Let’s start off the year with a shot of pure chemistry, and a neat-looking molecule. This new paper details a surprisingly easy synthesis of a cage-type molecule, and making these things is generally a lot more work than this.
Admittedly you first need the tris-furanyl starting material, but that’s not hard (Suzuki coupling), and the bis-isopropenyl partner is commercially available. The yield is suprisingly good (>30% on a good run), and if you want a demonstration of how different acids (both Brønsted and Lewis) can give you different yields, then look no further. The rather exotic combination of Ca(NTf2)2 / nBu4NPF6 gives the highest yields for the reaction, but good ol’ trifluoracetic acid gives 20% and stannic chloride gives 18%. But the classic Friedel-Crafts acid, aluminum chloride? Nothing. Sulfuric acid, nothing. BF3 etherate, nothing. Never stop trying acid conditions for these cation-driven reactions, folks, you never know what’s lurking out there. Personally, polyphosphoric acid got me out of a Friedel-Crafts tight spot once years ago, and I’d like to see if that’s any good here (and how about triflic acid?)
But you can get that cage-compound product in good quantity, anyway, as a white solid after chromatography. No melting point given, though – what’s the world coming to? But it’s well-characterized by NMR and mass spec. It forms hexagonal crystals, which makes sense given the structure, I’d say, and there’s a good X-ray structure. It also forms obvious host-guest complexes with pyridinium-type compounds, sandwiching in the middle space with an equilibrium that is clearly seen in the NMR spectra. Two of these formed X-ray quality crystals, which I note were red- and yellow-colored, and clearly show the host-guest behavior with distortions of the cage structure as it interacts with the guest molecules, as expected. That gives the crystals different symmetries, too, if you’re into that sort of thing (who isn’t, though, right?)
The authors say that “This method has proven to be a general approach for synthesizing a variety of cages“, but give no further details (future papers, no doubt). That’s one of the first things I’d do on discovering a reaction like this, too – see what else it would make! To be honest, I don’t necessarily expect any great discoveries out of this work in the end. Host-guest complexes are well-known in general, and the sorts of interactions that these take advantage of (donor-acceptor, cation-pi, etc.) have also been studied many times. But I appreciate this paper for the neatness of the reaction that forms the cage compound and for its pleasing level of symmetry. It doesn’t have to do anything particularly useful; it’s already brightened my day!
