Well, here I was the other day going on about automated chemistry when this paper was waiting in my RSS feed. It’s from a group at Pfizer, and they’re using an automated microscale flow chemistry rig for reaction optimization. Inspired by this work from Merck, which demonstrated evaluation of 1536 reactions in a plate-based system, this new paper moves from microscale batch mode to continuous flow.
The key objectives were to develop a fully automated system for HTE screening with flow chemistry technology that would (i) integrate inline high-resolution LC-MS analysis for real-time reaction monitoring; (ii) use diverse volatile and nonvolatile solvents; (iii) use ~0.05 mg of substrate per reaction to enable broad parameter space exploration with minimal material consumption; (iv) enable the preparation and analysis of up to 1500 reaction segments in a 24-hour period; (v) establish the capacity of the platform to directly scale up preferred conditions via multiple injections to produce 10- to 100-mg quantities of a specific compound; and (vi) show translation of nano-HTE conditions to both larger-scale batch and flow synthesis.
Objective met, it looks like. They’ve worked up an ingenious system to assemble five-component mixtures (two reactants, a metal catalyst, a ligand for it, and a base) for evaluating Suzuki-style coupling conditions – the five are combined in concentrated form and then injected into a carrier solvent (which can be varied as well), and checked at different temperatures. Injecting in a bolus like this also gives you the ability to send the next one in after a suitable delay, instead of waiting for the first one to get all through the tubing. Five or six dimensions gives you a lot of potential variations, which (unfortunately!) is just what you need for metal-catalyzed couplings. On the plus side, it’s long been a belief of mine that any such coupling can be optimized to a high yield if you’re just willing to spend enough of your life doing so. This setup takes that idea and runs with it.
It’s not a cheap assembly – there are two Agilent systems waiting at the far end of the flow apparatus, to handle the reaction bolus injections that are coming out every 45 seconds. The whole idea took a lot of careful validation as well, to figure out what concentrations to use for the injection stocks, what solvents to have them in, their behavior on hitting the carrier solvent and being diluted by it, the extent to which the injections would spread out as they came down the reaction tubing, the efficiency and accuracy of the LC/MS analysis at the far end, and so on. Without all this groundwork, though, it would have been easy to use the fancy robo-rig to generate pile upon pile of crappy, hard-to-reproduce data, which is a temptation that has to be avoided. Measure fifty-three times, cut once, as the old saying goes.
They ended up screening a total of 5760 reactions, which evaluated Pd couplings across eleven ligands and seven bases (organic and inorganic), all in four different solvents, for most combinations of a matrix of four electrophile and four nucleophile partners. As mentioned above, the machine ran 1500 reactions per 24-hour period, on a 0.4 micromole scale per reaction. A heat map of the results are shown, although don’t feel as if you have to work your way through all of it. You can see immediately, though, that there are some combinations that have a much better success rate than others. Xantphos, for example, seems to consistently underperform the “No ligand” control category for these transformations, whereas good ol’ triphenylphosphine is your friend. The 6-chloroquinoline is a tough customer. The trifluoroborate partner (2c) is just not reactive enough under these conditions, unless you use methanol (probably because it has to hydrolyze before it reacts?).
It’s possible to plot these conditions out in several different ways, naturally – the authors show, for example, that if you’re looking for conditions that always seem to deliver product, no matter the combination, then X-Phos or S-Phos in acetonitrile is the way to go. So if you’re going to turn around and set up a big parallel synthesis run, you’ll be very glad to have scouted all these things to improve your success rate. The paper shows that they could use the exact same conditions (but just injecting the same combination over and over) to provide 50 to 100mg of a specific product within 75 minutes, and translating the same conditions to a larger traditional batch reaction worked just as expected. (They tried a few winning conditions and a few losing ones in batch, actually, and the trend held up every time, which is encouraging).
Interestingly, the group then turned to Pfizer’s traditional parallel synthesis efforts, taking a particularly challenging aryl bromide/pinacolborane combination and optimizing it (the standard parallel conditions for such couplings had given no product at all). Running 576 different combinations across 8 hours showed that there were painfully sharp cutoffs in the reaction landscape. Only two catalysts seemed to give any product at all: CataCXium A was unusually effective, particularly in in THF/water, AmPhos showed some reactivity, but the rest of the catalysts (including the XPhos and SPhos combinations in the main screening run) were completely useless.
That is indeed metal-catalyzed coupling as I have experienced it, and until we get an utterly thorough understanding of the reaction details – don’t hold your breath – the only way to deal with this situation is this sort of brute force experimentation. And this is the best brute-force technique I have yet seen. Believe me, setting up five hundred and seventy-six Suzuki couplings in a row is not fit work for a human being, not when there’s a machine that can do it instead. Great stuff.