Friday, August 16, 2013

Friday Fun: Super-Calcium!

Source: Niggemann Group, RWTH Aachen
I like to stroll through journals on quiet mornings, over coffee. After a few months, you start to see trends crop up: The Gold Rush. Carbon-carbon bond cleaving chemistries. Fluorine and boron everywhere.

A new trend almost snuck in under my radar: Calcium catalysis. In the past, a few groups had played around with amino-ene reactions, arylated tertiary alcohols, and made some enantioselective calcium pincer complexes. But I couldn't honestly tell you that I had branded any specific group with the "calcium" label, as opposed to the "palladium" or "organocatalysis" badges worn by many.

Well, the Niggemann group in Aachen, Germany appears to want that distinction. Prof. Meike and her team have released a slew of interesting reactions - Friedel / Crafts, [3+2] cyclizations, cyclopropanations - with more popping up seemingly monthly. But...calcium? The stuff ingrained in our bodies, stapled in the phosphate matrix of our bones and teeth? The stuff I eat in yogurt, milk, and cheese is now a catalyst?

Source: Niggemann Group, RWTH Aachen
Let's dig a bit deeper. To start, Niggemann's group uses a weakly-coordinated calcium complex, calcium (II) bistriflimide. Next, they exchange anions with a quaternary ammonium source, producing the "mixed" catalyst Ca(II) PF6 NTf2, increasing organic solubility. The group claims that this complex exhibits both high selectivity for olefin coordination and stability against air and moisture - both important properties if you're exploring new reactions!

This new catalyst combo, dubbed "Super-Calcium" (with mascot, above), reacts like a wild hybrid of alumnium, gold, and palladium. It activates alcohols as leaving groups (Al). It permits [1,2] hydride shifts (Pd). It's a hard enough Lewis acid to unzip donor-acceptor cyclopropanes, but soft enough to permit hydroarylation (Al / Au). Checking some of the historical calcium-catalysis reactions (above) reveals even more head-scratching reactivity reminiscent of magnesium, titanium, or vanadium.

So, what's really going on here? First, I'd say it's early days: Some deuterium-labeling studies were done on the older reactions, and molecular modeling on this latest batch, but several steps (Vinyl cations? Hydride shifts?) make me wonder exactly how intimately the central calcium atom gets involved. Second, no one yet knows the exact structures of these reagents in solution; look how long it took to figure out LDA!!! Third, Meike's battle cry rings mostly true: reactions exploring the reactivity of early alkali metals (potassium? barium?) remain largely terra incognita.

More reactions will lead to more interest; perhaps a Calcium Craze looms over the horizon? Time will tell.

Happy Friday, Everyone!


  1. Hi, it's Jake Yeston. Don't know how to link comments to Twitter. I started trying to tweet this but decided it was just too long. Anyway, this is a GREAT post, but it seemed like a good example to discuss in the context of how to sell chemistry to the public. In theory, this is an easier sell than some topics, because folks have at least heard of calcium ("the stuff ingrained in our bodies, stapled in the phosphate matrix of our bones and teeth? The stuff I eat in yogurt, milk, and cheese is now a catalyst?"--Yay, exciting!). But the reason this particular set of results is interesting is that chemists know calcium is usually inert. If you start trying to explain that to the public, you need to give a primer on valence and redox potential. Then, if you're lucky, readers will have a vague idea of what catalysis is, but they'll probably remember "something that doesn't get consumed in the reaction" rather than how it assists the process. You completely lose them when you start talking about "amino-ene reactions, arylated tertiary alcohols, and ... enantioselective calcium pincer complexes." They have no idea what's going on with the bistriflimide. Even if they get past that, they don't understand HSAB theory, and what it means that gold is soft (like, easy to bend?). If you explained all of that, you'd lose the chemists who just want to click over here and find out what's actually new. That's what I meant about the public lacking the vocabulary to read about chemistry. I could launch into a whole spiel about how chemistry gets taught, but frankly I'm not an education expert. Anyway, like I said at the outset, this is a great write-up for chemists--nothing wrong with that!

    1. I take your point, thanks for the comments. Perhaps, as an exercise, I'll try writing the same post back-to-back, but for general audiences. Wait 'til the weekend, I guess.

  2. Super Calcium seems to be a ripoff of Peter Puck: