begin: finding out what on earth they're good for. You'd have to think that there are interesting reactions and catalysts which can be prepared from calcium derivatives, since they're bound to have their own character. But where to start? An obscure element needs a champion. Boron had H. C. Brown, and Sharpless brought vanadium into vogue for a few years. A host of people lifted palladium from the back shelves to indispensability. Who speaks for calcium?
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1. milkshake on March 5, 2007 1:08 AM writes...
Aryl calciums described in the Angewandte paper are worthless from a preparative point of view - they have to be kept at low temp because they decompose readily at room temperature and their preparation involves activated Ca prepared by dissolving chinks of Ca metal in liquid ammonia and then quickly distilling ammonia out (because unlike Na or Li, Ca solutions in liquid ammonia are quite unstable). There are so many good mild and stable alternatives thanks to Knochel and others. One would have to have extremely good reason to mess with organocalciums.
As illustration of an extremely good reason: Corey group people have been using allyl bariums (Ba metal + iodine = BaI2 solution, BaI2 + LiNaphtalenide = Riecke Ba) because allyl baryums react with electrophiles exclusively on the less hindered allylic carbon. But it was still pain to use them.
Permalink to Comment2. jokerine on March 5, 2007 5:49 AM writes...
Westerhausen came to give a talk at our Institute and seemed pretty convinced of his Calcium chemistry. His main argument was, that Calcium is absolutely nontoxic. If it were a bit more stable, it would be interesting.
Permalink to Comment3. Jose on March 5, 2007 11:59 AM writes...
Another wholly nontoxic element that does some truly staggering chemistry- Bismuth. Barton was on to something there- why has no-one followed up in the past 20 years?
Permalink to Comment4. Chemist of Sorts on March 5, 2007 1:16 PM writes...
It seems that the key question is: 'What can calcium do that other reagents can't?'. Milkshake hit on this point and I have to agree. Toxicity is just not really an issue until you work on large scale. Look at the continued popularity the Stille reaction.
Permalink to Comment5. Cryptic Ned on March 5, 2007 3:32 PM writes...
I hear about calcium more than any other metal because I read a lot of things about the activation of lymphocytes, in which one of the steps they use as a marker is the increase in cytoplasmic Ca2+. The cell biologists have certainly got accurate systems for measuring changes in calcium ion levels in solution, if nothing else.
Permalink to Comment6. datadriven on March 7, 2007 11:44 AM writes...
Derek,
After reading this entry, I looked at my PhD thesis (now 12 years old!) and found that I used calcium metal to do birch chemistry to fragment a 1-keto, 2-arylcyclobutane ring. Why? I can't remember... In the text, I wrote that the sodium and lithium conditions gave a mixture of product, starting material and over-reduced alcohol and that employing calcium metal gave mostly product with a very small amount of alcohol. Unfortunately, these were unpublished results that I would never have remembered had it not been for your entry -- thanks!
Permalink to Comment7. neo on March 9, 2007 9:37 AM writes...
Anybody remember the Benkesser reduction? Calcium dissolved in ethylenediamine reduces simple naphthylenes and indenes to the bicyclic compound with one lone olefin left at the ring junction. Don't know any other slick way to do that, although I'm sure someone could find a nice expensive hydrogenation catalyst to do the trick. That's about the only useful thing I know of that calcium metal can do.
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