The Birch reduction – there’s an old-school synthetic transformation from you. I thought that when I first did one in 1983, so it must be even more so now, right? You condense liquid ammonia and dissolve a reactive metal in it (sodium or lithium are the usual), giving you a rather unexpected blue solution. That color (weirdly) is due to solvated free electrons, which means that this is one powerful reducing agent. The Birch is famous for being able to knock aromatic rings back down to cycloalkenes, and there are (still) not too many ways to do that.
This new paper (from a group at China Agricultural University) describes an ammonia-free (and indeed, amine-free) variant of the reaction – you use commercial sodium dispersion with a crown ether in isopropanol, and you can do it all at zero degrees C, as opposed to the -30 or so you have (at most) in liquid ammonia. Not having to break out the ammonia tank does sort of lower the barrier to running the reaction, it’s true, although I always regarded that as a sign that one was about to do some serious Birching. And condensing ammonia always seems like a magic trick, as the clear liquid just appears on the dry ice cold finger and drips down into the flask. You get that same something-out-of-nothing effect when you do a distillation, of course, but I think it’s the lack of visibility of the source in the case of the ammonia tank that gives it more of a flourish. And of all the gases that I’ve seen condensed on a cold finger, ammonia is definitely the one that I’d rather work with (as opposed to liquified HCN, for example, which I’ve seen once and have no desire to encounter again).
Another advantage to the Birch is that you typically just let the reaction warm up to get rid of all the ammonia, letting you switch to whatever solvent system you want for the workup. In this case, the reaction is done fairly concentrated (more so than the ammonia ones), and the authors just do an ether/brine extraction without evaporating the isopropanol away. The advantage of that is that the crown ether actually goes into the brine layer, and can be recovered by a second extraction with dichloromethane. This recovery helps, but still is unlikely to make this method of industrial interest (no one wants dichloromethane on that scale). But for bench-scale reactions, this should be pretty convenient. It looks like this reaction does pretty much what the classic Birch does. Of course, sodium in an alcohol solvent is yet another old-school reaction (the Bouveault-Blanc reduction, in the case of esters), and if you don’t add the crown ether, that’s all you’re going to get. But complexing the sodium ions changes the mechanism (outer-sphere versus inner-sphere), and that gives you Birch products.
One thing I wonder is if you can make sodium amide with this system, if you add some catalytic ferric salt at some point in there. That prep starts out just like a Birch, but the iron sends it down a different path. If so, I might have been able to spare myself a painful episode (and I mean that literally) when I first starting doing this sort of chemistry. I’m just glad that there was someone else in the lab to unwind me from the apparatus, is all I can say.