Chemicals In Capsules Make Synthesis A Snap

Anyone who’s ever had to use a glove box to handle air- and moisture-sensitive reagents knows the experience is something akin to playing piano in boxing gloves. To make it easier to work with such compounds, chemists at MIT have devised paraffin capsules that can hold premeasured doses of chemicals and be dropped into reaction vessels outside the glove box (Nature 2015, DOI: 10.1038/nature14654).

The group, led by chemistry professor Stephen L. Buchwald, was inspired by work from University of Delaware chemistry professor Douglass F. Taber. More than 10 years ago, Taber invented a uniform dispersion of oxygen- and moisture-sensitive reagents such as potassium hydride in paraffin wax that can be added to solutions. When heated, the wax melts, releasing the reagent into solution. The paraffin doesn’t interfere with reactions and can be removed from mixtures via filtration and chromatography.

But when Buchwald and coworkers tried a similar approach with reagents for an aryl fluorination reaction, they weren’t able to get a uniform dispersion of the chemicals in the wax. That’s when they hit upon the idea of creating a capsule with preloaded amounts of reagents.

To create the capsules, one simply dips the end of a glass stir rod into melted paraffin several times to create a hollow tube with one sealed end. Once the open-ended wax capsule cools, it can be removed, loaded with reagents, and sealed at the other end with a heated metal spatula, describes postdoc Aaron C. Sather, who came up with the capsule-creating technique.

At the moment, the method doesn’t entirely unchain chemists from the glove box. After all, the reagents must be loaded into the capsules in an inert atmosphere. But Buchwald envisions that such capsules will eventually be commercially available. “It simplifies things, and it prevents a lot of waste,” he says.

“It is particularly exciting that some of these capsules have been shown to maintain activity for many months under ambient conditions,” Taber comments. Indeed, the MIT chemists demonstrate that the capsules can sit on a benchtop for more than a year in some instances and can even be submerged in water overnight without losing their potency.