There are exactly five regular polyhedrons that can be made, and as they were first discussed in detail by Plato, they’re sometimes known as the platonic solids. Now, you might not have heard of them under that name, but I’m pretty sure most of them are familiar to chemists. In order of increasing size, the series starts with the tetrahedron, the shape of stereogenic centres at carbon, and the source of asymmetry in life. In fact, Jacobus Henricus van 't Hoff won the very first Nobel prize in chemistry ever back in 1901 for being one of the first to notice this. Although the parent hydrocarbon has yet to be synthesised, a number of tetrahedrane derivatives have been reported. Next comes the cube, the corresponding hydrocarbon of which, cubane, was famously first synthesised by Eaton back in 1964. Third up is the octahedron, more of an inorganic chemist's shape and rather unlikely to ever exist with a carbon skeleton due to the crazy C-C bond angles required. Fourth, the dodecahedron, has actually been better studied by organic chemists than most people realise and, after much competition, Leo Paquette was the first to synthesise the corresponding hydrocarbon in 1983. Last and largest in the series is the icosahedron, which I can’t think of a way to link to chemistry, but we’re certainly unlikely to ever see a carbon based version as all the atoms in the skeleton need to have five bonds.
In this post and the next two I’m going to discuss three syntheses; those of the two platonic solids made to date (cubane and dodecahedrane), and that of the non-regular polyhedron pentaprismane, because it’s also pretty cool. Why do this? Well, when if you consider, say, dodecahedrane in a retrosynthetic sense then unless you lived through the era when these compounds were fashionable targets, have studied them, or are a bit of a genius then it's not obvious where to start so hopefully we can learn a bit of chemistry. I also enjoy a bit of chemical history and some of the methods used were pretty neat as we'll see.
On Friday, I found myself discussing the use of Eaton's reagent with a coworker. Many people know it as a handy alternative to polyphosphoric acid (PPA) for acylations, Friedel-Crafts reactions and the like. It's even endorsed by Milkshake. A simple 7.7 wt% solution of phosphorus pentoxide in methanesulfonic acid, it's not as viscid, viscous or unpleasant to work up as PPA. And it's commercially available. But neither my coworker, nor many other people I know, are aware of Eaton's other contributions to the synthetic world. He's still an emeritus professor at the University of Chicago, and you can read about his interests on his website, but what he's most famous for, unbeknownst to many younger chemists, is his synthesis of the cubanes.