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The Problem with Oxygen

“Development of safe and scalable continuous-flow methods for palladium-catalyzed aerobic oxidation reactions” Ye, X.; Johnson, M. D.; Diao, T.; Yates, M. S.; Stahl, S. SGreen Chemistry, 2010, 12, 1180-1186.  DOI: 10.1039/c0gc00106f

We’ve had a pair of posts recently about using oxygen as an terminal oxidant in cross-coupling and biomass degradation, and as a green oxidant, it’s pretty hard to beat.  So I was a little surprised to learn that of the many cool aerobic synthetic methods that have been developed in the last decade, very few are used in industry.  The big drawback, especially on large scale, is safety – oxygen is usually the limiting reagent in the combustion reaction, and things can get pretty crazy when you have an oxygen-enriched atmosphere (and much crazier with liquid oxygen – check out this awesome video, and this one that Marty had in his last post).  So while stirring 100 mL of toluene under a balloon of pure oxygen might be fine, doing the same thing with 100 L is problematic.

This setup doesn't scale up very well

Safety aside, these reactions suffer because proper gas-liquid mixing is more difficult to achieve as you scale up.  All of this prompted a collaboration between Eli Lilly and Shannon Stahl‘s lab to develop a scalable continuous-flow method for aerobic alcohol oxidation, which avoids these problems. (more…)