Here’s one to add to the “weird mechanosynthesis” pile. According to this paper, you can do hydrogenation reactions in a stainless-steel ball mill, without any sort of noble-metal catalyst. The hydrogen is produced when you add some n-alkane or diethyl ether to the mix (these actually get converted to gaseous methane and hydrogen under the milling conditions). Some pretty severe reductions are realized, for example, taking biphenyl all the way down to 1-cyclohexylcyclohexane. Olefins go to alkanes under these conditions, of course, and ketones and alcohols tend to just get erased: it’s a bulldozer.
And although it’s surprising, this reaction isn’t quite as much voodoo as it might appear. Well, not more than any other hydrogenation, anyway. The ball mill is, of course, a sealed vessel, and the authors show that the lower-boiling additives seem to be more effective, presumably because they increase the internal pressure. And it’s not like there’s no catalyst present, because you have the stainless steel. If you use zirconium oxide balls instead of the steel alloy ones, for example, you get no hydrogenation at all. Adding bits of different metals back in showed that the chromium is what’s producing the hydrogen from the alkanes or ether, while the nickel is acting as the hydrogenation catalyst itself. The stainless-steel alloy itself (304 steel) was the most effective combination, though.
The balls do get eroded a bit under the conditions, so what you’ve got is finely divided metals, under some heat from the mechanical energy of the milling, and under hydrogen pressure from the decomposition of the co-reactants. No wonder things get reduced! But it’s not a reaction that you would have predicted up front. And even if you’d stipulated that hydrogenation would start under these conditions, I don’t think you would have guessed how powerful it would be. The last time I hydrogenated down a whole aryl system (it’s been a while), I was running at something like 1600 psi (110 bar) at 120C with a rhodium catalyst, and the stuff came out looking – and smelling – like old lawnmower oil. This reaction probably doesn’t get above 60 degrees, by contrast. Admittedly, there’s no word in the paper about the appearance or aroma of the crude products, but I’m willing to bet that that part remains the same. . .