Friday Fun: Super-Calcium!

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!
-SAO