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March 4, 2007
Calcium: A Backwater
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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