I noticed this paper when it came out, and filed it in my “Useful Lab Tricks” folder. Organic chemists who have used triphenylphosphine-driven reactions will want to do so, too. For the non-chemists in the crowd, triphenylphosphine (TPP) is a cheap, crystalline substance that is pretty easy to oxidize to triphenylphosphine oxide (TPPO), a noticeably lower-energy species. There are a number of reactions that take advantage of those thermodynamics, setting things up so that the TPP-to-TPPO reaction is part of the mechanism and letting that drive the whole thing forward. It’s not exactly the most atom-efficient way to go, but it gets the job done at the bench. (Process chemists will make a little mark next to your name if you suggest doing this on scale, though, so be warned).
The oxide is an even more crystalline compound, but there are problems. It’s soluble to some extent in a number of common organic solvents, and there tends to be a lot of it around in a reaction that’s using triphenylphosphine in this fashion. The classic Wittig reaction throws off a lot of triphenylphosphine oxide, as does the Mitsunobu reaction, and you can end up trying to purify your product out of a big mound of white stuff. Sometimes it’s straightforward, and sometimes it’s a real pain. It can form very nice crystalline complexes with other organic molecules, which can help you out if you want to grow those, but you can lose material that way if you don’t.
What this new paper has found is a straightforward way of causing all the TPPO to fall out as an even more definitively insoluble mass. Treatment with zinc chloride apparently forms a complex between the two that crashes out of most everything, and I can tell you that I’ve had a few occasions where that knowledge would have come in pretty handy. I wondered at the time how this was discovered, but then I came across this story (courtesy of Chemjobber on Twitter).
The inventor of this trick is 85-year-old Donald Batesky, who used to work at Kodak back in the 1960s doing custom synthesis for non-catalog orders, work that will expose you to a lot of different kinds of chemistry, for sure. After Kodak, he worked for Aldrich (along with some other ex-Kodak chemists) doing contract synthesis for them in rented lab space in Rochester. So it’s safe to say he knows his way around a synthetic lab; by this point he’s seen it all. And now he’s on his third lab career at the University of Rochester, helping out as an assistant in the chemistry department. That’s where he remembered some metal-complex work from back when and hit on the zinc chloride idea.
Long experience at the bench, if you’re any good at all, gives you a fund of knowledge that’s hard to pick up from the literature. What solvents to try first for a crystallization, what reactions have exothermic inductions that you have to watch out for, which reducing agents have the easiest workups, how to get rid of metal contamination in the final product, when to deoxygenate rigorously and when not to worry about it so much, ways to get rid of Common Byproduct X or Pesky Solvent Y. Donald Batesky has just contributed another one of those to the trade, and good for him. This guy is clearly a chemist (he’s already stipulated that he wants to be buried in a lab coat), and I would very much enjoy sitting down and talking with him. It’s always worthwhile to listen to people who are really good at their work or to watch them do it, and I’m glad to hear that he’s still sharing his knowledge with his co-workers at Rochester.