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More Stahl Aerobics

“Highly Practical Copper(I)/TEMPO Catalyst System for Chemoselective Aerobic Oxidation of Primary Alcohols” Hoover, J. M.,; Stahl, S. S.  J. Am. Chem. Soc. 2011. ASAP. DOI: 10.1021/ja206230h

To quickly follow up yesterday’s post on aerobic alcohol oxidation, I thought that this new paper from the Stahl lab on the same topic was worth mentioning.  While their continuous flow process for alcohol oxidation was a pretty big improvement over many existing methods, the reagents necessary were not ideal.  Toluene and pyridine are both toxic, and palladium is not extremely abundant, especially compared to 1st row transition metals.  So there was plenty of room for improvement, which is why I was really psyched to see this new catalyst system for primary alcohol oxidation that was published a few days ago. Virtually all of the reaction components have been replaced by greener reagents:  acetonitrile instead of toluene, N-methylimidazole instead of pyridine, and catalytic TEMPO/(bpy)Cu(I) instead of palladium acetate.  Unlike most aerobic alcohol oxidations, an atmosphere of pure oxygen was not necessary – the oxygen present in ambient air was enough for the reaction to run efficiently.  And the reaction is run at room temperature to boot.  It’s hard to imagine that this reaction would be more difficult to scale up using their flow reactor than the Pd-catalyzed version, although you never know I suppose.

There’s loads more in the paper on their catalyst development studies, and on the chemoselectivity of this process for primary alcohols versus secondary ones – definitely worth reading!

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…)

Oxygen, Nature’s Oxidant for Nature’s Feedstocks.

“Selective catalytic conversion of biobased carbohydrates to formic acid using molecular oxygen”R. Wolfel, N. Taccardi,  A. Bosmann, P. Wasserscheid, Green Chemistry, 2011, DOI: 10.1039/c1gc15434f

Graphical abstract: Selective catalytic conversion of biobased carbohydrates to formic acid using molecular oxygen

All of us have a very personal relationship to the oxidizing power of oxygen. We use oxygen to turn our food into energy, CO2 and water. There are a number of enzymes and pathways that aid this process, each aiding the reaction of food and oxygen toward the creation of CO2 and water.  Now the key to turning complex biomass into usable small molecules is the ability to control this reaction so that we can extract usable chemical building blocks without ending up back at CO2 and water. As you can see in this video over-oxidation can be a real concern.  This paper demonstrates the use of a polyoxometalate (POM) catalyst to promote the oxidation of biomass to formic acid.

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Learning from past mistakes – Dioxins

“Dioxins: An Overview and History”  Hites, R. A.  Environ. Sci. Technol. 2010, ASAP.  DOI 10.1021/es1013664

One of my main reasons for drinking the green chemistry kool-aid is that I believe it allows me to still do cool chemistry as well as to contribute to building a more sustainable civilization (at least that’s the idea).  The focus, in my eyes, is on the positive things you can do with chemistry.  However, as the Spanish-American philosopher George Santayana first said, “Those who cannot remember the past are condemned to repeat it.”  With that in mind, this post is about one of the most well-studied mistakes in the history of the chemical enterprise, polychlorinated dibenzo-p-dioxins (PCDDs), often referred to collectively in the literature as dioxins.  In Ronald Hites’ recent ES&T feature article on dioxins, he tells the story of the most toxic of the 75 dioxin congeners, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD).

The most sinister aspect of dioxins, from a chemists’ perspective, is the (more…)