Photoreactors For All

Includes: SIAL
by: Derek Lowe

I've been hearing about this for a while now, so I'm glad to link to the manuscript (open access, now with a working link!) on a new photoreactor that should find wide use in organic synthesis. It was developed by the MacMillan group at Princeton and researchers at Merck, and it's now commercially available. It has higher-power LEDs than most homebrew setups, and the reaction chamber is optimized for exposure, so in many reactions it leads to shorter times and higher yields. Some of the differences are dramatic - 3 minutes versus an hour, 20 minutes versus 3 hours, etc. A variety of transformations are shown to improve: N-arylations, alkyl decarboxylative coupling, fluorination, trifluoromethylation, and more.

I think that this is just what the photoredox field needs - an inexpensive, standardized piece of apparatus that people can just throw reactions into and not worry about. This should lead to more experimentation and reaction discovery, and there's a lot to discover. It's a young area, so there are still things being worked out: for example, one of the reactions (cross-electrophile coupling of alkyl halides and aryl halides) actually seemed to work more poorly in the new reactor. But the team found that the problem was that the reaction was generating HBr faster than the inorganic base (sodium carbonate) could neutralize it. Switching to a soluble base (lutidine) and turning down the intensity of the lamps led to a faster, higher-yielding reaction than the literature comparisons.

The ACS Pharma Roundtable companies have been evaluating this system, and have endorsed it as a standard to improve reproducibility of published procedures. Penn Optical Coatings is commercializing the device (and is taking orders for it), and I'm told that Sigma-Aldrich (NASDAQ:SIAL) will also be selling the units. If you haven't run one of these photoredox transformations, try it out and get ready to make bonds in ways you never have before.